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FUEL PRESSURE ENGINE OFF 40 DROPPED (10 PSI AFTER SITTING FOR AN HOUR) ENGINE RUNNING 32
MANIFOLD VACUUM IS CHECKED IN SEVERAL PLACES AND I GET 20" OF VAC
SMOKED CHECKED FOR LEAKS AND NONE WERE FOUND. PLAN ON CHECKING AGAIN (don't know if it is the intake manifold as I read it does not show up in a smoke test.
BYPASSED EVAP SYSTEM WITH NO CHANGE.
ADJUSTED THE TIMING. NO TIMING POINTER SO DID IT BY SOUND/FEEL. BETTER BUT PROBABLY STILL NOT RIGHT.
CLEANED ALL GROUNDS AND FUSIBLE LINK POST
PULLED THE MAF WIRE AND THE ENGINE DIES, NOT PROPERLY CHECKED
TOOK THE WIRE OFF OF THE IAC AND THE IDLE WENT TO 1600 RECONNECTED IDLE DROPPED TO 650
NO ENGINE CODES
Last edited by larryhobman; 09-06-2019 at 09:46 AM.
Take a look at my bad video with the hesitation and pressure readings and let me know what to check next.
It seems to idle fine, but I can tell the hesitation once the throttle is opened up that you are concerned about. Here are some thoughts:
Originally Posted by larryhobman
ADJUSTED THE TIMING. NO TIMING POINTER SO DID IT BY SOUND/FEEL. BETTER BUT PROBABLY STILL NOT RIGHT.
Did you do it with the EST disconnected? Why do you not have an timing pointer? You should... do you mean you don't have a mark on your balancer or do you not have a timing tab on your timing chain cover?
Originally Posted by larryhobman
MANIFOLD VACUUM IS CHECKED IN SEVERAL PLACES AND I GET 20" OF VAC
This is right where it should be or a bit strong. Probably eliminating any leaks around the manifold. What is your Idle RPM?
Originally Posted by larryhobman
FUEL PRESSURE ENGINE OFF 40 DROPPED (10 PSI AFTER SITTING FOR AN HOUR) ENGINE RUNNING 32
This is in range. Your gage isn't the most accurate thing in the world, most aren't and +/- a few PSI isn't going to make or break a mostly factory engine. Looks like it said over 40 when the pump first came on. That is KOEO pressure, but what is you idle pressure with the vacuum line removed?
Originally Posted by larryhobman
PULLED THE MAF WIRE AND THE ENGINE DIES, NOT PROPERLY CHECKED
This should not happen. The car should run in a Pseudo Speed Density mode where it using set tables based off RPM and Throttle position. Won't run ideal, but it should run and get you by.
Originally Posted by larryhobman
BOSCH 3 INJECTORS
Known issues with Bosch III's. Some will sing their praises, some report they need tuning. Mostly what I have read is voltage offsets and Constant. What is the number on the injectors BTW? Bosch III's have different flow ratings, they are not all the same.
Originally Posted by larryhobman
THROTTLE POSITION SENSOR VOLTS .58 TO 4.8V TESTED
After you changed all the components and checked the TPS, have you done the IAC reset procedure to set minimum idle and then make sure you TPS voltage is correct?
Did you do it with the EST disconnected? Why do you not have an timing pointer? You should... do you mean you don't have a mark on your balancer or do you not have a timing tab on your timing chain cover? yes the EST was disconnected when set. No timing pointer and I have not looked at the dampener well enough to find the markings. none found yet. Idle is about 650-700 and the fuel pressure with the VAC line removed is higher (raining out today, will have to check the actual number tomorrow)
What you are saying with the MAF it should run connected or disconnected, so I might have a bad MAF. Wish I had one around to swap and test.
NO ENGINE CODES show up.
I have not done a IAC reset (I did clean out the port when the throttle body was off). Sorry what is BLM's at idle??
Last edited by larryhobman; 09-06-2019 at 09:45 AM.
Did you do it with the EST disconnected? Why do you not have an timing pointer? You should... do you mean you don't have a mark on your balancer or do you not have a timing tab on your timing chain cover? yes the EST was disconnected when set. No timing pointer and I have not looked at the dampener well enough to find the markings. none found yet. Idle is about 650-700 and the fuel pressure with the VAC line removed is higher (raining out today, will have to check the actual number tomorrow)
What you are saying with the MAF it should run connected or disconnected, so I might have a bad MAF. Wish I had one around to swap and test.
NO ENGINE CODES show up.
I have not done a IAC reset (I did clean out the port when the throttle body was off). Sorry what is BLM's at idle??
Please check the Vac off Fuel pressure. You should see similar to the KOEO pressure except the fuel pump will actually still be running. Looked like your gage initially shot up to about 43 and then dropped to 40 after the prime cycle was complete. Just want to verify, but doesn't look like fuel pressure is your issue.
I have never worked on a TPI corvette, but all my TPI F-Bodies had the timing tab on the cover and timing mark on the hub. Not sure how you can set the timing without it. Possibly there is a way I just haven't done.
Typically, if you have a bad MAF, it will run unplugged just fine, but then have issues plugged in. This is not a definitive test that nothing is wrong with the MAF, but a fairly good indicator.
You need a way to see ECM data if you want to work on these cars and not just throw parts at them or do trial and error. The IAC counts will tell you how far open the IAC is to hold Idle RPM. I like to see them below 50 at idle warm, but lower is better. You do not want to see below 10 IMO because then the ECM is not really controlling the idle, its just shutting the port off and you are running off the throttle set screw. BLM is a what the computer uses to increase pulse widths based off of feed back from the 02 sensor... IIRC it actually stands for Block Learn Multiplier (that may be wrong) but when it sees lean, the number goes higher to add fuel, when it sees rich, the number drops to pull fuel. 128 is the sweet spot or Stoichiometric Air/Fuel mixture.
I would suggest you do an IAC reset procedure and see if it helps. I think your RPMs may be a little low. Do you have the same hesitation if you were to lock it in 2nd gear and hit the throttle at 1500RPM? Not enough to cause a down shift to 1st, just enough to see how quick the engine reacts. Without knowing the BLM and IAC its hard to pinpoint where you should be looking.
If you break it down, you have Air, Fuel, Spark.
You know the ECM has control of the Air based off the IAC counts
You know the ECM has control of the Fuel based off the BLMs
You know the ECM has control of the timing when your base timing is set correctly as it expects. If it is too far out off base the ECM won't really have control of it because it advances based off the expectation of the base timing being set correctly.
I use my Laptop with WINALDL or TunerPro RT (Both free software) and a USB to ALDL cable from MOATES.net (~$85). Lets you see what the ECM sees, read codes, and TunerPro can be used to burn chips.
In my 85, if I disconnect the MAF with the engine running, it stalls. I can restart it, but it runs like pooh. Totally undrivable. I know that the perception is that disconnecting the MAF puts it in limp mode, and should run well enough to drive. But that is not the case with the car I've owned since new, and have had MAF issues with.
Never experienced that.... 7 different MAF TPI cars over the years. All would run disconnected. Not saying they ran great.
Read my post again. I did NOT say they would not run disconnected.
Originally Posted by ihatebarkingdogs
In my 85, if I disconnect the MAF with the engine running, it stalls. I can restart it, but it runs like pooh. Totally undrivable.
Originally Posted by ihatebarkingdogs
I said mine stalls if disconnect with engine running.
I CAN restart it.
It does not run well enough disconnected to drive. (The perception is that they do, and this is a valid diagnostic technique. In my experience, it is not.)
Stalls/would not run/does not run well enough to drive - Splitting hairs a bit Whatever... Mine were able to run and drive and by design they are supposed to... all this is completely irrelevant to this thread anyway. If you want to question the validity of unplugging the MAF, start a different thread.
re-read what I posted:
Originally Posted by KyleF
Typically, if you have a bad MAF, it will run unplugged just fine, but then have issues plugged in. This is not a definitive test that nothing is wrong with the MAF, but a fairly good indicator.
I didn't tout that as an exact test either. MAFs are actually very hard to test without some form of specialty scope TBH.
Originally Posted by ihatebarkingdogs
The ECM "resets" the IAC every time the key is turned to RUN.
It sends enough counts to completely close the IAC, then opens it a predetermined number of counts to facilitate starting.
What is this "reset" procedure you're advocating? The ECM does it every time you start it.
Set base timing to 6*, Force ECM to close IAC, diconnect it, set minimum idle speed with throttle screw, adjust voltage from TPS, plug IAC in, check IAC counts. This is not the detailed procedure... details are missing.
I am not advocating, it is something that should be done whenever the IAC is unplugged, replaced, or removed for cleaning. You want the IAC in complete control of idle speed the ECM is calling for.
The ECM can't do this every time it is turned to run (obviously)... and what you are talking about is not the "Reset" the ECM performs on the IAC anyway. It resets itself during cruising when it can completely close the IAC and know where "closed" is so the pre-determined steps open can be used at the next start.
I pulled the plugs today and compression checked. Average number was 162PSI +/- 3psi and I see that it is very rich running. Ordered some new plugs today and look deeper into the IAC and MAF. Let me know what else to look at. How often is it the MAT sensor if at all. Ordered the right plug too as these are wrong. Ac Delco R43TS
What I heard when listening to the video was an engine "hunting". After seeing those plugs I am almost sure of it. The problem might be your Oxygen sensor but you said you replaced it. When an O2 sensor starts to fail the signal gets weaker and does not change fast enough for the computer. For the L98 to run in "Closed Loop" you need the following three parts:
1. Oxygen sensor
2. MAF sensor
3. Coolant Temperature Sensor
Those three are the ones who allow you to go 60 mph with a smooth ride and no bucking or hesitation. Any one of them can make you not happy with your Corvette. You need to check to be sure that your O2 is operating properly. If the O2 was working properly the plugs should be tan/brown. The O2 works very fast when new and their signal is wider as well. As they age the make a lot of problems so I replace mine before they die on the road. They told us to replace all O2 sensors every 2 years or 50,000 miles when I was working on them.
In my experience I was taught to replace all eight injectors with a matched set for flow and resistance. Only on a diesel have I replaced individual injectors. You might have a leaky injector still.
When you rebuilt the EGR did you verify it was working when you were done replacing parts? Did the EGR Vacuum Solenoid still work? My EGR solenoid is under the thermostat cover nut on the front of the engine. The EGR needs to have a good source of vacuum and it needs the vacuum solenoid to operate to turn it on and off. The power controlling the EGR is PWM so it is harder to see with a voltmeter. When my EGR failed I had a idle that wandered and other weird things that were happening. It was plugged with carbon before I took it apart and afterwards you can test the EGR using a vacuum pump to pull the diaphragm up.
After seeing those plugs I would not be surprised to see a partially or fully plugged catalytic converter on your engine as well. You can check for this by removing your O2 and see what is happening. It should be steady and pulling air into the exhaust, if blowing out the hole then you have a restriction somewhere in your exhaust system. The catalytic converter used on these cars was not intended or designed to last the life of the car. I put a cat back exhaust on my C4 and installed a new Hi Flow catalytic converter and gained almost 14 hp for the three hours of work. The newer catalytic converters flow so much better than the old ones so they help performance.
If your coolant temperature sensor went bad and thought it was really cold outside the car would be getting additional fuel due to the temperature being incorrect. This would also explain why it runs rough and stumbles as it is way too rich. Check your CTS and compare it to the chart from the FSM to see if your CTS is sending the proper temperature. If the CTS failed the other direction and thought the car was HOT all the time it would be harder to start cold as the car would not add any "Choke" to the mixture.
In your case the CTS might be stuck thinking it is colder than it really is and that might help explain some of the behavior. That would make the car run richer than it should and certainly explains the plugs you showed us. Check the output and verify it is working or replace it, it is fairly easy to change out. There are three wires going to it and be sure they all have good clean connectors.
BTW I see that your plugs had no anti-seize on them, that is a place you can save yourself a big headache by putting a bit of anti-seize on any fastener going into a dis-similar metal. Always use anti-seize on spark plugs on aluminum heads or you will find out why. I use anti-seize on my spark plugs in my cast iron heads even. For those of us who like having the right chemicals when you need them Champion Spark Plug Company makes a thinner anti-seize without as much graphite in it as the over the counter type. It is thinner but works great.
I currently have almost exactly the same problem with my 86 but the hesitation is worse. I've replaced everything you've done along with a few more items and the car idles super smooth but still has a super bad hesitation. Mine has 34k miles on it and is all stock. It also had the wrong plugs in it which looked a little lean but I would expect that when running a much shorter plug. With a new fuel pump, filter, fuel pressure regulator and relay my fuel pressure does not increase from 32psi when I remove the vacuum from the fuel pressure regulator or when I accelerate hard. No codes in mine either. Took me 4 MAF sensors before I got a good one so who knows if parts I've installed are good?
I currently have almost exactly the same problem with my 86 but the hesitation is worse. I've replaced everything you've done along with a few more items and the car idles super smooth but still has a super bad hesitation. Mine has 34k miles on it and is all stock. It also had the wrong plugs in it which looked a little lean but I would expect that when running a much shorter plug. With a new fuel pump, filter, fuel pressure regulator and relay my fuel pressure does not increase from 32psi when I remove the vacuum from the fuel pressure regulator or when I accelerate hard. No codes in mine either. Took me 4 MAF sensors before I got a good one so who knows if parts I've installed are good?
I was wondering if I put an adjustable pressure regulator in to maintain a running pressure of 43PSI my problems would go away. I do not want to buy 4 MAF like you did. I do not want to keep throwing parts at it either. I did just order a IAC and coolant temp sensor as both were cheap to do and with the throttle body off it made sense to do both. I hope my problems are fixed with this. If not I was thinking of adding like 2 thick washers inside of the pressure regulator to make it work like the adjustable one. Adjustable ones just have a screw that pushes the spring down. Has anyone done this, let me know.
Is the fuel pump receiving full battery voltage? If there is corrosion in your electrical system it can cause havoc. When trying to figure out my own 1988 I was surprised to see the low voltages everywhere downstream of the fuse panel. The fuse panel was not seeing the proper voltage either and that is when I discovered the post with the fusible links. After testing each fusible link and cleaning the connectors the battery voltage re-appeared at the fuel pump relay and actually at the pump it self the voltage was very close to battery voltage.
Before modifying parts to work makeshift lets try and fix it correctly. Have you checked out the CTS? Is it accurate or not? If it thinks that it is cold then you will burn a lot more fuel.
When diagnosing a problem you need to be somewhat methodical and You are getting pointed in many directions that are not causing the primary problem. You mentioned that some of the emissions system equipment was bypassed, By whom? The fact that your car dies when the MAF sensor is unplugged indicates another problem.
Like mentioned above by others I don't think that the fuel pressure is your problem. I would not stay focused on it very long. Your fuel mixture is off by a serious margin, the plugs were way too black. What controls the amount of fuel? The O2 does and they wear out and fail, don't wait till they fail to replace them. I bought a MAF and it has worked ever since I put it on the car. Since the MAF or the MAP both measure the volume of air ingested by the engine, any leaks downstream of the MAF will make a mess out of your car's drive-ability as the airflow is not accurate anymore. MAP measure the number of GRAMS of air going into your Engine. Whenever a car has bucking, surging or just starts to miss under a heavier than normal load.
Check the basic stuff first and then go from there. No throwing parts at it. Just take your time and go measure the CTS and verify that it is accurate. I really do think that the CTS is likely to be a big player in your Corvettes problems. Please check the big three like I asked earlier and let us know the answers. These cars are fairly simple things to work on, it gets worse when you have multiple items working against you.
The car running the way it is will damage the catalytic converter. The catalytic gets plugged easily and then you have another problem. Why is it running so rich like the plugs suggest if you really believe that the fuel pressure is the problem? Too little fuel the car would run leaner. If only one or two plugs were black I would suggest a leaky injector/s. Your mixture is not being adjusted properly because I believe the CTS is off and thinks it is colder than it really is. It is easy to find out with a decent multimeter in a matter of minutes. If you don't have the cross reference chart I can try and post one for the L98 C4 Corvettes.
Is the fuel pump receiving full battery voltage? If there is corrosion in your electrical system it can cause havoc. When trying to figure out my own 1988 I was surprised to see the low voltages everywhere downstream of the fuse panel. The fuse panel was not seeing the proper voltage either and that is when I discovered the post with the fusible links. After testing each fusible link and cleaning the connectors the battery voltage re-appeared at the fuel pump relay and actually at the pump it self the voltage was very close to battery voltage.
Before modifying parts to work makeshift lets try and fix it correctly. Have you checked out the CTS? Is it accurate or not? If it thinks that it is cold then you will burn a lot more fuel.
When diagnosing a problem you need to be somewhat methodical and You are getting pointed in many directions that are not causing the primary problem. You mentioned that some of the emissions system equipment was bypassed, By whom? The fact that your car dies when the MAF sensor is unplugged indicates another problem.
Like mentioned above by others I don't think that the fuel pressure is your problem. I would not stay focused on it very long. Your fuel mixture is off by a serious margin, the plugs were way too black. What controls the amount of fuel? The O2 does and they wear out and fail, don't wait till they fail to replace them. I bought a MAF and it has worked ever since I put it on the car. Since the MAF or the MAP both measure the volume of air ingested by the engine, any leaks downstream of the MAF will make a mess out of your car's drive-ability as the airflow is not accurate anymore. MAP measure the number of GRAMS of air going into your Engine. Whenever a car has bucking, surging or just starts to miss under a heavier than normal load.
Check the basic stuff first and then go from there. No throwing parts at it. Just take your time and go measure the CTS and verify that it is accurate. I really do think that the CTS is likely to be a big player in your Corvettes problems. Please check the big three like I asked earlier and let us know the answers. These cars are fairly simple things to work on, it gets worse when you have multiple items working against you.
The car running the way it is will damage the catalytic converter. The catalytic gets plugged easily and then you have another problem. Why is it running so rich like the plugs suggest if you really believe that the fuel pressure is the problem? Too little fuel the car would run leaner. If only one or two plugs were black I would suggest a leaky injector/s. Your mixture is not being adjusted properly because I believe the CTS is off and thinks it is colder than it really is. It is easy to find out with a decent multimeter in a matter of minutes. If you don't have the cross reference chart I can try and post one for the L98 C4 Corvettes.
This is all true and I was basically saying check the same stuff in a different manner. While you can probe under the hood, this does not always mean that is the same signal the ECM is getting. Before tossing parts or rigging things like the FPR... spend the $80 on a cable, grab an old lap top, and get some free software so you can see what the sensors are telling the ECM. It is good to know both, reading at the sensor and reading what the ECM is receiving. You can see the O2 reading and what it is causing the BLMs to do, you can see what the ECM thinks the coolant temp is, and you can see how much air the ECM thinks the car is getting.
The other one is, since it looks like you are running rich on your plugs, is the cold start injector staying open? The ECM does not control this. Unplug the cold start injector and see if the problem is still there. Imagine if your BLMs are low (Trying to pull fuel) but the 02 is still showing rich because your engine is indeed, actually rich?
The ground side of the cold start injector runs through its own coolant temperature switch that's located in the intake manifold below the thermostat.
The cold start injector turns on only when the ignition switch is in the start position and the coolant is cold, the temperature switch is closed and grounded which completes the electrical circuit. When the coolant is hot, the coolant temperature switch is opened.
I have actually seen a Corvette with it's cold start injector try to operate when it was 75* outdoors on a cold engine. It can happen but I would bet 80% of the cold start injectors are frozen up anyway. I had to buy a bunch to find a good working unit. Unless it is used regularly they go bad or rust up thanks to the ethanol in our fuel.
The additional enrichment fuel when the CTS thinks it is cold would be enough to cause the plugs to turn black all by itself.
KyleF,
I agree with your suggestion about getting a scanner or a PC program to allow him to monitor his engine. I recently bought a Snap On MT 2500 from another forum member and am still learning all it can do. It works on the older OBD1 and with OBD2 and the European CANBUS system as well. I am trying to find the right plug in for the Bosch ABS system diagnosis and bleeding. It allows such easy access once you learn how to do it. I am still planning a move to Tuner Pro so I can data-log and solve moving problems easier. The MT 2500 was a smart move and it is amazing how many people will ask to use it once they know it exists.
You are correct about the circuit of the cold start injector, it was an afterthought and you can tell when you look at the system itself. I have found many of the thermo switch's on the cold start circuit to be shorted closed causing it to be powered up all the time. Mine works well enough to be able to start a 20* L98 right up in the middle of the winter and settle into a nice fast idle. When it turns off the idle returned to 650 rpm or so.
I am still very suspicious of the CTS on this engine. A defective CTS would do precisely what he is seeing. A too rich mixture can cause an miss here and there at idle. Even dirty connectors can cause a CTS to be inaccurate.
What is the sensor in the front of the intake manifold just behind the thermostat housing? Has a vacuum line that goes back to the fuel pressure regulator or EGR I think. There is also a sensor on the under side of the plenum towards the distributor that I'm not sure what it is, has a 2 wire connector on it?
The EGR has a vacuum solenoid that is mounted on one of the ears of the thermostat housing. The EGR Vacuum solenoid in not mounted on the intake itself but on a plate that attaches to one of the thermostat housing nuts. The vacuum needs to be supplied to the EGR solenoid in order for the EGR to work. The ECM uses PWM to control the EGR so it is hard to see a steady voltage on the EGR solenoid. Once the EGR is activated by the two wires the vacuum will be directed to the vacuum Using the vacuum hose going towards the EGR you can put a hand held vacuum pump on it and test the EGR with engine running. The EGR Vacuum Solenoid also requires a two wire connector on it that the ECM uses to control the EGR. Be sure all connections are clean and tight.
I am not sure but I THINK the sensor you might be referring to is the Intake Air Temperature (IAT) sensor. I am not positive of this so lets wait for someone to verify or correct me on this one...
Check all your vacuum lines and if they are questionable, trims the ends of the lines. Be sure that your EGR has vacuum going to it or it won't work and the engine will get plugged up with carbon and have higher combustion chamber temperatures which makes for worse emissions. One leak that is a real PIA is the one where the brakes are de-activating the cruise control and there is a tricky vacuum line to replace due to its location.
Check the vacuum going to your Fuel pressure Regulator and be sure it is working properly. If you smell any fuel in this line you might have a faulty FPR.
Did I answer your questions regarding the sensor or are you describing the sensors on the intake manifold casting. On the front of the Intake manifold there is a themo switch for the cold start system with 2 wires, and there is the Coolant Temperature Sensor but that has three wires.
09-05-2019, 08:57 PM
1986 Low Fuel Pressure or Vacuum leak?
Not saying they ran great.
Whatever... Mine were able to run and drive and by design they are supposed to... all this is completely irrelevant to this thread anyway. If you want to question the validity of unplugging the MAF, start a different thread.
Then carry on... saw 86 and assumed Aluminum, you know what they say about assuming...
