V8 Stingray

I purchased my '89 Vert (L98 Auto) about a year ago and imported it from Ohio to here in Ireland. Since I got her she's been running a bit rough. On a fellow C4's advice I opted for the upgraded Bosch injectors and have since replaced the EGR, MAF, IAC, spark plugs, spark wires, Flamethrower Coil and Module, TPS, and nearly all of the engine sensors. The only one I havent replaced yet is the knock sensor and 02 sensor (have it in the box).

I've done the timing properly and its currently set at the base 6BTDC.

The car takes a few cranks for her to fire and when she does she runs rough - constant surging etc.When I drive you can also notice the surge and you can also feel a vibration coming through the car from the engine. Sometimes its really noticeable and its feels like the same sensation when you dive over rumble strips.

i've had it in with two mechanics and still no joy. Checked for vacuum leaks and replaced gaskets etc.

I purchased a USB OBD1 cable from 1320 electronics and figured I'd get TunerPro on it to see what was going on. I'm no expert so this is where I really need help as I've no idea what these figures mean. Log attached and
I was wondering if the knock sensor was bad, but based on the readings it seems to be running nomally. I see the erratic value of the 02 sensor - but I've no idea if these are "normal" readings so really looking for guidance. I did think the MAP sensor reading should have changed when I revved the engine - but it stayed static?

Oh, and I noticed something really weird... when I connect the OBD cable the car revs begin to surge to about 1100-1300rpm and when I pull it out it drops - I'm guessing this is not normal either!
Here is a link to the log from TunerPro when she is warmed up: https://1drv.ms/u/s!AqT2s3D1Ho7WkfI-...v0kaQ?e=61PRZC

thanks in advance guys!

V8 Stingray

Could anyone even refer me to a list of acceptable ranges for each of the readings so that it could help me identify a possible culprit?

ctmccloskey

Have you tested the fuel pressure regulator yet? Pull the vacuum line to it. If the lines smells of gasoline then that is part of the problem. If you smell gasoline you replace the fuel pressure regulator.

MAF sensors are frequently the cause of bogging in your TPI engine. I see that you replaced it but that means little in this world anymore with the high number of brand new defective units we find. Whenever you replace the MAF sensor you should replace the two relays that control it.

But then having the proper fuel pressure is critical to these cars. Have you verified that your fuel pressure is between 42 and 44 psi after the engine is primed? I assume you changed the fuel filter as well.

The O2 sensor is one, if not the most important sensor on your Corvette. Older oxygen sensors cause problems for the engine computer as they lag behind and slow down as they get older. That is one sensor you need to change out. The signal generated by the O2 is in the mV and as it ages the signal becomes weaker and lags. A faulty O2 can ruin the performance of a Corvette in a big hurry. The O2 sensor returns a signal of between .1 and .9 Volts and it oscillates back and forth very quickly and is hard to see without a scanner or your Tuner Pro.

Can you test your EGR valve for normal operation? If they get stuck because of carbon they can have an effect on your idle making it rougher. I test mine by using a vacuum pump on the hose connected to the EGR Valve body. When applying vacuum the diaphragm moves up and changes the idle speeds. The EGR is operated by a vacuum solenoid mounted near the thermostat. There are two wires leading to it to activate it. The wires have a pulse width modulation signal on it and are hard to read without proper equipment. Be sure the wires are secure on the vacuum solenoid and verify that there is a vacuum being supplied to the solenoid.

I don't know what fuel additives are available there in Ireland but I use something called B&G 44K as it is a very effective fuel system cleaner. It is available from Amazon if they ship to Ireland.

Do you have the Factory Service Manuals for your year Corvette? Having one of these is critical to a happy Corvette ownership. I assume that this is not your first Corvette? I saw the C3 in your Avatar...

Lets start here, this is enough to probably solve your less than smooth idle. Try a good fuel additive to clean out your injectors if possible.

Best regards,
Chris

KyleF

You are rich for some reason. BLMs should be near 128, I see you at 108 (pulling fuel).

Possible issue is your Bosch Injectors. Some have wanted to flame me on this, but some cars need tuned for voltage offsets at idle with Bosch 3's. If you want it fixed without a tune, buy the Dlephis. Go do some searching on Bosch 3s. It has seemed less people with Corvette L98's complained of issues, but in the TPI world, there is definitely data out there that suggest to expect idle issues and it looks like you are running rich!

Are you moving the throttle? I see your throttle position moving.

Idle goes up when it Diagnostic mode... completely normal. I have a MOATES cable with the switchable resistor. RPM goes up and down with the flick of the switch.

tequilaboy

TPS voltage at 0.67 volts is too high. This is preventing idle PID control and the extra airflow is causing engine to run in unstable region of spark advance table resulting in surge.

Readjust the TPS to approx. 0.54 volts. This will allow idle PID control again so that the IAC actually controls the idle rpm relative to target. This will reduce the idle rpm and IAC counts and should flatten the spark advance also.

BlowerWorks

Awesome knowledge !!!

VikingTrad3r

my bosch 3’s are perfect on my 86.

V8 Stingray

Guys,
Thanks so much for the replies!

I've had limited time to get near the car this week but here is what I've done to check based on the responses above.

1. Fuel Pressure Regulator - I don't have a fuel pressure gauge so I'm waiting on a loan of one. I did pull the vacuum lines and there is a smell of gas off the tube that sits on the regulator. I'm guessing this is pointing towards the fuel pressure regulator being bad. Assuming the readings are also not at the desired 40/45psi when I get the gauge what make of adjustable fuel pressure regulator would you recommend?
2. TPS reading - ok I think with my limited knowledge of the C4 and the TPI engine I may have got this wrong. When I replaced the TPS with a new one I followed a guide which didnt make reference to the need to disconnect the IAC valve and short the A/B pins on the OBD port - I merely set it when she was idle. Also, I went off the reading purely from the middle pin using my PowerProbe which read .54volts. I've read the article by Lars and I note it seems to suggest that the .54 volts is the value between the top and middle pins - am I getting that right? If so this probably explains why the reading from from my PowerProbe is .54 volts but TunerPro connected to the OBD port reads .67v !
3. TPS Adjustment - I tried Lars article to properly set the TPS but I could not get the car to run steady at the 350/400rpm rev range. I had to adjust the idle screw to get the revs up but she wouldnt stay running anything less than 700rpm. Also the idle was jumping up and down and very unstable. I'm guessing now the fuel pressure regulator being bad has to be fixed 1st before I attempt this again. I did note one member saying that the easiest way to set the TPS .54v value was to just use TunerPro and set the TPS to .54v when the IAC counts were between 15-20. Are they referring to the IAC Motor Position Steps value in TunerPro?
4. BLM Value - Could the fuel pressure regulator being bad be the cause of this? Kyle's comment is interesting above as I'm not too sure I'd rave about the Bosch injectors. I was getting a fuel leak on one of the injectors with new o-rings so I had to get collars made up for them so that they sat down properly without leakage.

I will probably need to redo/check the timing - if it turns out the fuel pressure regulator is the culprit should this be done before the TPS adjustment?


PowerProbe attached to middle wire of TPS connector.

ghoastrider1

Does a 89 use OBD 1 or 2 ? my 86 is type 1 I believe

KyleF

OBD1 until 96. That was the required year for all cars. However, GM in the corvette got a bit ahead and it's a bit of a Unique Child.

KyleF

Your IAC looks good at the RPM. I follow the procedure when I am setting mine - ESC disconnected, warm, and adjust throttle in P or N to about 750 or so. Jump in the car hold brake, drop in drive... and try to hit the 550-600. If not, put it in par, make adjustments, and then get in car to put in D again. I just don't trust the parking brake and someone setting in the car to do it all under the hood.

I use my laptop to read the RPMs acurately

After I have done this and plug it all back in, at warm idle I will make some final fine adjustments to the throttle screw. I like to see my IAC around 20 counts at a 750RPM idle warm. Last time I did this, I hit around 40 IIRC and just backed the screw out a bit more to end in the high 20s.

If you own a TPI, you need a fuel pressure test gage. Don't take anything apart until you have verified pressure and leak down. I understand you smell gas, but measure and confirm first. If you do need to replace it, I would suggest and AFPR from SouthBay so you can see if the car responds to a bit more FP.

V8 Stingray

Ok so got the fuel pressure tester and tried it out earlier today. Found 3 things, the 1st is that the fuel pressure is around 29psi with the needle fluctuating to a small degree. The 2nd is that when primed with the ignition on before start it doesnt look like the pump always kicks in to ensure 29psi before start. The 3rd is that it I spotted that the fuel pressure regulator is adjustable so I've upped the PSI to 39psi.



See 1st video below.


See 2nd video after adjusting the PSI to 39. The car seems to start and idle better initially, but then the lumpiness/erratic idle kicks in shortly afterwards


What should I try next?

tequilaboy

Record another datalog (at 8192 baud) idling in gear with warm/hot coolant temperature (to replicate the most common driving conditions).

This will document the desired idle rpm (vs coolant temp), current tps voltage setting, min air IAC position and help to determine if the rpm variation is due to the IAC (PID control), spark advance and/or fuel trim behavior, or due to a disturbance such as a misfire or voltage variation, or a combination of all factors.

Note: 39 psi is still a bit on the low side. Stock regulators often run 46-48 psi (no vacuum). Suggest you adjust fuel pressure based on the BLM data under steady state cruise conditions.

Deepa

I had an idle / surge at cold start. Ended up being a poor connection at the tps. Just double check all things.

these cars refresh so slow where my tech needed a breakout box to see the wiring issue.

V8 Stingray

Been a while since I managed to get to the 'vette to sort this issue and get parts, but this COVID crisis has given me a bit of time to play with her.

So today I did the following:

• I've set the timing properly (was set to 12deg BTDC) its now set at 6deg BTDC.
• I've set the the TPS properly to .54v (top wire value was .01v and middle wire was .55v). It was very hard to get the car to idle in 400rpm given the surge as she would cut out, but got it as close as I could.
• I replaced the fuel regulator with a new one (strong smell of fuel in the vacuum hose). Constantly at 42psi now.
• I replaced the O2 sensor with a new Bosch one.

The surge/idle is slighty better, but its still there! (arrggh!!!!). I took a video of readings with my OBD cable and TunerCat (Tuner Pro RT wasnt playing ball for me) - you can see the surge/erratic idle at 1:07 in the video.


I'm wondering if its a vacuum leak that's causing this so I'll get a can of WD40 tomorrow and stick the throttle at 1,250rpm and see if I get a surge when spraying.

So now, I've replaced all of the following in trying to sort this issue.

• 02 Sensor
• EGR Valve
• Engine Coolant Temp Sensor
• Mass Airflow Meter
• Throttle Position Sensor
• Air Intake/Charge Temperature Sensor
• PCV Valve
• Idle Air Control Valve (IAC).
• Pertronix Flamethrower Coil & electronic module

V8 Stingray

The new video above should show this, but how do you adjust the fuel pressure according to the BLM data?

krackenvette

One item that never gives you a code, because there isnt one, is the IAC. It has to be checked manually with an Ohm meter.. I fought idle and surging issues for a long time, till I change that damn thing.
FYI, the scanning software will show the IAC numbers changing as this is the ECM telling it what to do, not what it is doing as there is no feedback..

V8 Stingray

So guys I found out what the problem was and I'm embarrassed to say!!!

The previous owner had the plug wires going to the wrong cylinders to the firing order was all wrong..... It just goes to show you that sometimes it can be the simplest things!!!!

VikingTrad3r

good work. not the first time ive heard this! have fun with car!

krackenvette

Awesome. Now just think how much money you Saved. If you would have dropped this off at a stealership...