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Old 11-06-2012, 08:07 PM   #1
FletchUSMC
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Default 1987 L98 MAF Sensor Weirdness (long thread)

Hello all, have used and read this forum a lot since buying my 87. I have searched numerous threads between here and ThirdGen to try to figure out the problem I am having.

Here's what I can tell you about this car so far, L98 automatic, 141,000 miles. I have replaced plugs, wires, cap, rotor, coil, ignition module, fuel filter, and air filter when I bought it. The PO had no real history on the car so I did all that to know what I was starting with. I replaced the throttle body with a BBK 52mm after finding out on here how to check for a vacuum leak around the shaft, that cured the idle problem it was having. It has a 160* thermostat, A/C delete, I have removed the AIR system. Exhaust is factory manifolds and Y pipe, test pipe, to rear Y pipe and no idea what mufflers but stock looking. Air filter cover cut open and had a K&N filter when I got it.

From the beginning I had a SES light and a code 33. The car drove fine after the tune up and throttle body and I drove it most of the spring and summer like that. In August I decided to dig into the code 33. Went to local parts store and ordered a Microtech MAF. The car appeared to have the original Bosch MAF and original GM relays (both relays marked as 14089936). Installed the Microtech, started up and no more SES upon startup. Took her for a drive and 33 came back randomly over the next 2 months. Didn't come on at any particular temp, mileage, driving style, just whenever it wanted to. Right before the SES would come on I could just feel it in the car and then blink, SES is on. Still seemed to drive ok either on or off though.

Couple of months go by after I pulled the relays, ohm'ed them out, applied 12 volts, the contacts moved, ohm's out ok. Decided to go ahead and get new relays and made no difference, still SES at random times. Same weekend took the Microtech back and ordered A1 Cardone reman. Couple days later it came in and is a Bosch reman unit. Installed it, fired it up (on the original relays). Fired up ok in garage, let it run a bit but couldn't drive it due to weather. Next day drove it to work (about a mile away) and SES again. Started the car to come home for lunch and was wondering if I would make it, stumbling, backfire, almost die if given too much throttle. This time code 36. Cleared code, put the new relays back in. Didn't get a chance to really drive it due to relatives visiting and other stuff. Finally go to drive it, put in gear give little gas the car struggles, sounds like backfire through intake, runs like poo around the block. Pulled back in driveway, stared at it for a few idling like crap, smelling rich. Let it warm up some more, drives down the road just fine. Next day same thing, cold runs terrible, warm runs fine, but no SES light or codes even though it runs terrible cold. Disconnected the MAF and when it's cold she fires right up, runs fine cold or warm. Drove her about 20 miles Sunday with MAF disconnected runs like normal (with the SES light on).

I have printed the wiring diagram, chased every wire, all grounds are less than 1 ohm. Also check all wires for shorts to each other or ground. The only strangeness I can find by the diagram is pins C on the power relay and A on the fuel pump relay read just over 4 ohms to ground. The diagram I have shows these 2 wires tied together and to the fuel pump. Does this sound right reading through the fuel pump? Don't have a diagram for the fuel pump.

I know this is a really long post, I have been trying to figure this out for a while now. I know I will eventually figure this out, I've been an aircraft electrician in the Corps for over 19 years, schematics are no big deal, but I've never had to troubleshoot any fuel injection this far and this is the first Chevrolet I've ever owned. Couldn't pass it up when I bought her and it's been a fun project for my daughter and I. Would just really like to get this SES light off and stay off (and the car still run right!).

Bottom line, should I be getting about 4 ohms from the pins above to ground, did I get a faulty MAF, or any other ideas of where I need to look?

Thanks in advance for any help out there.
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Old 11-07-2012, 05:10 PM   #2
Corvette40
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Quote:
Originally Posted by FletchUSMC View Post
Hello all, have used and read this forum a lot since buying my 87. I have searched numerous threads between here and ThirdGen to try to figure out the problem I am having.

Here's what I can tell you about this car so far, L98 automatic, 141,000 miles. I have replaced plugs, wires, cap, rotor, coil, ignition module, fuel filter, and air filter when I bought it. The PO had no real history on the car so I did all that to know what I was starting with. I replaced the throttle body with a BBK 52mm after finding out on here how to check for a vacuum leak around the shaft, that cured the idle problem it was having. It has a 160* thermostat, A/C delete, I have removed the AIR system. Exhaust is factory manifolds and Y pipe, test pipe, to rear Y pipe and no idea what mufflers but stock looking. Air filter cover cut open and had a K&N filter when I got it.

From the beginning I had a SES light and a code 33. The car drove fine after the tune up and throttle body and I drove it most of the spring and summer like that. In August I decided to dig into the code 33. Went to local parts store and ordered a Microtech MAF. The car appeared to have the original Bosch MAF and original GM relays (both relays marked as 14089936). Installed the Microtech, started up and no more SES upon startup. Took her for a drive and 33 came back randomly over the next 2 months. Didn't come on at any particular temp, mileage, driving style, just whenever it wanted to. Right before the SES would come on I could just feel it in the car and then blink, SES is on. Still seemed to drive ok either on or off though.

Couple of months go by after I pulled the relays, ohm'ed them out, applied 12 volts, the contacts moved, ohm's out ok. Decided to go ahead and get new relays and made no difference, still SES at random times. Same weekend took the Microtech back and ordered A1 Cardone reman. Couple days later it came in and is a Bosch reman unit. Installed it, fired it up (on the original relays). Fired up ok in garage, let it run a bit but couldn't drive it due to weather. Next day drove it to work (about a mile away) and SES again. Started the car to come home for lunch and was wondering if I would make it, stumbling, backfire, almost die if given too much throttle. This time code 36. Cleared code, put the new relays back in. Didn't get a chance to really drive it due to relatives visiting and other stuff. Finally go to drive it, put in gear give little gas the car struggles, sounds like backfire through intake, runs like poo around the block. Pulled back in driveway, stared at it for a few idling like crap, smelling rich. Let it warm up some more, drives down the road just fine. Next day same thing, cold runs terrible, warm runs fine, but no SES light or codes even though it runs terrible cold. Disconnected the MAF and when it's cold she fires right up, runs fine cold or warm. Drove her about 20 miles Sunday with MAF disconnected runs like normal (with the SES light on).

I have printed the wiring diagram, chased every wire, all grounds are less than 1 ohm. Also check all wires for shorts to each other or ground. The only strangeness I can find by the diagram is pins C on the power relay and A on the fuel pump relay read just over 4 ohms to ground. The diagram I have shows these 2 wires tied together and to the fuel pump. Does this sound right reading through the fuel pump? Don't have a diagram for the fuel pump.

I know this is a really long post, I have been trying to figure this out for a while now. I know I will eventually figure this out, I've been an aircraft electrician in the Corps for over 19 years, schematics are no big deal, but I've never had to troubleshoot any fuel injection this far and this is the first Chevrolet I've ever owned. Couldn't pass it up when I bought her and it's been a fun project for my daughter and I. Would just really like to get this SES light off and stay off (and the car still run right!).

Bottom line, should I be getting about 4 ohms from the pins above to ground, did I get a faulty MAF, or any other ideas of where I need to look?

Thanks in advance for any help out there.
Sounds like you're on the right track, but I replaced both relays the first time my MAF went bad for insurance. I had to take back my remanufactured MAF twice before getting a good one.
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Old 11-07-2012, 05:55 PM   #3
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After reading problems others have had with finally finding a MAF sensor that work correct with their car, I will probably have to go down that road of taking MAF sensors back too. Unfortunately I haven't seen any other L98 MAF cars around or know anyone with one to be able to possibly swap parts with. I'm going to do some more poking around on this tonight.
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Old 11-08-2012, 01:12 PM   #4
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I don't know about '87s, but I know the '86s had Technical bulletin regarding the MAF relay and burnoff relay. Anyway IIRC the factory had different part numbers for the relays originally and if you changed one to the new PN you had to change both (to the new part number). 'Something like that....

http://forums.corvetteforum.com/c4-t...af-relays.html

For a Code 33 I would try another set of Delco relays.

Last edited by DanZ51; 11-08-2012 at 01:22 PM.
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Old 11-08-2012, 02:25 PM   #5
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This is the TSB for the 1987.

Refers to Code 36, not a Code 33.

Shows proper procedures for checking voltage.

Might help you in your quest.

http://www.corvetteactioncenter.com/...des+F+and+8%29
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Old 11-08-2012, 02:51 PM   #6
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huh uh-huh
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First of all, you REALLY need the FSM for this car. You'll never get anywhere without the right electrical drawings and the 2nd book is nothing but electrical...

next, you need an understanding of this system. Its not typical or normal. The biggest mistake folks make is the elec control. The C4 (all vettes ) are managed by grounds..not power. The power source is always present. So, having ground issues is very common and nobody looks for that as the source of their elec problems. C4s also have horrible wire harnesses...they insulation cracks and shrinks back and bare wire is exposed in plugs and in the sleeves.

Your ECM gets a trouble code., and turns the SES lite on to warn you of a code. At the same time the ECM makes a shift back to Open LOOP operation so the engine is running on pre-programed data stored in the Cal-Pak (calibration pacakage) in the ECM. Whenever you see the SES and a stored code, the ECM is not running on sensor inputs anymore. When you unplug the MAF that foeces the ECM into OL again, so it can run the engine. This is also known as Limp Home Mode. Its designed to allow the engine to run well enough to get the car home in case of a major sensor failure. Its not able to make the simple and constant adjustments to the air/fuel as conditions change, but it'll run, pretty good usually.

The MAF codes usually are relays. Most times its the wire or the relay. 87 used the same, so swap them around. One is burn-off that does not run all the time, the other is the MAF power relay...that needs to be ON whenever the key is.

Look at the power source connections on the jumper post behind the battery. Thats where all your engine control hot wires come from. Those are also all on fusable links under the battery. Some have a path thru a fuse, others not.

Get the FSM set of 2 books. its a must have thing for the C4. Those books will tell you more than 10 yrs of trial & error and cost you a LOT less. You can find used books for $50 on fleabay. Somewhere here there is a link to new sets. Do a search here for the FSM.
Good luck!

BTW...

you need to check that fuel pump wiring...that does not sound righht. Might even be dangerous.
Many people failt o understand the fuel system too...The pump only runs for 2 seconds when the key is turned to ON. It will not run again until the engine fires and runs on its own. Thats by way of a signal from the HEI distributer.
Fuel injectors all fire together. Back fired. Short one on that side they all short. Very confusing to someone thats unfamiliar with the system. Injectors stay HOT all the time as previously described. The ECM cycles the inj grounds to operate them on each circuit. Some sensors are variable resistence, some are simply switches..on/off. Ground path completion is how those operate the different systems as well.
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Old 11-08-2012, 08:09 PM   #7
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Thanks for the responses.

DanZ51 and TorchRedlt4man, I had stumbled across that TSB previously when I got the code 36 after installing this current MAF. Went ahead and replaced both relays and now no codes, just runs terrible cold.

leesvet, FSM and a scanner are definitely in my future (especially with Christmas coming up) but I have been able to download all of the diagrams for the MAF for now. All of the grounds in the circuit are reading less than one ohm. I have opened up all the factory harness, threw away the original outerwrap and electrical tape, checked all of the wires and repaired one frayed wire to an injector on the passenger side (used environmental splices, check youtube for *EXPERT LEVEL* Environmental Splicing Tutorial TTC Avionics, these things are awesome and all we can use on aircraft). All of the engine bay harnesses have new tubing over them now to protect what is in there.

Went out to the garage tonight to try something different, plugged the reman Cardone unit in but removed it from the throttle body. Started the car with no air being drawn through the MAF but electrically plugged in. Fired up and idled great (SES light came on). Turned the car off, put the MAF back in the air tube, this time when I fired her up it ran like crap again but no SES light. This tells me that the computer is not liking this MAF when it is connected electrically AND has air being drawn through it even though it won't throw a code. Tomorrow I'm taking this one back for another one. Nobody here keeps them in stock so not sure if I will get one by Saturday or have to wait until Monday for the chance to try another one.
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Old 11-08-2012, 09:06 PM   #8
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If anyone ever puts a remanufactured maf sensor on there car, it is imperative that you check it immediately with a scanner..Failure to do this can cause damage to your engine.... I say this because when I was buying remanufactured maf sensors for my car I was also in the middle of having my car tuned. Every time I put a different maf sensor on my car and datalogged , my tuner would call and tell me that the maf sensor was throwing my tune "WAY OFF"....I went through 3 maf sensors trying to find one that was even somewhere close to what the original one was, and never found one. They were taking my BLMs everywhere from 125 all the way to 160... My tuner and I finally came to the conclusion that they never even tried to get the remanufactured units up to spec, they just rebuilt them and sent them out the door. One of the units was so far off spec it wouldn't even let the car start... So, I will say this again... If you put on a remanufactured maf sensor, check your car with a scanner immediately to see how far off its going to throw your BLMs....WW

Here is the solution I finally decided on, it ended up being a very good choice...

http://forums.corvetteforum.com/c4-t...-i-got-ww.html
.
.

Last edited by WW7; 11-08-2012 at 09:27 PM.
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Old 11-11-2012, 10:28 AM   #9
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Update to this,

Took the MAF back to parts store yesterday morning, picked up my replacement this morning. Disconnected the battery while I went to get it to clear codes. Installed new MAF, hooked up battery, car fired right up, idled great. I've only driven around the base a few miles so far, stop signs, stop lights, up to 50mph for a short stretch and so far no SES and seems to run great. Going to let her cool down now and try again. Will post any more updates if anything changes. For anyone that needs any future reference to this I'll try to break it down as to how this has progressed:

MAF 1- Bosch, in the car when I bought it. Constant SES, code 33, drove fine, didn't write down the numbers off it, appeared to have factory relays by the AC part #'s.
MAF 2- Microtech, circuit board style inside, drove fine but random SES, could feel it in the car while driving right before the SES would come on. Code 33 randomly. Replaced relays, no change.
MAF 3- Bosch, numbers on it were 0 280 213 004 ... under that is : 14 094 712. Idled terrible, code 36 with AC relays, no code with new relays but still ran bad. When warmed up would finally run ok. Looked like this inside:
Click the image to open in full size.
MAF 4- Bosch unit, numbers on it are 0 280 213 009...under that is 10 055 877. Fired right up, no SES so far, only a few miles on it, will update if anything changes. Inside looks like this:
Click the image to open in full size.

Last edited by FletchUSMC; 11-11-2012 at 10:32 AM. Reason: Photos
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Old 11-23-2012, 09:27 AM   #10
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Since it's been a few days thought I would update this post. Weather has been cooperating enough for me to drive over 100 miles since I put this last MAF in. So far no problems and I'm going to finally call this fixed. One thing I would like to add, in my earlier posts when I said the car drove fine, that was before I had a good working MAF system. Now that it seems to be fixed I can tell a difference in part throttle response and feel.
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Old 11-23-2012, 11:22 AM   #11
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Interesting thread for me......I've been following this since I have run into a MAF code as well.....funny thing is it only is during cold, open loop (like sitting overnight starts) and the check engine light will remain on and run a little choppy at low RPM (light throttle) until roughly 2500 RPM and it then kicks in like it just got full spark or fuel burst all of a sudden. Once the engine is warm and you shut the key off, then restart, no check engine light and the car runs perfect. As I think about it, this all started when the original MAF went bad and was throwing a code and then was replaced with a Bosch. I know this a remanufactured unit as well. I would bet I have similar situation that you had.
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Old 11-23-2012, 05:04 PM   #12
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I've run into a lot of posts where people had to try multiple reman units before they got one that worked. I did notice that the parts store I got these from don't even offer the microtech anymore that I used the first time.
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Old 11-24-2012, 01:18 AM   #13
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I wrote down the numbers of the last one you used and now seems to work as a reference.....let me know if you run into any other issues with this latest unit!
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