My 89 L98 acted up the other day. I had driven less than a mile and it stumbled and stalled at a stop sign. Service engine soon light came on. The engine would restart and idle roughly, but struggle to take any gas. Stabbing at the throttle would bring RPMs up. Limped home and parked it. Started checking things over, but found nothing obvious to the naked eye. Called my mechanic and he agreed to look at it right away.
The car started and ran perfectly with no service engine light for the 2.5 miles to the shop. He stated that the ECM stored a 36 code. Shop manual indicates that the 36 is the Mass Airflow Sensor Burnoff Relay. I installed a new burnoff relay and the engine ran smoothly for one short trip. now the problem has returned exactly as before. I tried to read any codes stored (new to me) by jumping from a to b on the diagnostic port and counting blinks of the service engine soon light. It did not seem to present any codes.... just 12 over and over. ... Then I started the car and is ran and idled perfectly. This morning I started the car and the problem has return exactly as it was.
One symptom might mean something. When the problem is present, even though the engine is cold, the electric cooling fan kicks on as soon as the key it turned to the run position. Also, the exhaust is white smoke (too rich?) The couple of times I've started the car when the problem was not present, the exhaust was perfectly normal.
Would this all indicate a faulty ECM? or something else? Help me smart mechanical people!

 

the ECM's can do some weird stuff and give no codes, and then it becomes hard to diagnose.
Some people would immediately expect a sensor fault, but most sensor faults will give off a code.
If you are having these problems with no codes, then it may well be the ECM. How do you tell if it is the ECM ? put another computer in unfortunately !
you can find the '165 computers for around $50 - $100, or maybe you could find someone to borrow one from ?
my car had problems with the timing retarding and advancing huge amounts, car would run well then run rough, no faults were coming up, just the code 12, I borrowed another computer from a club member, plugged it in, problem solved !

 

Your response is very helpful.

My gut is pointing me to the ECM, but I'm not an experienced mechanic. I don't mind spending $100 on a replacement ECM, but I thought some other folks might have good advice for me.

Another thing happened that might be helpful information. When I initially installed the jumper in the diagnostic ports and turned the key on, the service engine soon light flashed and flickered randomly, and with varying intensity. I turned the key off and then back on, and it flashed clearly the 12 codes as expected.

 

i experinced the SES flinckering being related to ECM bad connection (connector or pins ) or a bad eprom (eprom pin bent after removal)

 

This is classic wire harness/ground problems.

Get under the car and find the ground bundle of 14-16ga black wires on a single bolt upwards of the oil filter. Take that apart and clean and reassemble. Those 4 or 5 wires are the ECM and Fuel injection system harness grounds. These are MORE important than the cable from the battery to the chassis....

I do not KNOW, but IF your car has the jumper pole behind the battery where there are several orange and red wires, same deal, those are EFI power source wires and I have seen a loose or dirty connection there cause misfiring and other engine behavior problems.
Remember, this ECM gets information if the form of variable levels of resistance. A loose or corroded connection makes it more difficult to conduct low voltage OR that precise amount of resistance the ECM wants to see. The connections MUST be clean and tight so the signals that sensors generate, are not altered by poor connections.

Grounds and Jumper post.

Take the battery tray OUT and do a complete visual inspect of the wire under the batter tray that are hidden from view, but NOT the fumes from sulfuric acid...

Good luck !

 

Ill look at that first!

cheaper too!

 

I can buy that. I will spend some time following your advice as well.
Thanks!

 

Could the ground issues you refer to also be effecting my HVAC control head and some dash lights? I have one section of the dash that lights/does not light intermittently. My HVAC electronic control unit is also pretty wacky.

 

Yessir.

STOP looking at the C4 electrical as you would ANY other car. IT is NOT and has nothing in common except that is has a positive and negative.

The C4 is managed 100% by the grounds. Power is ever present to almost every function. Some even with the key OFF. (orange IIRC) anyway, its completion of ground path either thru the ECM or relay that completes and manages the circuit or servo. Feedback via resistance is how the ECM collects the data necessary to make real time adjustments to things like AFR or timing etc. Some things also use a low voltage reference signal for the ECM to compare things to....5v IIRC. The TPS is one of those things.........many others.

if you have the opt 68 a/c, go to the BCM on top of the evaporator core and go thru those 2 big plugs and make note of any burnt wire in or at the plug,. this is where you will find MANY of the a/c control problems. On the blower motor as well. That plug is famous for melting the contacts inside the motor side so you get to buy a new motor when its only the damn wire in the receiver plug.... Other a/c trouble spot is the loose circuit board in the control head, finger board gets loose, or just display lites MIGHT be part of the dimmer switch in the headlite switch. You'd have to follow the drawings in the FSM to confirm that one...

 

Leesvet advice is a good starting point, checking the grounds and electrical connectors is something that takes a relatively short amount of time and is free if you do it yourself,

my bad ECM did the same thing with the flickering of the SES light, flashing fast and varying intensity when you turn the ignition on,

if the earths and connectors check out fine I would look at getting another ECM, go into the for sale section and if none are listed put in a wanted to buy, someone will have one at the right price for another member, and if it isnt the ECM , well you youve got a spare or you can sell it on to another forum member who needs one

 

....started going over the grounds last night. So far, they look pretty good. I still have to pull the battery and fish around in there to clean things up. No resolution yet.
Thanks for your advice. I'll post more as I move along.

 

Keep in mind that we are NOT talking about the ground cable to/from the battery. Although it does not hurt to ADD another when this adventure is over. I added one from battery to frame, and battery to engine. There is a single large and 1 small cable end on my positive post and a STACK of cable and ends on the negative !

I guess we're looking AT the individual ground wires condition at point of contact AND the wire as it enters its plastic insulation. They tend to corrode and rot where bare wire is exposed and this can lead to a connected wire that has almost zero ability to conduct a signal or carry voltage. It becomes a high resistance circuit that the tiny bits of signal to/from the ECM cannot pass.
The grounds that we are referring to are all over the car. Usually as close to the system servo as they can find a place to connect. Like the dash components...where there are working chimes, things like that in the dash, there will be a ground somewhere close by.
The single most important 'group' is the bundle of 5 wires on a single bolt somewhere over the oil filter. It may even be on the trans bellhousing. I've seen it under the oil temp sensor. That bundle of wires is critical. The other end of that deal is the harness section where they originate, under the wiper motor on the firewall. DO NOT tug on that too much. These ground wires are simply crimped ON the ground wire that lives in the main harness...its a VERY cheesy deal and the little aluminum fold-over crimp-clamp often lets the corroding wire detach from the harness wire. The ONLY protection that this "splice" was given is a wrap of black tape from the factory. So, its fragile at best.

You should also do the wiggle test.

Just last week we solved a similar problem with this sophisticated diagnostic procedure.

Start the engine, let it warm and idle. Go to the EFI wire harness on the driver side and where it runs to the rear of the valve cover and to the firewall, wiggle the harness and the individual sections that come to the injectors and around the driver side valve cover. Listen for a change in the engine tone and idle.
Do the same on the passenger side.

You are looking (listening) for some clue about the harness insulation. This wire tends to crack the protective insulation and that allows grounding, cross-over signals, blended signals etc. Wiggling the wires around while running will tell you IF this is a problem or not. Sometimes the engine suddenly starts to run RIGHT.....sometimes it stumbles and dies. Either way, it PROVES that there IS a problem in the harness condition.

I had to chop out the whole driver side harness and solder in all new wire and I added all new plugs for the sensors and injectors. The engine had suffered a misfire that moved around. #3 one day then it was #7 the next. New wire fixed it permanently.

If nothing else go thru all the plugs (sensor, harness ) and clean and check the wire crimps to be sure they are not broken or corroded and not making good contact.
Conductivity and a clean signal thru the wire AND the plug ins is the KEY to the vettes electrical system being healthy. Because this is a plastic car it has to ground where ever it can, and that means there are that many more places where a poor connection CAN take place,... The system cannot share a "common" ground like other cars. There isn't one. It has common 'hot wires' but not common grounds...and since everything is ground managed, it becomes a problem sooner or later.

Good luck!

As a way to make a point, simply by cleaning the jumper post and taking the plugs off everything and cleaning and assembling with silicone grease to seal it all, I gained over 1 volt charging on the alt, got the dash lites to stabilize, and lost a mild stumble off idle to WFO. THATS how much clean connections under the hood matter ! I've seen temp issues go away, low voltage, oil pressure readings become stable and consistent, all things electrical are effected by a corroded or loose route in a circuit.

 

I found the bundle of wires bolted to the back of the bellhousing last night, took them all apart, cleaned them all with fine steel wool, did the same to the stud they were connected to, and reconnected.

Will keep working on it until it is taken care of!

Thanks!

 

Focus on the problem and troubleshoot code 36.

http://members.shaw.ca/corvette86/Component%20Location%20View%2086.pdf

thanks Agent86 for link.

You may also want to visually verify if burnoff is ocurring after shutdown.

 

Thank you very much for that diagram! That is sweet.

 

Are you sure you replaced the burnoff relay? The MAF power relay is right next to it and has the same part number so it's easy to confuse them (this is NOT true for 1986 -- the two relays are different and NOT interchangeable). One thing you can do is swap the connectors from relay to relay.

Code 36 is set after the ignition is turned off. The MAF burnoff relay applies 12 volts to the MAF wire, which causes it to glow red hot. While this is happening the ECM monitors the signal voltage output from the MAF. If it is too low it sets error code 36 because it assumes that burnoff did not occur.

To visually check that burnoff did occur, you have to be in closed loop mode. With the engine still running, disconnect the coupler to the air filter. Put some kind of mirror in front of the MAF and turn off the ignition. You should be able to see the MAF wire glow red. It only happens for a second or two, so you have to be paying attention or you'll miss it. Maybe your car is different, but if I disconnect the MAF from the throttle body the engine will die immediately.

I have a spare MAF and I am planning to take a picture of the sensor wire glowing red. That won't happen until late next week at the earliest because I'm away from home.

 

I'm going to fiddle with that! At first, I was quite confident that I had replace the burnoff relay, because the two relays in the car did not look the same and I replace the one that looked exactly like the new part. As I look at the diagram given to me by the prior responder, it shows the burnoff relay being the left one. I replaced the right one. Life lesson repeated..... just because it looks different, doesn't mean it IS different.

 

Checking and cleaning grounds is always good to do. My 89 was acting similarly, but I set the code for "battery over voltage" so I followed the manual procedure by checking the voltages off and running, replaced the battery, then the alternator, and it still wasn't fixed so I cleaned all the connectors and grounds I could find and that didn't help either. I was going to get a replacement chip burned and in talking to the guy (sorry can't remember his name) he said check the connections to the computer (they were fine), then borrow someone elses computer and try that. Not having anyone to borrow from I just bought one, used my old chip, and that fixed it, no problems since. Turned out the computer in it had already been replaced at least once.

 

Hey everybody.

I just wanted to thank you folks for helping me with my problem. I did not search prior threads as I should have. I still have not had the opportunity to follow all of your advice. With your advice, I will get my car fixed. You are good people for taking the time to help out a newbie.

Respectfully,
Matt

 

Matt,
you're welcome and I appreciate you giving me the opportunity to be of assistance.
I just hope that my advice will lead to a solution...Hands on is always better, but hands on a keyboard is the next best thing I suppose.

I've had every odd electrical "thing" in the world happen and there seems to always be a new place to look, something else to take apart, another way for the system to confuse and baffle me.
On the code 36, I've had relays that were working intermittently, and then I've had that code when it was the plug TO the relay that was at fault. Another great place to look for a bad plug is the MAF plug. It has too many wires (5) stuffed into a very narrow plug and I've had to change mine 3 times because wires broke and strands contacted the next wire and caused problems. This plug is so small and hard to work with, that its easier to just solder in a new one instead of trying to fix or replace the individual pins inside. Like I said....got the 4th one in there now, 3 had to be chopped out and replaced, like the entire driver side injection harness. I still have not figured out why only that side rotted out and went bad...the passenger side is good as new. But that driver side had insulation broken and cracked everywhere and caused my mystery misfire that went on for months. Pure accident that I discovered the source of the problem when I tugged on the harness for something else and the engine suddenly started running smoothly for the 1st time in months !
With so much electrical magic taking place under the hood, there are lots of ways it can cause problems...

On your MAF and Burn-off relays, you can swap them and see if the problem continues to show up as a 36. If its a bad relay it will then show up as the maf code, not burn-off.
Keep in mind that Burn-Off does not effect how the car runs. I always clear the codes after each time I see them so I KNOW if a new code is set. I don't want to keep digging because I'm looking at an old code... that goes to the theory that if you LOOK for a problem, you WILL find one.

Hang in there. the more info that you post as you explore the more someone can help. Its a matter of time, that's all. Someone here knows whats wrong, its getting the info in front of THAT person so they recognize it and can help.