yessir...I argee with that. Its interesting how these very sensitive control systems are piggy backed in a sense where they can create a cascading failure...meaning that one system fault can cause or lead to another independent system that is not directly linked. Sometimes there may be a power source connection or a control ground...but they somehow find a way to start a domino effect when there might be a single part failure.
Thats why I tell people to learn how to interprit the codes and do not see them as an indicator...a clue, thats all.

In theory...a bad burnoff should not make the car run poorly since its not a control system...its housekeeping, automated maintinence.
BUT, if its been bad long enough the MAF can be so filthy that the sensor values are now way off...and THAT is a problem !

I just wanted to point out the harness issues. This harness is so fragile that I have a hard time believing that GM used it with its poor insulation and lighter than normal wire. Its Chinese at best, nowhere close to something you;d want to see in an aircraft....or a corvette !

With the continued effort and some time....you guys will find the problem...I can see there is an expert involved and whats even better, a student/owner that WANTS to solve the problem instead of whinning and pushing it off on a shop..the easy way out. Learning how is half the fun.

Good luck to the both of you !

 

He's not done troubleshooting yet. Like you say, the MAF burnoff may not be the big issue. But I think it can really screw up driveability if the sensor is not working properly.

I recently had a problem with my 85 not running worth a crap in open loop. It got better in closed loop but not like it should run. After a lot of troubleshooting, I found a wide crack in the plastic air intake neck the MAF seats in. The crack was on the bottom where I didn't see it. That crack caused enough turbulance in the air flow to screw up the MAF signal. So these things can be pretty delicate.

The EGR could also be a separate issue. I had to replace the EGR valve and temperature switch in mine. But I don't remember the EGR causing any significant driveability problems. It just threw codes.

 

I tested the B circuit and it gave me .02v with the ignition on. Also touched both black wires and got the same number. Is this reasonable or too low?

What's next?

 

Perfect.

Now leave the voltmeter connected like it is.

1. Ground the ALCL Terminal E like you did earlier.
2. Turn the ignition on for 10 seconds.
3. Turn off ignition and within 4 seconds check the voltmeter. The voltmeter should read greater than 2.5 volts for 1 second.

If this checks out OK, the problem likely is in the MAF sensor.

 

Ok, I had my neighbor help me hold and check numbers with the volt meter while I worked the ignition and the diagnostic part.

For less than a second after I disengaged the ignition I got 11v.

Too high or just right?

 

Are you sure the voltmeter was connected to terminal D and not E on the MAF connector?

 

I'll quote the shop manual.

"When conditions are met to activate burn-off, a 3 to 5 second delay will occur. After this delay the control circuitry will cause 12 volts to be applied to CKT 994 for 1 second and this will cause the sensing wire to glow red hot (about 1000deg F) and the cycle is then complete".

Circuit 994 is terminal D on the MAF connector. So it looks like the 11 volts you read is within range. The diagnostic chart says you need to see greater than 2.5 volts. 11 is definitely greater than 2.5.

Your troubleshooting results point to the MAF sensor. It appears the burn-off circuitry is working properly. If I were you, I would repeat the earlier tests when you checked for the MAF wire sensor glowing. If you get the same result, replace the MAF.

I would take the MAF back where you got it armed with the troubleshooting results. I still have the original MAF in my 85. But I have heard from others that remanufactured MAF sensors can be problematic. Maybe someone else can chime in.

 

Good to know, but also troubling. I dealt with 3 remanufactured MAFs and ended up with the current "new" one, which has been working aside from the conditions that you're reading about, which still puzzles me why it works better under warm conditions. Definitely still under warranty so I'll see what I can do, but I fear the same results.

Going to look into the harness problem, although I dread the idea of resoldering everything.

 

I'll guess that it runs better under warm conditions because the ECM is in closed loop. That means that the ECM is adjusting fuel based on the O2 sensor readings. It can compensate for MAF readings that are slightly off.

 

Is there any way that this could be related to a faulty ECM? I've also been lead to this by a nearby member, but his only spare ECM was sold. Before I start slicing and dicing I'd want to see if another ECM would fix the issue, or still show it occuring.

 

I have come across bad remanufactured MAF sensors, I'm not surprised when they turn out bad. If you have your original sensor you should try to test it as you have done with the current one to see if it tests the same. As stated you can also do a wiggle test with the harness to see if the car stumbles at all. I also have given the sensor a couple taps with the end of a screwdriver to see if the car stumbles.

 

Tapped the MAF with my biggest screwdriver while running and no effect.

Can't use the old one because a certain auto parts chain screwed me out of it due to "core exchange" procedures. I got my money back after all those replacements, but no MAF. That's when I bought new.

Wiggling the harness is next on the list. Right now I'm figuring out how to trick the ECM to run on a stored memory whenever it gives me trouble.

 

I wouldn't slice any harness at this point. So far the circuits have checked good.

If the MAF is not burning off as it should it will produce bad readings. You demonstrated it is not burning off even though the burnoff module is sending the proper voltage. So far, troubleshooting has not led to the ECM. I understand why you are doubting the MAF but testing says otherwise.

 

Well, I found a way to make it work for the time being.

I started her up and didn't get the "check" so it would have had all the symptoms described earlier. Since it seems to work better running off of stored memory, I unplugged the MAF, started the engine which forced it to run off stored info (which brought up the "check"), let it idle for about 30 seconds, then plugged the MAF back in and took it for a drive. It was a little rich at first but started to behave normally and even had normal gas mileage. I gave it WOT twice and it still had the performance that it had before.

Obviously, this isn't a permanent fix but I can live with popping the hood once in a while when the issue comes up. Honestly, when isn't it cool to lift half the car and look into the engine.

I'm ordering another MAF since I'm under warranty, but it'll take a few days to get here.

 

The fact that it runs better with the MAF unplugged is an indicator that the MAF is the problem. Good luck.

 

Very true.

At least I've learned something today...
Cardone and Duralast mass air flow sensors are both crap for this car.

Anybody know what the stock C4 came with?
AC Delco or some other brand?
All the remans I got were rebuilt in Bosch units.

 

Bosch was the original. As I understand it, in 1984 GM was up against the ropes to get the TPI out in time and meet smog requirements. They borrowed the Bosch technology from Porsche.

 

Yep,
Good JOB !

yessir...unplug yer MAF to force the ECM into LHM so it runs off the Cal-Pak. This isn;t optimal but it'll get you around...

I've seen as many BAD aftermarket MAFs than good ones. The Corvette MAF is very precise. These "clones" are as generic as tap water. Most are NOT calibrated anywhere close to GM/Delco standard..or Bosch.
You DO get whatcha pay for in the MAF isle at the Corvette store.

Look at the MAF plug to make sure there are no bent over pins. The PLug itself is suspect as well. I have replaced the plug on one of my cars 3 times since I've owned it. The pins are fragile, the wire going in tends to rot and break inside the plug where you cannot see it. Only clue is a car that won;t act right. That wire rotting inside the weather-tight plugs and in the insulation goes back to my earlier complaints about the harness.
Its just cheap wire that does not meet automotive spec...As part of the GM R&D team, I fully expect my paycheck (or a new ZR-1) (2014 model will do) for working thru this issue along with the many many other folks (vette owners) that do GM R&D in order to maintain the highest standards for their cars.

Sounds like you got it handled...
Again, Good job !

 

Good and bad news...

Bad news... Got my new MAF that was warrantied (Duralast) and it gave me the same results. Ran nicely for a bit, but after the second start up it ran like crap again. Returned it within the hour and got a full refund.

Good news... just bought an original Bosch MAF off ebay and it should be here by next week. Luckily I paid less than the Duralast.

Fingers crossed that it works. If not, I'm tackling the harness.
Leesvet, I'll let you know if I need your help on that one.

 

I started a thread regarding which MAF to buy. Opinions are mixed, but I think the original Bosch is a good choice. Mine is 28 years old and working. I got a message from a forum member that he has a used but working original MAF for sale for $75. That's if you need a backup plan.