Yesterday my 91 ran great all day just as it has for months. Today i get in to go someplace and started quickly (normal), but ran very rough and would not take throttle. I let it idle to see if it changed as it warmed up, but no change. Connected scanner and no codes. I then recorded data as it ran. For some reason I can't get my Actron scanner to properly connect to my computer.

Until I can download information to computer do all of you have any thoughts of what would cause the perfect one day and horrible the next??

 

..... Unfortunately , sometimes S**t happens … try disconnecting the battery neg cable for a few minutes and then see if it repeats … if it does repeat , then start checking the usual suspects (the ones that won't set a code) … CTS , FPR , ICM , Distributor cap & rotor , burned/melted spark plug wires , rodent action , sensor connectivity issues , grounds , etc. …..

 

this is exactly what tunerpro rt, an aldl cable and a windows laptop is for.

its most certainly one of the sensors. will this actron thing scan the car while running?

 

The Actron does but for some reason my computer is not connecting so I can't load the readings.

 

It sounds like you just found your problem, bad ECM.

 

check fuel pressure

 

do all the interior gizmos and lights and cig lighter work?

 

Yes, everything else works except that engine ran great when i drove it on Thursday but like crap when I went to drive it yesterday.

 

You can pull the same codes without scanner with a paperclip.

 

google corvette c4 aldl plug and short the a and b with a paper clip.

does the fan turn on?

does the ses light begin a series of flashes?

 

I did check for codes and it's clear. Fan turns on if i turn A/C on and when engine temps get to proper level. Everything

 

When you entered "diagnostic mode" what exactly happened. What code flashed (what count did you get)? Did the fans turn on IMMEDIATELY or after car was warm or a/c turned on only?

 

^^^

when u shorted a and b with a paperclip, did the fan turn on. thnx.

 

Here are readings i was able to take pictures of because still can't get scanner to link to my computer.

 

One thing i have just noticed is when first started it takes throttle (revs) normal but within a minute it starts missing. I turned it off and on a number of times and it happens each time.

 

Throttle % 38 ????

seems like it’s cause a drivability issue is it’s supposed to be closed

 

I'm pretty sure that was because i was playing with different amounts of throttle when scanner was recording. The other recording I did shows throttle % 0. Also just returned from a drive around the neighborhood and still has a miss, but not as bad as yesterday. I have checked all the spark plug wires for connection at both plugs and distributor. On data i show MAP is at 2.43 which i believe is within specs. If that is incorrect I'm sure one of you will advise.

 

Your readings from the Actron are all over the place and likely not correct. The (MAT) Mass Air Temperature is showing -40 and that alone would cause problems. Just because the Actron was acting up I would certainly not replace the ECM. I read someplace that if the ECM is bad the cooling fans will start and run the whole time to indicate a faulty ECM.

In the past when a vehicle has drive ability issues where the pedal does little to help it is frequently a MAP sensor or the MAF on the older L98 engines. When they fail the volume of air going into the combustion chamber is measured incorrectly and this makes a mess out of the Corvette.

Did you buy gasoline someplace different or anything?

Does the engine really think it is -40 degrees or is it the default number when not connected properly? At -40 there would be a lot of extra fuel being injected which could explain the long pulse width time for the injectors. This might explain why the O2 is seeing a rich mixture as well.

In "closed loop" an engine needs to see 3 devices to make the correct air-fuel ratio for those conditions. One is the Oxygen Sensor, the second is the Coolant Temperature Sensor and the MAF or MAP. This is why we start with those three parts and start looking .

Your oxygen sensor was reading .324 which is a bit off the median of .450.

The MAP is reading 17.88 which seems high but this was found to help understand the MAP:
A contaminated or failed mass air flow sensor cannot measure the amount of air flow correctly. This causes the engine computer to miscalculate the amount of injected fuel. As a result, a bad mass air flow sensor causes various drive ability problems, including a no-start, stalling, lack of power and poor acceleration.Feb 18, 2020

The knock sensor seems to be active at the low RPM's when you ran the Actron and that might indicate an "overactive" knock sensor trying to solve the knocking by pulling your timing back. If the knock sensor fails it will retard your engine a great deal, it pulls the timing back to prevent damage to the engine. I had one fail on my 1988 C4 L98 and it really cut my power back until I fixed it. Check it to see if it is overactive, while the car is idling there should be no knocking sounds to pick up on so take a metal wrench and tap on the metal surfaces of the block or cylinder head. As soon as you tap the metal it should automatically cause an idle change. A knock sensor is like a Tuning Fork and at specific noise levels it will produce a small voltage that triggers the ECM to pull back the timing.

The pulse width for your injectors was like 15.7 Ms and that would indicate that your engine is probably having an issue with low fuel pressure. You will need to verify the actual fuel pressure with an external gauge to see what your Corvette is getting for fuel pressure.

Check your Fuel pressure and then check your Knock Sensor as both of these could make for some bad drive-ability issues. I would also verify that the ECM is seeing the accurate temperature and not -40.

 

The one thing that puzzles me is at initial start up regardless if cold or hot it revs freely, but after a minute starts missing. Wouldn't that indicate it is not an injector since if it was it would probably cause a problem soon as started?

Also, both the CTS and Map sensors are fairly new. The bad CTS was causing car never going into closed loop so would stall because stayed rich and old MAP was making car not idle correctly.

The oxygen sensor reading in other data recording was 431.

 

I went through similar with my 91 years back. A delco reman ecm fixed my issue. Open loop was fine. Closed loop it’s fall flat on its face.