In addition I still think it could be the PCM. The reason is for no reason I now am kicking codes 81,82,84, and 90. All transmission solenoid related issues. The transmission shifts great when the MAP sensor is disconnected! When it is connected it does not!

 

I did a vacuum test today from the power brake booster port and also from the throttle body port as well. The vacuum was at 14 at each location. Went to 5 on acceleration and snapped to over 20. I believe that is too low so I an going to do more tests to see if I have a leak . I have researched that it could need a valve adjustment or the timing is off. The timing is controlled by the PCM so I still my be looking at a faulty PCM? I will redo the test again to confirm the results.

 

You need to data log.

 

I disconnected all the sensors one at a time. No changes in engine performance. The only sensor that changes the performance is the MAP sensor. As soon as I disconnect it it runs very well! Plug it in it goes immediately to ping, backfire, no power etc. the new MAP sensor connector came in and the new map sensor seal but I am positive they will not make any difference at all! I also rechecked the engine vacuum. At all but the Purge valve port the vacuum is around 14-15. At the canister purge port at the throttle body is does come up a bit to about 15-16.

 

I apologize for doing most of the posting on this issue but I can be OCD at times and this problem has me really going crazy! I came up with an interesting finding in testing today. I retested the map sensor by removing it from the car. I back probed the signal wire and applied suction. The MAP sensor tested with these specs. Turned ignition on and signal wire voltage was 4.8 I applied vacuum and the voltage went down to 1.0. That is a normal response! I put the sensor back on the intake plugged it in and tested it with a volt meter with the engine running. Still back probing the signal wire. The voltage started at 1.7 at idle as the throttle opened the voltage started to rise but then dropped at wide open throttle it dropped to 1.0. It should go consistently back up to 4.7. Now what does it mean when it does not act that way? The vacuum is building in the intake and not dropping correctly. Does anyone have any thoughts as to why this would be happening? This would explain my problem. If the vacuum builds at WOT the computer get the signal of idle and leans out the fuel.

 

Today is a new day and I have some new ideas! I am going to try to find the reason that the map sensor signal wire is dropping voltage on acceleration. Today I am going to test the signal using a different MAP sensor and a different vacuum source. While keeping the original sensor in place and the engine running. I will also test the vacuum from a different port at full throttle to see if the vacuum increases or decreases on wide open throttle. What is my thinking? There may be a problem with the MAP sensor port. It could be causing the false reading going to the PCM. If the signal wire drops in voltage on acceleration the PCM will restrict fuel and adjust timing at a time when it should be doing the opposite. This would cause the engine to be starved for fuel creating performance problems.

 

One simple test and I confirmed that the vacuum is increasing as the throttle is opened. With a quick snap of the throttle it drops to five and when released goes up over 25, as it should. What is strange is with a gradual increase to open throttle the vacuum goes up not down. Just a small idle increase and the vacuum goes to 19 and holds steady. I will confirm these tests later today. Either the air coming in or the air going out is somehow incorrect? This causes the MAP sensor to read closed throttle when in fact it is opened, cuts back the fuel supply, changes timing and creates the lean condition the O2 sensors have been picking up. Now the big question is why? I have had the exhaust off checked the cats for restriction and they looked good. So are there any suggestions or additional tests I can do to narrow down the cause? I have not had the throttle body off this car yet!

At this point as by now everyone who reads this can tell, I am not a mechanic! I am retired and bought this car to learn and so far I have learned allot. The biggest thing I have learned is I do not know enough to fix it!

 

You might have to bite the bullet and spend the $100 for the FSM.

 

Just came back from the NCRS road tour. Had a great time on the tour but did not go to the convention. Now to continue on this 1994 corvette. I believe the reason the MAP sensor disconnect make it run better is when the MAP is disconnected the car goes into open loop and runs great! So now where to go?

Tested vacuum and it was 14 which seems a bit low? I tested all the ports and got the same results. I borrowed a scanner and took some readings:

Mass Air flow 10 grams at 650 RPMs increases with acceleration.

O2 sensors when warm are reading 840 right and left. A rich reading

MAP sensor 4.84 volts with key on. When engine running the volts are 2.33 volts

Throttle position sensor reads .58 with key on not running. When the throttle is pressed it reads up to 4.1 open throttle

Nothing I can see in the data would be that far off to make the car run like crap. backfire, stumble no power on acceleration.

So I am back to the drawing board!

 

https://www.ebay.com/itm/1994-1995-C...UAAOSwYf9bI0-s

40 bucks and there is a paper one too
I bought mine on a CD as well and print what I'm working on when needed.

 

The same CD manual is $20.79 on Rock Auto...

https://www.rockauto.com/en/catalog/...r+manual,10335

Not sure how good it is. I bought the BISHKO CD manual for my 95 last year from either Rock or eBay, and it is the exact shop manual (2 volumes) on CD, licensed by GM. I think it was about $35 IIRC. Best of luck with your efforts, but I strongly suggest getting the manual to preserve your sanity. These systems are pretty complicated.

Regards,
John

 

That where I got mine too.

 

Hi Richard, I have nearly exactly the same symptoms on a complete different car: Runs perfect from cold start until it reaches normal operating conditions when warm. No engine codes. Applied the same method as you to trace down the problem. Still no success after about 5.000 miles and two months of searching and still used it as a daily driver. Checked for air leaks, sensor problems, disconnected nearly everything, physically and electronically, checked the ETM, changed the MAF sensor, changed the TCV, cleaned the kat, the precat was melted down, which means a far to lean condition, did not solve the problem though, being just a consequence of the lean condition.



Had to drive the car out of Morocco a couple of days ago so the problem is still not fixed until now. But of course I am still thinking about it all the time.....It does not seem logic.....

But: If you disconnect or change a sensor and it does not change the situation, does not mean that this is not the cause. If it is just slightly reading wrong, under normal operating conditions, the values still being within the limits registered in the ECM, no codes are thrown. The cause for the slightly wrong signal may be a resistance problem in the wiring or connector. If the O2 sensor for example is giving a 'rich' signal (though not the case), the ECM is injecting not enough fuel. No codes are thrown, but the engine has less power.

Assuming the resistance problem in the wiring appears only when the engine bay is warm, and only when in closed loop this slightly wrong value (still being within the registered limits, so no codes are thrown) becomes in closed loop one of the regulating parameters for the ECM, changing or unplugging the sensor does logically not eliminate one possible cause for the lack of power when warm.

Cheers, Michael

 

Hi Micheal,

Thank you for your reply to my post!!! Please respond again.

I have been through everything I can think of to fix this corvette. The key is it runs good in open loop. When the car is warm I can disconnect the MAP sensor which forces the car into open loop and immediately the car runs great. That in my opinion narrows down the causes. The main sensors seem to be responding OK, within limits. I have scanned the car several times and the O2 sensors, MAP sensor, Throttle body sensor are all responding within limits. At least in my opinion, limits that would not cause this serious poor performance condition? I thought about removing the throttle body for cleaning, that is all that has not been done, however since it runs well in open loop I feel that is a wasted effort. I am going to check the grounds under the battery to the PCM today, I have read this can throw off the PCM signal. I am going the hit the PCM with a screw driver as well, read that as well.

After that I have decided I am going to replace the PCM and see what happens? This car is a very nice low mileage example with good paint, tires, and interior. I can not give up on it! I will sell it when it is fixed because I am seriously thinking of ordering the new C8!

Thank you again for your feedback!

Rich Harris

 

Yes, I am also sure that is has to be an electric/electronic (wiring) related problem. Either a ground problem or, as I stated, a resistance problem, in the wiring or in the PCM unit itself.
If possible, you should check the resistance of all wiring (when everything is warmed up/hot) going from those sensors which values are used in closed loop (IAT, MAF, ETM, O2, Crankposition sensor, Camshaft position sensor, etc.), to the PCM before replacing the PCM.


My 1996 C4 LT4 for example has a problem with the ASR (ASR light on, DTC65, which only occurs when the car is really warm). This is surely because when everything is warmed up the resistance in one point of the wiring (ground connection or cables and connectors) gets to high, and the signal is lost or wrong or the ground is shorted out.

Good luck!

Cheers, Michael

 

As I am doing some further testing these are the findings. The key turned on and with the car not running, the TPS sensor tests voltage is correct. Five volts to sensor, good ground. As the throttle is opened the signal wire reads closed throttle at .58 volts with open throttle at 4.01. These are all normal readings. When I scan the TPS with engine running the TPS reads at .58 closed but only goes to .84 at 3500 RPMs.

Summary:

When the car is running the TPS signal wire fails to give the PCM the needed voltage as the throttle is opened. Could the signal wire become affected when the car runs?

These results lead me to believe there is a problem with the signal wire. It is wrapped tightly with electrical tape at the sensor so I will try to expose it from the sensor and inspect the wire! Hopefully I am on to something!

 

I exposed the TPS signal wire almost to the fire wall. I do not see a problem with it so far. I ordered a new connector just in case but I do not think that it is the problem at this point. I am back to the PCM as the problem. All the sensors are in acceptable ranges and It runs well in open loop. I am going on vacation soon and work will come to a halt for awhile on this car! I am going to have the PCM repaired while I am away so I can make ensure that the PCM is not the faulty part!!!!!

 

Whatever happened on this car it was an exciting read? Did you ever fix it?

 

what was the issue im having the exact same issue with a 1994 Vette

 

The poster was last on the forum
Last Activity: Apr 3, 2023 09:11 PM
Seems that the issue may not have been resolved and he quit.
You should make a new post with your issues so others may be able to help you.