Still experiencing performance problem 1994 corvette
 

Hi All and thanks for all the help!

I am still not getting to the fix for my performance problem. I replaced the knock sensors and now I have an H43 code. I believe it is because I installed cheap knock sensors and did not torque them properly. The old sensors tested at 3.7 and 3.8 OHMS, which is in the range. I did not get code H43 until I put in the new sensors. I am going to do the job again and ordered OEM delco sensors and I will pay attention to the torque of 14 ft pounds. I will also test the wire for 5 volts! The car does run differently even though I have the service engine light on and the code. It does not stubble as badly and does not seem to backfire. The car runs good in open loop and starts to run poorly when warmed up and in closed loop.

One question I have is on a 1994 corvette the ecm is under the hood drivers side and is there also a ccm somewhere in the car?

Since the car runs good in open loop I do not think I have a bad ecm or if I have one ccm? So my problem is somewhere in a sensor?

I checked the voltage from the wire to the knock sensor. Unless I am doing something wrong there is no Volts at all to the sensor with the key on either side. I do not know how to check the wire back from that point. The wire looks fine going into the black harness and into the connector.

Yes the car does have a ccm. Believe it is behind the radio. And yes, the ecm/pcm is under the hood. Right in front of the left side firewall, by the brake booster.

Thank you for the info. Would the knock sensors send voltage to the ECM or does the signal go elsewhere first? I am assuming the signal travels from the sensor ,not to the sensor?

Since I have had few replies to this post I will keep whoever is ready this updated on my progress. I am waiting for the OEM knock sensors to arrive. I am ready with the car jacked up the knock sensors loose and a wrench set a 168 inch pounds, 14 ft lbs for installation. When they are in place I will see if code 43 disappears? While waiting I have removed and cleaned the mass air flow sensor. I still have it removed with and will attempt to start the car with it removed. I have read that if the ECM is good it should start and run and refer back to original factory setting. If it will not run my problem might be in the ECM module? I have also started spaying down the exhaust system for removing tho see if the catalytic converters are clogged. I do not think that is the problem because it does run well in open loop. Also since it runs well in open loop I do not think the ECM is at fault. That is where I am in this project any thoughts are greatly appreciated!

Thank you
Rich Harris

Running bad in closed loop would make me think o2 sensors. But yeah, install the new knock sensors and go from there.

Thank you for the reply, both O2 sensors have been replaced. Did not make a change although the H64 code did disappear!

In a 1994, the knock sensor signal goes to the ECM (technically called a PCM in these cars). The CCM is behind the radio front end in the dash, but it won't use that signal at all. The knock sensor is a piezoelectric device that creates voltage when a knock occurs. It sends that brief voltage signal to the PCM, which compares it to a table. If the signal is in the range that corresponds to knock, then the PCM should pull timing. I don't think there should be any measurable voltage on the wire between the sensor and the PCM - it should have no voltage unless it's actually sensing knock. You can test the sensors for continuity. This writeup indicates they should have more than 10ohms, whereas you found less than 4. However, I don't know if the linked article is accurate for our cars. You seem to have a spec that says 3.8ohm is good. So if it's an LT1-specific spec, then I'd go with that.

I hope this helps

It does help, thank you. I do not think my problem was in the knock sensors I replaced the knock sensors with OEM AC/delco and the check engine light went away! The car is still running lean. it pings and back fires although it has gotten much better but still not near to 100%.I have an extra MAF sensor and I may give that a try tomorrow? When I disconnect mine (MAF) presently the engine shouts down? After that my next step will be to see if I have a clogged catalytic converter. I am not sure of the easiest way to test this? I just replaced the o2 sensors and really do not want to remove them again so I will keep researching!

I replaced the MAF valve with one I had that was good. No change in performance, still pinging and backfiring and stumbling. Rechecked fuel pressure and it holds a 40 lbs. disconnected vacuum line to fuel pressure regulator and it backed off to 36 lbs as it should. No leakage and the vacuum line to the fuel pressure regulator is good. I have done just about everything i can thinks of and no change in performance. It is either fuel or spark! The opti spark was replaced by the previous owner as well as many other parts. Somewhere there is a fuel problem or a vacuum leak? Since it runs well in closed loop and starts to run badly in closed loop it points to a sensor somewhere that is not functioning! Oh well thanks for listening and any advise would be greatly appreciated!

I agree 100% with this. Best thing to do is probably try to data-log the car in open loop and see what inputs look suspect. The ECT, O2 sensors, and maybe a few others are common causes of this kind of thing.

the water temp sensor in the water pump will cause all sorts of weird problems

Well I planned my next moves. I am replacing the MAP sensor and the coolant temp sensor. They will be arriving on Saturday. I know I am throwing parts at the problem but allot of professional mechanics have tried to fix this car to no avail. These are two sensors that have not been replaced yet. I really believe it is a fuel delivery problem..

Well today is a new day, so being discouraged yesterday has passed to renewed hope. While I am waiting for the new Map and Coolant temp sensor I am going to replace the IAT. I read this morning on the forum that these sensors can cause my condition to occur. It is a cheap part and will keep the project moving forward?

Do you have a FSM? And before you go swapping out all sensors take a good look at all the connectors that plug into each one and make sure they have good contact. I chased a issue with the w/p sensor on my 94 for a long time until I realized he ground plug in the connector was bent and not making contact giving the ECM a false reading. I had other ones go bad as well

Great advise! I will check them tomorrow. I replaced the IAT today and it was a simple inexpensive job. It made a big improvement, not a 100% but a big improvement. The water pump sensor and the MAP sensor should arrive tomorrow. At this point I think the computer is adjusting the fuel mixture as I drive it, it seems to get better! The car still stumbles but does not ping as much or backfire. When it kicks in, it takes off!!!! Not there yet but on the right track!

Today I changed the MAP sensor. When I went for the test drive I almost did not make it home it ran so bad. It was hardly running on light throttle. Spitting backfiring etc. I slammed it down and it took off, go figure. On light throttle it would not go, had to hit it hard to get it to take off. It has to be gas or a fuel mixture problem. I am thinking it is not getting gas, throttle body block? I do not think it is the mixture necessarily? Tomorrow I have the pleasure of changing water temperature sensor. At this point I do not think that will make a difference but it is worth a try. It still runs well in closed loop for about 2 minutes, leaving the garage it takes off!

My goal with this car was to provide me with something to do and I wanted a learning experience, at least so far I achieved both those goals!

have you checked your fuel pressure and regulator

Yes the fuel pressure seems fine, I have checked it twice. It stays at 40 lbs as engine accelerates. It might be worth it to tape it to the windshield and check it under load. The fuel pump and fuel filter were replaced by the previous owner. I replaced the temp sensor and as I expected there was no change. I also disconnected the EGR vacuum line. It ran like it was on seven cylinders. The things that I know:

It runs well in open loop! Once it gets warm the problems begin. It does eventually get hot probably from the pre detonation. It has not kicked any codes since I replaced both O2 sensors. Before that a lean code did show up.
There has been allot of work done in an attempt to correct this problem. It has been to two dealers with no solution, only a big bill (previous owner) That is why he sold it!

New OPTI
New fuel pump and filter
MAF sensor was changed
New Map sensor
New air temp sensor
New Water temp sensor
The injectors were found to be good.
It was checked by the dealers for vacuum leaks

I can not think of much else to replace.

At this point I am lost.....I could remove the plugs and check compression. I may have a serious issue internally? But why would it run so well in open loop?????

I have decided to replace the egr valve and the egr selenoid. It is a quick fix, not real cheap but quick. The reason is I disconnected the wires to the EGR selenoid and the car ran smoother, it did not have power but was smoother. It also idled ruffer., The other thing I noticed was the car ran much cooler, did not get real hot. I know I am throwing parts at the problem!!!! I do this for two reasons, I like the C4 and a few more parts hanging around will be good for the next C4 I buy, secondly the car is 25 years old, it deserves some new parts, we all do!!