I have a 1994 z28 and everything works fine except for after 1-5 minutes on the road after a cold start, it will, seemingly, misfire. I have no idea what could cause this. We have been having some electrical issues with the throttle body so maybe it's an air issue? We just replaced the egr valve as well which was effectively a vacuum leak. The only after market thing on the car is a magnaflow cat back exhaust. Other than that everything is stock so I wouldn't think it would need a pcm tune or anything. We have also been fighting a high idle issue and had the tps replaced, and now the egr valve as previously mentioned. If anyone has any suggestions on where to start, that would be great, because I am totally lost.

 

Hi josh adams37,

Is the computer setting any codes or turning on the check engine light? If you don't have a code reader I urge you to start looking for one. These cars are very hard to diagnose without a code reader. I have a Actron 9145 that works great with my 1995 obd1 car and my 1996 obd2 car. I looked on eBay but none of them are listed right now. There are several Inova units for sale but I am not sure how they work for your car. Here is the link below for the inova. There is also a computer program that garydoug wrote that allows you to get the full info from the pcm and there is a link in the stickies that will tell all about that.

https://www.ebay.com/itm/25442836275...3ABFBMltrFku5f

 

I've tried the Innova way back when i first bought the car but it unfortunately does not work with my style of obd1. Also unfortunately, the misfires do not consistently throw check engine lights. Every time it misfires it seems to be about a 50-50 shot if it does so.

 

Avoid most Innova and Equus scanners. The internal database for LT1 codes is corrupted. The typical scan AWAYS produces a list of 7 bogus codes.

Scan9495, free downloadable software will pull LT1 codes for multiple systems - PCM, ABS, SRS - and resets codes. Also can produce a data log that shows what's going on with critical sensors and PCM parameters.

https://www.camaroz28.com/forums/com...95-lt1-874306/

Guide to interpreting scanner data:

https://www.camaroz28.com/forums/com...-guide-886891/

Your problem would appear to be related primarily to “open loop” operation - the way the PCM controls the engine from cold start, until the O2 sensors get hot enough to work, the coolant gets sufficiently warm, and a timer (typically 206 seconds) times out. Then it can enter “closed loop”, where the PCM uses the O2 sensor feedback to control the A/F ratio.

 

I appreciate all of the great insight! What are some common culprits of open loop misfires?

 

- faulty coolant temp sensor

- O2 sensors so far off the PCM can't correct in open loop

- leaky or dirty injectors the PCM can't correct for in open loop

- possibly IAC not maintaining elevated idle RPM programming calls for on cold start

- if it’s auto trans, incorrect converter lockup making the misfires more noticeable.

Probably some more that I’m not coming up with right now.

What is wrong with the OBD-1 connector on your scanner? 94 has a very common 12-pin connector. Unlike the 95 with its hybrid 16-pin OBD-2 style connector. If you're getting an SES light when it happens, even intermittently, your going to get a lot better indication of the cause. Less guessing necessary.

 

IMO this is huge. I made the mistake of treating my 1995 like an old car when I first decided to stop taking it to the shops and fix it myself. Too many systems interlock with each other through the pcm.

 

I don't even like to make suggested fixes before diagnostics are done.... because a lot of people run with it and start playing "parts replacement roulette".. Drives me crazy.

 

What is wrong with the OBD-1 connector on your scanner?

Nothing is wrong with it, I simply just do not have one. Next time it throws a code when it misfires I am going to have it scanned, however. Hopefully, that will be today but we will see. As I said, it throwing codes is seemingly random whenever it misfires.

 

It's misfired a few more times but no codes thrown so I can't get it scanned. Suggestions?

 

It has misfired a few more times but no engine light shows up. Suggestions?

 

Unless you pulled battery power from the PCM, all the old codes should still be stored. It takes a number of drive cycles without the problem recurring before the codes auto clear.

 

I had it scanned and they told me that there are no stored codes. I've got no leads here.

 

It sucks that whomever is scanning the car can't get you the info. Almost surely the pcm is noticing the stumble or missing that you are noticing and recording a code. Some codes do not turn on the check engine light but are stored anyway. I suspect that the scanner being used by the folks checking your car is not working for your car, i.e. not returning a valid scan.

If I were you I would work toward getting the scan94/95 software (or some other working scan tool) and just keep driving the car in the mean time (short trips, within your triple A tow radius, lol). If you like the car and want to keep driving it having the ability to read codes in the future is super helpful, I would dare to say necessary? Also by keeping driving the car what ever is causing the problem may get worse and reveal it's source in doing so. I am an electronics tech for automatic gates and people call me all the time with intermittent troubles. Many times I can not find the source of the problem until the problem gets bad enough to where I can see it malfunction in real time.

 

There's no misfire detection in 93-95 OBD-1.