YBSTOCK

Ok guys I have a 94 z28 M6 208,402 miles. What would cause a check engine light to come on but the car runs perfectly fine. Fine to the point I can spin the tires through 3 gears and hit 100 mph real quick. It a bone stock engine. Goes through about a quart of oil a week. And all fluid levels are good.

moparman

You need to find someone with a code reader or scan tool to pull the codes. A flashing SES light means there's a misfire. Steady light is something else, and guessing usually doesn't work.

parkers Z28

Agreed to many possibility's ..... the auto zone in my town will pull your codes for free!!!
post back with the codes and we will interpret them for you

YBSTOCK

Went to autozone and they tried to short the 2 pins in the 12 pin connector. Of course after reading here that it doesn't work like that, it was back to the drawing board. I have an update to the car though. I was driving it to work today (70 miles one way) and the check engine light stayed off for almost 20 miles. When I pulled up to a stop light after coming off the highway the idle dropped to around 600 rpm and was shaking. I reved it up a couple times and after about a minute of the low rpm and shaking the check engine light popped on and the idle smoothed out and went right back to around 900 and it ran just fine the rest of the way to work. I'm almost positive it needs a new coil and opti. The guy I bought it from had it since 95 and all he did was drive it and change the oil so I'm guessing it's still the original with 208,000 miles on it. I have new plugs and wires and will be installing them this weekend along with a new starter and all new brakes and rotors. I will also be removing the AIR pump this weekend. It is already unplugged and a new fuse installed so as not to throw a code. My next question is when I put the true duals on there with the cat delete, can I delete my O2 sensor or sensors without tuning?
Thanks guys

Injuneer

Your 94 only has pre-cat O2 sensors, so their operation has nothing to do with whether the cats are there or not. The pre-cat sensors are there to allow the PCM to control the A/F ratio in closed loop. If you delete the pre-cat sensors, your PCM will default to an open loop tune due to the codes. You can have the PCM tuned to run open loop without all the codes, but that will eliminate the ability to correct the A/F ratio for changes in the the engine components.

YBSTOCK

Ok cool thanks for the reply injuneer. Will keep those cause they serve an important purpose. Now once I put my pacesetter longtubes on with the xpipe will I have to fit the sensors to the longtubes or can I put them farther down the stream just after the header collector on the xpipe. Basically I'm asking does it matter where I put them in the system? The air pump will be gone tomorrow fyi.

Injuneer

They belong in the header collectors, a few inches after the merge. Even "no emissions" headers (no AIR, no EGR) usually include O2 sensor bungs. I did buy a set of Hooker long tubes that needed to have them welded in, however.

YBSTOCK

Quick update on this check engine light. I haven't had a chance to get the codes read yet but as I understand it OBD1 doesn't store the code? The gentleman that is going to read my codes is out til Monday so that is as good as I can do. Now down to what is going on.
When I start it and drive the check engine light will stay off and as it warms up it seems like it is missing or starving for fuel. It sputters and cuts in and out under a load (probably cruising also, just can't feel it) and no check engine light will come on. Once I get up to speed and settle in cruising usually around 65 (not using cruise control) the check engine light will come on and the stutter and cutting out goes away, no matter if I'm cruising or pushing it to WOT or anywhere inbetween. I'm am confused as to why this would happen, all these symptoms seem backwards. When the check engine light comes on it should run worse not better. I also would think that if it were a fuel problem it would also happen all the time it certainly shouldn't get better with the check engine light on. Same if it were a spark problem it would do it when the check engine light is on not when its off. Until I can get the codes read Monday anyone have any thoughts on this?

rngilliland

OBD1 does store codes.

Injuneer

As noted above, OBD-I stores the codes, and they stay stored until you clear them, or the problem goes away and they self-clear after a large number of drive cycles without the problem recurring.

The engine runs better when the code sets because the PCM (on many codes) will take the offending sensor or control device out of the circuit, and control the engine using a default portion of the program that does not require that particular sensor or device. Runs better on the default program than it will with a defective sensor. MAF sensor is a perfect example - if it fails the PCM will default to speed-density fuel control.

You have to get the codes read.

And don't let them tell you they can flash the codes on the SES light by shorting the ALDL pins - 93 was the last year that would work. And don't let them tell you they can't read the codes because the SES light isn't on. They can read stored codes at any time, and there are active codes that do not even turn on the SES light.

YBSTOCK

Very good info thank you. I was thinking along the same lines as what you said about taking the offending sensor out of the equation and the computer going to a predetermined fall back tune almost like a limp mode. Question off of that thought. Is it possible for the optispark to miss with the engine light off but when it turns on to go to a predetermined spark timing so that it seems like the miss goes away?

Injuneer

In OBD-I there are no codes related to the high voltage section of the Optispark distributor, and no misfire codes. If the misfire is due to a problem with the cap, rotor, plug wires, plugs, coil there will not be any codes. So there is no SES light and no default program.

There are two codes for the optical cam position sensor in the Opti. There is DTC 16 for the low resolution cam position signal, and if that code sets, the PCM shuts down the fuel system and the engine stops. And that code does not turn on the SES light.

The second code is DTC 36 for the high resolution cam position signal. This code sets if the crank rotates 10 revolutions (based on the low res pulses) and the PCM does not detect a high resolution pulse (normally, there are 360 pulse for each crank revolution). Irregularities (other than total failure) in the high resolution signal can cause erratic timing and an erratic tach signal. But the code only sets if there are no pulses. This code will turn on the SES light, and the engine will continue to run, using only the low resolution pulse signal to time the spark and the fuel injectors. The only result of this is a very tiny loss of timing precision - so small that you would probably never notice it.

Rather than speculating, you need to find a way to get your codes read. That will tell you EXACTLY what is causing the problem.

YBSTOCK

Thanks for the help. I went to O'Reillys yesterday and hooked up their scanner. It couldn't read the computer. It kept asking to check that the ignition was on. The guy that was supposed to do it Monday had to postpone it til Friday. Guess i can't complain too much when he is doing it for free. I'll report back when I have some codes for you guys.

moparman

It's doubtful any of the auto parts stores, such as O'Reilly, Autozone, Advance, etc, will be able to pull the codes, as the code readers they use are for OBDII. Need to find someone with a code reader or scan tool that can pull OBDI codes.

YBSTOCK

UOk guys just got back from the mechanic who said he could pull the codes. He printed off some directions (probably off alldata or something) and went to work trying to jumper the 2 wires together. That lasted for about half an hour with him going back and forth many times to the office. Then he comes out with a scanner and I think ok we might be getting somewhere now. Another 45 minutes goes by and finally he backs my car out and says the problem is that I need a new pcm cause his code reader won't communicate with the pcm and he proceeds to tell me "when you jumper these two wires it will do blah blah blah". So I tell him thats not how the 94 and 95 work and he interrupts me and says basically if you know how to do it then why'd you bring it to me. So needless to say I didnt get any info i didnt already know and between that and his rude attitude I dont think I'll be going back there ever. Also, let me say I do own a hotrod and custom shop where I work on old cars with carbs and such. I did graduate Wyotech in 2002 and do know alot about cars. Basically what I'm getting at is anyone can read a computer and do what it tells them to do if they have basic reading skills like this guy. That doesn't make you a mechanic. People like shoebox and injuneer and others on here are real mechanics. I know its not possible for a person to know every vehicle and I don't expect that what I do expect is what I'm not affraid to tell my customers "i don't know and you might wanna take it to someone that can help you better than I can". Honesty goes a long way don't tell me I need a pcm but have nothing to back it up. Just tell me I thought I could pull the code but I was wrong. Sorry for the rant but I guess I'll just get the cable and do it myself. Ill let you know what codes I get.