Nathan Plemons

Alright people, here's the situation. As many of you have read my posts before you know my mechanic has a 95 Z-28. The day we started working on my car, his started acting up. It had been running fine when it just started to get this terrible miss. We found one cut plug wire, which we replaced, no love, same crap. He tried a different coil, no love. He even tried a different electronic control module, you know the little thing the coil mounts to. At that, he took off down the street and about two minutes later came walking up the driveway. Well, he had managed to coast home, but we still had to pull the car back to the garage, being the driveway is uphill. It quit running period, wouldn't even try to fire. We had to assume the worst with the opti.

Anyway, he knows a guy at the dealership and managed to get a NEW one for FREE. So he puts this on and, no fire. He then thinks it's the coil again, changes that, no love. Electronic control module? Changes that, nada. We even did the screwdriver thing to determine that the COIL wasn't even doing anything, so it wasn't even getting to the opti. At this point we were grasping for straws. I just happened to throw out, "Fuse Maybe" Sure enough we started checking fuses and we found a ten amp that was toast. We replaced that and the car fired right up. It ran fine on part of our test run, but on the way back it started missing again, LIKE IT WAS BEFORE WE STARTED!!!

This time it was overheating because it had air in the system. We bled that off and filled it back up, let it cool off and tried again. Same story, it would run on the way out, but not on the way back. Not believing that the car would only run correctly while headed south we did a little more looking. As it turns out if you let the car idle, this is what we found. When you first start it, it'll run just fine until the computer gets out of diagnostic mode, then as soon as it sets it's service engine light, it starts running like crap. Now the service engine light has been on since he bought the car, and has always run fine. In fact it was running perfect until the one day it just started acting so stupid.

Having nothing else to try at this point, we thought maybe if we unhooked the battery and let the computer forget EVERYTHING it knows. So we try that, now it runs like crap ALL the time. It doesn't even run correctly when you first start it.

Now, to make it even better he has had someone try to scan it before and the diagnostic port doesn't work. So what now??? We are gonna try to investigate why the port doesn't work so we can fix it, probably gonna be a dealership thing.

Anybody got any ideas on this one? We are about to pull our hair out. His car missed race day after having worked on it all day. It's very irritating.

One thing I just thought of, if the port doesn't work with a code scanner, can you pull the codes manually with the paper clip method?

Any help would be greatly appreciated on this one.

nobodyunknown

Id change the o2 sensors I think theres 3 of them on the Z/28.
Then I'd try testing the ECM,Believe it or not the way the factory repair manual say's to test it is,With the motor running bang on the ECM with your hand or rubber mallet,If you notice any change its bad.
My wifes 94 camaro ALDL connector doesnt work either I took it to the dealer & basicly they ignored my questions about it changed my plug wires & sent me on my way.No love from them for sure but it fixed the car.

Nathan Plemons

I'm sure the dealership here won't do him any favors, it's a heavily modified, not close to emmisions legal car, they probably won't even thouch it.

BTW, he has replaced the PCM before, it could be bad, but I doubt it. We'll certainly give it a shot.

Nathan Plemons

Anybody else?

Rich_01659

the diagnostic port may not be the problem. it will not work with the standard OBDII cable. 94/95 GM require a special cable. what type of scan tool are you using? the Auto X-ray requires the BLUE cable ,green is standard OBDII.
Good Luck

Nathan Plemons

This was trying to be scanned at a shop, so I don't know what they were using.

It's still OBD I also, so it won't be the MOST informative thing.



[This message has been edited by Nathan Plemons (edited 07-26-2001).]

Nathan Plemons

Ok, so I have been informed that the manual way to extract the codes doesn't work, meaning the port is completely toast. Is there fuse on this? Could it be the computer, or just a fuse going to the computer? Is it really a wiring issue. SOMEBODY HAS TO KNOW, THIS CAR IS GETTING TRULY IRRITATING.

ChrisB

Okay, without knowing that code you are working yourself for no reason. First thing I would do is try simply running a new set of wires for the aldl from the pcm

His should have the 16 pin OBDII style connector.
The OBDII connector has the following
pinout (for OBDI implementation)

01 02 03 04 05 06 07 08
09 10 11 12 13 14 15 16

The ones that matter to us are

04 = Power Ground
05 = Ground
06 = Mode Select
09 = Serial Data
16 = +12V


The PCM has the serial data on Blue 30 (D30).

Mode select needs to go to Blue 20 (D20).

ALDL Ground needs to be tied in to Blue 1 (D1).

Mode select typically isn't used on our cars for the laptop based scan tools or the LT1_Edit program. It's for diagnostic modes that only the Tech2 will allow. So, it's good to wire up if you think one day you'll hit it with a Tech2, but you don't have to have it wired up.

I have the pinout for the pcm at

http://shared.slowcar.net/PCMpinout.doc

which should answer any other questions there.


IF you can't get the ALDL port to work after running new wires, then something is definitely fried in the computer. See if you can borrow another one out of a similar car (94/95 with the same tranny type).


If you/he just wants to try replacing parts, I would replace

1) O2's (as suggested above - your car is in "open loop" until the o2's heat up, and this would coincide with your description of the car running fine initially).
2) Opti-spark.
3) PCM


Good Luck!

Nathan Plemons

Thanks everybody for your input, we think we have it figured out now. He replaced the o2 sensors and purchased quality extensions for them. When we first fired it up, it would backfire every few seconds. He decided that was the timing, but let it run because we were going to try to fix his A/C. It had never worked since he bought the car, we just figured it was completely empty. Sure enough we filled it up with a R-134a recharge kit and it started working. It cooled fairly well, but I told him it should cool better if the vehicle was moving. We went to take it for a spin, then we noticed something. It had kinda quit backfiring at some point. In fact, the car was running great.

We drove it around a little and it never backfired again. We finally came to the conclusion that the backfiring was a result of the stock computer still trying to re-adjust to his cam and such.

ChrisB, thank you very much for your information about the diag port wiring. We are going to try and fix that in the next day or so. He'll need it sooner or later and your info looks to be top notch.

One more quirk I thought I might mention. The heater/A/C blower motor on full blast equates to like the second setting on my car. The motor makes a little noise too, kinda like bearings or something. So here's the question, fixable? I know it's replacable, but how much? Also, where is it? Somewhere up under the dash? Why can't it be like a vette where it would at least be easy to find?