Nellsw88

I've been having a random misfire/surge problem in my 95 z28. Its starting to get worse to the point where its affecting drivability.
If I'm sitting at a red light, the tac will jump and the car will all of a sudden stumble then begin to surge similar to a bad iac or vac leak and it will throw a check engine light. The surge has become so heavy that it begins pulling the car forward(automatic). and sometimes if you mash the throttle at a low rpm or cruising along and get into the pedal a little to much but not enough for the trans to kick down it will miss.
Other times it wont stumble/surge at all. Also, if you go WOT from a dead stop or even normal driving nothing 90% of the time. All this happens whether the car is cold or warm.

The car currently has 75k miles on it, I got the car with 67k. As soon as i got it (car did a lot of sitting) I replaced the fuel pump, filter, and regulator. all the vacuum lines were replaced. Plugs, wires, IAC, ICM, and an MSD cap and rotor (not the opti unit itself) and a new coil. From my research it seems like the opti is crapping out. Just looking for some feed back here before I spend the time and money replacing the opti.

Injuneer

Since the SES light came on, it would seem logical to scan it for codes.

Nellsw88

Will it hold the codes after the light goes away? The ses light only comes on when it begins to surge and then goes away once it levels out again.

Injuneer

Yes, the code that turned on the SES light is stored in the PCM until a fairly large number of "drive cycles" have been driven without the same problem recurring. Removing battery power (taking a cable off, or pulling the "PCM BAT" fuse) will clear the codes, so don't do that.

Since you have a 95, scanning for codes is a problem. You have an OBD-1 PCM and a 16-pin OBD-2 style ALDL connector. If you have an older OBD-1 scanner with 12-pin connector, you can connect it using jumper wires as shown by Shoebox:

http://shbox.com/1/xraycable.jpg

Some newer OBD-1/OBD-2 scanners will work, but some will not. Avoid the Innova scanners, they have a bad data base, resulting in false codes. The typical auto parts store will only have an OBD-2 scanner, and they will try and plug it in, and will not be able to connect. And, 94 and up, you can't flash the codes on the SES light by shorting the ALDL pins. Avoid the $30 Actron "code scanner".... all it does is short the pins and won't work.

Best solution = Scan9495, free software download, written by a member here:

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

Nellsw88

I just scanned the car using the software you recommended. It threw a few codes:
13: bank 1 (left) O2 sensor circuit open
18: Injector circuit
29: air pump circuit
32: Egr
34: Map sensor low-high vacuum

Nellsw88

I just started chasing down some leads. The air pump relay was bad so i replaced that, lucky i have a bunch of these parts laying around. Cyl 2 injector connector wasn't on all the way, maybe that was part of the code 18. I reset the codes, took it for a drive and the only code to appear now is number 34 the map sensor. As of right now the car is idling fine between 650-700 rpm no jumping. Im gonna pull the map sensor and check it. Hopefully its just that sensor.

Injuneer

Check the harness connector for the MAP sensor.... they dry out and crumble, getting loose and giving intermittent bad readings. May only need a new connector/pigtail. Check it using a volt meter. No need to take anything apart. Shake the wires and connector while testing. I can give you the voltages for each wire if you need them.

Yes, DTC 18 indicates a faulty circuit on one of the injectors.

The AIR pump fault does not cause the problems you are experiencing.

Nellsw88

The MAP sensor was going bad. Luckily a good friend of mine works at advanced and had one in stock and gave it to me dirt cheap. The car runs much better after clearing the codes and replacing the MAP.

Unfortunately it still has the idle problem. I noticed right before it begins to surge something will click and you'll hear almost like a fan. Then it will stop for a few seconds. And it throws the air pump code but nothing else.

Injuneer

Next step is to run a data log with Scan9495.

I've seen at least three instances of erratic signals from the optical cam position sensor in the Opti causing sudden RPM surges. Two of them were MSD's, one of them was an AC Delco.

The log typically shows an erratic single frame high RPM reading, and then the PCM responds by increasing the IAC counts and the surge happens.

Nellsw88

In line 60 of the log it has a sudden rpm drop about 50 rpm and the IAC spikes from 32 to 62. Then the rpm jumps upwards of 1000 and then both numbers are all over the place. I attached a screen shot of the log where it begins to happen.