Hey y’all!

Recently became the owner of an old 1994 Camaro Z28 4A with 122k miles. I’m having a problem where the car will start to buck violently between 2k-2500 rpm, it feels almost like a fuel cut. Goes away at full throttle when the RPMs get higher, but very occasionally also happens at high rpm’s around 4500. It only happens when the car is around operating temp, and only sometimes. It happens in every gear. Verified fuel pump is ok, ignition coil ok, spark plug wires all ok, what else could I check? Replacing plugs in a few days, but otherwise should I check opti, injectors, o2s? Ive read similar posts but nobody seems to have a definitive answer. Not sure anybody will see this but any help at all will be much appreciated!

 

A few thoughts:

Did you verify fuel pressure with the engine under load?

Have you had the ICM tested, making sure it got good and hot?

SES light come on? Any codes?

Did you check the ignition wires for arcing in the dark, sprayed lightly w/ water?

Have you checked the rubber intake elbow for cracks, tears, or being off the bottom of the throttle body flange?

 

Thanks for your reply! The ICM appears to be working fine, the plug wires do not appear to be arcing at least not while parked, and the SES light does not illuminate though I have no way of reading codes at this time. The intake boot does have a noticeable leak which I will be addressing soon, and I haven’t tested fuel pressure at load which I will be doing soon. The car was sitting a bit before I got it so my first thought was the fuel pump, but it does seem to be working so I’m not sure. Thank you!

 

The ICM can be OK, until it overheats, picking up heat from the cylinder head. Runs fine 'til it heat soaks, causes issues, and returns to normal when it cools down.

Fix the intake boot temporarily, even if it takes some duct tape. Air bypassing the MAF sensor forces the PCM to use the long term fuel trim corrections to keep it from running lean. When the throttle opens suddenly, the boot leak covers up the sudden inrush of air and instead of an enriched mixture for the inflow in air it runs lean and falls flat on its face.

Scan9495 (free download) plus a decent cable allows you to monitor the PCM, data log, pull codes, clear codes, activate actuators, etc. The author is a member here, and can help with install problems.

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

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

 

I appreciate it! Will be applying your advice asap.

 

Actually, to be more precise, the problem only happens after the car has already accelerated to speed. Like, from a stop, even when warm, it revs fine throughout, but then if I try to apply 50% throttle from a roll then it won’t accelerate well at all, basically at all speeds from 2nd to 4th gear. It’s like the 50-80 mph acceleration that causes the most problems. That’s why I initially wasn’t thinking the ICM was the issue because wouldn’t that stay warm once the engine reaches temp no matter what speed the car is going? It’s been a frustratingly intermittent issue.

 

Are you using premium (at least 92-octane) fuel?

 

I’ve tried 89 and 91, but the problem persists with both 89 and 91, though it does seem to happen slightly less on 91. I don’t have access to anything higher than 91 unfortunately, but was the LT1 designed for that high octane? I have tried to find an answer to that but haven’t.

 

Your best bet of being able to track down issues without wasting money on parts is going to be a scanning solution as Injuneer suggests with scan9495. Effectively troubleshooting a modern car demands the right tools.

 

if only the car was about 2 years more modern… thanks for the help though, I will definitely be doing that asap.

 

The 1994 Owner's Manual indicate:

Avoid fuels with more than 10% ethanol.

The octane levels indicated above are for vehicles operating at or near sea level elevation. If the elevation where you live is significantly above sea level, the octane requirements are lower, and as a result is gets harder to find 91 octane at locations at higher elevations.

If you run less than premium often enough, the PCM switches to a low octane fuel ignition timing setup. Run low octane once in a while, the knock sensor picks up the knock (before you can even hear it) and the PCM retards the timing. You will feel it. But it isn't going to be "car will start to buck violently between 2k-2500 rpm, it feels almost like a fuel cut.". Will just feel more like a flat spot.

If there is a fault on the knock sensor circuit, the PCM defaults to significant ignition timing retard all the time, even at idle. You will notice it.

Do you have a manual or auto trans?

If you are trying to accelerate in the wrong gear at low RPM, the combustion chamber pressure and temperature builds. That is exactly when the PCM uses the exhaust gas recycle (EGR) system to cool the combustion temperatures. A problem with the EGR system may cause issues. EGR is only used below 3,500 RPM, not used at all at WOT. The other problem with lugging the engine is the fact it increases the voltage required to fire the spark plugs, and that's when any weak components in the ignition system will break down, and you will get arcing, misfires, cross-firing, that won't show up when the engine is idling or under low load.

The cap of the Optispark distributor is prone to carbon tracking (just like a normal distributor). 93/94 LT1's used an "unvented" Opti, and they are prone to problems with moisture and due to the ozone generated by the high voltage, causing corrosion that can mess with the optical cam position sensor in the rear portion of the Opti. If the car still has the original Opti at 122K miles, could be a problem. Cap and rotor can be replaced just like a "normal" distributor.

To me, the intermittent problem has to be related to ignition problems or A/F ratio problems.

With an auto trans, there can be issues with the torque converter incorrectly locking and unlocking under stress, internal trans problems, etc.

Scan9495 is a very effective tool for analyzing problems like this. It was written specifically for the 1993-1995 LT1 engine. GaryDoug (electrical engineer) developed it, I (mechanical engineer) have - in the past - provided reviews of the data logs to look for problems. I've kind backed off on reviewing logs, because of the reasons stated in the second post in my "LT1 PCM Scanning Guide" linked above. But I can make exceptions.

 

I have the 4l60 auto, but it appears to be working well. PO states it was replaced recently and visual inspection seems to corroborate this. It’s entirely possible it’s dying but I’m hoping not. The Opti is not original to the car, once again PO stated it was replaced in the last couple thousand miles, but there does seem to be a bit of moisture in the general vicinity of it, but it’s hard to tell if it’s just old dirt considering the engine is filthy as of right now. I would love to know the most effective way to diagnose EGR issues, as I would love to be able to rule that out.

 

Something else to mention is am having abnormally long cranking during semi-warm starts, like if I drive the car to work then go out and start it for my lunch break after sitting for a little while it’ll crank for like 30 seconds longer than normal before starting. This doesn’t happen at cold starts. Could it be related? Thinking it might help narrow down the issue but i can’t find how it would be related considering fuel pressure is fine and doesn’t drop too quickly after the engine is shut off.

 

Might be leaking injector(s). When you try to start it warm, put the accel pedal on the floor BEFORE you turn the key to “START”. Turn key to start and continue to hold the pedal on the floor until it starts. That puts the PCM in “clear flood” mode. If it starts quicker, it's flooding. Could be an injector, or could be a problem with the coolant temp sensor telling the PCM the engine is colder than it actually is.

EGR is tricky. If the valve is sticking open when it's supposed to be closed, it can cause the engine to stall at idle, or run rough at higher RPM. The PCM runs a test on the EGR system by commanding the EGR valve to open, and watching for the expected change in manifold absolute pressure (MAP). If it doesn’t see the expected change it sets a code (DTC 32). Problem is the OBD-1 test used 93-95 is not very sensitive. People block off EGR and the diagnostic often doesn’t pick it up. The OBD-2 test is much more accurate.

PCM also checks the circuit for the EGR vacuum solenoid, and sets DTC 27 if there is a short or open circuit.

 

So I did the plugs (took almost 2 whole days ) and discovered that 7 of 8 appear to be original after 122k miles (?) and the car is running MUCH better. The problem hasn’t surfaced at all since, even when intentionally trying to make it happen. Crossing my fingers that a simple tune up is all the car needed as it seems a bit neglected.