Prospect62

Hey guys. 1994 Camaro Z28 with an LT1 and T56. Has a hot cam, headers, aftermarket torque arm, aftermarket driveshaft, bunch of suspension mods. Car has 107,000 miles on it and probably 10,000 miles on the suspension mods.

I've been trying to diagnose a long-crank issue thinking it's the fuel pump or aftermarket FPR that the previous owner installed. When I took the car out today, a new issue presented itself. I noticed a really bad "shake" when taking off in first gear, car felt like it "wasn't all there" so to speak. Accelerating through the gears I detected a shake in just about all gears at varying RPM's under acceleration. Power felt slightly lacking over normal. Tried it with AC on and OFF, didn't make much difference. I had the car hooked up to Scan9495 and not one single misfire was detected (which this really felt like a misfire to me) so I was thinking maybe a trans mount or motor mount? Maybe a u-joint?

If I really gave it the beans the call still pulled very strong up over 120 mph, no high RPM skipping or missing. Just that weird "wobble" almost under load at relatively low speeds. If I put the car in 6th gear at like 2000 RPM's and gave it some throttle it almost shook me out of the seat it was so bad...yet still not one single misfire code over probably a 30 mile test drive. Hard acceleration and crusing at 65 mph in 4th or 5th gear where the RPMS were over 2000 it was relatively smooth.

Just feels a bit weak but maybe that's just psychological. That shake never happened before until today. Every single thing in my scan data looks perfectly normal to me, but I'm a novice.

Context:

The last time I had the car out I was pretty hard on it, did a couple clutch drops and burnouts.

The previous owner burnt the plug wire on cylinder #5 numerous times until he finally bought high-temp sleeves for the plug wires - those all appear in tact I can see no immediate evidence of a burned wire but I haven't pulled any off. Still - if it was a burned wire wouldn't I be getting a misfire code??

New OPTI about one year ago.

Thanks for any help.

Here's links to my scan data, it looks like I lost the files from when I first started the car. These are all I could recover.

​​​​​​https://drive.google.com/drive/folde...usp=drive_link

Injuneer

What version of Scan9495 are you using? There is no "misfire detection" in OBD-1.

Based on a very quick look:

Did you look into DTC 48 for the MAF sensor? It disappeared a few seconds into the log.

In the idle cell (#16) your long term fuel trims (LTFT) are bottomed out (108 = subtracting 15.6% fuel) on both banks ay idle, and it's still running so rich, particularly on Bank 1 that its using the short term fuel trims (STFT) to pull out even more fuel (as low as 95 = subtracting 25.8% fuel). Since the fuel trims are multipliers, the engine is drowning in fuel, and has to pull out about 1/3 of the normal fuel to keep it from running pig rich. That's an extreme fault. Might account for the "shake". Similar problem in other operating cells. Probably one of the worst results I've seen in 20+ years of looking at data logs.

Knock retard, even at idle.

What's the difference between the two files? I only looked briefly at the first one.

Prospect62

Thank you for that opinion sir! The two logs are the same trip, just interrupted by a quick stop for gas.

I had a feeling it was running rich. And your explanation confirms that. But the “shake” is new. I have about 200 miles on the car since I bought it and the shake just happened today. Before, even though it smelled rich, it never exhibited the shake.

The MAF code is literally because I disconnected it because it looked loose (but wasn’t).

Good to know OBD 1 doesn’t show misfire codes. It’s probably misfiring then.

What would cause it to run so rich? It’s got an aftermarket fuel pressure regulator on it, might that be an issue?

Injuneer

Check fuel pressure. Hopefully the aftermarket regulator is not from Aeromotive. They fail all the time.

- fuel pressure too high
- incorrect MAF sensor readings
- faulty O2 sensors
- issues with O2 sensor extensions
- incorrect injector spec programming
- leaking injectors
- leaking fuel pressure regulator
- PCM not tuned or incorrectly tuned for the modifications

Who tuned it for the cam?

Check the spark plugs for carbon build up.

Prospect62

I’m not sure who tuned it or if it was ever tuned. I bought the car like this.

I have a fuel pressure gauge on the way. Will report results.

Was considering getting a new OEM style regulator, as the one on the car is adjustable and I’ve heard nothing but bad things about those.

Injuneer

Does the AFPR look like this?

https://www.summitracing.com/parts/aei-13107

An adjustable FPR is not inherently bad, but they really have very limited practical uses in most builds. The Aeromotive AFPR has been a problem since they introduced it 20 years ago. Odd, because most of their fuel handling products are built to very high standards.

Prospect62

No, it looks like this...

Prospect62

Could a faulty FPR be causing the rich condition and ultimately this shake that feels like a misfire? Likewise, if the adjustable FPR on the car is still good, could I adjust it to the lean side to reconcile the rich fuel table that Injuneer points out? Or should I just grab a stock FPR and toss it on? Not really sure why the previous owner installed this adjustable FPR, all the other mods were very well thought out. This I don't get - but I'm not even certain this is the root of my problems so I won't villify it yet.

Also, can anyone tell me HOW to adjust this fuel pressure regulator? Can't find any resources online.

Also, I have read that this car calls for a non-vented gas cap which is what's on it. But when I open the fuel cap, a LARGE burst of vapor escapes. I have read that too much tank pressure can cause issues with fuel delivery and even shorten fuel pump life. Should I have a vented gas cap on this car?

Prospect62

Could this be caused by the aftermarket roller rockers? If so, I assume an LT4 knock module was never installed and may be on my list (although at $200+ I'd have to be convinced this is worthwhile at all).

Either way, I'm not sure how this would cause a rich condition OR the "shake" under load which only recently appeared.

I'm just befuddled that the car can run like this and not a single code has been thrown. I know OBD1 isn't as comprehensive but I expected SOMETHING to show up.

Injuneer

Don’t replace the FPR until you test the fuel pressure.

You mentioned extended cranking to start the engine. That can be due to fuel pressure problems. I dug deeper into the first data log and every “cell” actually used in your drive to store the long term fuel trims (LTFT) is showing the 108 (or close to) value. Could be a fuel pressure problem.

For now pull the vacuum control hose off the FPR and check the line for wet fuel or a strong smell of fuel.

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

When you get the test pressure gauge:

- attach gauge line to the Schrader valve. Do not over-tighten. That connection is easy to break off the hard line.

- have someone turn key to run while you watch the gauge. Pump will prime for 2 seconds and shut off

- pressure should reach at least 40 PSI, and hold fairly steady. Pressure should not drop rapidly, say no more than 10 PSI in 10 minutes.

- start engine, let it idle. Remove vacuum line. Pressure should be 43.5 PSI. Per GM spec, anything in the range of 41 - 47 PSI is OK.

- attach vacuum line. Pressure should drop about 8 PSI below the “no vacuum” value. You may see less drop because the data log shows your intake manifold vacuum is a bit weak, even for the HOT cam.

- if the hose on the gauge is long enough, tape the gauge to the windshield and watch while driving. Pressure will vary with throttle pressure. At WOT pressure should be at least 40 PSI, and never above the pressure you got at idle with no vacuum line.

Injuneer

Also, see if you can get a part # off a fuel injector.

Prospect62

OK, gauge attached (hose is not long enough for me to do a driving test).

Turned key on, pressure climbed (not spiked) to 48 and then immediately dropped back down to 0. Repeated this just to make sure I was seeing it right. Checked to make sure no fuel was leaking - didn’t see any.

Started car, pressure went right to 40 and stayed consistent. Revving the car made it dip ever so slightly but it compensated immediately.

Car now warm and running, I disconnected the vacuum hose from the FPR and it was soaking wet with gas, gas was pouring out onto my hand. Pressure seemed to stay consistent whether vacuum line was connected or not. Strong vacuum at the vacuum line.

No time to pull an injector today but something tells me based on what I found you might have some insights.

By the way I took the car for a ride and the shake and rough running still exists. Got some black smoke on startup also.

Chimera96

if gas was running out the vacuum nipple the FPR is bad. I have that adjustable one. Its made by borg warner. There is a alan screw on top to allow FP adjustment.

A FP gauge (ideally one with a hose long enough so you can tape it to windshield to monitor while driving during testing), some form of scan tool ability and an IR temp gun are a must have in your tool box to diagnose issues on these cars.

Replace FPR before anything else and see if problem is resolved.

There are several other reasons the motor can run rich but first resolve the FPR. You can just get a stock replacement

Ditto on Freds note on Aeromotive FPR. Mine was DOA out of the box.

Injuneer

FPR diaphragm is ruptured. Replace FPR with GM part if possible. That's why checking that hose was in my first paragraph.

All the fuel leaking out of the FPR was pouring into the intake manifold, dumping into the cylinders with incoming air. Hopefully the extremely rich condition didn’t wash down the cylinder walls and damage the rings.

Prospect62

AC Delco 217-3295 on order.

Could this issue lead to the "shake" that started this thread? The car always smelled rich since I've owned it, but it never had this issue until a few days ago. Hopefully the fuel washdown didn't create any new issues? Perhaps one or more plugs have been fouled badly?

I took a cursory look at the plug wires today and could find absolutely no evidence of any of them being burned (an issue the previous owner battled). I also took a stethoscope to the injectors with the car running and verified (to the best of my ability) that they are al clicking along nicely.