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I've tried to solve this and nothing. I had an exhaust leak at driver side exhaust manifold gasket. Fixed that. Manifold was not cracked, luckily. When I accelerate at certain RPM's, I can feel it pull timing. When the car warms up, you can hear the exhaust popping at idle. It goes away if I turn the AC on. I checked compression on all cylinders and every one was 140-150 psi. I checked it with a cold engine. Should I check compression with the engine warmed up? I haven't datalogged the car since I had the exhaust leak. At that time, my left BLM had a max of 150. Right BLM is 124. But I will datalog it again tomorrow. Car gave me a code 42, so I put a new AC Delco ICM. I assume my ECM is good because when the key is in the ON position, the SEL light is lit up. My question is, can a bad opti cause an LT1 to run lean? I'll post a vid of the exhaust popping at idle tomorrow.
Here's what's new on my car
Opti(about 1200 miles on it)
O2 sensors
Plugs, AC Delco
ICM, AC Delco
Wires, Taylor
FPR, AC Delco
Fuel Filter
Bosch III Injectors
I might be forgetting some parts.
"IMPORTANT NOTE: part of the ICM tests include verifying that one of the ICM connector's connection is receiving between 0.5 vAC and 4.5 vAC while cranking. This simple test verifies the ECM is working **AND** that the OPTI is working. THIS IS A GREAT TEST TO PREVENT UNNECESSARY OPTI REPLACEMENTS." I saw this info in another thread, also in my FSM. How do I know if I'm getting .5 vac and 4.5 vac? With an ohm meter?
Both times I had opti failure, the car would turn off. Mine doesn't turn off. Back then, I would check for pulsing at the injector when I suspected a bad opti. Sorry for the long thread. Just want this problem solved already. Any help is appreciated.
Fuel pressure goes to zero in about 2 hours. Key on, engine off, fuel pressure is at 42 psi. Running, about 38 psi.
I dont like the compression numbers. My '94 when i measured compression on the original motor (while i had the plugs out & replacing the opti) was all 180-190psi except for my only bad cyl #8 which was like 140psi. Kinda send me on a search for an engine to rebuild but i drove it another 10k mi at least w/o any issues. U really need a leakdown test to see if all the cyls are leaking.
Yes the compression test should be done at norm op temp but who the heck can get their hands in there to remove the plugs w/hot exh manifolds? More important is ensure the throttle blades are held open.
Now the BOSCH III injectors raise a question or 2. Did u upgrade the injectors from stock? Did u have it retuned for a new injector pulse width/constants. I think this was an issue in your previous thread(s).
I think i will wait for your answers here before making any suggestions.
Yes a scan will tell u the condition of your O2 sensors and any knock issues. Gives u a look at the timing adv too. Always scan if u can.
Yea I had mentioned my problems in another thread. The injectors are 24lb, which Jon said won't need a tune. I'll scan the car tomorrow morning. I believe my spark advance was 22 at idle. Didn't see any knock retard. I thought that was good compression, but apparently not. You're right, pretty impossible to do a compression test with the engine warm. That'll burn me good
OK, here's the video. Hope you guys can hear the pops.
Scan when cold:
Left bank BLM 132
Right bank BLM 124
IAC 48-50
Scan when warm in closed loop:
BLM Left bank 136
BLM Right bank 121
Spark advance at idle, 22. 30 when revved.
IAC went to 0.
Didn't see any knock retard.
O2's were cycling between 100 and 900 excellent.
That popping is not very loud.
Does it do it cold? or just hot?
I ask this because my 93 does this a bit too. Never cared to look into it much because the car runs 100%. Does not do it cold, and not with the A/C on. Just hot with no load at idle. Honestly, if it runs 100% I would not worry about it. Its a 23 year old car (in my case by build date 10/92) and it has batch fire fuel injection....its not going to be perfect.
Now if it was not running 100% I get that being a problem.
No CEL, and that's what blows me away. Because IMO, I'm pretty lean. BLMs should be 128. So Left bank is lean and Right bank is slightly rich. When it warms up, the popping sound stars. Never when cold. Does it when coolant hits 190-200 fahrenheit. When warm, it's hard starting and has a rough idle. You can see the wheel shake slightly, barely. Idle smoothens out with AC on. My dad swears that popping sound is a miss. So I asked a mechanic and he said it is. He said it's the distributor. I've driven cars that are misfiring, and mine doesn't feel like that.
The weird part, before fixing the exhaust leak, the popping was only on the driver side and louder. Now on both sides, but quieter. I don't remember if my previous 93 C4 did the same pops.
No CEL, and that's what blows me away. Because IMO, I'm pretty lean. BLMs should be 128. So Left bank is lean and Right bank is slightly rich. When it warms up, the popping sound stars. Never when cold. Does it when coolant hits 190-200 fahrenheit. When warm, it's hard starting and has a rough idle. You can see the wheel shake slightly, barely. Idle smoothens out with AC on. My dad swears that popping sound is a miss. So I asked a mechanic and he said it is. He said it's the distributor. I've driven cars that are misfiring, and mine doesn't feel like that.
Ok, so its not 100%. Got 'ca.
What do the BLM's do with A/C on? I bet it richens up a bit.
A tool that I've started playing with is a inductive spark paddle probe. Its part of my UScope Occiliscope kit. They make stand along units. just search for paddle probe. They also make a plug wire lead that would work on the LT1.
You can "view" and check spark with it running and see just exactly what the issue is. I'll let Dan explain.
This might help to see if its an ignition miss or fuel miss.
Like 93rubie mine also pops a bit at idle and always has. Sounds like you did some good diagnostic work finding the leaky manifold gasket which would for sure cause some false lean issues.
Your BLM's honestly are not that bad. It's running a tad lean, but your car is correcting based on o2 feedback and adding a small amount of fuel. Therefore, it's completely safe to drive and no harm is being done.
0 IAC counts at idle is usually indicative of a vacuum leak. Do they jump when you turn a/c on? what is the idle speed when warm and does it have a hard time settling down?
Has someone messed around with the throttle stop? Its on the throttle body on the drivers side. It SHOULD have a metal cap over it. I've seen a few C4's missing this b/c people mess with it to get the idle decent when they have NO idea what they are doing.
Like 93rubie mine also pops a bit at idle and always has. Sounds like you did some good diagnostic work finding the leaky manifold gasket which would for sure cause some false lean issues.
Your BLM's honestly are not that bad. It's running a tad lean, but your car is correcting based on o2 feedback and adding a small amount of fuel. Therefore, it's completely safe to drive and no harm is being done.
0 IAC counts at idle is usually indicative of a vacuum leak. Do they jump when you turn a/c on? what is the idle speed when warm and does it have a hard time settling down?
I'll check to see what it does with AC on. I'll have to check what warm idle is when warm, to give you an accurate reading.
Has someone messed around with the throttle stop? Its on the throttle body on the drivers side. It SHOULD have a metal cap over it. I've seen a few C4's missing this b/c people mess with it to get the idle decent when they have NO idea what they are doing.
IAC counts at 0 at idle is odd.
I left my car for a diagnostic with a mechanic, and he said he adjusted my idle speed. Pretty sure he did it through the IAC. I read up on how to reset it, per the FSM. But how do I set it 'correctly'?
Went out an drove the car. Left BLM had a max of 142 at 1600 RPM. Coasting down the road, both BLM's were at 128
Honestly, the car is running better than I remember. I did find this, but with the engine running I put my finger up against it and it didn't suck it in. So I guess it's not a vacuum leak? Idle with engine warm, in drive, foot on brake(auto trans) is 625 RPM. In park, about 700-725.
That's for sure a problem right there. Fix that and your IAC counts should come up and most likely your BLM will come down. An easy way to tell if that is pulling in unmetered air is using an UNLIT propane torch. The leak will suck in propane and you'll see fluctuation in idle speed. The same can be doing using brake cleaner or carb cleaner, just be careful and do it on a cold motor.
That's for sure a problem right there. Fix that and your IAC counts should come up and most likely your BLM will come down. An easy way to tell if that is pulling in unmetered air is using an UNLIT propane torch. The leak will suck in propane and you'll see fluctuation in idle speed. The same can be doing using brake cleaner or carb cleaner, just be careful and do it on a cold motor.
Will do, thanks
Originally Posted by TorchTarga94
Brandon,
Did FIC recommend the 24lb. injectors for your 93'? Aren't the 92'/93' LT1s 22lb. from the factory? Are you still using the stock FPR?
If all else seems to be well fix the crack in that coupling and start putting some seat time in that thing.
Yes, he told me to buy the 24lb over the phone. Stock FPR. And yes, will fix the coupling.
Anyone with 93's running 24lb on a stock engine with stock tune with no issues?
I think they sell an injector just for the 93 in the rebuilt section, 93s need specific ones and I don't think 24lb are right. I think you got the wrong injectors in there. I know this cause I've been lining up to get a set myself.
I was actually thinking about just having my originals cleaned and gone through.
Last edited by touyech2883; 11-04-2015 at 10:40 PM.
11-01-2015, 05:13 PM
LT1 Running lean
I did find this, but with the engine running I put my finger up against it and it didn't suck it in. So I guess it's not a vacuum leak? Idle with engine warm, in drive, foot on brake(auto trans) is 625 RPM. In park, about 700-725. 
