1993C4LT1

Runs rough as it heats up. Idle is not smooth. Also, letting it sit for 10 minutes or more when engine is warm, it's hard starting. And, on a hard hot start, the car feels very weak. Idle is low based on what I always saw, about 800 rpm seem to be normal. But on the hard hot start, idle is at about 600 rpm.

Car has new:
Opti(last summer)
Taylor wires(month ago)
AC Delco plugs(last fall)
FIC Injectors, 24lb(2 weeks ago)
AC Delco FPR(January of 2015)
Coolant sensor(Summer of 2015)
Both O2 sensors(work fine based on the scan, replaced in May of this year)
I might be forgetting something, I will post if there is something missing.
EDIT, new Fuel Filter(about 2 months ago)
Oh, I'm lean on the left bank. BLM is 140-150. Almost spot on the right bank with BLM at 124.

So what can be causing my car to run rough when the engine is warm?

Here is a plug from cylinder number 3, just for the hell of it.


I've done everything I can, so I took it to a shop my family has been using for 20 years. Mechanic found an exhaust leak at the driver side exhaust manifold. He says the opti is the cause of the rough idle. I doubt it's the opti. Sorry for the long post. Car is a 93 LT1 auto with just under 79k miles. Hopefully I am making sense

When I turn the AC on, the idle is noticeably smoother.

antfarmer2

see if your looseing coolant, smack your egr and put some antizies on your plugs

Kevova

Did you have exhaust leak corrected? If not I needs to be done 1st. Exhaust leaks can cause O2s to read lean. You can try power balancing. while engine is running at idle disconnect IAC. Then disconnect injectors one at a time noticing RPM drop. You may weak cylinder if RPM doesn't change when injector is disconnected. Another possibility intake gasket not sealing causing vacuum leak. Bolts maybe loose.

DGXR

Vacuum leak could easily cause rough running.
Lean bank is one possible indication of vacuum leak.

1993C4LT1

I'll give this a shot. Exhaust is getting fixed first.
I'll tell the shop to check for vacuum leaks. Even tho I replaced the bad ones I saw.

Antfarmer, I don't have overheating problems. Car doesn't leak or burn coolant.

reg_255

Based on others input on a Vacuum leak, I believe if the intake manifold gasket is bad there will be a leak there too.

The LT1 is prone to an intake gasket failure.
Check the back of the engine block for oil leaks.

A small leak here and a small leak there all become accumulative.

Regards,
Roy

cardo0

Any codes? Always check for codes first. Always use the diagnostics first. I like to look at vacuum so have a vac gauge installed right on the dash where most mount their tach. Any but a steady vac needle usually means cyl leakage of some type. But a misfire could make it shake some also.

U have some fancy spark plugs there. Did u have those before the rough running started. Try a return to stock plugs again. Measure the resistance of all your plug wires and look for arcing in the dark w/eng running.

Try a real time scan. I found a AutoXray scanner for my '94 OBD 1.5 at a swapmeet brand new in the box for $40. I can real time monitor most paramenters (cant print or store them though). I believe there is some "trial" software u can use if u have a cable for your OBD I connector and and laptop w/RS232 serial port.

Let us know how the exh leak changes things.

1993C4LT1

I had that problem July 2014. Used permatex right stuff and felpro gaskets. I double checked it to make sure.

1993C4LT1

Only DTC's I have are for VATS, which I bypassed and a DTC for the radio. Believe it or not, those are factory AC Delco plugs. The plugs were in there before the rough running started. I do have a real time scanner. I use 1320elctronics with ALDL Droid. No arcing from the plug wires. Haven't ohm'ed them though.

1993C4LT1

Got code 42 which is this -Code #42: Electronic Spark Timing Circuit Grounded.

Opti right? If so, I guess Petris is the way to go?

Kevova

DTC 42 is more ICM - ECM thing. Ecm controls icm for spark control. Opti sends signals reference signals to ecm for FI. DTC 16 and 36 relate to opti

1993C4LT1

Thanks. I figured codes 16 and 36 were opti related. Just double checking.

Do I use thermal, or di-electric grease?
What brand ICM do you guys recommend?

Just pulled this from the FSM. Hopefully to help someone in the future,lol

1993C4LT1

Dumb question, does hard starting foul plugs?

Kevova

It can and fouled plugs can cause hard starting. It is still possible to AC Delco for a little over 100.00 from rock auto. Look at connector first, use di-electric grease

1993C4LT1

Bought an AC Delco ICM today. The grease it came with is very little, probably the size of a quarter. Is that enough?

Den14

It only requires a thin layer. For the record, dielectric grease and thermal paste are not interchangeable. Without the thermal paste you will cook the ICM. If I remember correctly, the FSM calls for thermal paste in between the block and ICM plate mount as well. It will not hurt in any way for sure.