Now that the high rpm oil pressure problem has been corrected, the car has developed a hard start issue, which it never had in the past.

Anytime I disconnect the battery, it takes a while for the car to figure out how to idle. So, in the beginning, I thought this was this issue and it would go away. The car is idling, almost like normal, and the car is still hard to start, especially when warm. Once running, no problems at all. No codes either.

When cool, the car starts easier. When warm, I have to crank the motor way too much. It smells really rich until it starts running. When cranking, I hear that nice "woofing" sound as if timing is off.

Installing the oil pan, I had to remove the O2 censor, crank the motor backwards to check for clearances, and disconnect the battery for a few day. Everything else that was done would have no effect on how the car is running or starting. I checked the O2 censor and connections, all looks good there.

GRRRR, this car is moving to my bad list!!

 

Years ago a friend of mine had a car that wouldn't start for anything. Turns out he had the valves adjusted too tight, and they were basically staying open. He got the noise you were describing because fuel was actually burning as it got pushed back into the intake or down the exhaust pipe.

I'm thinking that maybe if your valves are right on the verge of too tight, when the engine is cold and the lifters have bled down it allows the cylinder to seal well enough to start pretty easy. Once it's warm and the lifters are pumped up and / or the pushrods expand slightly due to the heat, it's holding your valves open a hair and causing your headache.

Seems like if this were the case though the car would be noticeably down on power.

BTW you mentioned an oil pressure problem. A change in oil pressure could also affect hydraulic lifters and possibly cause this.

 

Pull the vacuum line off of the Fuel Pressure Regulator (FPR). If it has any fuel in it, the FPR is bad. Hard starting and rich when hot is a typical symptom.

 

Not to thread jack, but would a bad FPR cause the car to kinda "studder" when you go to get on the gas?

I only ask because my car has the same symptoms as the OP, with the exception that mine will idle rougher than normal (nothing really bad, just kinda choppy after it warms up a little) and kinda studders when I get on the throttle.

 

I had that issue with the AFPR. I had to much pressure going in and once I only gave it 1 PSI above normal the issue went away.

 

FP is good; regulator seems good (I didn't see any gas in the vacuum line). Once the car is cold, it fires within 1/2 a second. When hot, near impossible to start. Let it sit about 15 minutes, hard to start, but I have to pump the gas pedal. When it starts, I can smell raw fuel…

ICM??

Once it starts, it runs great!

 

Check the fuel pressure. Should hold steady. If it bleeds off quickly, could be a leaky injector or a bad fuel pump check valve.

It's not the ICM. The either work or they don't.

Raw fuel smell says too much is getting into the cylinders somehow.

 

I have an on board FP gage and it holds steady at 38 - 40 PSI. I pulled the IAC and TPS last evening and the IAC was very dirty. I ordered a replacement and hope this is the issue. Went out this morning and the car fired within 1/2 a second cold. Hot, it cranks and cranks. I have to hold the gas down about 1/10 throttle before it will finally start. Once started, it runs fine.

Do others agree that this sounds like the IAC?

 

It could be, but it sounds kinda backwards. It takes more air to start the car cold than it does when it's already hot. If the IAC was stuck it would make sense if you had to give it throttle when cold, but not when it's hot.

It certainly won't hurt. Be sure to clean out the passages well and be sure to go through the IAC relearn / reset procedure in your FSM.

 

Sounds like the FPR to me. If you have to open the throttle to get it to fire, you could have too much fuel, and you need more air to start it. I had this issue before with my LT-4, and I never could pinpoint it.... until I blew the engine (unrelated) and when dismantling it, I removed the vacuum line from the fpr and out came a bunch of fuel.

Check the ECM coolant temp sensor. If it has failed you could be running lean or rich all the time. You can test it with a digital volt meter, thermometer and a cooking pan of water on the stove. There is more than one coolant temp sensor on this engine.

The other thing that comes to mind is the oil pressure start switch/fuel pump relay. Not sure on the C4, but other GM vehicles of the era used the oil pressure sender/sensor as a back up to a failing fuel pressure relay. When above 8psi, the oil pressure sender connects the circuit for the fuel pump relay.

Easy way to eliminate the ICM, test the spark when its not starting... An osciliscope would be ideal. But as other have said, when they go... they go. You would not beable to start at all. (and btw the 96 lt4 icm is specific... and not the same as the ones they will give you off the shelf at a parts store, dont ask me how I know...)

 

I took the car out and wanted to let you know what I found out.

Car starts just like normal when cold. As it warms up, it gets harder and harder to start. I could get it started each time. The warmer it got, the more I had to push the gas to get it going. When warm, I had to hold 1/8 throttle. When hot, about 1/4 to 1/3, while slightly pumping the gas. When it started, it would jump to 2200 rpm and hold for about 10 seconds, then idle down to normal. After that, ran as normal.

It's got to be some type of idle control. I purchased the IAC, so hope that is what is going on.

 

Clear Flood mode(integral program in pcm) is more than 50% tps voltage during cranking... fwiw

it start either because
1. there is more air going in (open throttle blade)
2. Clear Flood Mode is cutting injection pulses, reducing the amount of fuel

or 1 & 2

Check for busted diaphragm in FPR (I dont think you can do this with out destroying the FPR on a 96). Correct fuel pressure has nothing to do with correct VOLUME of fuel flow.

Check for bleeding injector, or stuck open injector. (take out fuel rail with injectors still attached, prime the fuel pump several times, or jumper the fuel pump relay to stay on, and see if an injector is leaking)

Conduct fuel leak down test.

Check PCM coolant temp sensor.

It cannot and will not be the IAC. IAC is the IDLE AIR CONTROL, it is a stepper motor that is not active until the car reads its initial status (after/during starting). The amount of air it lets in is no where close to 1/4 throttle. its nothing more than a controlled vacuum leak.

Get a FSM. do the procedures for this problem. Throwing parts at your car and incorrectly trouble shooting is going to hurt your wallet! I dont mean to come off as rude, but if you just want to replace parts blindly, then go ahead, but if you want to fix your car... you are going to have to trouble shoot properly.

a failed or slow/sticky IAC will cause erratic/high/low idle, or stalling. its a 4 lead stepper motor, if it doesnt retract, you will stall. if it doesnt extend, your rpm will be high. If it doesnt move as commanded, your idle will fluctuate.
the engine would die after de-acceleration fuel cut off (DFCO), or any time you slowed down from higher rpm or came to a stop light-if the IAC motor failed to open the pintle properly
The engine would hunt for an idle and likely buck on acceleration from idle...

As a test, remove the iac, but keep it plugged in. Start the car (hot or cold). the idle should be sky high. this shows that the IAC was working to control the idle.

Put the IAC back in, and disconnect it. Start the car cold. The idle will not raise (and if cold enough the car will flood out or stall). You will also get a CEL.(check engine light)

Remove the IAC again, twist the pintle out far enough to close off the bypass port in the throttle body. Re-install. Disconnect the IAC. Start the car. The car will stall unless the throttle is opened. This shows the IAC is opening. plug the IAC back in and it should idle no problem.

Remove the IAC, twist the pintle in as far as it will go down. disconnect the iac. start the car. it should rev way up. this show it was closing. Plug it back in, and it should idle down.

Bottom line, if you are driving the car just fine, and you dont have a check engine light and code for the IAC, then chances are, its just fine and your problem lies elsewhere.

If you have no CEL, then your problem is probably not a failed sensor. (with the exception of a temp sensor, they can fail by not changing their value, but still be within tolerance for the pcm, and a CEL will not be set. I have seen that several times)

Just trying to help, take it as you will.

edit: also if the IAC fails mechanically (not a problem on the electrical side of the IAC, say the pintle head broke off or became disfigured), it sometimes causes a MAF or MAP code, cause the engine has commanded a certain set of conditions for idle and they are not happening, it will see an increase of air flow or decrease of manifold pressure than it should be with said conditions...so it will think that the maf/map is reading incorrectly.

 

help for you....

 

I don't take offense to your comments at all and appreciate your passion.

Tell me a little more about the PCM coolant temp sensor. Where is the sensor located? The car ran flawlessly before installing the Canton oil pan and having the battery disconnected for a few weeks. While installing the pan I had to move several of the electrical connections under the car. Maybe one was the PCM coolant temp sensor?

Thanks!

 

There are 2 coolant temp sensors. One is on passenger side cylinder head, the other is on the front of the water pump housing.

The one in the cylinder head drives the analog gauge and nothing else. The one on the water pump housing feeds the ECM and the digital dash readout. Check your coolant temp on the digital guage to see if it is reading reasonably (IE shouldn't say LO when the car is warm).

 

What brand injectors are you running?

 

As Nathan has said, but I am not sure which is which. But... the one on the passenger side head, is right next to the dipstick tube. (I broke mine when putting headers on the car, had to replace it, but dont remember if it was for the guage or not.) Likely you had to remove the dipstick when putting in your new pan... food for thought.

The easy way to test is to disconnect one while the engine is warm and see which is for the guage. Or maybe search the forum. Or take Nathans word for it.


I like what STL has asked... and to go on that... What other mods have been done recently? A canton oil pan usually means you have more going on than a stock LT-4.

I see Nitrous in your signature... checked the fuel solenoid? It could be dumping fuel.

 

STL, I don't know. I've never had a problem with them, so never pulled them to tell. I am the 3rd owner and not sure if they have been replaced or not.

Hmmm, good point about the fuel cell. It is a wet system and does have separate fuel and N20 cells. What is an easy way to check this, pull the fuel line from the NX plate and see if fuel is ejecting with the N2O system off?

 

Or just disconnet the line from the fuel side and plug it. Then see if you still have the "hard to start when warm" condition.

 

Yep, good suggestion. I'll try that.

Other than replacing the oil pan, I had a fried MSD Dig 6AL Plus, that was replaced, changed the plug wires, and replaced the plugs. I did use NGK TR6's, which are a little cooler, but have not had this problem before.