So i have a recent problem where my c4 idles rough almost like its cammed. Also when i start it it take a good sec to actually start firing up, but only after its been sitting and the engine is still hot. cold starts its starts up almost too fast and if i turn it off after its been running and then back on it has no problem but some where in this certain time frame it take a while to start up and its never acted like this before. Only thing i did before this problem is undo the battery, fill oil because it said it was low and i accidentally over filled it by about half a qt drove it around a couple days then drained it to the proper fill. Then a day or two after these things it has these problems. I checked one spark plig and it seemed fine so i dismissed that but should i check all? It doesnt seem to be miss firing i dont think? Also its not a bad gas problem. Is there certain censors i should clean or check that might be throwing of the computers ratios?

 

First thing to check is your profile. We have no clue what you are working on and where you are. If you are nearby and I can lend a hand with expertise or tools, I might do it. If you are 500 miles away, probably not.

Check out the filters first. Are they all new? After that, change the plugs if they are old. I buy a used but new to me car, "it flows, it goes". All fluids. Are the plug wires good? Is your fuel pressure good? See below for a test. Cap and rotor? Is the timing spot on? Air filter?

1. Get a fuel pressure gauge on the schraeder valve.
2. Turn the key to run but not crank. What is the fuel pressure? Does it hold for a while or drop quickly?
3. Start it. Pull the hose off the fuel pressure regulator and see what you get.
4. Tape it to the windshield and do a Wide Open Throttle run. How is the pressure during the run?

Clean the Throttle Body and clean it with brake cleaner

1. Take the TB off. This is a good time to do the coolant bypass where you put a pipe to connect the hoses coming out of the TB
2. Take the IAC solenoid off
3. Take the IAC Solenoid housing off and clean it
4. Take the TB plate at the top off and clean the passages
5. Clean the TB and the passages
6. Clean the IAC solenoid by spraying a fine mist of brake cleaner and GENTLY wiping the pintle down

Get that all done and we can start checking the Idle Air Control Solenoid

1. Buy a scanner. It can be a handheld unit or a bluetooth unit. It is NOT a code reader. If it is cheap, it's a glorified paperclip, aka codereader
2. With the engine at full operating temp, see what the Coolant Temp Sensor is and put an IR thermometer on the CTS or as close to it as possible
3. Does the CTS agree with the thermometer? If not, change the CTS
4. Assuming your timing is spot on, how many counts do you have with everything but the motor off? You are aiming for about 20-30 counts.
5 If you have more than that, turn the adjustment screw in. If you have less, turn it out
6. Rev the motor and repeat

If you cannot adjust the IAC and it is always 0, you might have some sort of air leak since it is trying to compensate. This is a decent test of your system

If you have a speed density car, there is no more adjustment. If you have an MAF car, change the Throttle Position Sensor setting. TPS setting is about 0.54V which the scanner can verify.

Don't worry. You will always need that scanner. If not now, later so might as well be now. Yes it cost more than a code reader but it will pay you back many times over if you are keeping the car.

 

Thanks for the detailed reply. I got the engine rebuilt with new enternals stock. Ive cleaned the air filter and tb is cleaned. Fuel injectors are new. Its a 91 c4 which is maf i believe which i cleaned or replaced i cant remember. The distributor burned out on me and i replaced it, also got custom spark wires. All this stuff thats been done almost a year ago. Fuel filter was replaced.

 

Not sure why you did that but I'd have sprung for a 383 if I was getting it rebuilt. Not that much more.

Did you clean the passages and the pintle or just spray brake cleaner on the opening?

New what? Stock size? Larger than stock? Smaller than stock? Did you get good ones or something made by a fly by night company?

I don't see how. I have a 91 and it is Speed Density. 89 and below are MAF. 90 and 91 are SD in the L98 version. 92-96 is LT1. So what exactly did you clean or replace?

Assuming you got a decent replacement for the distributor, what exactly do you mean by custom wires? I can get a set of Duralast Gold wires from Autozone that are supposed to be fitting for me. Well, I too replaced my distributor to a small cap one which necessitated harness modification and remote location of a coil and custom plug wires. I had to decide how I want them to run and what looms I wanted and cut the length to fit my situation.

If the filter is replaced, I'd check the fuel pressure at the rail like I said and report back. In the meantime, buy yourself 2 things

1. Factory Shop Manual unless you have more money than God to pay
2. Scanner. This or That or the Other

 

Thanks for the help guys, it turned out to be the fuel pressure regulator. All is put back and good!