96' S-15 Sonoma - Rough Idle, Hard Cold Start

[Burnsside42]

I have a 1996 2.2L 4 Banger w/221,000 miles;

Since I have owned this truck (June 2010) it has always had a rough Idle - since then I have replace the plugs, wires, Air temp sensor with harness kit, Purge Valve by fuel tank, clutch kit w/slave cylinder and cleaned the IAC Valve and MAP sensors with Electrical Cotnact cleaner.

Truck idles very poor - and is clearly loading up the cylinder(s) with fuel - if you rev it up after idle for awhile it back-fires. It has coil packs - and being it does not skip a beat under load or driving I see nothing left in the ignition system to be at fault.

Looks like previous owner replaced the cat, o2 sensors, throttle position sensor, cylinder head, water pump, alternator, tensioner, radiator.

What controls the idle/fuel on these trucks? I want to put a new MAP sensor in the truck - but I see no reason for that to affect the idle and the fuel trim @ idle in the way it's acting. Is there a sensor which would control the cold starts? it does start very rough when cold and needs the throttle to be blipped once to keep from stalling on the initial start of the day.

Any help appriciated. Thanks

 

[Molson02536]

I have a 1996 2.2L 4 Banger w/221,000 miles;

Since I have owned this truck (June 2010) it has always had a rough Idle - since then I have replace the plugs, wires, Air temp sensor with harness kit, Purge Valve by fuel tank, clutch kit w/slave cylinder and cleaned the IAC Valve and MAP sensors with Electrical Cotnact cleaner.

Truck idles very poor - and is clearly loading up the cylinder(s) with fuel - if you rev it up after idle for awhile it back-fires. It has coil packs - and being it does not skip a beat under load or driving I see nothing left in the ignition system to be at fault.

Looks like previous owner replaced the cat, o2 sensors, throttle position sensor, cylinder head, water pump, alternator, tensioner, radiator.

What controls the idle/fuel on these trucks? I want to put a new MAP sensor in the truck - but I see no reason for that to affect the idle and the fuel trim @ idle in the way it's acting. Is there a sensor which would control the cold starts? it does start very rough when cold and needs the throttle to be blipped once to keep from stalling on the initial start of the day.

Any help appriciated. Thanks

You PCM (ECM) controls all thew above you have mentioned, the other thing you may want ti inspect or replace is your fuel pressure regulator. It's located on the fuel injector rail.

When the fuel pressure regulator fails, fuel will leak in to your engine through the vacuum loading up your engine's intake with fuel causing a rich condition. You can pull the vacuum line from the (FPR) and see if there is any fuel leaking in to it, if there is replace the (FPR $15.00)

Good luck and please keep us posted

 

[Burnsside42]

Well - the other day I did check the hose to the FPR while the engine was idling very rough and it was as dry as it should be. I still intend to replace it and the CTS eventually with new - but I am going to see if I can find a good used PCM to swap out first. It does seem to be ECU related - it goes from rough to really rough at times. If I have to idle long I just shut it down b/c I can't stand to listen to it.

 

[Redbeard]

You also might want to check the EGR Valve vacuum lines and the EGR Solenoid vacuum lines, for SMALL cracks.
Check the EGR to make sure it is opening and closing properly.
Inspect the coils (they are a pain to get to, i know) for cracks.

I had these problems show up on my truck. Fixed 'em and it runs just fine now!

 

[Burnsside42]

Well - when I replaced the spark plug cords I did check out the coil packs and they actually looked more recent then the wires I was pulling off. They looked to be in good shape anyways. The EGR does look original - and as a previous Saturn SL2 owner I am all aware of how those stupid Digital EGR valves lack to operate, but again - no CEL and those USUALLY toss a code very quickly from my experiance with them. I would like to remove it and clean it at somepoint - but I will re-inspect those specific hoses to and from it.

I have a used ECU on order however from fleabay which comes with my VIN programmed - $72 shipped seemed very reasonable - so I'm really hoping that cures it.

It's getting worse at initial start - it does not want to idle, just bounces around 450RPM unless I shut it down and restart, then it's "OK" It is just acting more and more like an electrical problem and less mechanical.