are you having hesitation issues also? Same symptoms? did you try fuel pressure testing?

let me know if you have any luck with changing the O2 sensors.

 

I'm haveing hesitation issues also. So far I have done new plugs,wires,cap and rotor, and cleaned the intake and throttle body along with MAF and a new air filter couldnt't hurt. Tomorrow I'm changing out the fuel filter on it. I've been reading these post for a few days and I keep hearing about this ICM, What is it and where it it located. Is there anything else I should check for hesitation issues?

 

the ICM is the Ignition control module. its located right next to the throttle body(passenger side) on an aluminum heat sink attached to the ignition coil. its about 1" x 2", black and has a 4 prong plug connector to the wiring harness.

and like I found out, any decent auto parts store can test it for you. you'll have to disconnect it to have them test it. but its really easy. there are 2 small bolts holding it down to the heat sink. Be careful, you may need to use some thermal grease on it when re-installing it so that it will continue to work correctly in the future.

hope that helps.

PS there are some other threads about hesitation issues. make sure to check them out. you may have other issues.

Are your symptoms exactly like mine? or do you have cold start- or idling issues? Have you done a fuel pressure test? if so what are the results?

 

The o2 sensors i installed did seem to help the car run better. As did the fuel filter; but the same issue remains. If i gently squeeze the pedal i can accelerate up to highway speed but if i press harder or floor it the revs drop, i hear a backfire occasionally and/or it will stall. Sometimes even when driving! i have to drop it into neutral and start it up again while i'm rolling. To reiterate, the truck has about 220,000mi so as a matter of course the plugs, wires, cap, rotor will be changed soon. However I don't see that making much of a difference with this issue according to everyone elses experiences.

also, i have not yet done a fuel pressure test. but that sounds like a good idea. tomorrow is another day, its a bit dark at the moment.

 

Cap and Rotor changed. still same thing. The old one wasn't too bad, but I changed it anyway to see if it would help.

What other electrical parts would it be Swartlkk? I'm about to order another fuel pump soon.


how long do you run your engine before you notice the symptoms? it only takes me about 10 minutes or so. It sounds exactly like what I have.

good luck, and keep us up to date if you find out what it is.

 

Could be a broken wire that is causing the fuel pump or coil or ... to lose voltage intermittently. Could be a corroded electrical connection. Could be a bunch of different electrical gremlins due to age.

The reason I point to an electrical fault is that you can reset the system (turn it off) and it will act appropriately for a seemingly set amount of time before returning to the bad running... This points to an electrical fault.

It could be a problem with a specific sensor in that when the PCM is in open loop mode when you first start it, the sensor data is ignored. Once it goes into closed loop mode, using all sensor data to calculate various engine performance criteria, a sensor or combination of sensors feed the PCM with bad information resulting in poor performance... Without being there, experiencing what you are experiencing, then using the tools I have available to watch what is happening with the various sensor data, I am having a difficult time directing you...

 

Thank you for your input. I'm sure you have valuable experience dealing with problems like this. I am leary of taking this to a mechanic, because my experience shows that a majority of them simply replace parts until they fix it. I can do that...

Do you have any suggestions for me to tell the mechanic if I take it to someone?

What is open loop mode? how is it different than closed? If there are sensors that are bad, wouldn't it throw an engine code? are there any tests i could do that would let me know if a sensor is bad, or if the engine is operating in either open/closed loop mode.

Surely there must be some other diagnostic tests I could try?

thanks again.

 

Replaced the fuel filter the other day, still didn't help. I have not done a fuel pressure test. It starts up fine and it idles anywhere beween 500-650 rpms depending on how long it has been running. My problem is is that when you throttle down it bogs out and hesittates but if you hold it down long enough it'll start going. I can't make sense of it.

 

Today i had the truck scanned again to see if any new codes popped up. Same code, same problem. But heres the weird part... after i had him clear the codes (which probably had nothing to do with it) it wouldn't start. Finally got it to start and it ran great!! no hesitation, no chug a lug bog down nonsense... perfect. throttle response was right there. check engine light stayed off. coincidentally, i was also almost out of gas. Next time i got a chance i put in about 3 gallons to get around (no money on me) still ran fine... WHEN I FILLED UP IT STARTED HAPPENING AGAIN!!!

 

Weedenski,
I am sure there are other tests that could be run, just can't think of them at the moment (strapped at work). This problem sounds so familiar to something I have ran into in the past, but I just cannot remember that particular instance... (damn I need sleep...)
PCM Modes:
OPEN LOOP - this is the mode your PCM is in until the ECT and a couple other sensors get to operating temperature/values. The length of time after initial when your vehicle is in this mode could either be based on sensor values in the case of the ECT sensor after a cold start or time after starting the vehicle after it has already been at operating temperature. This is also true when you are at WOT or may be forced when certain codes are set.

CLOSED LOOP - the PCM uses information primarily from the HO2S and adjusts fuel delivery and timing to achieve minimum emissions, better gas mileage, and power.

To gravitychk, remove the charcoal canister and see if you get fuel to drain out of it. Your problem almost sounds like fuel is getting into the EVAP system, causing your problems.

To Chevyboy28, there are a number of things that can cause this problem. MAP sensor plugged up, EGR valve stuck open, insufficient fuel supply, engine timing off, and the list goes on.

It gets a bit difficult to diagnose three vehicles in one thread especially when trying to determine if all vehicles are similarly equipped. To everyone in this thread, please put your vehicle information into your signature and attach it to all of your posts. This will help greatly!