I have been researching on here for a couple of days looking for people with similar symptoms to mine, but I haven't found any concrete answers. For the past couple of days my car has been surging and stumbling when it is fully warmed up and has been running for between 10-20 minutes. Usually the stumble is when sitting still in drive and when slightly acclerating under 3000 or so RPM's. It may possibly happen above 3000 but I haven't floored the car once it starts stumbling.
The car runs perfectly until it is fully warm, in fact today I went WOT with no problems when the car wasn't fully warmed up. I have no trouble codes coming up and I have replaced my Opti about a year ago. When it starts stumbling it sounds like the engine just dies and then catches back up. When I was sitting in the car yesterday I could hear the whine from the fuel pump very loud and when it would surge the sound from the pump would also quiet down. I have also noticed that when the car surges that the MAP reading goes up at least one full point higher could this be a sign of the pump failing. From all the other posts I have read when the fuel pump goes out on most cars it is very hard to start and then it just won't start, but my car starts very easlily. I haven't checked the Fuel Pressure yet but I will do that tomorrow on my day off.
Also in a post from last year sometime there was someone with a problem that sounded almost exactly like my problem and there never was a clear answer but I think it ended up being the Coolant temp sensor.
I am sorry this is a long post but I just wanted to give all the details. My car has 114,000 miles on it and many of the sensors are factory, but it has ran perfectly for about a year.

Thanks for your help, Dan

 

Put 2 fat washers or nuts between the coil and the cylinder head. The coil might be overheating. If that doesn't work, disconnect your MAF and see if the car misfires in open loop.

 

I have the ICM cooling mod done, I will try unplugging the MAF sensor and see what that does. The last time that I did that the car wouldn't crank with the MAF unplugged, do you have to unplug it when the car is already running?

Thanks for the Suggestions

Dan

 

unplugging while car is running will stall the car, has to be when engine off.

 

Thats interesting. Your car SHOULD start even with the MAF unplugged.

Reset your IAC motor. http://shbox.com/1/4th_gen_tech2.html#iac_reset

How is your oil pressure?

 

I will try the IAC reset as soon as I get off work. My oil pressure is 25 at warm idle and 40 at initial idle and about 40-45 at normal crusing speed and my average coolant temp is between 170-175*. My oil pressure was also the same that it has always been when the stumble started. I have a scan master so I can tell you any other readings if you need.

Thanks for your help grip.

 

O2's could be on the fritz. a low RPM stumble is known to occure when your O2s go bad. Hope that helps.

BIZ

 

I thought about the O2's and I am going to replace them this summer, but why would it only do it after about 10-20 minutes, I thought that closed loop only occured after about 5 minutes, I was monitoring the 02 mv's and it started shortly after the car went into closed loop. I guess it could definetly be the O2's , I know I am past due for changing them.

 

it sounds like the ignition module to me, have seen several do the same thing, i see a lot of them fail when they warm up but do alright when cooler, most parts stores have testers for them i know advance auto parts does

 

I have a spare from my other car I will try that today.

Ohh and Grip resetting the IAC Had no affect. When I was driving it around after work the belt started sqealing like a stuck pig and I noticed that one of my heater hoses was pissing coolant on my belt, I fixed that this morning. I freaked out thinking I had killled my 3rd opti but it was completely dry. I am about to go test drive it again.

Thanks

 

My car was squealing and idling low. Turned out that the PS pump had ceased.

How is the car under WOT acceleration? Does it stutter or hesitate?

Check for a loose vacuum line on the intake as well.

 

Gripe it runs well at WOT which is one thing that confuses me. I took it on several WOT blasts today and it ran like a champ up to 80-100 mph when I let off. It seems that the problem is mainly at the lower rpms, although one time when I floored it, I noticed that it stumbled right before it fully acelerated.

I changed the belt and fixed the leaking hose this morning, the squealing is gone . All of the pulleys are turning freely I checked all of them when I had the belt off. I checked all of the spark plug wire connections going to the opti and they were good and tight. The problem seems to be less today but it did still happen. I had the fuel pressure checked today and it held a constant 38-40 at idle and looked to be around 45 psi at WOT. The stumble is mainly in the 1500-2500 range and I did notice that when the stumble occurs the fuel pressure appeared to flutter some but no more than +/- 2 psi from what I could tell, lol it is kind of hard to look at the guage when you are driving and it is on your windsheild. I still haven't tried changing over to my spare ICM, I may to that this weekend. I am about to take it for another drive and see how it acts and see what else I can observe when it stumbles. Maybe it won't leave me on the side of the road.

Thanks again
Dan

 

Well I seem to have somehow cured the problem, I have driven it for the past 3 days and haven't noticed any stumbles. The things I fixed were the following.
*Heater hose leaking coolant by the AC compressor and Serpentine belt(could have been getting some near the Opti
*Cleaned the Throttle Body and All Passages
*Cleaned the IAC Spindle and changed to IAC from other car
*Changed TPS to one from the other car
*Changed the Gasket under the top TB plate (old one was in bad shape from misinstalling at an earlier time)
*Repaired split in hose coming from Valve cover to TB
*Changed Opti Wiring Harness (Had a Small chip in male end going to plug on top of opti by one of the prongs.)
*Also changed to MAFS from other car (cleaned that MAFS)
I am not sure which one was the problem I was going to do all of these this one at a time to see which was the problem but it was raining off and on all day so I didn't have time and most of this I was needing to do anyway. Just thought I would let you guys know that I had made progress in my stumble situation. It sure does help having a parts car that had good electronics on it.

Thanks for all your help.
Dan