Intermittent Hestitation w/No CEL

[oneloudskunk]

Soo....my sister-in-law just purchased an '01 S-model w/5spd. I've been driving the car for a few days now, checking and inspecting things. She had mentioned a rough running scenerio during acceleration and when shifting from 1st to 2nd. I haven't noticed the problem until this morning. I was at a traffic light and attempted to accelerate. The car just fell on its face until I unloaded the engine, revved it and slipped the daylights out of the clutch. As soon as I shifted into 2nd gear, same problem; severe hesitation until engine is unloaded. The spark plugs appeared almost new when I checked them out. Air filter is good and fuel tank is clean. NOTE: this is the first morning it's been really rainy and cold since I've had the car. Unfortunately the CEL light is not on, so there are no codes to check. I've sifted through many of the forum threads and it seems that everyone else with this problem has an active code of some-sort. I dumped a can of SeaFoam into the tank and am awaiting results of its operation.

Thanks,
-rob

[CorollaTRD99]

I have had the same problem and it ended up being a bad exhaust leak and an o2 sensor. Also you should still connect a scanner to it regardless if you see the check engine light of not. I say this because I removed my bulb from the cluster and I no longer see a cel. Connect it to a mac tool scanner or a launch scanner.

[oneloudskunk]

The CEL works because it cycles on and off as it should when the ignition is turned on. There's no noticeable exhaust leak anywhere. After longer periods of driving today I noticed that it has a hesitation every time the accelerator is pressed. If I push the pedal and wait for the RPM's to rise it's fine, but the RPM's don't rise immediately, there's a definite abnormal delay.

[sam333]

Try cleaning your MAF

[oneloudskunk]

Just cleaned MAF and still no change in operation. Once the engine is warmed to where it will run at normal idle, the car will stumble before it revs whenever I hit the accelerator or blip the cam on the throttle body. Anywhere above 1000 RPM it responds immediately to throttle inputs. Still no CEL.

[fishexpo101]

Did you reset the ECM between those cleanings? If not, the ECM could be confused because the new signal ranges it may be reading.

Since the stumbling is just off-idle - I'd suspect that there is an issue with the throttle body being excessively dirty (gummed up throttle plate), vacuum leak, exhaust leak (incorrect readings from O2 sensor), bad or lazy O2 sensor (excessively rich mix), faulty TPS sensor, sticking injector, and/or weak or intermittent spark. water in some electrical connector somewhere (nightmare to diagnose this).

What was the maintenance history of the car? Original owner provided any information on the car, past receipts? Could be a simple case of just needing a good tuneup and hard run on the highway to clean it up a bit. I would pull the plugs and read them - even with newish plugs, could still have an ignition problem.

[oneloudskunk]

Originally Posted by fishexpo101

Did you reset the ECM between those cleanings? If not, the ECM could be confused because the new signal ranges it may be reading.

Since the stumbling is just off-idle - I'd suspect that there is an issue with the throttle body being excessively dirty (gummed up throttle plate), vacuum leak, exhaust leak (incorrect readings from O2 sensor), bad or lazy O2 sensor (excessively rich mix), faulty TPS sensor, sticking injector, and/or weak or intermittent spark. water in some electrical connector somewhere (nightmare to diagnose this).

What was the maintenance history of the car? Original owner provided any information on the car, past receipts? Could be a simple case of just needing a good tuneup and hard run on the highway to clean it up a bit. I would pull the plugs and read them - even with newish plugs, could still have an ignition problem.

1) I did not reset ECM. Is that as simple as disconnecting the battery for a short time?
2) I cleaned the TB when I did the MAF sensor.
3) What's vacuum pressure supposed to be on this one? I've ordered a service manual but have not yet received it.
4) It has Autolite Iridium plugs in it. All appear to be burning identical and lean (white, just like all these newer cars. My wife's '06 has identical burn.)