Car bucks/jerks going onto throttle, coming off throttle, and holding part throttle after warmup?

92GS603 · Aug 15, 2014

Let me just say that I've had this issue for a long time now, but it's become such an annoyance that almost 2 years after it started, I'm finally sick of it and seeking answers. Title says it all--If I quickly go onto or off of the throttle, or hold the throttle between 0-15% (guessing, but basically just enough to hold a steady speed), the car bucks. It does it in every gear, and is most pronounced when slamming the gas pedal to accelerate hard. It starts out during early warmup with a delay (let off throttle, then bucks after a 1-2 second delay). There is also an occasional instance of rough idle, but its fairly rare.

Since it started, the following parts have been replaced due to failure or out-of-range operation.
-MAF (IAT sensor was way off) --didn't help bucking at all
-IACV (flat-out died) --didn't help bucking at all
-Coolant temp sensor (was reading way off) --moderately reduced the severity of bucking
-Plugs and wires 2 times (20k miles, first set of wires rubbed on other parts til they started arcing) --little to no change
-O2 sensor (old one was dead) --little to no change

I have also replaced the PCV with a new OEM one, and have replaced the fuel filter and ran BG44k through twice. I have not had a chance to check the fuel pressure, but the fact that the bucking is delayed during the early warmup period makes me believe it's a sensor and also rules out it being drivetrain/transmission lash.

As for the other things I've tested, here is what I got:

TPS: 600-5000 ohms from 0%-100%, no weird spikes or dips but the low bottom end makes me suspicious, as it's supposed to be ~1100-5500.
With the harness attached it's putting out .48v at closed which is within the range that the Haynes says it should be, though I'm very tempted to adjust it up above .50v just to be safe.

Coil Pack: Primary resistance is around .9-1.0 ohms when cold and just slightly higher when hot. Secondary for both coil sets is around 13k cold and 15.5k hot. Seems slightly out of range.

Closed throttle switch: Operating properly

Vacuum system: No leaks that I can find, idle is VERY smooth for a DSM.

I have yet to check the CAS or power transistor, but my understanding is that either of these going bad usually causes you to lose 2 cylinders. Keep in mind that this is also a non-turbo daily driver, so I don't have any A/F gauges or access to tuning software to let me see all of my sensor values on the fly.

bmxfelon420 · Feb 4, 2015

Hmm have you checked your battery voltage? It's possible that under certain conditions the voltage might drop low enough to change injector deadtime. I'd verify that. Barring that, I'd guess the ECU, or possibly fuel pressure regulator although i'd say it'd show itself in more ways than just throttle open/close. If you dont wanna drop the money on a different ecu and you can let the car sit for a week or two you can send it to ECMtuning and have them verify it for you.

wickedboost92 · Feb 4, 2015

does the car have a cat in it still? far reach but a clogged catalytic converter could cause a lot of issues. giving the o2 sensor false reads thus messing with fuel delivery.

92GS603 · Feb 4, 2015

Battery was replaced last year, and AAA checked the alternator the same day after the dead battery left me stranded. This, of course, all happened after this started happening, and made no difference with the condition.

I was thinking it might be FPR as well, but it makes no sense why it would only show up after the car was warmed/warming up. Seems to me that the only possible things that could cause this are electrical sensors/components that are sensitive to heat or adjust the engine operation based off the engine's temperature. I suppose it could also be a vacuum leak appearing after the engine parts expand, but I've eliminated just about everything that could cause one short of the injector seals.

92GS603 · Feb 4, 2015

Car has a high flow cat, and has had two of them since this started happening (first Magnaflow one fell apart internally from hitting a rock). No change in the way it ran between the two and the OEM cat.

wickedboost92 · Feb 4, 2015

i not sure a vacuum leak would cause the problem you are having especially since your are na. but crazier things have happen. have you rebuilt throttle body? there are seals in there that wear out around butterfly and may cause said issues.

wickedboost92 · Feb 4, 2015

i would just check all wiring from injectors make sure nothing may be broken. have you altered the maf at all?

inthemidst · Feb 4, 2015

Get a multimeter and see if both your o2 sensors are reading correctly.its giving you problems when it's in closed loop/warmed up so I would check that first.

92GS603 · Feb 5, 2015

I only have one O2 sensor--this is a 92 Eclipse GS with the non-turbo 4G63. I replaced the O2 last year when I first attempted to fix this problem, but it made no difference.

jimsgsx · Feb 5, 2015

Replace your fpr. And verify fuel pressure.

92GS603 · Feb 5, 2015

From my understanding, if the FPR was bad I'd be getting issues like poor gas mileage, puffing black smoke out the exhaust, and having trouble starting the car in the morning. I get none of these.

92GS603 · Mar 3, 2015

Had some free time today, so I decided to try experimenting again.

Turns out my O2 sensor--Denso brand that I installed about a year ago--is giving no output voltage at all whatsoever, regardless of temperature or engine revs. Just a dead flat 0.00v across the board. I'm beginning to wonder of it was DOA and has not been functioning this whole time, much like the dead OEM one that it replaced.

What's strange about that, though, is that neither my old OEM O2 nor this one is causing a CEL, despite them both appearing to be completely dead.

luv2rallye · Mar 3, 2015

Make sure it's wired correctly:
http://www.dsmtuners.com/threads/oxygen-sensor-wiring.244376/

inthemidst · Mar 3, 2015

Last thing to check ecu

Turboterd · Mar 4, 2015

As long as there is a sensor plugged into the connector there usually will be no CEL on OBD1 vehicles. I like to call them OB-dumb. They dont like to account for sensor degradation.

Any boost leaks?

Since you have adjusted the closed throttle position switch and replaced o2, have you cleared the learned memory from the ECM and tried to relearn?

92GS603 · Mar 5, 2015

luv2rallye said:
Make sure it's wired correctly:
http://www.dsmtuners.com/threads/oxygen-sensor-wiring.244376/

Wired correctly.

inthemidst said:
Last thing to check ecu

It's crossed my mind more than once. I opened it up to check the IAC caps last summer and the board looked clean. Still could be it, I suppose.

Turboterd said:
As long as there is a sensor plugged into the connector there usually will be no CEL on OBD1 vehicles. I like to call them OB-dumb. They dont like to account for sensor degradation.

Any boost leaks?

Since you have adjusted the closed throttle position switch and replaced o2, have you cleared the learned memory from the ECM and tried to relearn?

All vacuum lines were replaced, intake is free of leaks, and couldn't find any leaks in the TB or IM. ECU has been reset from battery removal several times since the O2 and throttle switch were done.

I managed to pick up a new OEM O2 sensor--still in it's dealership shipping box from 1995--for $16 on ebay. We'll see how things go with that and go from there

iceguru1114 · Mar 5, 2015

It sounds like you are running thin on hardware issues left to run to ground.

Honestly, when I read your first post the immediate thought I had is that this sounds like a calibration/controls issue. Do you have any way to log data to look at tip in enrichment?

92GS603 · Mar 6, 2015

iceguru1114 said:
It sounds like you are running thin on hardware issues left to run to ground.

Honestly, when I read your first post the immediate thought I had is that this sounds like a calibration/controls issue. Do you have any way to log data to look at tip in enrichment?

It's a N/T, so I don't have any sort of datalogging and can't get dsmlink or anything like that.

iceguru1114 · Mar 6, 2015

Then it's really hard to narrow my hunch down to something you can base off of data, which is ofcourse the right way to do it. However, I maintain that what you are describing sounds to me like a calibration/controls issue. You are probably having a tip-in enrichment problem. If it's sensory related, that would most likely be a TPS problem. If it's a calibration issue, then you may be out of luck if you have no ability to alter you tip-in enrichment and/or transient enrichment tables.

92GS603 · Mar 6, 2015

iceguru1114 said:
Then it's really hard to narrow my hunch down to something you can base off of data, which is ofcourse the right way to do it. However, I maintain that what you are describing sounds to me like a calibration/controls issue. You are probably having a tip-in enrichment problem. If it's sensory related, that would most likely be a TPS problem. If it's a calibration issue, then you may be out of luck if you have no ability to alter you tip-in enrichment and/or transient enrichment tables.

I replaced the TPS last month, adjusted it everywhere between .48v and =.52v at closed and it was no different than the other TPS.

Turboterd · Mar 7, 2015

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Does this little goober matter? There is some on the other base too..

I'm with Iceguru, definitely seems like a control issue. You can ohm check the signal wires going back to the ECU. That will verify wiring is not creating a voltage drop/change

413mudman · Mar 7, 2015

Where in NH are you kid!! I just got back from the west coast, so I'm sitting on my hands doing nothing! My 91 seems to like the cold weather and better gas.

92GS603 · Mar 10, 2015

New O2 sensor didn't make any noticeable difference.

Based off what I've checked and/or replaced, the only real parts that could still be causing this are:
-ECU
-Coil pack?

I should probably mention that my original IAC died while on this ECU, but it APPEARS that the driver is still intact due to the movement of the replacement IAC that's on there now when cycling the key. Could it still have possibly caused damage to the ECU when it shorted?

As for the coil pack, my only real question is if it could act abnormally at low loads and be fine everywhere else?

413mudman said:
Where in NH are you kid!! I just got back from the west coast, so I'm sitting on my hands doing nothing! My 91 seems to like the cold weather and better gas.

I'm in the southern NH area around Amherst and Milford!

92GS603 · Mar 13, 2015

I went and rechecked every sensor and all of their harness connectors per the repair manual's specifications, and they all check out fine. Also went searching for any vacuum leaks I've been unable to find so far and didn't find anything.

Still have the same questions as my last post.

I might just buy a re-cap'd ECU and see what happens. Worst case, I'd have a worry-free ECU in there.

92GS603 · Mar 18, 2015

Tried another ECU and there was no change. I did find out I mis-adjusted the idle position switch, though, so I'm going to readjust that, the TPS, and BISS tomorrow to see what happens.

If that doesn't solve it, I really have no idea where to go next.

92GS603 · Apr 12, 2015

Quite embarrassingly, I'm pretty sure I solved the problem.

Solution: relocated my front license plate.

For the entire time I've owned this car, the front license plate has been sitting smack in the middle of the grill--right in front of the radiator. The stock mount had been removed at some point, so I put it down there instead. My best guess is that it was blocking enough airflow to make the coolant exceed the ECU's ideal temperature and cause it to pull timing. This would make sense of it getting worse on hot days or the longer the car was driven.

Today was the first time we've had 60 degree weather in 5 months, and the car never hiccuped once at part throttle. Coolant needle was a sliver lower than it has been before, and the engine felt much peppier. The on/off bucking is still there, but it's so minute now that I could happily live with it.

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