Engine hesitation, running rich [Archive] - GrandAmGT.com Forum

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dissident
09-06-2013, 11:36 AM
2000 Grand am GT 3.4

Here's my problem... I have a slight engine hesitation... I'll notice it the most in low torque situations, for example, when I'm going up a slight hill and if I don't give it extra gas/throttle to compensate and just hold my foot steady on the pedal, I'll notice it hesitate during this time. Similarly with the cruise on going up slight grades, car gives it a little more throttle, but it will hesitate. There is no hesitation at idle after startup, no problems starting car, and no hesitation at idle while parked with engine hot.

Hard acceleration is no problem, and nor is coasting at slow speeds. Transmission shifts fine.. pan has been dropped and fluid changed about 50K miles ago.. planning to do that again soon.

I also have a negative long term fuel trim.. it will tend to be around minus 10%, go up to around zero when accelerating, and drop to minus 20% under low throttle situations such as slowing down for a stoplight, suggesting I'm running rich.

No relevant codes being thrown.. only an evap leak code p0440/p0442 which I suspect is being caused by needing a new fuel filler neck and that's not relevant to these symptoms.

I have done the following things (myself,except for the fuel pump) :

Changed spark plugs, wires.

Changed PCV valve

Changed upstream O2 Sensor

Changed Air Filter

Cleaned off MAF sensor with sensor cleaner.. it did have a layer of dust on it, but not enough where I would say it would block airflow but what do I know. :) ordered a new 'used' one for 25 bucks just in case but it will be here next week.

Cleaned off throttle body

New fuel pump, filter... old pump had been dying for over a year before it finally died for good and I had it changed (for what it's worth I did not have this problem with the old pump/filter, however the fuel pressure looks good... 50PSI while at idle)

The fuel pressure regulator was replaced 2 years ago and I don't believe it to be the problem but I have not tested it in any way.

Also: I get a 'rotten eggs' smell when first starting up the engine, suggesting a rich condition.. I did not reset the fuel trim values yet by unplugging battery after doing all this work yet, maybe it just takes time to fix itself? I just did the MAF sensor and throttle body yesterday, the O2 sensor a few days before that, with not much driving done since.

I tested catalytic converter with infrared thermometer and it appears that it's 100 degrees warmer after the converter, and the noise profile sounds good, but I do suspect the cat might be going after 13 years and 150k miles, but that shouldn't cause a negative fuel trim.... I have a cheap backpressure tester which I can hook up to the upstream o2 sensor port coming next week to check for back-pressure.

Thought I would post here and get suggestions as to what to look at next. I thought to maybe check fuel injectors for leakage or test fuel pressure while driving by hooking up sensor to port and taping to windshield. Learned quite a bit about cars and was able to make little repairs for a lot cheaper than it would have cost at a mechanic so I'm happy with the whole process thus far.

sleepyalero
09-06-2013, 12:38 PM
What about down stream o2? Do rpms bounce on slight incline when you give little gas?

dissident
09-07-2013, 12:07 AM
yes RPM's act as expected. Did not change downstream sensor yet. (upstream easier to access) The hesitation feels like an intermittent hesitation.. like a sort of subtle jerkiness. One theory I had is that there is exhaust back-pressure and hard acceleration will push the exhaust out whereas light acceleration won't so it hesitates... combined with the rotten egg smell the cat is simply going out. I'll know next week. I suppose I could simply remove the upstream o2 sensor and drive it but the sensor is located just under the spark plug coil pack so I don't know if that's the best idea.

No codes are thrown for downstream sensor, cat, or anything like that so I'm not suspecting it as culprit since air fuel ratio is determined by the upstream sensor. The fuel trim thing isn't totally explained, but maybe it's a side effect of the back pressure combined with the sensor's location near the engine.

sleepyalero
09-07-2013, 12:53 AM
If your rpms bounce up and down on slight acceleration and you can hear the car go vroom vroom vroom vroom and you feel the car jerking, it could be a torque converter, my car did the rpm jump on slight acceleration and after i put a new transmission in it, it stopped doing it.

dissident
09-07-2013, 10:29 AM
the RPMs don't do that... I don't hear the car do that either.... they remain exactly where they should, rock steady, depending on where I'm holding the throttle.

It's more like the jerkiness and hesitation will occur without affecting the RPM. It's not an up down RPM situation, it's at a steady RPM, it is jerky.. and not very jerky.. it's subtle.. the jerkiness occurs multiple times in a second and lasts until I either let off the throttle or accellerate harder. if that helps describe it. It's also random.. it doesn't follow any pattern like it might be a specific cylinder causing it.

Like I said it seems to occur in low throttle situations such as going up a hill at a steady speed with the cruise on. It should be noted that in these light throttle situations when I feel the hesitation the manifold vacuum pressure gets down to around 10-11 in/hg.. I have no idea if that's normal for the vacuum pressure (as measured by OBD reader) to go down like that when steadily holding the throttle or light acceleration up small hills.. thought I would mention that. When throttle is released or it's idling it's back up to 18-20 in/hg... if I floor it of course it's way under 10 in/hg.

http://www.freeasestudyguides.com/engine-vacuum-test.html