Thread: Trying to figure out a slightly uneven acceleration

This will sound nit-picky, but it's the kind of thing that bothers me.

I have a 2004 GP GT, NA. A few months ago I did a small tune up. The belt was wore, so while I had it off I also replaced the tensioner. At 145K miles, I figured I'd just be proactive about it. I aslo picked up a PCV valve and fuel filter. I have forgot about the fuel filter for about 80K miles, so I really thought I should add that on. The tune up went fine without any issues other than the fuel filter being pretty rusted on. After taking it for a spin, my butt dyno said it had more pickup and felt a little faster.

Fast forward 45 days or so. I've noticed that my acceleration just isn't as peppy as it was. On top of that, when I accelerate a little harder (like pushing up around the 3K RPM range and higher) it no longer sounds like a smooth arc. I hate trying to describe sounds, but to greatly exaggerate the sound, it goes like this: waaaaaaaaAAAAaAAAAAaAAAaAAAaAAA.... It's just these minor dips. It's not like a miss, but as if it just slightly dips in the acceleration curve. Those dips also get faster as the RPMs get higher. I can also almost hold the noise if I put the car in 2nd and drive at ~3000 RPM. It is not audible when revving in neutral. It seems to need a load on the engine to do it.

So it's not enough to trigger a check engine light, unfortunately. But it's audible if you shut off the radio. Part of my wanting to chase this down right now, is that I'm thinking of selling the car and I don't want to pass off a problem to someone else. The tensioner feels good, the PCV valve has it's o-rings... I can't say much about the filter expect it's not leaking. Any ideas?

Any codes?

First step. Check your air intake. Air filter, temp sensor, throttle position sensor, idle air control valve, maf sensor, butterfly plate, and your throttle body innards.

Clean these with electronics cleaner. (do NOT touch the maf wire, it WILL break, just spray it off)

Do this, then get back to me.

Yep, time to pull the throttle body and clean it till it shines. Common issue.

Sorry, I forgot about a few things.

1) no codes, no lights.
2) I did pull and clean the TB at that time. Also cleaned as much of the intake as I can get to.
3) Cleaned the MAF at that time with MAF cleaner.
4) The 2004 is a Series 3, so it's an electronic control, no IAC.
5) Just had the whole intake (up to the TB) out a little while ago when I had to pull the rad fans. I found a mouse nest on the computer, but all else is good and clean.

So that leaves the TPS and temp sensor. Do you mean the air or coolant temp sensor? I guess the easiest thing is to try and clean the TB again and make sure the EGR isn't stuck open. Is there a diagnostic for the TPS, or temp sensor? I assume the the TPS should just read a nice smooth range as I open and close the butterfly. Let me know what to check off these and I'll just do it all at once.

Thanks for the tips and ideas.

I hope I'm not missing anything here, but did you check/replace your spark plugs and wires? I had a similar uneven acceleration only at low rpms, and one night when I opened the hood in the dark, before I turned a light on, I saw one of my plug wires was arcing to the oil dipstick and block. Had I not looked in the dark, I would never had known.

Either way, if you did a tuneup, plugs and wires should be right at the top of your list.

As for the TPS, the easy way to check for correct operation is to use a scan tool or Torque for android, and just watch the throttle position reading as you slowly depress your pedal.

If you don't have a way to look at that data, then you're going to need a multimeter. I don't recall what you need to look at though, either voltage or resistance of the TPS.

I did plugs and wires in the summer of 2012. I did the wires because a mouse nested under the engine cover and chewed them, so I've already had that thought and checked it out. The factory Iridiums at 120K miles still looked new, and were only a tiny bit over gap. I was tempted to keep them, but I went ahead and replaced them with new AC Delco Iridiums. I figured this way I wouldn't have to do it again.

Yeah, I don't have anything more than a code reader, but I do have a DMM. I was thinking it'd be a measure of resistance, but it might be voltage. If anyone knows for sure, I'm going to try and take a peak at it tomorrow. Maybe I'll try a wet down of the wires just to be sure.

dont forget the coil packs. Mine had a misfire that started very gently and gradually got worse until i replace the coil pack that runs cylindar 6 and i forget which other cylindar it runs...... just a thought.

Well, the noise seems to be gone. Unfortunately I did too much to be sure which fixed it.

I started with spraying off the MAF sensor again. I set that aside to dry and went to removing the throttle body. It wasn't bad, but the back side was black from the EGR. I imagine that happens pretty quickly with a little idling. I cleaned out as much of the intake as I could reach, which really isn't that much. Someday, I'll just take the upper intake off and give a proper cleaning. Next I removed the EGR and checked it out. The EGR looked like it functioned just fine, but I couldn't remember the pinout to actually test it. the plunger moved easily, and it seemed to hold a lot of air pressure. I went ahead and scrapped/brushed it out anyways. I didn't have a spare gasket, so I didn't go crazy. I also scrapped/brushed the holes on the EGR mount. I blew them out too so I didn't add any crap right back in. I also pulled the PCV valve and rinsed out the oil in it. It rattles, so I assume it's still good. For the first time, I stared at the MAP sensor and thought "Hey, I'll bet the oil gets up in there". So I used a tiny bit of the MAF cleaner and kind of rinsed that out too. Lastly, I pulled all the coil packs and checked them. I expect to see the grounding plane under the coils, but it's not quite like I remember from my last 3800. These packs appear to ground only through one of the mounting bolts. And these bolts were corroded too. I used the wire wheel on the bolts, but has to spend some time on the coil packs with steel wool. Once everything was dry/aired out I just put it all back together. She started right up and sounded good on a test run. I think the PCV valve rattled some at first, but that it probably got oil on it again. I've drove it for two days now without hearing that worrisome noise.

Thanks for the all ideas. I'm not sure which one did it, but I'm glad to have it gone.

Oh, one other thing. I must have been thinking of another car as I never found an air temp sensor. It must be part of the MAF.

Air temp sensor is on the top of the plastic elbow from your airbox to your throttle.

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Originally Posted by Thompsju

Air temp sensor is on the top of the plastic elbow from your airbox to your throttle.

Thanks, but that's not the case on a Series 3. There is no sensor on the snorkel other than the MAF.

Sorry, I'm a Series II guy, and still learning.

Glad we could help.

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