Rough idle during warm up, loss of power.
#1
04-26-2009, 09:41 PM
Rough idle during warm up, loss of power.
I have a 2002 A6 2.7t with 178k miles. The previous owner took EXCELLENT care of her and I have tried to maintain that same attitude. Recently I have been experiencing some issues with my warm up and a noticeable loss of "oomp" in my power band.
On a cold start I see my RPMs shoot to 1500, then to 1000, then settling at 750 after about 15 seconds. During this whole affair, my engine shakes noticeably. I have a P0421 CEL on and have replaced a bad Air Pump and MAF within the last thousand miles. I checked my vacuum lines, spark plugs, coil packs and they are all good. I am going to replace my post-cat O2 sensors tomorrow, but I'd like a second opinion if it fails to fix my problem.
My vehicle is also real sluggish when I hit the gas. Stutters occasionally. Would O2 sensors screw with the Air/Fuel mix that much?
Thanks!
On a cold start I see my RPMs shoot to 1500, then to 1000, then settling at 750 after about 15 seconds. During this whole affair, my engine shakes noticeably. I have a P0421 CEL on and have replaced a bad Air Pump and MAF within the last thousand miles. I checked my vacuum lines, spark plugs, coil packs and they are all good. I am going to replace my post-cat O2 sensors tomorrow, but I'd like a second opinion if it fails to fix my problem.
My vehicle is also real sluggish when I hit the gas. Stutters occasionally. Would O2 sensors screw with the Air/Fuel mix that much?
Thanks!
#3
04-27-2009, 06:48 AM
I had a co-worker scan my car and read the voltages on all sensors. The pre-cat ones were low, but in tolerance, the post-cat ones were reading a flat zero.
I forgot to mention that the shaking is bad when the car had been sitting a while. When it warms up, it drives okay. Still some hesitation when I hit the throttle, but rpms go right to 750 on a warm start. Would an ICV perform better warm?
I forgot to mention that the shaking is bad when the car had been sitting a while. When it warms up, it drives okay. Still some hesitation when I hit the throttle, but rpms go right to 750 on a warm start. Would an ICV perform better warm?
Last edited by doomwalrus; 04-27-2009 at 06:55 AM.
#4
04-27-2009, 02:23 PM
This may help you a little.
The idle air control valve regulates the amount of air entering the engine during idle and other times during an engine's run period (such as deceleration). There are actually two idle settings on most fuel injection engines: minimum idle and running idle. Minumum idle is the idle based on the throttle blades alone with the IAC passage closed. Running (or commanded) idle is programmed in the chip.
A GM engine's IAC is measured in steps. For example, 0 steps is all the way closed, and 160 steps is all the way open. So, for instance, if your commanded idle is 600 rpm, and your actual idle is 500 rpm, the IAC will open a certain number of steps to obtain 600 RPM.
Now, remembering that the IAC (at least GM's) works on a range of 0 (fully closed) to 160 (fully open), you want the IAC to be about 40 steps at warm idle. It is also set on a curve based on coolant temperature, with cooler temperatures allowing more steps to help warm up the engine. This is so that it has room to move both back and forth, to account for various loads, etc. This helps to keep the engine from stalling. The IAC also has different step settings for Park/Neutral, as the engine naturally requires less air when it is not under load. Think of the IAC as a smart air choke.
The IAC also provides another function called "throttle following". This function helps keep the engine alive when you floor it and then suddenly let off and close off the throttle valve. It temporarily opens to a certain step (also programmable in the chip) to curve the drop-off in incoming air. Some people disable this function, as they feel it hurts their braking power (as they are still receiving minimal engine power right after they let off the throttle).
The IAC also has a "Keep Alive" function. If it detects a drop in RPM, it will open to help "keep alive" the engine. This is why you want your warm idle setting to be at or below its half open range. For instance, if at idle the IAC was at 140 steps (almost fully open) it wouldn't have much room to open up to keep the engine running.
A GM engine's IAC is measured in steps. For example, 0 steps is all the way closed, and 160 steps is all the way open. So, for instance, if your commanded idle is 600 rpm, and your actual idle is 500 rpm, the IAC will open a certain number of steps to obtain 600 RPM.
Now, remembering that the IAC (at least GM's) works on a range of 0 (fully closed) to 160 (fully open), you want the IAC to be about 40 steps at warm idle. It is also set on a curve based on coolant temperature, with cooler temperatures allowing more steps to help warm up the engine. This is so that it has room to move both back and forth, to account for various loads, etc. This helps to keep the engine from stalling. The IAC also has different step settings for Park/Neutral, as the engine naturally requires less air when it is not under load. Think of the IAC as a smart air choke.
The IAC also provides another function called "throttle following". This function helps keep the engine alive when you floor it and then suddenly let off and close off the throttle valve. It temporarily opens to a certain step (also programmable in the chip) to curve the drop-off in incoming air. Some people disable this function, as they feel it hurts their braking power (as they are still receiving minimal engine power right after they let off the throttle).
The IAC also has a "Keep Alive" function. If it detects a drop in RPM, it will open to help "keep alive" the engine. This is why you want your warm idle setting to be at or below its half open range. For instance, if at idle the IAC was at 140 steps (almost fully open) it wouldn't have much room to open up to keep the engine running.
#5
04-27-2009, 02:43 PM
Ok Chefro..
No the o2's should not mess with the fuel mixture. I have zero o2's because of my gutted pipes and I have zero problems. Also I have the code still showing for the o2's. They are there to read the mixture or temp build up from possible bad cats. Lets start with make sure you click your gas cap a few times each time you put gas in. I see you live in Texas... has it been hotter the past couple of weeks during the driving issue? Has it been happening in a certain rpm or under boost only? Under boost do you hear any whistling, clicking or anything strange under boost? What fuel have you been using? It sounds pretty normal about your cold start. I have asked my tech about that previously worried about it. Get back to me and hopefully we can find out the issue
No the o2's should not mess with the fuel mixture. I have zero o2's because of my gutted pipes and I have zero problems. Also I have the code still showing for the o2's. They are there to read the mixture or temp build up from possible bad cats. Lets start with make sure you click your gas cap a few times each time you put gas in. I see you live in Texas... has it been hotter the past couple of weeks during the driving issue? Has it been happening in a certain rpm or under boost only? Under boost do you hear any whistling, clicking or anything strange under boost? What fuel have you been using? It sounds pretty normal about your cold start. I have asked my tech about that previously worried about it. Get back to me and hopefully we can find out the issue
#6
04-27-2009, 04:45 PM
On a cold start I see my RPMs shoot to 1500, then to 1000, then settling at 750 after about 15 seconds. During this whole affair, my engine shakes noticeably.
However, after rereading that sounds like a normal cold startup, indeed... less the shaking noticeably part. And so far he only talks about idling, right?
He said he checked the spark plugs, the coil packs, and the vacuum lines, which all are ok. One other thing that could create problems may be the fuel filter, but if it was plugged/clogged, I highly doubt the car would start "normal" (with the instant spike to 1500RPM, then slowly winding down).
The vigurous shaking is not a good sign, that's for sure. I never had a bad cat, so I have no idea if a bad (clogged or else) cat would cause that misfire shaking.
Maybe you know better, wammy. You're more an expert on these Audi than I'll ever be.
As far as the O2 sensor question, it was as I specified - out of sheer curiosity since I believe that the downstream O2 sensors are a waste. The upstream ones are critical.
That's all.
Thanks for putting me in the corner for the O2 blasfemy...LOL
#7
04-27-2009, 04:54 PM
I appreciate it Chef. So if my actual valve is a go, my chip might still be out of whack a tad? Just got her back from the dealer and they told me the rear senors were fried. Coated in carbon and just messed up. So even if it doesn't fix the problem, at least it's one more thing I don't have to worry about 
To answer your questions Wammy:
I replaced my gas cap about 4k miles ago to get rid of a pesky CEL. Car acts the same, regardless of outside temp, or so I've noticed. Runs better after the engine heats up a bit, but still acts sluggish as opposed to a few months ago. Not rpm specific, just feel the car hesistate when I push down the pedal (not flooring it, normal driving ;P)
Other than normal turbo whistle, no noises under acceleration. Being using 91 since I've owned it. With the exception of one garbage FINA fill-up, always a name brand gas.
When I first got here, the rpms would settle to 750 within a few seconds with no shake. Now it takes 30 seconds to get there, with the car sputtering. After she warms up a bit, and after the first looooong shift to 2nd, she drives better.
To answer your questions Wammy:
I replaced my gas cap about 4k miles ago to get rid of a pesky CEL. Car acts the same, regardless of outside temp, or so I've noticed. Runs better after the engine heats up a bit, but still acts sluggish as opposed to a few months ago. Not rpm specific, just feel the car hesistate when I push down the pedal (not flooring it, normal driving ;P)
Other than normal turbo whistle, no noises under acceleration. Being using 91 since I've owned it. With the exception of one garbage FINA fill-up, always a name brand gas.
When I first got here, the rpms would settle to 750 within a few seconds with no shake. Now it takes 30 seconds to get there, with the car sputtering. After she warms up a bit, and after the first looooong shift to 2nd, she drives better.
#8
04-27-2009, 08:38 PM
I've had the fuel filter changed 1k miles ago so that's not an issue 
And if it was a cat, it wouldn't run better the warmer it got. I'm thinking perhaps a faulty fuel injector....an ECT sensor...or even(gulp) the timing belt. Timing belt is newer, so probably not.
And if it was a cat, it wouldn't run better the warmer it got. I'm thinking perhaps a faulty fuel injector....an ECT sensor...or even(gulp) the timing belt. Timing belt is newer, so probably not.
#9
04-28-2009, 05:40 PM
Just an update. Had some guys at work look at it. They said it might be an injector issue. My exhaust is not rotten-egg, but it's got a sour-ness to it. I also have an oily substance coming from my exhaust tips. Like oil almost, but no smell. Would a bad injector do all this?
#10
05-01-2009, 06:51 AM
I've gotten a bunch of advice from work, but none of them are of much help. Don't think I've got a bad valve, I DON'T want to put a little diesel in my gas tank, and driving for two hours to get to the nearest Audi dealer doesn't sound enticing. I'll keep ya'll updated on what local mechanics find, if they can find anything 