'02 K2500HD 6.0 Idle Issues...

[Bartonmd]

OK, so my truck (2002 Chevy K2500HD 6.0L gas, NV4500) is having idling issues. It's just as easy as anything to quote what I posted on a fullsize site:

OK, so we've been having this crazy cold weather (for us). It's been -10F or lower for the last day and a half. Drove the truck yesterday a few times and all was good. Drove into work this morning at -17F, and it started up fine and ran fine for the 2 hour drive to work (43 miles in the packed snow/ice). I went to start it to go to lunch, and it popped over, then died. Did this about 3 times, then I put my foot in the throttle and it sprang to life and stayed there. I let my foot off the throttle, and it died. Start it again with my foot giving a little throttle, and it wouldn't idle by itself until ECT was around 100F, then it idled fine, but would drop down to 300-400rpm, before coming back up to 500-600rpm to steady state idle. If I rev it sitting in neutral, it would come down to right at 5-600 like normal, with no blip below it. Did the "dip" thing on 2 warm restarts, as well, but didn't actually die.

There isn't very much to go wrong on this, vacuum leak-wise, but I checked the couple hoses that there are, and everything seems hooked up.
Intake is tight between MAF and TB with no obvious leaks.
Vac line from intake to fuel pressure regulator was dry, and did not have fuel in it.

My only thought is that the idle air control valve isn't liking the cold, all of a sudden?

Thoughts?

Also, it has 150k on it.

It also runs fine at 500rpm when cold, when I'm holding my foot there on the throttle.

Temp shows correct.

Vac leak theory died. It was at 120f when I started it a few mins ago and it all seems to work perfectly without any of the dips before it recovers or anything. It did this from the start, all in open loop.

Bought carb cleaner, MAF cleaner, and an IAC at AutoZone a minute ago and will see what I see when I get home.

Still no codes when I got home (1 hour drive).

Restarted it when I got gas near home, and it did the "almost die then catch itself" thing at every stop on the way home. It was still doing it when I got home and I sprayed carb cleaner on everything that I thought could possibly be a leak point, and no surging or anything. So, I cleaned the throttle body and IAC ports with carb cleaner and a toothbrush, cleaned the MAF with MAF cleaner spray, and put in a new IAC. I restarted it, and of course it needed throttle, because of the carb cleaner and such that had to go through, but after I revved it a bit, it idled fine and didn't do anything weird. I shut it off and restarted, and it caught itself and then acted normal. I expected this because it'll have to do a bit of relearn on the cleaned MAF and new/cleaned IAC. However, it didn't do anything strange after it caught itself and brought itself to speed. I'll try it in a couple hours when it's cooled down some, then in the morning.

Interesting... It never died this morning, but when it was cold, any time I'd let off the gas (free revving, or pushing in the clutch after coasting), it would go down to 300rpm, then up to 600rpm, then down to 300, then back up to 600 and settle out. Once it warmed up, free-revving it would act normal, but when coasting then pushing on the clutch, it would drop to 300 once for a second or 2, then back up to 600 and act normal.

IMO, that kind of thing is normally a MAF sensor, from what I've seen, but a bad MAFs usually doesn't idle steady, ever. I didn't unplug the battery when I cleaned the MAF and put the new IAC in and cleaned the TB, but I would have expected it to learn in 3 starts, with the last one being an hour and a half of driving, this morning.

Still no codes.

So I got home from vacation and did a capture with HPTuners. See the following:

Here's the startup. See RPM slowly find its way to leveled out after the first dip, and the idle adjust slowly comes way up to correct.

A cold rev, out of gear. See the RPM slowly find its way to leveled out... Idle adjust slowly comes way up to correct.

Coasting down in gear, them putting the clutch in. Almost dies, then idle adjust just barely starts to come up, right before I hit the throttle.

Revving at idle after it gets mostly warmed up. Idle adjust doesn't have to do much...

Looks to me like there isn't a vacuum leak, because the fuel trims in both banks are about the same and both banks look roughly the same to the O2 sensors.

Nothing looks really out of bounds to me. Then again, there still isn't any SES light or codes, so nothing is likely REALLY out of bounds.

ETA: It's not in these captures, but the TPS seems to be working fine, when I turned it on in the log after I took these captures.

Thoughts?

Mike

 

[Bartonmd]

I switched the MAF out from the TB onto the truck, and it didn't change anything.

Put a fuel pressure regulator on it, and it seemed better (though it's kind of intermittent, so it could just be a fluke), but not 100%. Went ahead and did the spray around the intake valley with the brake cleaner while data capturing... You can see the bank 2 O2 sensor go way rich, then the bank 2 long term and short term fuel trims lean out. I know what I'm doing on Sunday afternoon (the only day in the next week and a half above freezing)... lower intake gaskets...

 

[Envoy_04]

Oh boy, sounds like you're in for some fun!

I did lower intake manifold gaskets on a buddy's 3.4 liter Impala in a parking lot with limited tools inside of one day back in the fall. On that truck though it should be a lot easier and I doubt you'll have issues, it's really not too bad of a job - having the correct tools and a good place to work is half the battle.

 

[Bartonmd]

Oh boy, sounds like you're in for some fun!

I did lower intake manifold gaskets on a buddy's 3.4 liter Impala in a parking lot with limited tools inside of one day back in the fall. On that truck though it should be a lot easier and I doubt you'll have issues, it's really not too bad of a job - having the correct tools and a good place to work is half the battle.

This isn't anywhere near the job that the lower intake is on the 60-degree V6! I've done like 5-6 of those, and they take all day! This should be an hour and a half job or so.

Mike

 

[Envoy_04]

I figured it would be a lot easier on a bigger V8 with more room under the hood to work. The 3.4s have so much crap you have to remove before you can even get in there to work that it's not funny. Good luck with it, and hopefully that fixes your idle. Dad's 09 work truck is a 2500 with the 6.0, and from day one the one complaint he's had with it is the idle quality just generally sucks - it dips and jumps all over the place.

 

[Bartonmd]

I figured it would be a lot easier on a bigger V8 with more room under the hood to work. The 3.4s have so much crap you have to remove before you can even get in there to work that it's not funny. Good luck with it, and hopefully that fixes your idle. Dad's 09 work truck is a 2500 with the 6.0, and from day one the one complaint he's had with it is the idle quality just generally sucks - it dips and jumps all over the place.

It's just easier to do on the 4.8/5.3/6.0 than on the 60V6 engines. Not as much stuff to remove, pull the lower manifold without removing the upper part from it, don't have to pull the valve covers, etc..

Mike

 

[Bartonmd]

This setup actually worked pretty well. The shop heat kept it around 40-45F in there the whole time.

It didn't start out idling well at all, but as it learned the new air characteristics, it got better, "normal" almost, even. I won't know until the next few heat cycles if it's actually fixed, or if it just had a good cycle.

 

[Bartonmd]

Well, it's better, but it's not fixed. It seems to idle well and not search for idle while sitting still. I still have cold start issues, though it seems to be able to idle for itself much sooner than before. I've pinned down something really interesting that it does. When you are coasting and push in the clutch, it drops to 300 and hangs out around there, until the instant that you come to a stop, when it jumps right up to a steady 600rpm idle. I coasted like 3/8 of a mile down a hill and it stayed at 300-400rpm the whole way, until the instant that I stopped, and like I say, it jumped right up to 600. It always used to hang out at 1200rpm expecting you to go into another gear, then drop to 600 when you came to a complete stop. It's like the 1200rpm settings (starting and idling while rolling) have been replaced with 300-400rpm.

I did the "spray brake cleaner and watch the computer for rich conditions" thing again, and there are no vacuum leaks that I can tell with the new intake gaskets. I also put a throttle position sensor and a MAP sensor on it today, and plugged off the input to the canister purge solenoid. I also cleaned and dielectric greased all the grounds that I could find. Still does it.

I'm starting to wonder if I've got a damaged PCM... The only other thing I can think of is a cam position sensor...

Mike

 

[Bartonmd]

Well, it's not the alternator. (had the 8 month old Delphi alternator start acting funny, so got a warranty replacement)

It's not the clutch switch.

The symptoms kind of fit a stuck open EGR, but I removed mine and it moves freely and spring returns. To be safe, though, I cut a thin piece of sheetmetal and sandwiched it between the EGR and the base, to totally block the flow. Still the same thing. Not the EGR...

Took my Dad for a ride this morning, showing the logging software and what it is doing. He agreed that it is the strangest thing he's ever seen a vehicle do, and that it may be something in the ECM, or some other little oddball thing that nobody's thought of... After the snow this coming week, I'm going to take it to the dealer and get it done to me, I guess...

Mike

 

[Bartonmd]

OK, I never would have gotten this one... $900 at the dealer later, it's fixed. It was the throttle body.

Apparently, in the throttle sticking (which mine wasn't doing, BTW) TSB, there was a voltage spec saying that if the TPS voltage at closed throttle was more than 0.61V, don't do the TSB, and replace the throttle body. Mine was 0.63V. From what my buddy (dealer mechanic) was saying, it would seem that if it's above .61V, the computer thinks you are slightly on the gas when it's moving, and closes the IAC.

I didn't think it really fit, because that's a wear issue and this happened all at once, but this happened 1 heat cycle after I cleaned a bunch of grounds. The only thing I can think of is that the high resistance grounds (which ended up causing an ABS light, which is what caused me to clean them) were taking away a couple tenths of a volt from the reference voltage in the computer, so everything was OK. I cleaned those grounds, then BAM, it sees a problem that's been developing for a while...

Mike

 

[Texan]

OK, I never would have gotten this one... $900 at the dealer later, it's fixed. It was the throttle body.

Apparently, in the throttle sticking (which mine wasn't doing, BTW) TSB, there was a voltage spec saying that if the TPS voltage at closed throttle was more than 0.61V, don't do the TSB, and replace the throttle body. Mine was 0.63V. From what my buddy (dealer mechanic) was saying, it would seem that if it's above .61V, the computer thinks you are slightly on the gas when it's moving, and closes the IAC.
Mike

I have a question that somebody smarter than us might be able to shed some light on.
I retired about the time that the unattended acceleration issue came up, and I watched
the hearings about this issue in Congress on C Span. The long story short version was
that the acc. pedal has two variable resistors that send two signals that go from .1v to .5 v +/-
to the PCM and TPS had to similar feed back signals to the PCM. Japan version of "Drive by Wire".
I got my TB shop manuals out and looked our system. Similar, except the GM system two
signals go in opposite directions. If either signal is out of whack, it will set a CEL code,
such as P0120-124 or P0220-229. I gave my shop manuals to my nephew who has a 2004
TB, I do not remember what the exact voltages were. Any comments appreciated.

 

[Bartonmd]

I have a question that somebody smarter than us might be able to shed some light on.
I retired about the time that the unattended acceleration issue came up, and I watched
the hearings about this issue in Congress on C Span. The long story short version was
that the acc. pedal has two variable resistors that send two signals that go from .1v to .5 v +/-
to the PCM and TPS had to similar feed back signals to the PCM. Japan version of "Drive by Wire".
I got my TB shop manuals out and looked our system. Similar, except the GM system two
signals go in opposite directions. If either signal is out of whack, it will set a CEL code,
such as P0120-124 or P0220-229. I gave my shop manuals to my nephew who has a 2004
TB, I do not remember what the exact voltages were. Any comments appreciated.

I'm not sure I see a question in your post. Either way that has nothing to do with this. The TSB is for cable-actuated throttle models. The 5.3L was cable-actuated up to 2000, and the 6.0 was cable actuated up to 2002. This is not an electronic DBW throttle issue.

Mike