K2500HD 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.

Thoughts?

Mike

[Sugar16]

I've been looking at your parameters and I couldn't really tell of something being wrong but there may be an issue that isn't being mapped above. The only thing I can think of is to just keep looking at the parameters of the sensors and see if anything is not normal, like you said even though the SES light isn't on doesn't mean something isn't right. it could be as simple as some corrosion at a connector causing additional resistance to a sensor or maybe even a ground wire not making good enough contact.

[bartonmd]

Yeah, already moved and cleaned all the grounds I could find. I did have an ABS issue the other day, which was quickly fixed by cleaning up the grounds on the frame. I also disconnected, checked, and reconnected all the connectors that have anything to do with anything that would probably cause something like this.

I'm half tempted to put a MAF sensor on it, and just keep the old one as a spare if that's not the problem. I hate to just throw money at stuff, though.

Mike

[dvanbramer88]

flaky throttle position sensor on the throttle body?

Similar symptoms on my dads saturn when the TPS crapped out. A new one fixed it. IIRC it did throw a code though.


Or flaky IAC on the throttle body?


I'd clean the MAF before replacing it if you haven't already. I ran with a "bad" MAF for a couple days and it never acted that bad. It threw a "Low input voltage" or similar code and it took me a couple days to get a new one.

[bartonmd]

flaky throttle position sensor on the throttle body?

Similar symptoms on my dads saturn when the TPS crapped out. A new one fixed it. IIRC it did throw a code though.


Or flaky IAC on the throttle body?


I'd clean the MAF before replacing it if you haven't already. I ran with a "bad" MAF for a couple days and it never acted that bad. It threw a "Low input voltage" or similar code and it took me a couple days to get a new one.

I thank you for the reply, but did you read anything I posted above? The first things that I did were replace the IAC and clean the MAF.

Also, it's not on this graph because I replaced it with something else, but when I turn on the graph for the TPS, it gives correct values. Also, before I graphed it, the throttle % looked pretty OK on the scangauge.

Mike

[dvanbramer88]

I read the first part and skimmed the rest. I see it now, I missed that.

EDIT: Ok I read the whole thing again.

Looks like you ruled out intake gaskets. They're somewhat common on the 4.8/5.3/6.0 motors but are much easier to change than on my motor.

I guess MAF is the next logical choice than. When I replaced mine, i got lucky with a $15 junk yard MAF that I've been running for a few years now.

[Sugar16]

I'm half tempted to put a MAF sensor on it, and just keep the old one as a spare if that's not the problem. I hate to just throw money at stuff, though.

Get out a voltmeter and check some values and just see whats going on with it? If you're convinced that it is it?

[bartonmd]

I'm half tempted to put a MAF sensor on it, and just keep the old one as a spare if that's not the problem. I hate to just throw money at stuff, though.

Get out a voltmeter and check some values and just see whats going on with it? If you're convinced that it is it?

With the MAF doing the (seemingly) correct readings shown in the data logs above, a DVM wouldn't show anything useful. The only thing a DVM will show on a MAF sensor is whether the wire is broken or not. If it's giving any information, the wire is not broken. The readings of a MAF are resistance of a heated wire, so it won't show anything but a dead short when it's off, unlike a lot of sensors that are essentially variable resistors.

Mike

[bartonmd]

I read the first part and skimmed the rest. I see it now, I missed that.

EDIT: Ok I read the whole thing again.

Looks like you ruled out intake gaskets. They're somewhat common on the 4.8/5.3/6.0 motors but are much easier to change than on my motor.

I guess MAF is the next logical choice than. When I replaced mine, i got lucky with a $15 junk yard MAF that I've been running for a few years now.

I've seemingly ruled out lower intake gaskets, but that doesn't completely rule out unmetered air coming from somewhere. I think (hope) I've ruled that out by doing the spray carb cleaner around everything, but am not sure. I guess I do need to do a data log while spraying it, and see if I can tell anything from the O2 sensors...

Mike

[dvanbramer88]

I read the first part and skimmed the rest. I see it now, I missed that.

EDIT: Ok I read the whole thing again.

Looks like you ruled out intake gaskets. They're somewhat common on the 4.8/5.3/6.0 motors but are much easier to change than on my motor.

I guess MAF is the next logical choice than. When I replaced mine, i got lucky with a $15 junk yard MAF that I've been running for a few years now.

I've seemingly ruled out lower intake gaskets, but that doesn't completely rule out unmetered air coming from somewhere. I think (hope) I've ruled that out by doing the spray carb cleaner around everything, but am not sure. I guess I do need to do a data log while spraying it, and see if I can tell anything from the O2 sensors...

Mike

'



here is a video i set aside for if i ever need it for my dad's truck. He shows you what to look for on the scan gauge while data logging and spraying at the same time.

And I was privately reprimanded by a moderator this morning for "post skimming"

[bartonmd]

Good video! It also showed that since his LTFT was like 25, he had intake leaks, and mine is 0, so I'm pretty sure I don't have unmetered air issues... Eh... I guess I'll put a MAF sensor on it... Looking at the graph, it looks like it gets lean when it does the idle issues, so I wonder if the MAF sensor is having trouble at low flows (which is usually where they start), so I guess I'll put a MAF on it, unless somebody has another take...

Mike

[bartonmd]

Good video! It also showed that since his LTFT was like 25, he had intake leaks, and mine is 0, so I'm pretty sure I don't have unmetered air issues... Eh... I guess I'll put a MAF sensor on it... Looking at the graph, it looks like it gets lean when it does the idle issues, so I wonder if the MAF sensor is having trouble at low flows (which is usually where they start), so I guess I'll put a MAF on it, unless somebody has another take...

Mike

Wait... Not ordering one... The TB and the truck take the same MAF sensor... Switching that shit out, tonight, to see what it does...

Mike

[dvanbramer88]

Good video! It also showed that since his LTFT was like 25, he had intake leaks, and mine is 0, so I'm pretty sure I don't have unmetered air issues... Eh... I guess I'll put a MAF sensor on it... Looking at the graph, it looks like it gets lean when it does the idle issues, so I wonder if the MAF sensor is having trouble at low flows (which is usually where they start), so I guess I'll put a MAF on it, unless somebody has another take...

Mike

Wait... Not ordering one... The TB and the truck take the same MAF sensor... Switching that shit out, tonight, to see what it does...

Mike

NICE! having similar vehicles that share parts is always great for diagnosing and in pinches. Interested and waiting for the results.

If it fixes your problem, you just got to convince the wife that her TB needs a new MAF "all of a sudden."

[bartonmd]

Good video! It also showed that since his LTFT was like 25, he had intake leaks, and mine is 0, so I'm pretty sure I don't have unmetered air issues... Eh... I guess I'll put a MAF sensor on it... Looking at the graph, it looks like it gets lean when it does the idle issues, so I wonder if the MAF sensor is having trouble at low flows (which is usually where they start), so I guess I'll put a MAF on it, unless somebody has another take...

Mike

Wait... Not ordering one... The TB and the truck take the same MAF sensor... Switching that shit out, tonight, to see what it does...

Mike

NICE! having similar vehicles that share parts is always great for diagnosing and in pinches. Interested and waiting for the results.

If it fixes your problem, you just got to convince the wife that her TB needs a new MAF "all of a sudden."

Eh... I pay for all the maintenance and gas for everything with wheels at our place, so who cares...

Mike

[Trail X]

Mike, its getting rich when you have your issues, not lean. It's being starved for air, which I agree, rules out a vac leak.

The reason it's only doing it on cold starts has to do with the closed loop vs open loop modes. I would bet that if you could force it into open loop once the engine is warm, that it would react similarly. Can you do that via hptuners? Open loop (engine cold), relies heavily on the MAF and IAT settings. While closed loop relies mainly on the O2 sensors.

I don't really suspect your MAF. It seems to be registering correctly, but it would be nice if you could scale it a bit more so we could see the graph better (but then again, the flow at idle is pretty low). I'm also not sure how much the ECU depends on the MAF for idling. I'd bet you could disconnect the MAF and it would behave similarly at idle. I believe the ECU depends more on the IAT, coolant sensor, and brake switch.

Your IAT shows +14 and +16 degrees... is that correct? I thought you stated it was -17. What's the coolant temp sensor reading during cold startups, the correct values?

[bartonmd]

Mike, its getting rich when you have your issues, not lean. It's being starved for air, which I agree, rules out a vac leak.

The reason it's only doing it on cold starts has to do with the closed loop vs open loop modes. I would bet that if you could force it into open loop once the engine is warm, that it would react similarly. Can you do that via hptuners? Open loop (engine cold), relies heavily on the MAF and IAT settings. While closed loop relies mainly on the O2 sensors.

I don't really suspect your MAF. It seems to be registering correctly, but it would be nice if you could scale it a bit more so we could see the graph better (but then again, the flow at idle is pretty low). I'm also not sure how much the ECU depends on the MAF for idling. I'd bet you could disconnect the MAF and it would behave similarly at idle. I believe the ECU depends more on the IAT, coolant sensor, and brake switch.

Your IAT shows +14 and +16 degrees... is that correct? I thought you stated it was -17. What's the coolant temp sensor reading during cold startups, the correct values?

Actually, looking closer at them, the one of them it's rich, and the one it's lean, then gets rich 4-5 seconds later. I was only looking at the one where it did it worse, which was the third one. But yes, looks like there's likely not a vacuum leak, especially given that my fuel trims are around 0, not 25.

It does it worse during open loop, but it still does it during closed loop, even after an hour of driving. When I started it up at lunch, today, it wouldn't idle at all until it got to 160F, then it would loop up and down so much that it would finally die. I didn't have time to get it fully warmed up in the parking lot at lunch.

Closed loop relies on O2 sensors, but only when it's at steady state. Otherwise, it relies on MAF, MAP, TPS, RPM, and ECT. Where it has problems is in transient, like where I'm coasting, then push the clutch in, like in the third graph. When it gets warmed up, it usually comes back from a rev OK, but that's because the IAC is controlling it all the way down. The real issue when it's warmed up is when the fuel is off and the engine is turning above idle, then you push the clutch in and it has to catch it.

The other MAFs that I've had start to fail would present as unsteady idle that would slowly drop down to almost die, then would shoot back up to high idle, then slowly fall down again. This isn't exactly that, but when a MAF starts to go, idle is where they have trouble.

Yeah, it was -17 when I first had the issue, but when I did the data log last night, it was around 14F. Any sensor that shows temperature is roughly correct, as far as I can tell.

I'll know about the MAF tonight, when I swap them out and see... The one in the TB is known good.

Something that happened at lunch today was when I revved it hard a couple times, the speedo would jump to 5-8mph. I don't know if that's just it being cold, or if that's indicative of a ground issue, which could cause everything. I checked and cleaned the grounds between the body controller and the frame, the engine and the body, and the engine and the frame, when this first happened. I'll look at the ECM to body ground, tonight, as well.

Mike

[Trail X]

Ground issues suck to chase down. We had one on a unit today at work that still baffles me.

Do you have access to the fuel pressure data/sensor if there is one? I was talking it over with a guy at work and he said the fuel regulator could be giving you problems.

Sorry I don't have any better suggestions... just throwing more shit into the wind.

[bartonmd]

Ground issues suck to chase down. We had one on a unit today at work that still baffles me.

Do you have access to the fuel pressure data/sensor if there is one? I was talking it over with a guy at work and he said the fuel regulator could be giving you problems.

Sorry I don't have any better suggestions... just throwing more shit into the wind.

Fuel regulator is one of the first things I checked, because it's an issue on these, with the E10 that we all run. No fuel or fuel smell in the vacuum line to the regulator. I will check that in the log when I get home, though. I'd think it would be more of a problem running, not just at idle, but what do I know?

No problem, I'm happy to talk about it. At this point, I'm throwing shit into the wind, as well. There are about 10 things it could be, and I'm trying to spitball as much as I can, without just throwing money at it. I don't have a problem spending the money on it, but I just hate throwing parts at something without the proper troubleshooting...

Mike

[bartonmd]

Well, that saved $80... Switched MAF sensors and no change...

I plugged the scanner (HPTuners) back in, and while I can't get it to tell me actual fuel pressure, it says the fuel system is fine (there's a "fuel system OK" bit). I'm half tempted to put a fuel pressure regulator on it, because they're $35, and it's a really common failure that I'd rather get in front of, rather than behind.

I'm just baffled, really... There's only a certain number of things it can be...

Mike

[bartonmd]

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...