<<<6.0L rough idle HELP!!>>>
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From: Huntsville, AL
<<<6.0L rough idle HELP!!>>> I have a 2003 Silverado 1500 that I put an LQ9 in several years ago. Last year, I was experienced some rough idling, and I got a flashing CEL while driving. However, I logged no codes. I ended up changing the fuel pump and fuel filter which seemed to fix the issue for a while, but now it's back and worse than before. Still no codes. I have a scanner (HP Tuners), and I've done some digging, but I'm not really getting anywhere. A couple things I've noticed while scanning...(1) while idling in open loop, my MAF sensor reading is ~2lb/min (~15g/s) which I think is really high, (2) during closed loop idling, I see ~30% STFT on bank 1, and (3) while scanning, I'm seeing misfires that jump around on both banks. I'm determined to get this fixed, but I could really use y'all's help on what to look for so I don't simply throw parts at this. Any help is greatly appreciated.
FYI: I changed the intake manifold gaskets last year, and I did a smoke test on the intake. So, I don't believe I have any significant vacuum leaks.
FYI: I changed the intake manifold gaskets last year, and I did a smoke test on the intake. So, I don't believe I have any significant vacuum leaks.
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From: Huntsville, AL
I did more scanning today, and I noticed that the MAF sensor reading leveled out at ~0.8lb/min (~6g/s) once the truck was warm. That seems about right. I checked the fuel pressure, and I'm getting about 48psi with the truck running and about 58 psi with the fuel pressure regulator vacuum hose unplugged. So, I don't think it's the fuel pump. I have no idea what to check next especially since the misfires seem to move from one bank to another.
Last edited by Weathers5; Mar 1, 2025 at 03:59 PM.
What I have found on many 6.0s coming in the shop with a rough idle and upwards of 150 on the clock and sometimes wiith and without a mil is weak valve springs. This is after all the other checks have been done and everything is in order. Usually stick a set of LS6 springs in em they clear on up and ship em.
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Thread Starter
Teching In
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From: Huntsville, AL
What I have found on many 6.0s coming in the shop with a rough idle and upwards of 150 on the clock and sometimes wiith and without a mil is weak valve springs. This is after all the other checks have been done and everything is in order. Usually stick a set of LS6 springs in em they clear on up and ship em.
The engine is a reman'd LQ9 that I put in a few years ago, and that engine has <25k miles. Should've been outfitted with new springs, right? I have some new injectors that I'll be putting in today. If that doesn't help, I'll have to keep searching.
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Should've been outfitted with new springs, right?
Shouldn't we have been outfitted with antlers? Iunno. Frankly, I doubt it.
Repeat after me 10 times: When in doubt, swap it out.
The symptoms you describe are EXACTLY like a bad intake gasket; which of course, IS a fuel issue, since it allows unmetered air into the plenum which the ECM can't "see", with it not having passed through the MAF, and therefore it doesn't know it's there, to add fuel for it, therefore it runs impossibly lean at all times that the vacuum is high (basically, all the time except at WOT). I would suggest re-visiting that, using the metal (more expensive), rather than the plastic, type of carrier for the O-rings. Check the intake itself carefully for cracks. Or for that matter, since those are SO common - same on 4.8, 5.3, & 6.0 from like 02 to 06 - grab one at yer local buzzard nest and just ... you know the drill ... while you're also doing the valve springs.
I would note also, they used the same injectors in all those motors as well. They are essentially TOTALLY maxed out in a stock 6.0. Larger ones often help those motors ALOT, and especially if they have any other mods, such as headers.
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From: Huntsville, AL
:shrug:
Shouldn't we have been outfitted with antlers? Iunno. Frankly, I doubt it.
Repeat after me 10 times: When in doubt, swap it out.
The symptoms you describe are EXACTLY like a bad intake gasket; which of course, IS a fuel issue, since it allows unmetered air into the plenum which the ECM can't "see", with it not having passed through the MAF, and therefore it doesn't know it's there, to add fuel for it, therefore it runs impossibly lean at all times that the vacuum is high (basically, all the time except at WOT). I would suggest re-visiting that, using the metal (more expensive), rather than the plastic, type of carrier for the O-rings. Check the intake itself carefully for cracks. Or for that matter, since those are SO common - same on 4.8, 5.3, & 6.0 from like 02 to 06 - grab one at yer local buzzard nest and just ... you know the drill ... while you're also doing the valve springs.
I would note also, they used the same injectors in all those motors as well. They are essentially TOTALLY maxed out in a stock 6.0. Larger ones often help those motors ALOT, and especially if they have any other mods, such as headers.
Shouldn't we have been outfitted with antlers? Iunno. Frankly, I doubt it.
Repeat after me 10 times: When in doubt, swap it out.
The symptoms you describe are EXACTLY like a bad intake gasket; which of course, IS a fuel issue, since it allows unmetered air into the plenum which the ECM can't "see", with it not having passed through the MAF, and therefore it doesn't know it's there, to add fuel for it, therefore it runs impossibly lean at all times that the vacuum is high (basically, all the time except at WOT). I would suggest re-visiting that, using the metal (more expensive), rather than the plastic, type of carrier for the O-rings. Check the intake itself carefully for cracks. Or for that matter, since those are SO common - same on 4.8, 5.3, & 6.0 from like 02 to 06 - grab one at yer local buzzard nest and just ... you know the drill ... while you're also doing the valve springs.
I would note also, they used the same injectors in all those motors as well. They are essentially TOTALLY maxed out in a stock 6.0. Larger ones often help those motors ALOT, and especially if they have any other mods, such as headers.
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Is there any way to check whether the springs are actually bad or not?
Not to be an *** or anything; butt...
When in doubt, swap it out.
There is no way of really knowing what you have in the way of valve springs on a reman engine. Sometimes they are new most of the time they are cleaned up springs that came in on the rebuilder core motor. If nothing else it puts you a few steps ahead if you ever decide to cam it.
One other thing is did you run the crank cam relearn when the new engine was put in. This procedure must be ran anytime motor or valve train is touched. It allows the pcm to determine variance in sensors cam gears crank gear etc and determine true 0 location. It uses this information to determine when there is a misfire based off torsional acceleration differences that occur when a misfire is present.
One other thing is did you run the crank cam relearn when the new engine was put in. This procedure must be ran anytime motor or valve train is touched. It allows the pcm to determine variance in sensors cam gears crank gear etc and determine true 0 location. It uses this information to determine when there is a misfire based off torsional acceleration differences that occur when a misfire is present.
Last edited by 01WS6/tamu; Mar 13, 2025 at 09:34 PM.
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From: Huntsville, AL
There is no way of really knowing what you have in the way of valve springs on a reman engine. Sometimes they are new most of the time they are cleaned up springs that came in on the rebuilder core motor. If nothing else it puts you a few steps ahead if you ever decide to cam it.
One other thing is did you run the crank cam relearn when the new engine was put in. This procedure must be ran anytime motor or valve train is touched. It allows the pcm to determine variance in sensors cam gears crank gear etc and determine true 0 location. It uses this information to determine when there is a misfire based off torsional acceleration differences that occur when a misfire is present.
One other thing is did you run the crank cam relearn when the new engine was put in. This procedure must be ran anytime motor or valve train is touched. It allows the pcm to determine variance in sensors cam gears crank gear etc and determine true 0 location. It uses this information to determine when there is a misfire based off torsional acceleration differences that occur when a misfire is present.
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From: Huntsville, AL
So, I changed the injectors and I'm still seeing misses on cylinders 2 and 7. I'm also seeing misses occasionally on cylinders 1 and 8. These are cylinder pairs, right? Does that help narrow down the issue? In the related thread, the issue was the crank position sensor. Maybe that could be my issue. One more thing, the misses come in what I would consider small numbers...like <50 at a time.
This is getting frustrating, but I'm not willing to quit yet. I'm definitely open to changing the springs, and I can check the intake again. I'm still getting LTFT values on both banks around 10%.
This is getting frustrating, but I'm not willing to quit yet. I'm definitely open to changing the springs, and I can check the intake again. I'm still getting LTFT values on both banks around 10%.
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From: Huntsville, AL
Update: I smoke tested the intake again and the exhaust. I didn't see any vacuum leaks, and all exhaust leaks were after the upstream O2 sensors. I replaced the upstream sensors anyway since they've never been replaced. I also changed the crankshaft position sensor and did the crank position relearn procedure again. I also measured the fuel pressure again which is ~45psi with KOEO and while running...goes to ~53 psi with the fuel pressure regulator vacuum hose (when running). After that, I blew out all the lines and put a new fuel filter in. Performance-wise, nothing has really changed. Still seeing high LTFT values, intermittent misfires, and the occasional long start and rough running. I ordered the valve springs like some on here have suggested, and I'm waiting on the spring compression tool before I install.
Question for the experts: is 45psi fuel pressure low enough to cause the kind of problems I'm seeing? A quick google search seems to suggest so. The fuel pump is new, but it's a Precision brand I think. I'm leaning toward buying an AC Delco, but I'd like to get another opinion on whether it could simply be that I didn't buy an OE pump? If I simply needed to put an OE pump to fix this, that would be awesome but would suck at the same time.
Question for the experts: is 45psi fuel pressure low enough to cause the kind of problems I'm seeing? A quick google search seems to suggest so. The fuel pump is new, but it's a Precision brand I think. I'm leaning toward buying an AC Delco, but I'd like to get another opinion on whether it could simply be that I didn't buy an OE pump? If I simply needed to put an OE pump to fix this, that would be awesome but would suck at the same time.
KOEO fuel pump commanded on via scantool
Vin Z 48-54
all except vin Z 55-62
This is after all the other checks have been done and everything is in order.
May as well be in the damm corner talking to myself around here these days sometimes!
Vin Z 48-54
all except vin Z 55-62
This is after all the other checks have been done and everything is in order.
May as well be in the damm corner talking to myself around here these days sometimes!
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From: Huntsville, AL
I'm not sure what you're getting at when you say "after all the other checks have been done and everything is in order". I think something is getting lost in translation. I'm trying to do all of the checks, and I'm asking for y'alls advice on what to check. If you're talking about the springs, I have new ones in the garage, and I'm about to install them. Seems that has nothing to do with hard starts and low fuel pressure.