2001 5.3l Sierra - Miss At Idle On Cyl #2 - I'm Stumped

treblehook · June 24, 2010

I have a 2001 Sierra extended cab with the 5.3L, a stepside bed, and bucket seats with 160k miles. It's been relatively trouble free except for replacing 3 driver's side front hubs (two for bearings, one for the stupid ABS sensor) and an unfortunate 4L60E rebuild at 120k miles. It's still in great shape and I'd like to keep it for awhile longer before I get a new one.

The truck started idling rough (but runs fine above about 1500 RPM) a couple of weeks ago and threw the SES light, so the first thing I did when I got home was pull the DTC with my older Predator....the dreaded P0300 random/multiple cylinder misfire. So I pulled all of the plugs and they were all light tan in a cone shaped pattern on the insulator, except for the #2 plug, which was black in that same area.

So I got a hold of a Nemisys scan tool, verified is was logging a #2 cyl misfire, then switched plugs, wires, and coils between cylinder #s 2 and 4, started it up, and still got the misfire on cyl #2 (but again, only at idle, it doesn't record any misses above 1500 RPM.)

Next I go to fuel. The pressure bleeds to 0 psi after just a few seconds, so I assume the check valve in the fuel pump is bad, but don't think this is causing the miss. It runs about 51 psi at idle, and goes up to 62 psi after I unplug the vacuum from the regulator, so I assume it's ok. I also checked the scan tool at both 51 and 62 psi,and the misfire counts on cyl #2 are equal at both fuel pressures, so am I safe in assuming that low fuel pressure is not the issue? I also changed the #2 fuel injector (that entire bank 2 rail was pretty dirty, I didn't pull the bank 1 rail) with no difference. It was a cheap e-bay injector, even though it's a Delphi, so I can't totally eliminate that injector although it's resistance is ok and I can hear it clicking.

I also recorded the short term fuel trims on the Nemisys and get the following:

............................................Bank 1....Bank 2

Cold idle...................................+6.........+18

2000 rpm at approx 1 minute..+21........+16

Return to idle............................+2.........+14

Tomorrow I'll do a compression test and give it the old Sea Foam treatment.

Does anyone have any ideas? I've already gotten a lot of good ideas from reading posts in this forum and y'all are a lot of help to a lot of people...it's great.

Any help is greatly appreciated, thanks.

gmtech121 · June 28, 2010

You need to have someone put a leakdown tester on it...

treblehook · June 25, 2010

I ran about a half can of Seafoam through the brake booster vacuum line, let it sit about 20 minutes, then drove it out of town, with fast starts every time, and keeping the RPMs above 2,500 for about 10 miles. When I got home and let it idle, the miss was still there.

I got out the compression tester, screwed it into # 2 cylinder, and got 125 psi. When I added a little oil into the cylinder, I got about 130 psi.

# 4 cylinder was 170 psi

# 6 cylinder was 175 psi

I quit at that point, heartbroken.

Since it runs fine during normal driving conditions except idle, I'm thinking that a sticky valve is the problem.

Since Seafoam didn't do the trick, I'm thinking of replacing a quart of oil with Marvel Mystery Oil.

Any suggestions or comments?

Thanks in advance for any help you can give me.

jro909 · June 26, 2010

don't use snake oil, whats the fuel pressure at? hows the injector? hows the coil?

treblehook · September 4, 2011

It's been awhile (over a year,) but the problem was the exhaust valve wasn't sealing on the # 2 cylinder. The # 8 valves actually looked worse (carboned up.) Since a rebuilt head was quicker and cheaper than a valve job locally, I replaced it with a rebuilt from Fliteline Engine Supply out of Fort Smith, AR.

It was about an eight hour job, probably would have been much longer on the driver's side head. Definitely go for the "problem solver" intake manifold. It's worth the extra $20.

As far as those who would say you need to replace both heads if you're going to pull one, I understand there are probably more arguments for than against that, but it's been 25,000 miles and still running great. Just yesterday, I checked the fuel trim, and both short term and long term numbers were nearly identical for both sides of the engine.

Thanks to everyone on this board for the help, and sorry for the delayed report.

redvett · June 26, 2010

Low compression like that most likely is a burnt valve or a weak or cracked valve spring.

jwh463 · September 5, 2011

It's been awhile (over a year,) but the problem was the exhaust valve wasn't sealing on the # 2 cylinder. The # 8 valves actually looked worse (carboned up.) Since a rebuilt head was quicker and cheaper than a valve job locally, I replaced it with a rebuilt from Fliteline Engine Supply out of Fort Smith, AR.

It was about an eight hour job, probably would have been much longer on the driver's side head. Definitely go for the "problem solver" intake manifold. It's worth the extra $20.

As far as those who would say you need to replace both heads if you're going to pull one, I understand there are probably more arguments for than against that, but it's been 25,000 miles and still running great. Just yesterday, I checked the fuel trim, and both short term and long term numbers were nearly identical for both sides of the engine.

Thanks to everyone on this board for the help, and sorry for the delayed report.

Thanx for the update I have the same problem except it is on my #5. Stealership told me $2500 to repair if the heads were bad. I keep driving it and outside of the rough idle it is all good

treblehook · June 28, 2010

Yes, I took out all of the plugs and blocked the throttle plate open.

Thanks for the info.

I've never thought about valve cooling before, so what you're saying is that the valve is cooled by contact with the head? And if there's buildup on the valve and it's not seating, # 1 the buildup is acting as an insulator and not transferring the heat from the valve to the head, where there's water flow internally in the head that takes heat away from the surface where the valve seats, #2 the buildup is causing the valve not to seat, which doesn't allow the greatest possible surface area of the valve to be in contact with the head, where it can be cooled and # 3 if the valve doesn't seat, instead of being in contact with the head, the top part of the valve is excessively exposed to the heat of the combustion chamber, eventually burning it.

treblehook · June 27, 2010

I'm going to pull the valve cover off this week and see what's going on. I sure hope I won't have to pay for a valve job.

Since the scan tool isn't recording any misfires except at idle, and it does feel like it runs on all 8 above 1500 RPM or so, wouldn't that point to a valve spring?

treblehook · June 27, 2010

I'm going to pull the valve cover off this week and see what's going on. I sure hope I won't have to pay for a valve job.

Since the scan tool isn't recording any misfires except at idle, and it does feel like it runs on all 8 above 1500 RPM or so, wouldn't that point to a valve spring?

Doug_Scott · June 27, 2010

I'm going to pull the valve cover off this week and see what's going on. I sure hope I won't have to pay for a valve job.

Since the scan tool isn't recording any misfires except at idle, and it does feel like it runs on all 8 above 1500 RPM or so, wouldn't that point to a valve spring?

Nope, it would point to a leaking valve. If the spring was causing this, it would be worse as revs go up. A leaking valve (burnt, crap on the seat etc..) will not be able to leak enough to cause a miss as revs go up, simply not enough time to leak enought to cause a misfire. Since the compression is merely low, I think it is the valve currently is not seating properly. I say currently because if it is not seating properly, it is not being cooled properely, and will be burnt soon. Once it is burnt, you will have near 0 for compression. I assume you pulled all plugs for the compression test as well, and did with throttle held wide open.