Suburban miss fire..

[gn69z28]

2005 Z71 suburban 5.3 vin code z. Engine rebuilt with less than 50,000 miles on it. Started a random miss fire at idle. Check engine light on. Scan and get several codes. On is random miss fire code but no specific cylinder. Mass airflow sensor code and lean 02 sensor code. So I replaced mass airflow sensor,map sensor,all plugs,plug wires and all coils and throttle body since it has the throttle position sensor in it. Suburban has 370,000 miles on it so I figured it was time to replace the sensors. New 02 sensors and fuel pump was installed when engine was rebuilt. This is my daily driver so I don't mind putting new parts on it. After all this the check engine light goes out by itself. But the random miss fire is still there. Only at idle and a slight stumble off idle. Driving down the road it's fine. Foot to the floor it's fine plenty of power. Only at idle. Seems to be idling around 400 to 500 rpm. Would low fuel pressure cause this? I've ordered a guage so I can check it. What should I look for? If I pull plug wires off one at a time it makes no difference. Still has same random miss no matter which wire I pull off. So it's not one specific cylinder. Any ideas? Thanks and sorry for the long post.

[Racer71]

Good scanner will show individual cylinder misfires live in the data stream. Were the injectors replaced, have a seen a few with clogged ones causing some driveability issues at higher mileage.

[gn69z28]

It's the original injectors. If one was clogged wouldn't the miss be constant and on just one cylinder?

[BillK]

How about a vacuum leak ? The lean code might be leading in that direction ? My part time guy has been fighting a random misfire on a Chrysler 300 Hemi he just did and it tuned out to be a split pcv line where you could not see it.

[CGT]

Intake gaskets

[rebelrouser]

Sounds like a vacuum leak, first on your scan tool what is the long and short term fuel trims? If it has a vacuum leak they should be showing that they are adding fuel, an increase in fuel trim numbers. Also Take some carb cleaner and watch the O2 readings, a little squirt in a vacuum hose should drive the O2 sensor to at least .9 volts if they are functioning correctly. A blocked converter will sometimes cause the O2's not to read right even if new, not enough exhaust flow past them to work right. Most new engines will not rev up if you spray carb cleaner looking for a vacuum leak, but watch the O2 sensors while you are spraying on the intake.
Also a leaking EGRT valve can cause what you describe, again look at the )2's they will be lean at idle with a leaking EGR, and hook up a vacuum gauge, good engine should have 18 to 22 at idle, leaking EGR will knock that down to 15 or 16, needle should be steady, a bouncing needle indicates valve train issue, weak spring, leaking valve and seat.
A weal fuel pump can also cause the fuel trim numbers to go rich, adding fuel.

[1980RS]

3 things at the dealer I worked for when we had random mis-fires were, bad spark plugs, dirty Maf sensor and a plugged cat convert. on one side.

[dannobee]

What kind of scan tool are you using? The cheaper ones will give the generic "P0300" code while the better ones will provide you with a live date stream where you can watch the cylinders rack up the misfires in real time. I'd check that before throwing parts at it.

[gn69z28]

The scan tool is one from snap on. I don't know how to use it but my boss pulled the codes. I'll have to drive it to work next week and do some more investigating.

[1980RS]

2005 Z71 suburban 5.3 vin code z. Engine rebuilt with less than 50,000 miles on it. Started a random miss fire at idle. Check engine light on. Scan and get several codes. On is random miss fire code but no specific cylinder. Mass airflow sensor code and lean 02 sensor code. So I replaced mass airflow sensor,map sensor,all plugs,plug wires and all coils and throttle body since it has the throttle position sensor in it. Suburban has 370,000 miles on it so I figured it was time to replace the sensors. New 02 sensors and fuel pump was installed when engine was rebuilt. This is my daily driver so I don't mind putting new parts on it. After all this the check engine light goes out by itself. But the random miss fire is still there. Only at idle and a slight stumble off idle. Driving down the road it's fine. Foot to the floor it's fine plenty of power. Only at idle. Seems to be idling around 400 to 500 rpm. Would low fuel pressure cause this? I've ordered a guage so I can check it. What should I look for? If I pull plug wires off one at a time it makes no difference. Still has same random miss no matter which wire I pull off. So it's not one specific cylinder. Any ideas? Thanks and sorry for the long post.

I'm sorry, I did not see how many miles were on the truck. We had a 5.3 back in the mid 2000's that had random misfires with 350K on it. Some wanted to replace a bunch of parts but the one tech who knew his sheet suggested an injector clean with BG cleaner off the rail and plug the return line. You know what? if fixed the truck and ran perfect after that. I would clean those first.

[Monzsta]

I'd be worried about a worn lifter roller axle. The Chrysler Pentastar 3.6l is bad about those, and they will develop a phantom misfire, that usually comes in 3's. In park or neutral, run it up to 1200 rpm and it'll skip like crazy. Put it in drive and foot brake it up to 1200 and it's baby smooth.

The axle becomes egg shaped and the lifter pumps up on the low spot. when the high spot comes around the lifter holds the valve off the seat ever so slightly.

Best way to check this is a running compression test and keep bleeding the cylinder pressure off and watch it pump up. If you get uneven pump ups after a gauge reset, you likely have a valvetrain issue.

[econo racer]

I would check the brake booster for a leak.

[gn69z28]

Ok I'm back to this misfire again. I was asked about fuel trims. At idle with engine at full temperature. Long term bank 1 is -5.47% long term bank 2 is -6.25% short term bank 1 is 0% and short term bank 2 is 6.25% . I also checked misfire history and no 1 cylinder had 2671 and no 8 cylinder had 1226. All others around 10 to 20. Strange that it's opposite corners of the engine. I did replace intake gaskets and when removing intake I noticed that the flanges weren't exactly flat. So took it to work and resurfaced both sides on the CBN mill and got it flat again. No change. Still random misfire at idle. P0300 code. Did a smoke test for vacuum leak and nothing. No smoke anywhere. Any more suggestions? Front two oxygen sensors are new.

[dannobee]

It's not that the two cylinders are opposite, it's that they're close in the firing order. The PCM uses a drop in crankshaft speed to determine if a cylinder misfires. Each cylinder should contribute enough to keep it idling, but if one cylinder doesn't contribute enough, the PCM "knows."

It's #1 that's misfiring and the counts on #8 are likely just phantom misfires. First things, swap plug, wire and coil with another cylinder and see if the misfires move to another cylinder. After that, do the compression and leakdown test on #1 cylinder. Then swap the injectors and see if the misfires change to a different cylinder.

[gn69z28]

It has all new plugs,wires and coils and that changed nothing. At this point the injectors are the only things that are original. And if they are they have 360000 miles on them.