Random misfire at low RPMs
Thread Starter
On The Tree
Joined: Sep 2015
Posts: 159
Likes: 0
From: Cullman, Alabama
Random misfire at low RPMs I posted in the PCM diagnostics section with no replies... so I keep getting a SES light and I have a scan tool that will let me read codes, sensor information, and even chart this stuff. I have been charting a cylinder 7 misfire, about 20-40 misfires every 10 second seconds. It only misfires at idle and at cruising rpm. Anything more and it doesn't misfire. Also, the #1 cylinder reads as misfiring, but doesn't throw a code. It misfire around 10-20 times every 10 seconds. I've changed plugs and wires, moved coil packs, checked the crankshaft position sensor, tested my injectors with 12v and cleaned them out with cleaner, checked compression, the MAF is maybe 5,000 mikes old, checked the ground bolt on the back of the driver side head, and went ahead and make a new ground fron the front of the engine block . None of that helped. 2 nights ago, at one point in an hour drive, every single cylinder misfired at least 10 times on different occasions. I am going nuts to figure out what this issue is. It doesn't misfire at WOT at all. Anyone have any suggestions on what to do??
Thread Starter
On The Tree
Joined: Sep 2015
Posts: 159
Likes: 0
From: Cullman, Alabama
I've checked for vacuum leaks and the only one that I found was on the PCV valve and I fixed it. I have a BT reader and I still pick up a constant #7 misfire at idle, but no misfire at any rpm other than idle. I checked compression again and got a reading of 150 from 1, 2, and 3, but a 90psi reading from #7. I put oil in the spark plug hole and retried it but it still read 90. I think it's a valve spring.. Checked for bubbles in the radiator and there was absolutely none. Anything else it could be??
On The Tree
Joined: Oct 2017
Posts: 182
Likes: 9
From: Orlando
I had almost the exact same symptoms. Look further into your PCV system for vacuum leaks. Pull the whole thing off to inspect it. Mine was deteriorated and I think air was seeping out through all the cracks. Then just cut a few bits of 3/8 hose and put it back together, quick added assurance.
Thread Starter
On The Tree
Joined: Sep 2015
Posts: 159
Likes: 0
From: Cullman, Alabama
I had almost the exact same symptoms. Look further into your PCV system for vacuum leaks. Pull the whole thing off to inspect it. Mine was deteriorated and I think air was seeping out through all the cracks. Then just cut a few bits of 3/8 hose and put it back together, quick added assurance.
Trending Topics
TECH Junkie
Joined: May 2012
Posts: 3,050
Likes: 716
From: Poplarville, MS
I am sure that with a compression of 90psi you have found your miss fire. Pull the rocker arms off on that cylinder and hook an air hose up to the spark plug hole and listen to where the air is leaking out of the cylinder.
Last edited by bbond105; Dec 5, 2017 at 12:42 PM.
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo
I've checked for vacuum leaks and the only one that I found was on the PCV valve and I fixed it. I have a BT reader and I still pick up a constant #7 misfire at idle, but no misfire at any rpm other than idle. I checked compression again and got a reading of 150 from 1, 2, and 3, but a 90psi reading from #7. I put oil in the spark plug hole and retried it but it still read 90. I think it's a valve spring.. Checked for bubbles in the radiator and there was absolutely none. Anything else it could be??
As you said, it's likely a valve issue, possibly rings. You need to air up that cylinder with the valves closed or rockers removed and listen for air to hiss out the exhaust, through the intake, or through the oil cap to see where you're losing it.
Thread Starter
On The Tree
Joined: Sep 2015
Posts: 159
Likes: 0
From: Cullman, Alabama
Thanks for all the suggestions and replies. I'm going to pull the valve cover off this weekend and check to see what it is. I'm hoping that it's just a valve spring but I'm thinking that it's more than that. It's weird because the car runs great other than idle and it really doesn't idle rough. I can't even tell it's misfiring until I hook the scanner up. Still pulls strong. I put 2 cars on a 2012 5.0 a couple nights ago from a 60-90 roll so it's not losing power but hopefully I'll get this misfire figured out.
Last edited by 97TAsom; Dec 6, 2017 at 03:45 PM.