el torro
Newbie
Ok before anyone yells, I did search for my question. I found quite a few threads but nothing that I think answers my problem.
No DTCs are being thrown for miss fires, but I can feel them. The engine has that familiar miss fire shake.
I’m fighting a miss fire issue on a 98 2WD C3500 single rear wheel 5.7 with 240,000 miles and I need some ideas on what else to look for. I have miss fires on 4,6,8 at idle up to 1000 rpms. From 1000 to around 2000 they all but go away. 2100 and up there are zero miss fires. Cylinders 1,3,5,7 AND 2 are fine. The truck had this issue when my son bought it in October 2023. I knew it was a money pit, and now he and his wallet are learning a valuable life lesson.
This truck has a sketchy past. Unknown how many previous owners and zero maintenance records. A year ago I replaced the original distributor. The billet unit came with a new cap and rotor. The plugs and plug wires were replaced at the same time. After fighting the P1345 code for months I figured out with the help of a YouTube video the new drive gear was installed 180° off, meaning the dimple on the gear was wrong. Knocked the roll pin out, spun it around, and the trouble code went away but the miss fires persisted.
Two weeks ago it started running rough, fuel mileage dropped to about 5mpg, and it kept dying. A new fuel pressure regulator, new upper intake plenum, and new spark plugs and it was back on the road. The plenum was replaced because I found a hairline crack in the OEM unit. Someone in the past did the spider injector upgrade with a genuine AC Delco unit so that was a nice surprise to see.
The O2 sensors, IAC, MAF, air inlet temp sensor, EGR, and purge solenoid were replaced at the same time as the regulator. The MAF, IAC, and temp sensor were all throwing codes before the FPR died and all four O2 sensors have been bad since he bought it. The EGR and purge solenoid I replaced because I was already working on it and they were original so why not just do it while I’m in there. All of that and it’s running better than it ever has, but the same misfires are there.
All cylinders have good compression, 2 is 165, 4 is 160, 6 is 170 and 8 is 165. 1,3,5,7 are all 160-170. The misfire cylinders do show some oil burn on the spark plugs but not to the point of fouling and the exhaust doesn’t smell like oil or show any oil smoke. I just changed the oil and dumped the old oil into the 5 quart jug the new stuff came in. It was just shy of 5 quarts, shy enough to assume it was still in the engine, old filter, and drain bucket.
I know this truck is supposed to take 5.1 quarts, the oil cooler lines have been deleted and the filter adapter replaced with a standard adapter. So full should be 5 quarts and the dipstick shows right on the money with 5 quarts.
The only thing I can come up with now is bad valve stem seals. If the valve seats were bad it wouldn’t hold compression. If the intake gasket was letting oil leak into the cylinders it would smoke. If the rings were bad it would smoke and have low compression.
Anyone else have any ideas?
No DTCs are being thrown for miss fires, but I can feel them. The engine has that familiar miss fire shake.
I’m fighting a miss fire issue on a 98 2WD C3500 single rear wheel 5.7 with 240,000 miles and I need some ideas on what else to look for. I have miss fires on 4,6,8 at idle up to 1000 rpms. From 1000 to around 2000 they all but go away. 2100 and up there are zero miss fires. Cylinders 1,3,5,7 AND 2 are fine. The truck had this issue when my son bought it in October 2023. I knew it was a money pit, and now he and his wallet are learning a valuable life lesson.
This truck has a sketchy past. Unknown how many previous owners and zero maintenance records. A year ago I replaced the original distributor. The billet unit came with a new cap and rotor. The plugs and plug wires were replaced at the same time. After fighting the P1345 code for months I figured out with the help of a YouTube video the new drive gear was installed 180° off, meaning the dimple on the gear was wrong. Knocked the roll pin out, spun it around, and the trouble code went away but the miss fires persisted.
Two weeks ago it started running rough, fuel mileage dropped to about 5mpg, and it kept dying. A new fuel pressure regulator, new upper intake plenum, and new spark plugs and it was back on the road. The plenum was replaced because I found a hairline crack in the OEM unit. Someone in the past did the spider injector upgrade with a genuine AC Delco unit so that was a nice surprise to see.
The O2 sensors, IAC, MAF, air inlet temp sensor, EGR, and purge solenoid were replaced at the same time as the regulator. The MAF, IAC, and temp sensor were all throwing codes before the FPR died and all four O2 sensors have been bad since he bought it. The EGR and purge solenoid I replaced because I was already working on it and they were original so why not just do it while I’m in there. All of that and it’s running better than it ever has, but the same misfires are there.
All cylinders have good compression, 2 is 165, 4 is 160, 6 is 170 and 8 is 165. 1,3,5,7 are all 160-170. The misfire cylinders do show some oil burn on the spark plugs but not to the point of fouling and the exhaust doesn’t smell like oil or show any oil smoke. I just changed the oil and dumped the old oil into the 5 quart jug the new stuff came in. It was just shy of 5 quarts, shy enough to assume it was still in the engine, old filter, and drain bucket.
I know this truck is supposed to take 5.1 quarts, the oil cooler lines have been deleted and the filter adapter replaced with a standard adapter. So full should be 5 quarts and the dipstick shows right on the money with 5 quarts.
The only thing I can come up with now is bad valve stem seals. If the valve seats were bad it wouldn’t hold compression. If the intake gasket was letting oil leak into the cylinders it would smoke. If the rings were bad it would smoke and have low compression.
Anyone else have any ideas?
