Misfire at idle - Renix 2.5 TBI

[WeezyBlue]

1987 Comanche has what definitely sounds like a misfire, only at idle. Often when cold it starts up and idles smooth, then soon thereafter, maybe when it comes out of open loop and the revs drop to where they should be, it starts missing once every couple seconds sometimes, sometimes more frequently. It still idles strong, and under any amount of throttle it goes away entirely. Or at least, it seems like it goes away entirely. It drives totally smooth, under power or even at slow cruise.

 

Makes me think it's something that's just barely acting up and the lower revs just make it obvious.

 

I've done plugs, wires, cap and rotor. All my Jeeps in the past have been 4.0 HOs so I'm not really sure what to check next.

 

I've done some searches and read a lot of threads here and other places but most people seem to have a rough idle that causes stalling, or a misfire under load, etc. Any general ideas?

[eaglescout526]

I had an issue similar that made me think it was a misfire but it was the valves sticking on the third cylinder. 

Since you said everything else checks out fine except at idle leads me to think that because for me everything was fine while driving except at idle it would miss. I had my head rebuilt and the problem went away. 

[WeezyBlue]

8 minutes ago, eaglescout526 said:

I had an issue similar that made me think it was a misfire but it was the valves sticking on the third cylinder. 

Since you said everything else checks out fine except at idle leads me to think that because for me everything was fine while driving except at idle it would miss. I had my head rebuilt and the problem went away. 

 

I was considering that a possibility. Maybe seafoam, an oil treatment and crossed fingers should be on the list. Motor has something like 168k miles

 

 

[eaglescout526]

I tried sea foam. Not to say it didn’t work but with the previous history my engine had of having a blown head gasket and coolant all in the cylinders led me to where none of the snake oil could’ve cured the issue. Give it a shot. Might be just some crud in the valve guides. 

[WeezyBlue]

Hoping. It doesn't burn oil or anything, no smoke, coolant looks good. Who knows what happened before I took ownership though.

[87MJTIM]

You should get a Renix Engine Monitor from Nickintime. Search YouTube or Facebook. He sells a Renix scanner (REM) that will give you engine information. This can help locate issues problems 

[JustEmptyEveryPocket]

2 hours ago, WeezyBlue said:

Maybe seafoam, an oil treatment and crossed fingers should be on the list

For snake oil, I use Marvels Mystery Oil. I put a quart in on every oil change and add 4-6 ounces on every fuel tank fill. If your problem really is junk in the head, it will take several oil changes and several tanks of fuel until you notice improvements. It takes awhile to break down sludge buildup.

 

47 minutes ago, 87MJTIM said:

Renix Engine Monitor from Nickintime.

This should be required for all Renix Owners. It will give you actual data on how your truck is running. Really helps diagnosis. 

[cruiser54]

Clean ground at firewall. Bare metal.

 

Check the vacuum lines at the intake manifold.

 

Check the vac line to the MAP sensor.

 

Check the intake manifold bolts to see if they've come loose.