Hey guys, 89 302 auto/4wd again. Plugs gapped to .060 and timing advanced 16°.

I was driving around this afternoon and realized the truck started to develop a rough idle when I would come to a stop. I'd come to a stop, say in a parking lot, and would watch my vac gauge swing from steady vac to almost nothing, then back up again. Also, the voltmeter would swing back and forth with each 'bump' in the idle.

It started up normally after I came back out, and I didn't think any more of it until I had been driving it a little longer and it started to get rough again at idle. Also, started smelling the exhaust like I was running way rich. I'm thinking it's a misfire at this point.

Just got back in from my third stop, where I pulled into a lot and the truck was really running rough as I coasted in, to the point where it stalled out. I started it back up and moved it a little bit, and then cut it off again. Came back out and it started normally again, seemed to accelerate fine as I left, but as I got closer to home I noticed it started losing some power and I could feel it start to hiccup or get jittery as the driveline would develop some slack then jerk when the engine got power again. When I'd pull up to a stoplight, the idle would fall lower and lower til it almost died, then bump back up again. I put it in neutral and kept on the gas to keep the RPMs up until the light would turn green, then shift it back into drive (the truck would almost die as I let the idle fall and then shifted gears) to get it back home. Never stalled out, but I think it would have if I didn't watch it closely.

When I got home it was still idling rough so I turned it off and went to get the fuel pressure gauge. When I connected it to the rail and started it up again, it idled smoothly and the gauge read 37-40 psi at idle, right where it usually does. Revved it a few times but couldn't get the idle to start dipping again, so I think I'll have to take it for a drive and then check the pressure when I get home and it starts to try and stall. Waiting for it to cool right now to pull the spark plugs and check to see if it might be a misfire.

Anyone got ideas? Thanks!

 

Update: pulled the plugs tonight and all were fouled heavily with soft/sooty carbon. Looks like it's running rich or I've got a weak ignition, but I don't think that's the case with the Accel coil, 8.5mm MSD wires and MSD rotor/cap.

Here's #1, all looked exactly like this one. Sorry for the crappy pic

 

Thy don't look too bad. They are gapped to 60 right? Does it smell like it runs rich? Are all the contacts on both ends of the plug wires clean and corrosion free? Are the connections on the coil clean? The contacts on the rotor/cap clean? Are their any cracks in the wires?

 

Checked the gaps as I pulled them out and they're all still at .060", the wires are in good condition and all ends are corrosion free. Been running the wires about a year and been using dielectric grease.

When the truck starts and runs normally it doesn't smell rich, but after I drove it today and it would try to stall on me it would smell extremely rich until I shut it off, as the idle was loping and struggling.

 

Hmm. It's got me stumped. Sounds like once the engine warms up a few pistons don't fire. I'm not 100% sure on that though. I was almost certain it was the ignition system. But you said it all checks out good.

 

I was wondering if it might be the fuel filter starting to clog. When I run it for a while, maybe pressure forces particles to start choking the filter, then turn it off and flow stops, filter opens back up. Although when it smells extremely rich and with the loss of power I think it's a misfire.

I think it's strange that I can shut it off, then immediately start it up again and it idles fine.

 

Might be a computer problem. I'd change the filter also they attend cheap.

 

Another update: Was able to take it for a short drive with the fuel pressure gauge hooked up to the rail. At cold start it was around 30 psi, but went up to 37-40 within a few seconds of idling. I drove it down the street to a parking lot (about 200 yards away) and circled it a few times until it started to act up. I'm guessing it hardly got to operating temperature, if that far. When it started to lose power and stumble/idle roughly, I jumped out and opened the hood to check the gauge. The first time the pressure was around 80 psi. I hit the relief button on the gauge and it shot back up to 80 psi as the truck was struggling to run. At this point the exhaust was smelling very rich.

I shut the truck off, started it up again and proceeded to drive around a little more, including ~75% throttle from 0 to 30 mph. Slowing down to a stop, it started to stumble hard and I popped the hood to check the pressure. The needle was bouncing like crazy, almost pegged at 0 psi. I'm guessing it was in the 120-130 psi range as the gauge stopped reading at 100.

At this point, I'm guessing the FPR is going out. Maybe a seal leaks when it gets warm or a hole's opened up in the diaphragm.

 

There was just a thread with similar issues within the past week or so. IIRC, there was a vacuum leak that resolved the issue.

 

Thanks for the tip SC. Just went back about six pages and found a few people with rough idle issues but it looked like most boiled down to electronics, no vac leaks. I'll try to swap the vac line on the regulator and see if that helps, or hunt for the leak with a can of carb cleaner.

Also forgot to add, I've replaced the MAP sensor and TPS about a year and a half ago, and replaced the ignition control module last fall.

And if worst comes to worst, RockAuto's got several FPRs on closeout.