strange misfire question before troubleshooting

[jamie3529gq]

Before I spends a bunch of time....maybe someone else has seen this.

I drove my 01 mountaineer to Iowa and back this past weekend, about 1400 miles total. Pulled a uhaul out there, 94F outside, no issues, worked awesome.

Drove home through the night Sunday, it ran smooth as silk for 4 hours on cruise control, then suddenly started experiencing some stutters. For about 10 minutes it stuttered slightly (never lost 73mph cruise speed) and the check engine light popped on. Since I had my Bluetooth code reader with me, I checked the code (never turned the cruise off) and it was misfire cylinder 6.

It smoothed out again, and about 4 hours later it had another short episode. I should note that there were no hills involved, the road both times was flatter than water on a plate, engine not under much load.

I will do all the usual things, using help from this forum (check wires / coils / plugs / injectors / fuel pressure / compression etc etc etc) but before I start, has anyone had a misfire this intermittent, right in the middle of a nice smooth continuous cruise run??

Thanks.

 

[97-5.0]

I've had the same, still do

If not fuel related, dirty filter low pressure

, ide say minor crack/pin hole vac leak or plug wire on its way out

Do resistance test on the wire, wiggle and pinch the wire from end to end and look for the reading to jump around, should be steady solid reading if good

I have a vac gauge hooked up all the time on dash, nice for finding sweet spots on long drives

 

[jamie3529gq]

Thanks. I wonder about fuel. I have a recent new filter on it, but I don't know the fuel pressure. Be one of the first things I check!

 

[jamie3529gq]

i checked fuel pressure, its 68psi all the time. shut the truck off it drops to 65, then takes overnight to drop to 10psi. was still 10 psi 24 hours later. so i think the fuel pump is OK. truck fires right up, no hard starts, ever.

but i plugged the code reader in to clear the check engine light, and sometime after i checked it on the highway the code changed to 'random misfire' instead of misfire cylinder 6. maybe EGR? vacuum leak? i checked the EGR function with a manual vacuum pump, applying vacuum at idle almost stalls the truck, so it seems like its working OK. truck idles beautifully, smooth and quiet.

maybe i just got some bad iowa gas, too much corn in it or something. i haven't driven it since i got home, going to drive the 100 mile round trip to work and back tomorrow, see if the code comes back....