Suddenly running >>ROUGH AS CRAP<< below 2200RPM!

[fixizin]

DAY-um, just started out the blue the other day, made me think I'd gotten REALLY bad gasoline (+ water), but it had been 4 days/80 miles since I filled an almost empty tank... didn't think it'd take that long to manifest.

Like it's running on 4 cylinders, tiny bit of surging, but mostly cutting out type roughness, with plenty of torquing of the engine mounts.

Hard to drive, really gotta keep the revs up...


... NO MIL/CEL illumination... should I borrow code reader anyway?

... the condition improves considerably--but not completely--after driving for 10+ minutes.

Time to clean MAF sensor? Replace fuel filter? Listen to each injector with the stethoscope??

 

[RonD]

Bad gas(water) will usually effect things after truck sits for a few hours, water is heavier than gas so settles to the bottom, when you start up the engine the mostly water is sucked up and you start getting the misfires.
Yes, odd there is no CEL.

Dirty fuel filter or failing fuel pump would get worse as fuel demand went up, so not that.

Coil could be the problem, has it been extra hot lately, the weather.

Yes, clean the MAF, but probably not the problem, you can just unplug it and do a test drive, engine should run OK without MAF data not great but not like you describe.
Unplugging the MAF will also test the computers ability to set a code and turn on the CEL, it should come on very fast with no MAF connected, if no CEL then you could have a computer problem.

 

[fixizin]

Thanks much for the great troubleshooting points!

Yes, it has been extra hot lately, near record... but not in the mornings, when it seems worst.

But yeah, even though I'm not quite to 55k original miles, the coil pack--and everything else--did roll off the Twin Cities assembly line 16.5 years ago... stuff ages. Also time for new plug wires.

My truck is flex-fuel, so I'm guessing half a tank of E85 would clear out a lot of H2O, yes?


I live high-humidity land, SO... if it is WATER, and it didn't come in with a load of fuel, might that indicate a leaking fuel cap, vent line, charcoal canister, etc., and it's letting condensation form???

 

[RonD]

Just a can of HEET in the tank would take care of any water.

But yes, the ethanol/methanol can bond with water, gasoline can't, so pure water is minimized and doesn't effect engine operation as much when burned.
That is what HEET is, methanol.

EVAP system(fuel tank pressure monitor) is an emission system so any issues would turn on CEL.
And in any case you couldn't get enough water in the tank from condensation to effect engine as you describe.

 

[fixizin]

OK, so next day problem has abated from its worst, but still present... so I pump in 8.0 gals. of E85, on top of the remaining 7 gals of standard E10-87 unleaded reg... (side note: sticker on E85 pump says minimum 70% ethanol content, lol...)

... problem has abated even more, i.e. still worst when cold/sitting long time, and smooths out sooner... but day-um, how much H2O could be in there?...

AND/OR what else might it be?