Ford Ranger / B-Series :: 1999 - Intermittent Misfire On Startup - No Codes

Previously on "This Old Truck": [URL] ....

Mileage now 298,5xx. Done since last report: Oil change and filter. Trans oil change (merconIII, no leaks apparent). Tires. Front brake pads. Clutch safety switch (was interfering with cruise control operation) Coolant replacement. (see below)

Rat damage related overheat (near total coolant loss, but no MIL) at 288k. (Rat suffered acute lead poisoning later that night) No sign of rodents since.

Fuel filter change (after onset of symptoms).

NO MIL/CEL, no pending codes. CEL/MIL operates on KOST.

Temps normal (190F +/-2 deg during steady-state cruise operation, fuel flows and fuel mileage have not changed significantly. (fuel mileage has IMPROVED approx 5%, possibly due to gentler/slower driving)

M5OD with Limited slip 3.73/ payload pkg II (1650lbs)

Symptoms:

- Intermittent misfire on startup, seems to get better as the truck warms up. (intermittent= some days it do, some days it don't.)
- Truck starts and idles well, even with misfire.
- Intermittent loss of power, usually worse on days when no- or minor- misfire at startup/cold idle. From the driver seat it feels like accelerating over painted lines on a rainy day. . . a couple revs of no power, then it pulls hard for a second or two, then misses a couple of shots, lather, rinse, repeat.
- Usually worse in higher gears; (not noticeable in first or 2nd, bad in 3rd at high power settings, now almost constant in 5th, to the point that truck will not accelerate below 3000rpm in 5th even on flat ground.)
- Fuel flows above 4GPH/ OBDII "LOD" (engine load) reported above 75% are typical during malfunction. (Fuel flows have been as high as 9GPH with no problems on other days.)

Symptoms started at approx 292k and have been slowly getting worse over the past few months. . . . usually worse at beginning of trip, fading to very occasional after 50 miles of high-speed driving. Some days/trips no noticeable malfunction.

Thoughts:

1) Ignition wires tired? (did visual inspection, no obvious damage, but no water spray test performed, but seems likely given that the problem is most evident in conditions that would lead to high cylinder pressures.)
2) Fuel pressure? (not measured yet. . . original pump but possibility discounted due to intermittent nature of malfunction.)
3) O2 sensors tired/rat damage? (Original units still installed/ no obvious damage on cursory visual inspection under truck)
4) Injector going bad or injector wiring rat damage?
5) Plugged catalytic converters? (no sound of blown exhaust gaskets, exhaust flows well at tailpipe, with puffs at misfire.)
6) Cam sensor? (never been replaced; known issue with this model engine. Had intermittent mystery squeak that came and went at approx 250k-260k)
7) Clutch slipping? (never had one do that; always lose the throw-out first)
8) LSD problem? (fluid level/ appearance ok, last changed 150k; would expect worse in LOW gears)
9) Blown head gasket/s?
10) Cracked heads/ block? (Oil and coolant levels and appearance normal)

So: That's the plan of attack. What am I missing? (Clutch last done approx 170k; burned throw-out, did disk, flywheel, pressure plate, slave cylinder while the trans was down. I did not open the hydraulics or disturb the pedal linkage when I replaced the safety switch. )