4.0 OHV Misfire at all rpms - Need help

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crazyracer

Tres Cuatro Tacos
Sep 27, 2006
2,619
79
48
31
Pacific Beach, CA
#1
Yet another trip to Mexico on the horizon and the truck decides to throw a fit. I need help gents.
Video of symptoms.

Symptoms:
Starts up easy. Initial start is smooth then as soon as it tries to level at idle rpm, shit gets erratic.
The engine will miss, smells rich, and generally bounces around RPM. I have to keep my foot on it to keep it running.
At higher RPMs, the engine will smooth out then still have erratic misfires. Like it's being told to cut-spark for a cunt hair of a sec.

All the plugs show VERY rich/dark conditions. I think it's because they're not sparking intermittently

Replaced the plugs - no change

Replaced the MAF with a brand new one - no change

Replaced the Throttle position sensor with a brand new one - no change

Removed each plug wire from the coil to see if the symptom was cylinder specific - no change

Cleaned the Idle air control unit - no change

Checked fuel pressure - Nominal. I have good baselines to compare as well so I know it's good.

Ran sea foam through intake vacuum ports. No change.


To me, it seems like as soon as the engine is switching off start-up and looking to it's sensors, it's panicking and cutting spark. Seems to affect all cylinders, which now makes me think a sensor or computer.
Any ideas? What sensors "help" it run but are not crucial to it's ability to run? Any and all theories are welcome.

Update: I'm leaning towards crankshaft position sensor? Due to exposure & ease of replacement
 
Jun 9, 2005
252
0
16
escondido
#2
a bad coolant temp sensor could cause your rich condition or but for the misfiring, the cam position sensor could be suspect

You should scan for codes even if you don't have a check engine light on.
 

96f-u250

Senior Member
Sep 27, 2013
725
10
18
VenTurA 805!
#4
A similar issue I had was from a bad idle air control valve. When it is running and displaying previously stated symptoms, unplug it and see if anything changes. Thats all I got, good luck and keep us posted.
 

crazyracer

Tres Cuatro Tacos
Sep 27, 2006
2,619
79
48
31
Pacific Beach, CA
#6
A similar issue I had was from a bad idle air control valve. When it is running and displaying previously stated symptoms, unplug it and see if anything changes. Thats all I got, good luck and keep us posted.
Thanks for response. I've tried that already. Cleaned it too. That's not to say it isn't an issue still, but plugged vs unplug, it ran worse unplugged.
 

crazyracer

Tres Cuatro Tacos
Sep 27, 2006
2,619
79
48
31
Pacific Beach, CA
#9
Update:

Replaced the Crank Position Sensor - No change
Replaced the Idle Air Controller - No change
Sprayed Carb cleaner virtually everywhere looking for a vacuum leak - No change
Wiggle wires all across the engine bay - No repeatable changes (as I was wiggling, the engine kept hunting for RPM, but I didn't seem to cause any change)

I tried the blinky light method for obtaining codes, and after the first code it gets tough to read.

Looks like 133 and 211 are codes, but I can't be certain due to the frequency. Before I throw more credit cards at the problem, I'll wait for my scanner to get here.


My suspicion is that it's a wiring issue (corrosion or short), a PCM, or ECU. Leaning towards ECU since all the sensors seem to be feeding in, but bad info is coming out?

Anyone in San Diego have a 4.0 OHV OBD1 ECU from a 5 speed I could validate with?
 

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