Intermittent Misfire

[truckerdawg]

92 mildly built 4.0 5 spd in a 90 BII. I have a misfire that occurs once every 5 to 10 miles and sometimes the check engine light comes on then goes off(Shop could not pull a code). Changed fuel filter, plugs, ignition coil, and air filter. Runs perfect under load through 5000 rpm's(in all gears-did not try 5th of course)-it is just the normal load driving.Does this both when cold and when warmed up. 64,000 miles on motor with no ticks or rattles of valve train.

 

[RonD]

Most likely Spark issue if CEL comes on, that's the only system that is monitored that closely.

Is the CEL on and off quickly, like Flashing, which means Misfire occurring

Inspect spark plugs closely, I just changed mine, '94 4.0l, which had more pronounced misfire under load, found a hairline crack in center ceramic.

Test coil pack OHMs, it will test OK but make sure all 3 coils test exactly the same

 

[truckerdawg]

I changed the plugs after the misfire started happening(they all looked perfect-first time since rebuild) and the misfire continued with the new plugs. The same with the new coil. Old coil had over 250,000 miles so I figured it was time anyway. Cel may stay on for one second or 5 seconds, but misfire only happens for one misfire and engine light does not correspond necessarily to the one misfire at a time. I have had cracked ceramics but they always were more consistent, but that idea does make sense if I had not changed the plugs-unless I cracked a plug putting it in which I never have done(Pontiac dealer did to me one time).

 

[truckerdawg]

Also Cel may not come on for 20 miles then may come on again and repeat the process for the next 10 miles then quit again.Then it may not happen for the next 3 or 4 days.

 

[RonD]

Yes, mine started off as slight random miss then got steadily worse, but only took a few days

Tougher one for you

CEL only comes on near when misfire occurs, or has it come on with no misfire before or after it came on?
So you think misfire and CEL are definitely connected

If CEL comes on prior to a misfire could be a sensor issue, sensor rarely have issues, usually its wire connector, unplug MAF and TPS and plug back in to clean the connectors

 

[truckerdawg]

I think the misfire and CEL are connected but seemingly do not show a connection. Somehow the misfire problem seems recognized by the computer but not always with the untimely misfire-like there is a problem and sometimes the problem reaches a peak and then the misfire will occur once every several 10,000 plus revolutions. CEL will come on without a misfire but never used to come on till this issue started. I will disconnect and check sensors connections. Thanks.

 

[RonD]

If CEL comes on without misfire then computer is seeing a problem or it is a problem with computer itself, but no codes stored.

You should be able to get codes from 1992 4.0l computer using flashing CEL after installing the jumper wire on OBD connector in engine bay.
I would see if computer is storing codes first, start engine, unplug MAF sensor, CEL should come on and stay on.
Plug MAF back in, see if CEL stays on, it should if code was stored, I think, lol.

But in any case use the jumper and see if you get a MAF code
Not sure if the 1992 4.0l computer will use 2 digit or 3 digit codes?

If computer isn't storing codes then its more than likely you have a computer issue

The 3 Blue Capacitors seen here: http://www.auto-diagnostics.info/ford_eec_iv

Were known to leak after 20+ years and cause odd issues, they can be replaced, under $5 cost
So you could pop the top on the computer and have a look

 

[4x4junkie]

If computer isn't storing codes then its more than likely you have a computer issue

This.

Anytime the CEL lights, a code is supposed to be stored in memory, even if the CEL goes off afterward (the code should be retained for 40 (or is it 80?) warm-up cycles, then will be purged if the fault has not occurred again).


FWIW, my '94 developed a small misfire around 60K miles when under light load or when starting from a stop. Replaced plugs, no effect. Replaced the ignition wires, has never done it since.

 

[truckerdawg]

Thanks. Since I work for myself from 8am to 8pm most days, it will be this weekend when I will take I look at those things. Today for 30 something miles it ran perfect-makes no sense. Then when I went back out again it misfired one time in 8 miles. I do have a couple of extra computers.

 

[BlackBII]

Sounds similar to an issue I had with a 4.0 years ago. After replacing the plugs, coil, wires etc I finally figured out that the crank positioning sensor wiring had a short in it where the two wires go into the plug. The harness makes a 90-180 degree or so turn right there, and the wires were being twisted/pulled too much.

If a different computer doesn't help I would check the wiring on the cps and coil.

 

[truckerdawg]

Sorry guys it took so long. Well I did get a code, but no sense for the misfire which now just gets worse as time passes. It was a code 85- purge solenoid circuit. Heck my other 90 Bronco II does not even run one. No other codes came through. I checked the cps and yes it did have a bare wire which I fixed and that did not solve the issue either. I thought I would check the codes before changing out the computer. So on to the computer. I will let you know later today how that goes. Took off work today to work on the thing. Boy, I hope it becomes productive.

 

[truckerdawg]

The code test was KOEO and KOER that gave me the one solenoid purge code. Pulled the IAC, TPS, CPS, and MAF sensors and checked for bad wires and connections. All good. Replaced MAF with another one I had- no change. Checked IAC, TPS voltage-good. After the computer change, the same very intermittent misfire(that does not happen under heavy loads) happened(this time once in 8 miles)- just normal driving. Checked ALL vacuum lines. Truck idles very well at all times. I know I have not changed plug wires but such randomness and never under heavy load makes me think the plug wires are definitely not the culprit.

 

[BlackBII]

Quite the conundrum.

What's left to check now? Fuel pressure while driving and wiring harness shorts? Have you cleaned and checked all the grounds?

I think you've covered most everything else.

 

[truckerdawg]

Unfortunately, my thinking is there is a short in some wire while obviously minor now and could be anywhere in any loom. How to locate besides visible inspection all wiring on the engine side I do no know.

 

[jelinski]

TruckerDawg.... Did you ever find what was causing the occasional misfire?

My '94 B4000 with 245,000 miles just within the last few days started doing what sounds like the exact same thing.
Pulled the codes & got a MAF sensor voltage low, then another MAF sensor voltage high.
But..... just in case it is not the MAF (which I will dig into tomorrow)....
If you found the cause it might save me a bunch of chasing....... ghosts/wild turkeys/squirrels/random things.
Thanks in advance.
Jim