O/T: '91 Dakota misfire update

Original post at bottom for full details.

Update:

Put on a new coil a couple days ago, misfire cured! Coil states it's to be used with an external resistor(as did original coil), but I don't see it. Where did they hide the darn thing?

Tach still erratic and there's a slight hesitation just above idle. Most likely getting a bad spot in the TPS?

Does anyone have suggestions on the tach problem? Or a good wiring schematic for it? If the tach signal comes from the coil, why didn't the new coil resolve this issue also? Hmm, this could get interesting!

Thanks again!
Anthony

Original post:

My apologies up front to those who are offended by O/T posts. I understand their opinion, but the vast knowledge and helpfulness of this board is huge resource for a lot of us.

My thanks up front to those who contribute their time, opinions and suggestions.

My truck: '91 Dakota, 3.9 V6, auto, 219,000 miles

My problem: runs perfect cold, but has an intermittent misfire after it warms up. Been doing this for approx. 3 weeks now.

Light load(ie: maintaining speed) it does not miss.

When warm but not under load, it does not miss.

Any load over approx. 1/4 throttle, it will start to miss randomly.

Fuel filter, cap, rotor and plugs are new.

Timing is set right, new timing chain last year also.

Checked for vacuum leaks, found none. All hoses and connections good; all ports on intake and TBI unit sucking air, no blockage suspected.

The exhaust is cracked at the bottom of the crossover, in front of the catalytic converter. Heavy black stain here.

I am guessing a "too rich" condition exists, and is causing the misfire. But there are no diagnostic codes in the computer relating to this. Nothing referencing oxygen sensor, tps, etc. Only 1 code, and it's for the A/C relay circuit, probably because there's no relay in the under-hood fuse box for the A/C.

Additionally, tach is not working properly(recent behavior, but did not begin at the same time as the misfire problem). At startup, it rises quickly to about 1800 rpm, then slowly to about 2300 rpm, and there it stays until truck is shut off. It does drop sharply, and rises again, in time with turn signal operation. Ground problem? Instrument cluster circuit board problem? And which sensor generates the tach signal? There's no crank or cam sensor, so I'd guess it comes from the pick-up plate in the distributor. Right or wrong?

I've been told that the fuel pump is getting weak. I've never experienced this; only had them work or not work. And why would it only be "weak" after warming up? Please correct me if I'm wrong, but I don't think this is the problem.

Thanks again, and y'all have a great day!

Anthony

Maybe you should change the modual aswell. I had a bad coil and the engine shut down a few times. Ran again when it cooled off. Changed the coil and it still shut down.A bad coil will take more amps and get hot. The extra amps go through the modual and burn it out too. Might save you a break down on the road.
Later Bob

Do you mean the pick-up plate in the distributor? Or something else? That's the only other ignition part that I know of. Am I missing something?

Thanks,
Anthony

I am not familliar with Dodge but what I mean is whatever does the job of the condensor and points to control the current into the coil. The pickup coil will control that piece or it may be part of it. Have to see wiring diagram to be sure.
Later Bob

I had a 93 Dakota that had similar missing problems .
I took for granted that it had to be a spark issue and after trial and error method coil ,plugs and wires with no luck I ran it that way until it stopped moving.

It ended up being the torque converter coming apart and 1400.00 later never missed again.

no other evidence or noise tranny just went suddenly.

I did notice the miss only in gear so how is yours in neutral or park.

That would have to be the Hall-effect pick up plate in the distributor. No crank or cam sensors on this one. Haven't tested/changed this part yet, still on the list. Know a guy that had 3.9 in a full size Dodge, had an intermittent misfire, turned out to be rust & crud in the distributor, pieces going past the sensor would mess up the signal.

Thanks,
Anthony

In park or neutral(ie: no load) it's runs perfect. Yours being a '93, probably had a crank sensor picking up a signal off the flywheel? Mine doesn't have crank or cam sensors, uses Hall-effect pick up plate in the distributor.

Thanks,
Anthony

"In park or neutral(ie: no load) it's runs perfect."

Mine was the same way. and one day just as I made a turn onto another road tranny (just went away)

no noises or chattering other than losing the miss that was actually the bad torque converter giving out and metal all through every thing.

If you have no engine codes Id have the transmission pressure tested as they told me I should have done . torque converter cheaper than the whole tranny.

Put a fuel pressure gauge on it and observes if the pressure drops during miss fire.
Put a scan too on and follow the o2 sensor voltages. Too rich/missing spark will raise the voltage.
I chased a phantom voltage drop across a chassis return cable and connection for a while on the GN. Solved it by running a dedicated - return from the ecm and ignition back to the battery.

Anthony, you need to put a scope on the signal out of it to see. If it comes out even it's fine but if it's glitchey then you found the problem.
Later Bob