Posted by MeAnthony on April 13, 2010 at 16:50:27 from (208.100.201.111):
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.
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