2000 Dodge Dakota 3.9 Misfire

Hi, I have a 2000 Dakota that has an intermittent misfire when going up hill and letting the engine lug rather than stomp the gas and make it down shift. I have replaced the cap and rotor, spark plugs, coil, and spark plug wires. At one point there was a check engine light for misfire on a single cylinder, replaced that injector and the light has not come back on even when the engine misfires. I think there is a vacuum leak; the ac will stop blowing out of the main vents and only blow out the defrost vents. This does not always happen when there is a misfire and it does not misfire every time I loose ac. The fuel gauge is beginning to go bad as well, showing no gas when there is anywhere from 1/4 to 2/3 tank. Most of the time a key cycle will bring the gauge back. 303,000 miles on the truck. Do I start by finding the vacuum leak? Replace more injectors? Could the fuel gauge starting to fail be effecting the pump as that is all one unit?

Replacing those ignition components makes common sense, but suggest to hold off on replacing any injectors. First step, ask your shop to test the battery and charging system. Problems there could account for all the symptoms. If that tests ok, next test the cooling system. Overheating can cause both pinging and misfires, esp under engine load, and cause grief to the AC system. If you think you have a vacuum leak, and can’t find it w/visible inspection and using hand-held vacuum pump, a fuel trim test at idle makes sense, may show a lean condition confirming possibility of vacuum system leak.

If the vent system defaults to defrost, it usually means the vacuum check valve before the vacuum reservoir for the vent system is malfunctioning.


The battery died a month ago, replaced it already. Tested the new battery and the alternator about a weed after I replaced it, tested fine on both a computer test and a 100amp load test on just the battery. The truck runs cool, stays a little under the normal mark on the gauge. It practically has a new cooling system in the past 10,000 miles from fixing all of the leaks. I will try a fuel trim test this weekend.

The engine is worn out. That is your misfire and lack of vacuum. Takes more throttle to get up the hill which means less vacuum.

Run a compression test. I’d bet at least 2 cylinders are 20% lower than the other 4 and those are 25% lower dry than wet.

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There may well be a vacuum leak. But the problem w/that theory is that the symptoms for a vacuum leak usually show up most near idle rpm. Poor idle quality, stalling a stoplights, etc. Not so much when going at highway speeds uphill . That’s a max airflow into the engine situation, and would mask a small vacuum leak.