Hi, all, I desperately need some advice!
I have a 94 Dakota with the 3.9 V6, 5 speed, 4x4 that is getting older
(sigh), with just over 100,000 on the odometer. I've had it since it had
30K miles, and it's really never given me much trouble, except for one
thing. Since new, it has had a rough running characteristic when first
started up, hot or cold, where it backfires once or twice through the
muffler as the engine decelerates between 1st, 2nd, and 3rd gears. When it
was under warranty, there was a TSB out from the factory that specified
replacing the PCM to fix this condition. Well, two (warranty-covered)
replacements later, nothing changed. Several professional fuel injection
tuneups ($$$), and home-made maintenance efforts later, no improvement.
Once the warranty expired at 75K, I chose to live with it- warm it up, let
it sit after starting for a minute, problem solved. I mean it wasn't bad,
you just had to remember to let her sit and warm up a bit immediately after
starting, and then she ran like a top.
Well, about 10,000 miles ago I put on a Magnaflow XL free flowing muffler
and a K&N air filter in a nice chrome Edelbrock filter mount, and all was
well, it breathes much better, more power, sounds real good, etc. Put 31"
Michelins on it, slowed down a bit, but looked cool, and it still ran fine
(no reason for tires to affect how the engine runs anyway). She ran fine
for the longest time.
But, three weeks ago, it began to hesitate and stumble on hard acceleration,
and later, began to actually backfire loudly through the intake on half to
full throttle acceleration. It does it most on cold or cool days when not
fully warmed up, popping and hesitating quite significantly when you mash
the pedal to the floor or more than halfway (highway merging situations,
etc). Not acceptable!
The backfiring and hesitation eases somewhat when the engine's warmed up,
but it will do it every time, warm or cold, between 1900 RPM and 2800 RPM at
half to full throttle in second, third, and fourth gears. I took it to a
mechanic friend, and his diagnostic computer showed that all sensors were
according to the computer, working properly, and that the computer had not
recorded any codes that indicated any system malfunctions or maintenance
needed. The check engine light has never come on during all of this. I ran
an entire can of carb & choke cleaner through the throttle body (it's a TBI
fuel injection setup) just in case something was clogging the intake system,
no change at all.
So basically, when I tromp the gas, and she winds up to 2100 RPM, she pops
like a firecracker through the intake and into the air cleaner, hesitates,
and the overall power curve while accelerating feels a little bit lumpy and
uneven, like something's holding her back to some degree. It's still pretty
much driveable when warmed up, but I know something's not right, and am
afraid it'll leave me stranded somewhere, you know? It's much worse when
she's cold first thing in the morning, but the problem persists to some
extent even when she's fully warmed up. Seems like something's screwing up
the air-fuel mixture and/or the ignition (something's affecting the way the
computer delivers fire to the spark plugs???). Any ideas?
PCV valve? EGR valve? O2 sensor? Coolant sensor? Anything?!?
I definitely can't afford a multi-hundred dollar adventure with the local
dealer, and am very capable of most repair work on my truck, so any ideas
would be greatly appreciated!!!!
Thanks!
Rob
Any ideas?
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