I have a 95 dodge dakota with a 3.9L v6. I bought it as a project that needed head gaskets done. I finished that and got it going but it has an irractic idle. Seemed to drive fine around town and highway speeds for the first few drives. Now it started lurching at highway speeds. I have replaced the spark plugs and wires while doing the head gaskets. I also replaced the throttle position sensor and the Idle Air Control Valve. My old IAC was in pretty rough shape. The truck will die when coming to a stop most times and when sitting and idling it will jump between 400 and 700 rpm. After replacing the IAC it idled high at 2000 rpm then dropped to 1200 and then went back to 500 jumping down to 400 and up to 700.

-aj

 

a) You need to check for ANY vacuum leaks; with the age that could be a bad Evap line even, but being a Magnum I'd for sure check the plenum gasket (obvious sign is oil in the intake).

b) Did you clean the IAC passages in the throttle body, or just replace the valve? THEY can be plugged also.

c) I cannot repeat, check ALL possible vacuum sources.

d) Does it idle OK when cold but go flaky when warm? Or vice versa? Again, plenum gasket, but also check the O2 sensors (they're not used on cold, but after a length of time - 300 seconds IIRC, but that may be the wrong number! - the ECU will flip to using the O2s for air/fuel instead of a table.)

RwP

 

I cleaned the IAC passages with no differences.
I checked vacuum leaks and could not find any.
Idle is pretty much the same cold or hot. doesn't seem to matter. The plenum gasket was recently replaced as well.

 

If the plenum gasket was replaced but not the belly pan, it may have been damaged again, so that's still a suspect (until you either a) use the Magnum kit to fix it, or b) have the belly pan welded on).

Only other thing is to disconnect the battery overnight (at least 30 minutes; overnight makes SURE it's at least 30 minutes *grins*) and let the ECU re-educate itself on how the engine actually runs. If you do this, you may get a code 12 for at least the next 50 start cycles (defined as "Start and drive", not "Start, turn off, lather, rinse, repeat"), and that's normal. Also, be SURE before you do this to check the current stored codes (with the ON/OFF/ON/OFF/ON key dance; count the flashes of the CHECK ENGINE or PWR LOSS light); you'll end with a 55, if it's less than 50 cycles since the last time the battery was disconnected you'll get a 12; and any other codes will be helpful.

RwP

 

I did replace the belly pan gasket as well. I did some work on it last night. did find a cut o-ring on an injector that was causing a leak. Fixed that but it was still running rough, did show more testing and found that its missing on cylinder 4. Did a compression test and it's about 40-50 psi too low. Will be doing a leak down test tomorrow to find the source.

Did have a code for the throttle position sensor to high but I got that cleared and fixed.

 

Pat close attention to the exhaust manifolds.