92 Dakota Timing not right

It appears the timing on the 92 Dakota (3.9 L) is set by the ECM. My
timing seems to be off. My timing light shows it set at TDC while
idling. If I rev up engine (while in park) it backfires thru throttle
body. The timing chain is new. The truck seems to run ok but burns a
lot of gas. Recently replaced o2 sensor, cat converter and muffler,
cap, rotor, wires, spark plugs, PVC. THe vacuum (while at idle) is
about 15 inches, the needle is pretty steady.

Does any know why the backfire??? What can affect timing???
I'd appreciate any suggestions.

Thanks

 

The timing cannot be set with a timing lite, if you turn the distributor you
just knocked out the timing to fire the injectors, you have to use a volt
meter to set the distributor, A common problem for the 3.9 was the
distributor shaft bushing wearing out and causing extreme amount of play,
thus causing backfire, shutdowns and pinging, remove the cap and see if you
have alot of play in the distributor shaft.
Glenn Beasley
Chrysler Tech
SYMPTOM/CONDITION:
Vehicles may exhibit surging, light bucking, or intermittent engine
misfiring. This will most likely occur when the vehicle is at operating
temperature, and under a light load at approximately 2000 RPM. This
condition may be caused by a mis-indexed distributor. The following
procedure is an alternative to the indexing procedure outlined in the
service manual.

REPAIR PROCEDURE:
This bulletin outlines an alternative distributor indexing procedure.

1.. Connect a voltmeter to the distributor sensor connector by removing
the end seal and carefully back probing the connector. Connect the positive
lead to the sensor output pin (pin 3, either a tan wire with a yellow tracer
or a gray wire, depending on vehicle application). Connect the negative lead
to the sensor ground pin (pin 2, a black wire with a light blue tracer).
2.. Rotate the engine clockwise as viewed from the front, until the number
one piston is at Top Dead Center (TDC) of the compression stroke. The timing
mark on the vibration damper should line up with the zero degree (TDC) mark
on the timing chain case cover.
3.. Continue to rotate the engine slowly clockwise until the V6 or V8 mark
(depending on engine type) lines up with the zero degree (TDC) mark on the
timing chain case cover. The V8 mark is 17.5° after TDC and the V6 mark is
147° after TDC.
NOTE: DO NOT ROTATE THE ENGINE COUNTER CLOCKWISE. IF THE ENGINE IS ROTATED
BEYOND THE MARK, RETURN TO STEP 2 AND REPEAT THE PROCEDURE.

4.. Loosen the distributor clamp bolt.
5.. With the ignition switch in the ON position, rotate the distributor
slightly in either direction until the voltmeter switches between the sensor
transition point of 0 and 5 volts.
6.. Adjust the distributor as close as possible to either side of this
transition point and tighten the distributor clamp bolt to 170-230 in. lbs.
(19-26 Nm).