well here goes. I have a 92 dakota with a 3.9L 4X4. the truck was purchased from another gentleman with limited mechanical experience. the truck has an issue with a miss fire. it seems like an injector failure. the previous owner replaced timing chain, distributor, plugs and wires, water pump, radiator, ECM, one injector, and the O2 sensors. I went through and verified that all the injectors are working and they all seem to affect RPMs if I unplug them one at a time. I also have no movement from the tach. which leads me to believe i may have a problem with my crank position sensor. will the truck still start if the CPS is bad? also how common are valve and push tube issues with these engines? oh also the left bank rear cylinder exhaust manifold is leaking severely. I thought it may be due to that cylinder loading up due to a failed injector. Any help would be greatly appreciated. Thanks.

 

Got a new IAC and new pigtail for it and it now idles like new. isolated the #5 injector to be the issue by checking compression and inspecting plugs. trouble is the #5 injector is the one the previous owner had replaced with new. I'm wondering if the injector is to high a volume for the truck. like from a 318. Is there any differance between the two engines injectors. I'll be getting one from the junkyard tomorrow but am nervous about getting one from a truck that is newer if there are major differances.

 

Well to start, if the CPS is bad then the truck wont start, because the tells the computer when to allow the coil to apply spark, secondly I always tell people if one injector is bad they all are, replace them all and put no O-Rings on them to. As for the Manifold leakage, could need a new gasket or maybe one of the bolts broke, or the Manifold could be cracked, can't tell you unless I was there, the best thing to use is Brake Cleaner, blast alittle around to see if air is coming out from the Injector's (that can cause issues too) and same thing with the exhaust gasket (if it's leaking it will bubble up like no tomorrow) but with the injectors or Intake it will make the truck sound like it wants to shut off if it's leaking. Just a helpful hint.

 

Okay, this may seem stupid but my 1988, when it had the 3.9, had an issue that we thought a cylinder had died, like a misfire... just to humor ourselves my dad and i pulled the valve covers to check everything out. We pulled the pushrods and found the issue, it had bent a pushrod! After replacing it, the truck ran fine! I just thought id toss in my $0.02.


^just a note... i didn't really read anything besides what you said about having a miss

 

Thanks guys. Well I got into it some more this weekend and found that the last jerk replaced one injector with the wrong one. I guess the injectors for the 3.9 changed from 92-93 and 94-96. It looks like the newer injector stays open longer or is a differant size orifice cause it seems to be loading that cylinder up with fuel. I've unplugged that injector and it actually runs smoother. I'll be getting a complete set from a running Dak in a junk yard today and let ya know what happens. Also, broken bolts and a blown out gasket on the exhaust...no biggie.

 

No problem, glad to help you out and lead you in the right path. I kinda figured it was going to be a injector and a gasket.

 

Replaced #5 and #2 injectors and she seems to be running smoother. The only time she misses is at idle. Could be cause of the exhaust leaks... No back pressure...
Took it out back on some ORV trails and she's one sweet truck. Now time to get the A/C delete belt and change all fluids and filters.

 

well it came back. im still missing in no. 5 injector. checked injector with meter. 15.1ohms. checked harness continuity and its fine. checked codes and have a 21 & 27 code. 21 is o2 sensor and 27 is no signal to the ecm from an injector. i replaced the ecm with a good one i had and the problem didnt go away. all the cylinders have good compresion so i dont think it has valve issues. im at wits end and my wife is gonna kill me if i cant get this truck going. (traded her cherokee for it).

 

Sounds like a wire problem. All the injectors share a positive power source. Check the splice. The computer sends a negative to the individual injector. Check for continuity. If you have to bypass the correct wiring and connect it directly to the computer.

 

yeah. that sounds like the only thing left. i did some checking on the harness continuity before, but i will check for continuity between pins at the ecm connection. i did run the truck and check both pins in the injector connector and found the 12v b+ common and i found 0.15v on the ecm ground control side. i checked another injector connector and found only 0.3v on the ground side while running. so there is definitly some voltage transfer going on.