We have 1992 3.9L that is barely running and we are looking for suggestions. The vehicle has approx 80k miles.


To this point, the truck & engine have had following done:
1. repaired two harness splices between the power distribution box and firewall.
2. replaced the cap, rotor, coil, plugs, primary lead.
3. replaced cam position sensor,
4. swapped the PCM with another from a running 3.9L
( Some of these were for general tuneup, and some to troubleshoot the following issue.)
After reviewing the FAQ and searching posts and none of the above helped, i thought we had a cam timing issue, so today we pulled the front end apart and changed out the timing chain & gearset. Found a little slack in the chain, but no improvement from new gears/chain when running.

The engine will start and run (barely). However it backfires through the throttle body and must have the throttle feathered to get to 1200rpm. Cannot get it to rev past 1600rpm. It will idle briefly, but eventually will stall.

Tried to pull codes from the Check engine light, and got "3", "5", "5", (thats all).
Today i also tried a timing lite to see where the PCM was firing the plugs and found it was in the low 40 deg range BTDC at 1200+rpm and bouncing around quite a bit. Quite unstable. To me, this seems a little high, but from reading the manuals i have downloaded, it appears the timing is non-adjustable.
the fuel pressure runs approx 42psi with engine stopped, and holds low 40's when engine is running.

Tomorrow i am swapping the crank position sensor, as a blind shot in the dark.
The frustrating thing is that this truck was running one day, and the next day this issue started happening.

i am running out of ideas on what to look at next.
Anyone have ideas?

 

It does sound like a timing issue. Yes the timing is non adjustable. But the distributer is adjustable. If it is not in the correct place it will be off. Maybe the plug wires are wired wrong. Tons of more possibilities.

 

Agreed, it is appearing to be timing related. We are struggling with ideas to identify where the issue is.

Should have mentioned, that i found one older post that mentioned the Cam position "shutter wheel" had gotten loose on someone , so i checked this as well and it seems to be tight to the rotor shaft when we try to move it.
The distributor body has not been moved for as long as we have owned the truck.
I have checked the rotor shaft movement by hand and there is a small (1/16) rotation which i think is typical for the distributor drive gear.
The plug leads have been checked and rechecked for firing order and the spark is excellent at the plug connection.

The replacement PCM we have is a take off from the pick-a-part yard and was qualified as a working component back when we did the harness splice repairs. Engine shows the same symptoms with either PCM installed.

And as mentioned, the truck was running the day before. ??

I have tried to outline the things that we have checked, tested or replaced to this point. If anyone has ideas, please pitch in. Appreciate your help.

 

I dont know that the crank position sensor would have anything to do with it running bad. If it were bad then it just wouldnt crank. I dont know if your truck has a map sensor on it but it they are bad it will cause it to run like crap. I had a friend whos map sensor went out on his blazer and it did just about what you are describing on your truck.

 

I don't think these have MAP sensors. Maybe the timing chain jumped a tooth or two? Seen that happen before.

 

Thanks for the replies Folks.

i have also seen a slack timing chain jump a tooth with similar effects , (the backfiring out the throttle body made me think this was likely a cause).
We pulled it down and replaced the chain and gear set yesterday, no improvement. (the old chain was a little slack, but not at the limits listed in the manual.

Agreed that if the crank position sensor signal was absent, this engine should not fire or run in any manner. I was thinking that extra pulses or other noise to the PCM could cause the engine to fire at wrong times. Like mentioned in previous post, this is a blind guess.

i think that we will also drain the fuel tank and refill with clean fuel to be sure we know what is being pumped to the injectors.

It's pouring rain right now and we are doing this all in a ToysRus parking lot, so i think we will regroup till better weather.

Appreciate any ideas or suggestions.

 

You are lucky! I wish it would rain here. We are in some kind of drought and everything is dead and dry. 2 weeks ago someone threw a cigarette out the window in front of my inlaws and it caught the field on fire across the road. I would like it to set in a good rain here so the grass will grow and I can make my extra money for all my little projects.

 

Don't know if the 3.9's had a MAP sensor, but my 93 with 318V does...it will barely run if that is bad. Might also check your vacuum lines.

 

could compression issues cause these problems? just wondering, lol

 

Ok, we are back at this today.

tried disabling the fuel pump and feeding gasoline into the throttle body while cranking. The idea was to determine if the engine would run better, if it did this may suggest the fuel in the tank was affecting operation.
What we found was that the engine would fire a couple of times and then light off the fuel vapor in the intake manifold. to me this still suggests the pcm may be firing one or other plugs when an intake valve is open.
Next we pulled the throttle body to get the MAP sensor off, and we will swap this out as a test. I noted that the bottom of the intake seemed wet/oily, with residue in corners where you could see with a light. (this truck was literally owned by an older lady who idled lots and drove very slowly, so i am not surprised by this).
We have owned this truck for about 3 months now and except for the wiring splice repairs one month in, it has run good till last weekend.

We are open to suggestions here. Thanks for any input.