'94 4.0 computer issues

[mexgrizzly]

Been having some issues with the 4.0 in the scrambler.

The jeep was running a little rough, it is not too bad but the idle can be slow to correct and it runs a little rough. A few years ago when I swapped in the 4.0 I used a Hesco wiring harness that came with a lead for a check engine light (CEL). Up until now I used to connect a test light to it when I wanted to check/read codes and cover the end of the lead at other times. A few weeks ago I tried it and the light would not light up (tested light by connecting to battery). For grins I disconnected the battery for 10 min and the jeep ran better for a week or two. First question, in 94 +/- jeep fuel injection does the CEL come on when you turn the key to on without starting it?

I installed a light in the dash so I could see if the CEL ever came on while driving. No it didn't. I have a spare 94 YJ computer that I know worked two years ago and I tried swapping the spare into the scrambler. The pins looked OK though I did not have electrical cleaner. The light doesn't come on and the engine won't start. May have bounced around in a parts bin for too long....

In both cases the CEL does flicker when the starter is running. There is a chance the CEL lead to the computer may have grounded out over the past year or so. Normally I am using a computer from a 92 XJ but I think they are the same as 94 computers.

Other than tracing wires to check continuity. Any thoughts on what it might be before I get into that?

[johnsxj]

As I recall, the computer is the switched ground for the CEL. So your wiring should be batt+ to CEL to computer. In a regular Jeep the light is supposed to come on for 2-3 seconds when the key is first put into the "on" position. Not sure how Hesco sets up the stand alone harness.

Sounds like you have a short or a bad ground someplace in the wiring harness.

[ZAEDOCK]

When I was building my 2.5/SM420, I ran an XJ 4.0L/AW4. I had the same issue and couldn't figure it out. For poops and giggles, I ran a second ground wire from the ECU case to the body and she ran like a top for two years.

Hopefully, it's something as simple.

[mexgrizzly]

As I recall, the computer is the switched ground for the CEL. So your wiring should be batt+ to CEL to computer. In a regular Jeep the light is supposed to come on for 2-3 seconds when the key is first put into the "on" position. Not sure how Hesco sets up the stand alone harness.

Sounds like you have a short or a bad ground someplace in the wiring harness.

Thanks john. I also checked with the Hesco instruction and confirmed that the wire from the harness is a switched ground. In a way that is good because if the lead wire contacted the block under the hood it wouldn't hurt anything, right?

I tried connecting one side of the CEL to + and still no light. It was a switched + not sure it that matters. I have not tried with the spare computer yet.

Thanks,
-Andy

[mexgrizzly]

When I was building my 2.5/SM420, I ran an XJ 4.0L/AW4. I had the same issue and couldn't figure it out. For poops and giggles, I ran a second ground wire from the ECU case to the body and she ran like a top for two years.

Hopefully, it's something as simple.

The case is plastic I opened the cover and nothing spoke to me as a good place to connect a ground, thought? I did check a black wire what I assume is a diagnostic lead coming from the wiring harness. Zero resistance to the chassis so is it safe to assume the computer has good ground? I also checked the ground wires that attach to the block from the wiring harness, good ground too.

Thanks,
-Andy

[mexgrizzly]

Went on a run to Nevada this past weekend, about 200 miles east. After doing some quasi pre running (moderate speeds down bumpy dirt roads) I came to a stop shut off the engine and observed fuel dripping on the ground, it was coming from the charcoal canister (gas tank 2/3 full). It happened another time after bombing down another dirt road (gas tank 1/4 full). It has not happened during normal wheeling and on-road driving. On the trip back home the engine started sputtering and died near the top of Donner's Pass (elev. 7200'+, temps low 90's). It started again after a few minutes but then ran rough and died. The popped the gas cap and it had a lot of vacuum (I recently resealed a vent tube on top of the tank). It seemed to run better so we tried again, got a few miles up the road and it died again. We were close to the summit so I nursed it to the top and started down. At one point I have is a little gas and it started to stumble I gave it more gas and it ran better and didn't give me any trouble all the way home.

Initially we were thinking a fuel problem such as pump, filter, or some sort of vapor lock (fuel line runs under the radiator). Back in Sacramento (elev. 50'+/-, temps near 100) I ran it hard for a few minutes and no problem. Not sure if the gas coming out of the charcoal canister is related or not. I am thinking it may be a fuel injection system problem that may show up with a working CEL light and to start there. Thoughts?

-Andy

[johnsxj]

You shouldn't be having ANY fuel coming out of the charcoal canister!!!!!!!

[mexgrizzly]

You shouldn't be having ANY fuel coming out of the charcoal canister!!!!!!!

I completely agree with you, still not sure how it is happening. First step check connections and hoses.

[ZAEDOCK]

Have you ever changed the filter in the charcoal canister? If it clogs, the engine vacuum may pull from the tank.

[mexgrizzly]

Have you ever changed the filter in the charcoal canister? If it clogs, the engine vacuum may pull from the tank.

I had a former Mopar mechanic check for codes using a DRB II code reader and found no codes. We talked about the fuel from the emissions canister and he thinks the filter is plugged too. The engine may have stalled as extra fuel started coming up the vacuum line from the canister and running rich, exacerbated by fixing the leaks in the tank and the altitude. Before I start into this can someone verify the canister connections.

Facing the ports in the middle there are 3 ports, top is connected to manifold vacuum, middle is connected to the tube between the air filter and the throttle body, the bottom one has a vented cap on it. The line to the fuel tank is connected to a port on the left. The port on the right has a X inside it and is connected to nothing.

Thanks,
-Andy

[ZAEDOCK]

Andy, as a test you could pull & plug the engine vacuum line to the canister and leave the vent tubes open to atmosphere. Try it out a couple of days and see if it helps.

How do the plugs read?