Check Engine Light Fix + Running Rough

For a short period of time I was working at a mechanical repair shop. A fully certified mechanic and I took a look at my efi 92 b2200 and determined that it was running rough because it is running lean. I have also confirmed our suspicion that the check engine light has been tampered with; someone has cut the power source for it in from the charge light so that it comes on and goes off at the same time as it.

So to get a proper code read to help diagnose the truck I was hoping to start by fixing the check engine light, but whoever messed with it has done a good job of hiding it so I was hoping to see if someone knew how to identify which wire I should be looking at to find the chop job.

Thanks

First have a look under the driver's side dash, right above the kick panel area.........there is a reset jumper wire for the check engine light.......when the CEL comes on, it's usually so you know it's time to do maintenance on the engine, like change the timing belt.

Right above this area.....



You should see a bundle of wires.......



Here's what your looking for.......there are 3 single pin connectors in this area (2 white, one black or vise-versa) and 2 of them should be plugged into each other......and the last connector is just hanging there.





When the CEL comes on, you unplug the 2 connectors that are plugged into each other, and then plug in the one that was just hanging there. This turns the CEL back off again.

Maybe that is the area that someone has tinkered with.

Unless you can see obvious signs of tampering, the check engine light coming on with the charge light is probably normal. Almost all of the lights are fed by the same power source... and this includes the "Charge" and "Check Engine." Both are powered by the "Meter" (instrument cluster) fuse which is powered by the ignition switch. The lights are "turned on" by completing the circuit to ground.

On the 91 wiring diagram, it only shows the 2.6 truck cluster having the check engine light come on with the alternator light but not necessarily the 2.2 truck. HOWEVER, it's quite possible that the cluster has been swapped with a cluster from a 2.6 truck. In any case, it should have a ground side wire going to the ECM. So the ECM completes the ground to turn the light on and/or flash codes.

You know the bulb is good because the light comes on with the alternator... which is likely normal. Is that the only reason you think the wiring has been hacked?

If you want to make sure that the path from the light bulb ground to the ECM is still in tact, you can go to the ECM, unplugged it, locate the "White" wire on the small connecter, and jump it to ground with the key in the "ON" position. This should turn the check engine light on. If so, then your wiring is good to go. If not, then you can start following the wire between the cluster and the ECM. If somebody wanted to snip that wire to turn off the light, then it would probably be done in the harness just behind the cluster, or at or near the ECM. But I'm not convinced that your wiring has been hacked. If there are no codes being thrown, then it's not going to come on. And they seldom throw codes.

If you're certain that the truck was running lean, then I would look at these items:

1. ECM - pull it (passenger kick panel), open it, and inspect for signs of cap leakage. You can search the forum for what to look for.

2. Check fuel pressure if you have access to a gauge. Here are a few causes
Fuel Pressure Regulator
Fuel Filter (firewall)
Fuel Pump
Fuel Sock (filter in the tank)
Fuel line pick up leak - This is a new one I had never seen before. A member on MT was getting low fuel pressure. He went to replace the pump and found a rusted leaking pickup line on the pickup/sender inside the tank. Fuel was leaking back into the tank - so no signs or smells of gas. This was causing the the pressure to drop just enough to still allow it to run, but not hardly be able to accelerate due to being to lean.


3. You might also put a noid light on the #1 and then #4 injector connecter and confirm they are being fired by the ECM. Not uncommon to have a bad driver in the ECM take out a pair (or both pair) of injectors.

Any additional background on the truck would be helpful too. If it's been sitting a long time, it's not uncommon for the injectors to be restricted or clogged with fuel/varnish.

There are not many codes that the stock ECM will display, there is something like 12 codes total. I would search for a service manual, check out all sensors, stuff like a faulty coolant temp sender will cause a bunch of issues, since the computer uses this to determine fuel (eg choke mode when cold). This is a different sensor than what the gauge uses. Oxygen sensor if old, should be replaced, etc... Pretty much anything stock will run as lean as possible for emissions & fuel mileage, not run optimal fueling for power.

Wow thanks for all of the responses, I asked a few friends in the industry who would be a good mechanic and I got a recommendation to a guy who cut me a really good deal.

So the results are in and he did for me what he could on a budget (considering I'm an unemployed student at the moment). He did a lot of basic checks (timing, O2 sensor, vacuum leak, compression test, etc.) As he told me himself he could not diagnose anything specific without throwing parts at it which at over 324,000 kms (200,000 mi) on the odometer and an engine that's starting to slowly lose compression would just be ridiculous and out of my price range. But he did clean the combustion chambers and torque up the spark plugs (which he told me should be replaced) but other than that it is running at a satisfactory level again.

I will most likely replace said spark plugs and a fuel filter and finish addressing some other loose ends surrounding the air filter that I had installed that may also be affecting the idle.

All in all thanks for your responses the help is much appreciated and I look forward to getting back to driving my B2200 around town again