DFFECHSWU

A while back, my truck barely started and idled poorly. The smell of fuel was also in the air. To make a long story short, I tried changing the components ID'd by the code reader (MAP and EGR) but had no luck. I also inspected the IAC and TPS but found no smoking gun.

I then removed the throttle plate. The gasket was a mess and allowed air to enter near its top. This would obviously confuse any fuel system (especially one that is electronically controlled).

The truck now idles but isn't 100% smooth. It still runs rich. My hope is that the gasket was the root cause, but I have concerns the processor is still using settings when it was running very poorly.

I want to "reset" the computer but have seen various ways of doing that ... from removing the negative cable to touching the positive cable to the negative terminal to removing the negative terminal and turning on the lights. What is the preferred way? I suspect I'll have to drive it a bit to have the processor re-learn everything.

Thanks in advance.

Ty

any of the above mentioned meathods will work, I have undone the neg cable, and turned that lights on before, and it worked fine.

wde3477

May want to consider checking plugs for fouling.

There should be a test sequence available, initiated by the code reader, to identify weak cylinders in order to narrow the search. This sequence goes through and shuts off the injectors one-by-one, then flags the cylinders causing the least RPM drop.

If one cylinder is / has been mis-firing, that will tend to enrich the others (hmm, I think that's right?) - since the unburned oxygen from the faulty cylinder will affect the O2 trim.

Also, consider that you may not have found all of the vacuum leaks.

I agree with resetting the computer - find out where you're at now.

svt tech

Could not agree with you more.I do not think that you have entirely found the root cause to your concern yet.

NGM

I would attempt to find more vacuum leaks... Pull the plugs see if they are wet or black. If so change them. Do not go cheap and try cleaning them. Check your cap and rotor. Do an OHM resistance test on the wires.

DFFECHSWU

Thanks for the suggestions. I'll go through them once I "baseline" where I'm at now. I looked at 2-3 plugs early in my diagnosis (prior to gasket replacement), and they were wet and sooty. Nothing shocking there. I purchased a new set at that time and will install them prior to doing anything else. The cap and rotor are new, and I'll inspect plug wires as recommended.

DFFECHSWU

I reset the computer and re-checked signal or voltage references as defined in the Haynes manual. No matter what component I looked at (MAP, IAC, TPS, EGR, intake and coolant temp sensors), they were all 1/10 of what they should have been. They were ~0.5V instead of the typical 5.0V. I also disconnected the PCM connection on the engine side and probed a few connections back to the MAP and EGR. There were no resistive issues, as they were shorts as required. The ignition control module didn't seem to be an issue, either. It had 12V on pins 2 and 3 when the distributor base was used for signal return.

My question now is ... where does this 5.0V signal come from? Is it from a voltage divider between the battery and some other node in the circuitry? Finding the component the reduces the voltage would be key, since my problem appears systemic and not tied to one component. Any inputs would be appreciated.

DFFECHSWU

All,

Thanks for your help in this matter. Turns out the fault was in the ECM. There are (2) 47uF, 16V, electrolytic caps in the ECM. Both had "leaked" and one in particular had damaged a trace near it. This may be a common fail mode in the ECM's as some repair sites state they'd replace high fail items with more robust ones. It's just something to consider if this happens to one of you.

I thought about buying another ECM, but the fact I couldn't return it made me pursue other options. I wound up replacing both caps just to be safe and added some buss wire to the trace. I had 5.0V across the cap where VREF and SIG RTN are located when connected to a 12V P.S.. I installed the ECM last night and fired her up. It took only a few minutes to "re-learn" everything, so now it's time to determine where all those screws and bolts go ...

Thanks,
Mike

wde3477

Doggone - now that is troubleshooting down to the printed-circuit board level!

Good stuff to know and to file for future reference. Glad things are working out, and thanks for the update.

DFFECHSWU

Well, I have pictures of the work I had completed previously but not because I was ambitious. Turns out the computer gave up the ghost today. I pulled out the driveway, the truck stumbled a bit and two stop signs later died. The telltale fried electronic component smell gave it away. Luckily, I was only a few hundred yards from home and I had some friends push it back.

This all may be a cautionary tale, as back in 2007, I opted not to replaced the 3rd cap on the PCB since it looked solid. In the attached images, you can see the two smaller gray and white caps that I replaced (along with a jumper / buss wire near one of them). The third, green cap is awfully close to where the burn occured.

So I'm off to find a replacment, as this is unrepairable. From what I've seen so far, a replacement (not necessarily new) will be ~$200 ... a cap was $1 or so. Does anyone know of a reputable ECM supplier, i.e. they replace "high wear" items before re-selling. I would be disappointed if I pay $200 and the same thing happens shortly down the road.