help troubleshootinf pickup, 91 f150 6cyl

[SPIKER]

Hi Guys:

Worked on it for several hours today, still same issues I dont have a NOID light and could not find my O-Scope leads (it has not been plugged in in over 10 maybe 15 years so not sure it will even work anymore it was a 20 buck unit donated to the local school.)

I Pulled #1 plug confirmed TDC and Timing Mark, cleaned distributor cap pins and rotor, re-adjusted the distributor after pulling the jumper to set timing to off. Re-adjusted Timing to ~14degrees it was retarded about 5 deg. I pulled the plug to the distributor and re-inserted checking for possible corrosion no change.

Plugged jumper back in no change would not start, took a shot of ether to get it going would not start without it and still similar issues timing stayed the same throughout the RPM range & under quick throttle response I get no advance from the distributor advance. SHOULD have had an advance...

The PCM controls both the fuel pump OFF/ON relay & the Timing Advance so was next to look at. So I pulled the Plug/connector and re-inserted it to see if there were issues w the plug, did the same with the Fuel Pump Relay and PCM Power Relay, no change... SO pulled out the PCM it is in the trunk ready to head to junkyard for a replacement..

Still thinking it is SO aggravating not being able to FIX the dang thing.... Thanks for help so far.

I didnt look closely at the grounds on the truck/battery side will take a closer look there and will also put a meter on the injectors individually looking for shorts...

thanks guys

 

[SPIKER]

Hi ALL:

Well I got back from Junk Yard local (40 miles each way to Pick-a-part) with a 22 bucks for the PCM got it installed and started right up...

With Key on fuel pumps run for 10 or so seconds and shut off (one of the indications there was an issue as pumps didnt shut off.

SO lots of frustration but got it running. I've worked on cars & trucks a LONG time first time I've had a ECM/PCM failure where the vehicle would still run and 1st ever on a FORD...

Thanks for all the help ya all!

 

[LD1]

PCM failures are the biggest PITA to troubleshoot. And often times, the symptoms are intermitten making them that much harder.

I had a PCM go out on a 1993 firebird. Intermitten issue. When it would happen, it would absolutly NOT reb beyond 3000 rpm (manual car), and had barley enough power to move, (v8 lt1). But then all of the sudden it would clear up and be fine for awhile.

Took it to a buddys and hooked it to the scanner. The PCM wouldnt even communicate with the scanner. But if it werent for having a buddy that had a scanner I would have been at a loss. Didnt even show any codes. (old OBD1 where you could jumper two pins).

Glad you got it going:thumbsup:

 

[SPIKER]

LD1: ya, my 1st one in a ford, 2 or 3 in chevies/dodges tho. also 1st that would RUN with low power or starting issues.. I had a check eng light for months but started & ran OK, just could not clear the light using my own tool figured the tool was DOA as the old battery rotted it a bit.

Mark

 

[Tig]

Thanks for the update, but now I'm really intrigued. You had fuel and spark, so I guess timing was the issue or perhaps low ignition power. But if that's an indication of how well "limp home" mode works, then the mode sure lives up to it's name.
Did you try swapping the old PCM back or inspecting connectors for corrosion? Just wondering if more info on the nature of the fault can be gleaned from the parts.
I've scoped PCMs a few times while troubleshooting. Usually when the diags say "go buy one and see if it fixes things" I'd love to know the nature of the failure that makes things run so poorly, yet still run.

 

[SPIKER]

TIG:

I had the OLD PCM in and out 3 times on/off the connector inspected all the pins were good and all had a good greased contacts so not sure. I have not pulled it's cover off yet, will maybe take a peak inside it just to see if there are any solder joints or burnt board connections/chips. The Alternator shorted out against the exhaust manifold back in the summer which I'm guessing was what took out the PCM and up till cold temps it worked passable?>>??

Timing was flat to slightly retarded which should have let it start easier. fuel pressure was good but what I THINK was at issue was a short blurb in my book that seemed to state the fuel injectors fired in sets (3 or 4 at a time in 2 banks) not a true fully Multi-port EFI with 6 independent injector firing pulses. SO assuming that perhaps one set of injectors were not firing at all so no fuel in some ports... could have been un-luckly but the plugs I pulled seemed to have been getting gas... I'll post back IF I find anything on the PCM dissection.

 

[SPIKER]

Hi All:

I finally decided to pull the FORD 1991 Computer PDU covers off and do a quick visual inspection.

Found 2 CAPS 47uf both popped and leaked out the electrolytic goo onto the board. That stuff will often corrode anything it sets on like a 1.5v disposable battery does in your expensive code readers, scanners or meters.

The thing looks FIXABLE other than it is fully dipped in conformal coating for water resistance. I took a couple cell phone pics for ref the one BLUE cap fell right off when I touched it the other had one lead corroded thru.

^ that one fell off, should be just to right brown rusty solder spots

^ this one has one lead rotted thru seen on left of pic


sorry for poor pic quality but my cell does not have a MICRO setting for up close pics.

if I wanted I could pull out some caps or pick some up from RS and try & replace them. BUT the conformal coating would make re-soldering it rough and I dont really feel like disassembling the now operational truck to re-test the repaired unit...

Mark