New to the forum. 1st ford and 1st gas engine I've had in years so forgive my ignorance.
Just got a 91 f150 5.0 4x4 for a extra farm truck. It was in non running condition. It would crank, had spark and fuel but would not start.

I knocked the plug off the iac when pulling off the air intake hoses. After putting the plug back on the truck started but ran like crap and died. I took the two screws off the iac and pulled the part with the connector off to try and figure out what it was.

After reinstalling the truck ran pretty well but eventually idles itself down till it dies.
Could the iac be the only problem on this truck? The previous owner seemed to think the computer was shot. Should I replace it or is their a fix? Thanks any info would be appreciated.

 

Computers rarely ever fail. IAC solenoids were fairly common tho. If the engine starts and runs good but won't idle the IAC could be your problem.

 

A new development: I noticed that the fuel pump runs constantly with the key on and the truck won't start.

If pump runs a few seconds and shuts off as it should the truck starts fine. Pump only runs properly if the battery has been disconnected. Then goes back to running all the time after a few starts.

I'm going to check the relays, clean all the grounds, and test the battery for proper voltage and amperage. Anything else I should be looking for?

 

Fuel pump constantly running is a sign of a failing PCM. But it would be good to check the fuel pump relays and all connections to the fuel pump.

Otherwise, I feel that the capacitors on the mainboard have failed and are allowing for damaging voltage to cycle thru the internals of the CPU and other chips associated with the CPU of the PCM. Pretty much, dirty, unclean voltage is affecting the PCM. Gotta remember, anything electrical can and will fail eventually. Were are going on over 20+ years on capacitors and electronics.

 

Fooled with the iac some more today and now the fuel pump is acting properly but no start.

I guess I'm going to have to start throwing parts at it. I hate to do it because I haven't had it running long enough to tell if the drivetrain is even worth saving.
Any chance a bad iac solenoid would cause the computer to not let it start? If the computer is fried I don't see how I got those few starts.

 

As stated sounds like a computer issue, screwing with the IAC wouldn't have any effect on how the fuel pump acts none zero zip.

If the IAC was a problem say not functioning but not hung open no other issues it would start "Cold" if simply opened the throttle a little bit to allow a small amount of air to pass. Warm it would start and run but at reduced idle no need to open the throttle little bit.

Fuel pump running running constantly key in "run" position and not attributed to relay problems or PO engineering does point to the computer. The clock doesn't run fuel isn't injected.

One quick test would be introduce starting fluid into throttle body while cranking when fails to start, if it runs but stall out when starting fluid is taken away points to failure to fire injectors. A noid light would be used to verify/remove all doubt, that is provided it builds sufficient fuel pressure of course.

Starting fluid, doesn't take much if do so go easy with it if it will fire it will fire more of it will not help if it fails to fire right away.

Don't throw parts at it do some basic testing find out what the problem is first, everything you state does tend point to the computer, pump runs all the time runs like crap when it does run or at times runs fairly well, has fuel (assumes you checked the pressure) and spark but fails to fire but check first its not all that difficult to do.

 

I tried starting fluid. One time it started then no start since. Seems like every test ends up with mixed results.

What do you guys think of the autozone ecm's? Cheapest reman I've found online. Of course they have one listed for $93 and one for $198. Both show the same specs.

 

When it failed to fire with the starting fluid, did it have spark at the time?

If not when you remove the harness connector from the computer does spark return?

Keep in mind it is possible it suffers from more than a single issue.

It would be worth the few minutes it takes to remove the computer remove its cover and give it a visual inspection.

Example but there are others via forum search, posts with clear images of the problem.

https://www.ford-trucks.com/forums/1...nspection.html

Look close if find it suffers leaking caps you're better off having it repaired over buying another computer.
If not damaged beyond repair, odds are low that it is. Having the one it has now fixed either by replacing the caps yourself or having it done at a local electronics repair shop (costs around 20 bucks) is the best bet. You retain the correct computer rather than one some kid behind the counter someplace claims is, assumes has original (probably does) and one that won't fail from the same issue a few months down the road either.

Point is if the computer has issues you must address that problem first before moving forward.

Found one with clear pictures of the common problem they suffer from shared by many here.... including me! > https://www.ford-trucks.com/forums/1...fried-pic.html (keep in mind damage might not be that obvious, look at them closely)

 

Thanks for the good info. I hadn't considered it could be fixed. Haven't been able to work on it much today. Pouring rain and I wasn't smart enough to put it in the garage when it was running yesterday. I'll check back when I get the computer out.

 

Maybe that was a typo, but if not, what is a noid light?

Serious question, not poking fun. I'm always learning and part of that is asking questions about things you don't know

 

He means 'noid light, as in solenoid light.
These guys: OEM/6 pcs. noid light set (27161) | Injector Signal Tester | AutoZone.com

Since injectors are basically solenoid-actuated valves, I guess that's how they got their name.

ALSO: I would seriously NOT buy a computer/EEC/PCM/ECU/whatever you want to call it from AutoZone or any chain store. Most of those places don't actually rebuild them! There's a place that is recommended often on here, should turn up in a search. Or, if you can work with PC boards and you find that the problem with your computer is just bad caps, then they're easy enough to replace yourself. Assuming the electrolyte didn't eat the traces off the board or anything.

 

Kewl. Thanks.

Tried to rep you, but I've got to "spread it around" first. So for now have a beer:

 

Got the computer out. 2 out of 3 capacitors are leaking, one looks like a "leg" has burnt off. The rest of the board looks good.

I live in rural area in KS and haven't found anyone that does electronics repair unless it's smartphones. Has anyone replaced their own caps that could give me some advice?

I understand how their sized and polarity. It's the getting the old one out and new one in that scares me. Saw a couple YouTube videos but didn't show where the heat was applied. Basically just before and after shots

 

Thankfully soldering on these boards is fairly easy because of their age. Back when these were built components were a lot larger and mostly through soldered. Peruse Youtube and watch a few "how to solder" videos. If they start talking about "surface mount" go find a different video as you don't need to do that.

Another possibility would be to try and find a local ham or amateur radio operator. A heck of a lot of use know how to solder and enjoy tinkering with stuff. They might be more than willing to help.

 

Edit. Dumb question