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1990 EB, 5.0
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Discussion Starter · #1 ·
So yet again I find myself perplexed by the mysterious Bronco.

I recently replaced by IAC due to some idle issue on decelleration. Eventhough that fixed my issue I had already ordered a new ECM from Advanced Auto in case the IAC didn't do the trick. So I decided to swap it in because the truck is 32 years young and it's probalby only a matter of time before it goes. Plus, COVID boredom. I've swapped an ECM before in a 1992 and I must say that swapping the ECM for my 1989 was much easier.

Here's the issue: On the first start of the day the truck starts up fine. After 5 min it starts to idle rough. If I put it in drive and chug along a few hundred feet, roughly, then put it in park. Idles fine. Then go to drive again with no further issues. It is really weird. An idle issue at the beginning then goes away for the rest of the day. This wasn't an issue with my last ECM swap.

I'm telling myself it's just the new ECM trying to figure itself out, but I really have no idea.
 

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Yo Siggy,
Idle Air Control (IAC) Sludge; Poor Idle TSB 91-25-07 for 85-92 Bronco & F Series & many others;

"...Hard cold starts, hesitation and stalls on initial start-up or during idle or decel may be caused by sludge in the throttle body and/or idle by-pass valve. Sludge deposits or oil film on the throttle body bore and plate or the idle air by-pass valve may cause one or more of the following conditions. Hard Cold Start, Stall On Initial Start-Up, Stall During Idle, Stall During Decel, Rough Idle, Rolling Idle... Cleaning is not required on sludge tolerant throttle body designs released for 1991 and newer model years." Some owners have cleaned these anyway due to age and grime that lezks past intake.

Try a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19 @ Code Reader.....

See my Vacuum leak test in post #11 @ Help with dtc codes and idle

High idle with no applicable codes is usually caused by a vacuum leak,
 
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Charlie don't surf..
'92 Ford Bronco XLT
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Was there something specific that led you to think your EEC was bad..I mean aside from the fact that it was old..granted those capacitors may start leaking, but if it wasn't broken, I'd leave it alone..Having said that, I'm not sure where Advanced Auto sources they're EEC's..I've dealt with autocomputerexchange and have had good luck with them..If it were me, I'd put the old EEC back in(unless you had to exchange for a core) and see what happens.The EEC shouldn't take that much time to learn, and that more with driving. you shouldn't see any issues with start up.
 

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1990 EB, 5.0
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Discussion Starter · #4 ·
Thanks guys, I think the culprit might be some bad vacuum lines coming off of the throttle body. I replaced most of the others with silicon, but the throttle body escaped me. I'll go buy some new lines and see if that does the trick.

As for the EEC there wasn't anything wrong with it, but since i already ordered it I figured I might as well use it, vice return it. As for the all the Hard Cold Start info that was sent, I'm going to buy a can of sea foam spray for the throttle body butterflies. I usually do that once a year, and it's been over a year now.
 

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Was there something specific that led you to think your EEC was bad..I mean aside from the fact that it was old..granted those capacitors may start leaking, but if it wasn't broken, I'd leave it alone..Having said that, I'm not sure where Advanced Auto sources they're EEC's..I've dealt with autocomputerexchange and have had good luck with them..If it were me, I'd put the old EEC back in(unless you had to exchange for a core) and see what happens.The EEC shouldn't take that much time to learn, and that more with driving. you shouldn't see any issues with start up.
Agree on this. I would keep the old EEC in there until you run down every code first. I don't even see where you checked for codes.
And after you check for codes, you need to test the specified sensors from that code, and see what turns up.

Only after you've wrung all that out, and then tested almost every sensor (even one's that the EEC did not give you a code for), would I
consider swapping EEC.

EEC is kind of last resort. Now if you already have the old one out, I'd pop it open and check for damaged components
(anything you can see visually, burnt traces, etc..).
 

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1990 EB, 5.0
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Discussion Starter · #6 ·
Thanks guys, I think the culprit might be some bad vacuum lines coming off of the throttle body. I replaced most of the others with silicon, but the throttle body escaped me. I'll go buy some new lines and see if that does the trick.

As for the EEC there wasn't anything wrong with it, but since i already ordered it I figured I might as well use it, vice return it. As for the all the Hard Cold Start info that was sent, I'm going to buy a can of sea foam spray for the throttle body butterflies. I usually do that once a year, and it's been over a year now.
Well changing out the vacuum lines between the throttle body and the canister seem to have worked. No more choking throttle. Hopefully it won't resurface tomorrow. Stay tuned...
 
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AFAIK There are no NEW EECs in production, just rebuilt ones. And Reman companies are known to fix ONLY obvious problems on their parts. So it's very possible your rebuilt EEC could have failing caps or some other iffy component that checked out fine on their tester.
 
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1990 EB, 5.0
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Discussion Starter · #8 ·
Issue resurfaced again this morning. Sigh. I decided to change out the spark plugs, wires, distributor cap and rotor. It’s been on my list for a while cause I used cheap parts when I changed them about 18 months ago. Spark plugs were already looking like rusted crap, didn't expect that after only 18 months. Runs good as ever now, but the real test will be tomorrow morning. If it happens again I’ll look into a return of the ECM
 
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Discussion Starter · #9 ·
And it has happened again. Starts up normal and after 4 min starts to choke on itself. I checked the fuel pressure at the rail. With just the key it goes to 15 and slowly fades. With engine running I get 35 but once the engine starts choking it indicates lower. If I pull the vacuum line for the fuel pressure regulator the engine revs up fuel pressure goes to 40 and problem is solved.
 

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And it has happened again. Starts up normal and after 4 min starts to choke on itself. I checked the fuel pressure at the rail. With just the key it goes to 15 and slowly fades. With engine running I get 35 but once the engine starts choking it indicates lower. If I pull the vacuum line for the fuel pressure regulator the engine revs up fuel pressure goes to 40 and problem is solved.
Well I have seen this several times, and it is almost always 1 of 2 things... first is the Fuel Pressure Regulator, second is the MAP sensor.
 

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Discussion Starter · #11 ·
Well I have seen this several times, and it is almost always 1 of 2 things... first is the Fuel Pressure Regulator, second is the MAP sensor.
FPR was changed out in august so I’ll check the map sensor. Thanks
 

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A failing MAP sensor will not cause the fuel pressure to drop while the the engine is running. "New" in August may very well be bad by November as far the FPR is concerned.
 

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Discussion Starter · #13 · (Edited)
A failing MAP sensor will not cause the fuel pressure to drop while the the engine is running. "New" in August may very well be bad by November as far the FPR is concerned.
I really hate to think that the FPR has already gone bad, but I suppose it is possible. That'll be the last resort. I plan to replace the MAP today and see what happens first.

I Should also mention I ran a full spray can of seafoam through the throttle body this weekend while holding 2000rpm to clean out any gunk. And I changed out the fuel filter, which shouldn't be needed, but figured "why not?" since it was $14.
 

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Discussion Starter · #14 ·
Replaced the MAP yesterday. Also replaced the vacuum line going from the TAD to the smog pump. It had been melted by the manifold heat. Don't think that line really matters, but whatever.

Here's how this morning cold start went. Started right up with a high idle around 1500 rpm for one minute (i timed it) seems normal, then started to choke and struggle for one minute, then recovered to 900 rpm on its own and everything was fine. Going to take it out for an hour drive today and see how she runs.

Maybe I just got a bad ECM that can't figure out how to regulate fuel on start up? Maybe the Cold air intake is causing the issue? Or maybe the FPR really is the issue? Or perhaps I just need to do a super scrub of the throttle body as suggested by Miesk? I'll see if I get any issues on the road today.
 

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Discussion Starter · #15 ·
Still had some issues today. Got about 4 miles and had some issues. Misfires and stutter on acceleration. Unhooked the the vacuum line to the FPR. Rpms went up slightly and I drove 35 miles with no issues. Hmmmm.... what’s up?
 

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Post #12......
 

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Discussion Starter · #17 ·
Post #12......
Yeah, I read that one. Thanks for the input. If you feel certain that's the issue I can go buy a new one, but since this all started with the ECM replacement, still under warranty, I thought that I'd try to exchange it for another one first since that won't cost me anything. Seems to be the last thing I can trouble shoot cheaply before commiting to a new FPR.
 

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A bad PCM/ECM/computer will not cause the fuel pressure to drop like you stated in Post #9. Fix what you know is wrong before shotgunning another stab in the dark "rebuilt" black box fuse.
 

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Discussion Starter · #19 ·
Removed and replaced the FPR today. Things ran well for a few minutes then started to choke again. I shut down, removed all hoses from the tree and reinstalled. Swapped a few locations. Things ran smooth. But that makes no sense since the vacuum tree all goes to the same place. We’ll see how it goes tomorrow.
 

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'92 Custom w/ '95 MAF 5.0 M/T, 33's, 4.10 LSD
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I shut down, removed all hoses from the tree and reinstalled. Swapped a few locations. Things ran smooth. But that makes no sense since the vacuum tree all goes to the same place. We’ll see how it goes tomorrow.
It could be that the vacuum tree's barb to the FPR line is partially clogged, causing sporadic issues, and swapping that line to another barb gave it full flow. Whatever you switched over to the bad barb may be less sensitive to the restriction.
 
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