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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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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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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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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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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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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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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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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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Discussion Starter · #22 ·
Today’s cold start was good with no issues. Then about 10 more n later choked while trying to accelerate to 45. Stayed like that. Got home and popped the hood. Both vacuum lines on the throttle body had worked themselves off. I reconnected and all is well. Changed one of them to a smaller gauge line. We’ll see how tomorrow goes
 

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Discussion Starter · #23 ·
I noticed that the IAC connector harnes was a little loose. If I pushed the harness hard onto the IAC engine response changed. So I took it apart, cleaned it up, tightened it and reinstalled. No joy, truck still choked. So I decided to remove the new IAC that I installed last month and reinstall the old one. Hooked it up to the connector harness that I refurbished, and I had no problems. No choking at all. However, I'm suspicious because I'd already started the truck about 5 times trying to solve the choke issue. I'll let it sit over night and try in the morning. That'll be a valid test to see if the issue is really solved.
 

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Discussion Starter · #25 ·
So here is how today's try went.

Started right up. High idle for one min. Low idle around 900 for one min. I'm thinking it's fixed. BUT NO! At about 2.5 min its starts choking. I decide to let it play out. Takes a little over a min and then idle returns to normal. I go to take it for a drive and its choking and missing on acceleration for the first 3-4 min. Then the problem went away and didn't come back for the remaining 15 min of the drive. I just can't figure out what's causing this.
 

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Discussion Starter · #27 ·
Any chance you've got a bad tank of gas, i.e., it's been sitting quite a while? For that matter, have you checked your fuel injectors? (All the threads become a blur by page 2!)
No chance. I drive a little almost every day. I also ran a full can of seafoam through a half tank of gas recently. Then last week put in a bottle of seafoam fuel injector cleaner.

tomorrow I’m going to check the TFI on the distributor. Think it could be cracked.
 

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Discussion Starter · #28 ·
Today I thought I'd investigate the distributor since I've gone through everything else.

I pulled out the distributor and took off the ICM (not sure if that's the name, but its where the wiring harness connects to) It was pulling away from the distributor by the screw closest to the harness. Removed the ICM cleaned it up along with the backing plate it connects to. Put it back together. Decided to validate the timing since it was out. Set the balancer at 10 below TDC. Made sure the rotor was pointing just before the sensor. Reinstalled. Now it won't start. Turns over but doesn't catch. Even if I'm off on the timing it should still start up and rough idle right?

I tried multiple times. Reset the balancer at TDC, checked the rotor, installed the distributor. Cranks but no start. So I'm assuming the TFI (little box on side of the distributor) is bad and I need a new one. Any ideas.
 

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Discussion Starter · #29 ·
Fixed it ! So happy I could cry. It was the ICM. As soon as I replaced the ICM the trucks starts and runs smooth. No stutter, misfire or choking. The old ICM was coming apart on me.
Hard to believe it took this long to solve the issue.

Thanks to everyone for the help.
 
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