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91 Ford F150 trouble

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55 50 Ron
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PostPosted: Wed Mar 25, 2020 5:41 pm    Post subject: 91 Ford F150 trouble Reply to specific post Reply with quote

This has the big V8 engine. Upon first starting it runs for maybe 10 seconds OK then starts shutting off and on just like the ignition is being turned on and off. The intervals between running and chopping off and on are not even.

Then after maybe 5 minutes (haven't timed this exactly) it settles down and runs like normal. Seems like something related to the engine warming up.

Any help appreciated on what to look for that might be causing this.

Thanks
 
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guido
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PostPosted: Wed Mar 25, 2020 6:00 pm    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

Hello 55 50 Ron,

How about more info? Mileage, plugs mileage fuel, fuel filter to name a few,

Guido.
 
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marloweg
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PostPosted: Wed Mar 25, 2020 6:42 pm    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

o2 sensors
 
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55 50 Ron
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PostPosted: Wed Mar 25, 2020 6:47 pm    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

I'm the 3rd owner so I'm not sure of mileage, it's either 93000 or 193000. It's just a "sideline" vehicle and doesn't get used a lot. Fuel filter has never been changed by me. Plugs have never been changed by me. I know I don't take care of this truck like I should.

Things I have repaired not related to this engine problem are: Wiper motor, turn signal actuator (under steering wheel center), a brake line broken when I backed into a pile of brush - oops, dome light switch in the left door pillar, starter,

But I have been careful with engine oil and oil filter changes.

I'm sure this doesn't help much, sorry.
 
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Bruce(OR)
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PostPosted: Wed Mar 25, 2020 7:05 pm    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

Your 'big engine' appears to be the 5.8 and not the 7.3 of the earlier years.
Research shows the ign module bolted to the side of the dist.
Back in the mid 80's Ford used an inferior rubber on the ign pick up inside the dist. It was subject to breaking down and shorting out the wires for the ign pick up inside the dist.
That 'rubber' insulator was white in color.
Now if your ign pickup is not white in color where the wires come out of it and is NOT cracked and breaking apart, I would tend to suspect the ignition module instead.
Be sure to wipe off the base when installing and grease it with dielectric grease for proper heat distribution.
That ign module might be dist mounted and those were more subject to failure than the modules that were firewall located.
Parts book gives the dist mounted option subjecting the little parts to heat failure.
If mounted on the firewall, I would not really suspect it to be bad.
When it dies, Got spark?
 
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David G
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PostPosted: Wed Mar 25, 2020 7:09 pm    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

No, O2 sensors are ignored during warmup.
 
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JMOR
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PostPosted: Wed Mar 25, 2020 8:00 pm    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

trouble at only 30 years! what a piece of junk! :)
 
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55 50 Ron
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PostPosted: Wed Mar 25, 2020 8:18 pm    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

No - just 29 years! Have to keep it! I can't afford a $50,000 new one. :)
 
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Jaden
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PostPosted: Wed Mar 25, 2020 8:31 pm    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

Have you checked to see if it shows any trouble codes?
 
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RusselAZ
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PostPosted: Wed Mar 25, 2020 9:14 pm    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

Most likely you have a vacuum leak that the computer can still correct for when it gets into closed loop.
 
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Steve@Advance
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PostPosted: Thu Mar 26, 2020 4:32 am    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

Start with the basics, fuel pressure and spark.

You can get a fuel pressure gauge from the loan a tool at Autozone. Have it on the port before starting, see if it drops when the engine tries to die.

I think that has a distributor? If so pull the coil wire partially off so you can see the spark. Watch for intermittent spark when it's trying to die.

That should get you some idea what direction to pursue.
 
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ss55
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PostPosted: Thu Mar 26, 2020 4:32 am    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

You might be ahead to just have it towed to a shop and let them figure it out. The tow should be around $100, it does not take many unnecessary parts swaps to burn through $100 worth of parts and your labor.

I had a sudden "no start" problem with a '94 Chevy that I could not figure out. I sent it to a shop thinking they would replace the fuel pump. "Nope, the coolant temperature sensor is causing your fuel problem". It's been running just fine since then. I would never have found that.
 
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Hobo,NC
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PostPosted: Thu Mar 26, 2020 4:33 am    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote


I don't think a 91 gives scan data : (;

I would confirm grounds good at the battery and all leads that come off the battery ground are clean bright and tight both ends.

I would check for vacuum leaks most common where the plenum mounts to the intake.


I would run a KOEO test... Address any KOEO codes before I run a KOER test.... After warm KOEO codes repaired then run a KOER test.


Install Fuel pressure gauge and leave it on till its fixed...

Then hook up a break out box and check MAF, ECT, IAT, TPS, EGR sensor. The process is elimination to confirm sensors are good/bad are in range.

Those are a demon it takes some special equipment to diagnose them properly in a timely fashion. A good understanding of how the system works mainly because of no menu (scan-data)

I would not throw the parts cannon at it till I confirmed all of this... Its not uncommon for the fuel tank/tanks to be full of rust and the injectors restricted : (

On a 91 everything is suspect including the o2 sensors, ignition and basics... I have many stories where it was something simple it was a long road to get there... If you take it to a shop you will need a shop that cut their teeth on a ford with no scan data... A good question to ask do you have a ford 60 pin break out box and know how to use it...

There are no silver bullets until all basic have been confirmed : (
 
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ss55
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PostPosted: Thu Mar 26, 2020 4:46 am    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

As HoboNC said, find a shop familiar with your era of Ford trucks. The shop I went to had the problem figured out in a few minutes.
 
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Fred Werring
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PostPosted: Thu Mar 26, 2020 5:05 am    Post subject: Re: 91 Ford F150 trouble Reply to specific post Reply with quote

Pre 1995 (or maybe 96?) used the OBD1 system.

You can still get trouble codes, but it involves either an OBD1 reader OR jumpering out some connectors to get the engine warning light to blink out the code.

how to retrieve codes
https://www.f150forum.com/f10/how-retrieve-trouble-codes-obdi-10907/

List of codes
https://www.troublecodes.net/ford/

Found these with a quick google search, you should do some reading to verify this will work on your truck.

As an aside...doesn't that truck have dual fuel tanks? Does it do the same thing on both tanks? I think (IIRC) they each have a fuel pump in them.

Good Luck

Fred
 
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