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Old 11-24-2020, 10:23 AM   #1
kingranch
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Default Mustang foxbody 5.0 guys question

My 1990 lx 5.0 has trouble 1st in the morning to start it stumbles and will stall if I dont give it a tiny bit of gas.

Fuel pressure shows 40 psi primed and drops to 30 when car starts.

Funny thing is after it warms up for a second it runs great and restarts easily no stumble...only after it sits over 8 hrs.

I did notice if I unplug the vacuum on fpr it jumps to 40 and on a cold start in morning it doesn't stumble but idles higher like its rich...I replug the vac hose and it idles back down and runs fine then.

Could the fpr cause this early morning hard start condition? It's a ford stock fpr and holds good at 30 when running and it's not leaking that I can see or smell from the vac nipple.

Everything is stock as far as the fuel system goes.. 19lb injectors and new fuel pump and filter last year...also injectors were rebuilt.

I noticed the fuel pressure bleeds off overnight but from internet search this is fairly common.
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Old 11-24-2020, 10:38 AM   #2
kingranch
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I should add that I recently replaced alot of parts to make sure she was in reliable running condition..
New IAC, cleaned maf and throttle body, has newer bbk 70mm tb, new TPS sensor adjusted to spec.

Runs out strong I can rip it and drift all over the place with no problems whatsoever... just this cold start thing is driving me nuts as it just developed it in last month
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Old 11-24-2020, 10:48 AM   #3
Bama
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The fuel pressure bleed off is caused by a drain back valve on the fuel pump the only thing it should affect is a little longer start time once it starts you should not see any side effect. The idle you describe sounds IAC to me.even though its new?
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Old 11-24-2020, 10:54 AM   #4
kingranch
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Yep I installed it few months ago bc I was having a bit of low rpm when using the ac... it's a new ford part from lmr.com
But I guess it could be bad huh?
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Old 11-24-2020, 10:56 AM   #5
hopedale
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I had an issue like that with a Toyota. It was a dirty fuel system.

It was the carbon build up, which is what may have finally gotten to a point if this just should up out of no where.

It was the intake valve on my car. I kept adding fuel system cleaner with every fill up.

It took about a month and the problem went away as mysterious as it came.

Check out the before and after -


Last edited by hopedale; 11-24-2020 at 11:01 AM.
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Old 11-24-2020, 01:54 PM   #6
kingranch
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i guess the seafoam is worth a try
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Old 11-24-2020, 04:09 PM   #7
Bama
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Quote:
Originally Posted by kingranch View Post
Yep I installed it few months ago bc I was having a bit of low rpm when using the ac... it's a new ford part from lmr.com
But I guess it could be bad huh?
If you can tap the gas and it stay running I would lean towards IAC.
I'm the shop Forman at Prestige Ford I have seen my share over the years lol.
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Old 11-24-2020, 04:25 PM   #8
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No help, but that is a bad*** ride.
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Old 11-24-2020, 04:26 PM   #9
kingranch
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Quote:
Originally Posted by Bama View Post
If you can tap the gas and it stay running I would lean towards IAC.
I'm the shop Forman at Prestige Ford I have seen my share over the years lol.
ok that is enough for me to try.... im going to order a new one and install- ill report back w results

thx!
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Old 11-24-2020, 05:15 PM   #10
blfx4
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I had a 93 with a similar issue and it was the idle air control valve. An adjustable fuel pressure regulator wouldn’t be a bad idea if you happen to go with bigger injectors. That is a clean fox body. Something about the way those cars sound with a b or e cam and flow master mufflers. Good luck on the repair they are fun to tinker with.
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