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Fuel Question


Jarrett Bradford

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Hey all. So I had a previous thread about this issue but have new ideas as to what my issue may be. After driving my truck around a little, turning it off, letting it sit 20+ minutes and starting it again it runs real rough. Sometimes even dies. Starts back up, give it gas and eventually it will smooth out and it’s good to go. In my last thread people thought it may be the FPR. I have since replaced the FPR twice now with new Motorcraft replacements and there still seems to be fuel In the vacuum line even after letting the vacuum suck all the excess fuel out after replacing the FPR. And obviously it still starts rough. Could a stuck injector cause this issue? Thanks!
 


Andy D

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I would start with the coolant temp. sensor. Normally I don't replace a part without diagnosis, but the sensor changes resistance as the coolant warms up. It is difficult to rig a conclusive test. Need some back story for a better guesstimate :D

Also my '94 has had plenty of old age wiring faults.
 
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Jarrett Bradford

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I may sound dumb by asking this, but how would the coolant sensor cause those symptoms? I would like to add the the exhaust smells almost like raw fuel when I have to rev it up to smooth out the engine. Cold starts or if the truck has sat for an hour or longer it’ll start fine as well. Just when I run into a store or something real quick and come back is when it does it. If I knew how I’d, link my last thread to show what we went over there. Thanks!
 

91stranger

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The coolant temp sensor can very well easily be the culprit. It's a $12 part and about 10 minutes to change if that. If the sensor is not reading the proper engine temp then you fuel/air ratio wont be right. Its cheap and easy and could fix the problem. Have you tried replacing the fuel filter yet?
 

Shran

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My experience with a bad coolant temp sensor is that they make it run bad all the time, not just in the situation you describe.

I would suspect you've got an injector leaking fuel into the engine and flooding it.
 

Jarrett Bradford

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I appreciate all the replies. Last time I changed the filter was like 2 years ago, but I belive the truck was having it’s episodes back then as well. I’ll replace the temp sensor just to be safe, and as for the injectiors leaking which is what I’m thinking it is now, how would I check for that? And could that cause the FPR to leak into the vacuum line?
 
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RonD

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To check for leaking fuel injectors use Clear Flooded Engine mode

All fuel injected engine computers have this
Turn key on
Press gas pedal down to the floor and hold it down all the way<<this tells computer to turn OFF injectors
Crank over engine

It should not start, it should not fire at all

When computer "sees" WOT(wide open throttle) from TPS and RPMs are at 0 it will disable fuel injectors, but spark stays on

I use this every morning when starting my high mile 4.0l, I like to get oil pumped thru the engine BEFORE it starts
So I crank it for 10-15 seconds then release gas pedal and it starts up.

If engine fires then you may have fuel leaking in to the engine, from an injector, or from Fuel Pressure Regulators vacuum hose.

When you release gas pedal injectors will startup or if engine fires and RPMs get above 350 injectors will start

If engine does fire you can use same test to ID which injector is leaking
Cold engine
Disable Spark
Use Clear Flooded engine and crank engine over a few times
Now pull out the spark plugs
WET spark plug will be from cylinder with leaking injector
 

Shran

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Ron's method works great. However if there is some slack the throttle cable (common) it is possible that the throttle position sensor won't see wide open throttle voltage and will still fire the injectors - in which case you can do an easy fix with the zip tie mod - search on the forum will turn up about a million answers for how to do that.

Alternatively you can hook up a fuel pressure gauge to the test port on the fuel rail, cycle the key back and forth from off to on without starting the engine, and see if it will build up pressure and stay pressurized or if it leaks off after you stop cycling the key.
 

RonD

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Thanks Sharan

I forgot to mention "Throttle Cable mod" for stretched throttle cable

Stretched cable means no WOT so no Clear Flooded Engine mode
 

Jarrett Bradford

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Thanks guys. I’ll check all that this week and report back. Have a great Thanksgiving!
 

Jarrett Bradford

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I did peddle to the floor and it started up instantly. In the past when I’ve done it it’ll just crank and crank and crank. But today it started right up. I’ve got a pressure gauge on it now, cycled the key back and forth and the pressure has been as high as 40 PSI. And is slowly going down. Now I just need to figure out what injector it is. I may just need to go ahead and replace them all. Weird thing is tho. When I started it up after removing the gauge and everything, it started up smooth. I would have though the engine would have been flooded from the fuel leaking in and start up rough. But it didn’t.
 
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