Starting issue
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
Under what conditions did you measure that 7.5V out? The IAC is controlled by the computer via pulse width modulation. The duty cycle of On/Off is controlled by the computer so using a DVM set to Volts is pretty much useless other than verify the supply voltage is present.
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Cross-Country
Joined: Nov 2016
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Hey, i'm back. I was the original one of this post. After all of the advice i sent off my ECU and had it professionally rebuilt. I just got it back last week. I put it in on Sunday and the truck fired right up. I turned it off and fired it back up again. I shut it off again, then fired it up again and drove it to the store. It fired up great at the store, then when i got home i shut it off. I just went out today to start it and it did the same old thing it was doing before. Fuel pump kicks on and won't shut off and the truck won't start. I tried 4 times...now i'm stumped...Any chance it could be a bypass valve or anything else???
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
Yes, this is exactly why you do not start two threads for the same issue.
My answer in your other thread: https://www.ford-trucks.com/forums/1...l#post16915876
No.....the pressure regulator has nothing to do with this issue. The computer (PCM) controls the 1-2 fuel pump operation when turning the key from Off to Run.
My answer in your other thread: https://www.ford-trucks.com/forums/1...l#post16915876
No.....the pressure regulator has nothing to do with this issue. The computer (PCM) controls the 1-2 fuel pump operation when turning the key from Off to Run.
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Cross-Country
Joined: Nov 2016
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The reason for starting another thread was because I knew some people would open this one, and not want to read all the way through. I know, not the best way, but was hoping to make it easier to get fresh ideas. regardless...
There is nothing in the fuel regulator that can fail? Before sending it off for another 4 weeks to have them go through it again I just want to make sure I eliminate every possibility. It took months of research and hitting the right forum to find this initial answer of the PCM causing this issue...If there is ANY thing else that could cause this, even if it's a long shot, I want to test it first. And someone mentioned that if the regulator stuck open, it could possibly do this. I also talked to a mechanic and he said to test the diaphragm on the regulator. But then there's people saying that there is nothing else that could cause this...
There is nothing in the fuel regulator that can fail? Before sending it off for another 4 weeks to have them go through it again I just want to make sure I eliminate every possibility. It took months of research and hitting the right forum to find this initial answer of the PCM causing this issue...If there is ANY thing else that could cause this, even if it's a long shot, I want to test it first. And someone mentioned that if the regulator stuck open, it could possibly do this. I also talked to a mechanic and he said to test the diaphragm on the regulator. But then there's people saying that there is nothing else that could cause this...
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
The fuel regulator has no influence on when or how long the fuel pump is energized when cycling the key from the Off to Run position. That is controlled by the computer. There is no pressure feedback to "tell" the computer to shut off. It's a simple timer circuit inside the computer. Key On, timer starts to run for 1-2 seconds, then shuts off.
Classic failure mode is the clock inside the computer never triggers so there is no signal to shut off the count down timer. This keeps the fuel pump relay energized whenever the key is in the Run position (this position applies power to the fuel pump relay while the ground is applied full time via the computer).
The fact the computer did work, albeit for a short time, then died again still points back to a bad computer. Either a poor job repairing or the computer has failed in some other way.
Classic failure mode is the clock inside the computer never triggers so there is no signal to shut off the count down timer. This keeps the fuel pump relay energized whenever the key is in the Run position (this position applies power to the fuel pump relay while the ground is applied full time via the computer).
The fact the computer did work, albeit for a short time, then died again still points back to a bad computer. Either a poor job repairing or the computer has failed in some other way.
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Cross-Country
Joined: Nov 2016
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Ok, that makes sense to me know. I thought the computer was energizing the fuel pump and when it pressurized the system, the pump would shut off. I figured it was the responsibility of the pressure regulator to stay closed so it could build up pressure. What you are saying now makes more sense. Its weird that it is still intermittent...if I spray fuel in the intake, it will start. then it might start 5, 6...15 times after that...then it won't start again...but after your last post it makes sense that it is the computer...I wonder how they missed fixing that part...that's frustrating.
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Cross-Country
Joined: Nov 2016
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oh! since you seem to know what you are talking about...why will the truck not start in this situation? if the pump is running and there is fuel going...why will it just crank and crank? it seems to me that if the pump is running, then it should start...?
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
The computer also controls the injectors. So under these conditions the injectors are never told to open. You can get spark since the Ignition Control Module (ICM) is firing off the raw PIP signal just like it was designed to do as a "limp home" feature. That's why the engine runs with a dose of fuel or starting fluid, at least until that runs out.
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Cross-Country
Joined: Nov 2016
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This is making more sense...I really appreciate the info!
One thing though, is that if I get it to start by priming the intake, it runs just fine, and I can drive it around. Then it will continue to start the correct way after that (fuel pump kicks on and off), then it will randomly decide to fail again. It's just so intermittent. I have no doubt after what you've said that it must be the computer, but it just seems weird after your last comment "That's why the engine runs with a dose of fuel or starting fluid, at least until that runs out." Because if I prime it with fuel...it operates perfectly normal for the next 4, 5, 7...or 15 starts.
One thing though, is that if I get it to start by priming the intake, it runs just fine, and I can drive it around. Then it will continue to start the correct way after that (fuel pump kicks on and off), then it will randomly decide to fail again. It's just so intermittent. I have no doubt after what you've said that it must be the computer, but it just seems weird after your last comment "That's why the engine runs with a dose of fuel or starting fluid, at least until that runs out." Because if I prime it with fuel...it operates perfectly normal for the next 4, 5, 7...or 15 starts.
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Cross-Country
Joined: Nov 2016
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So I will prime the intake, and the truck starts. I shut it off, then I turn the key to the on position and listen for the fuel pump. it kicks on for 1-2 seconds then shuts off, then I know it will start. Then after maybe a day or two, i'll go to start it and it won't start. I turn the key off, turn it back to the on position and listen for the fuel pump and it's just running constantly. It's like if I prime the intake and get her to start, it resets everything and everything functions as it should, for a few starts or a few days.
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
Still points to a bad computer. Cold solder joint, heat related issue, etc. The telltale sign, as you stated, is the fuel pump running continuously.
Just to be sure I suggest you remove and clean grounds G100 and G101. G100 is located on the drive side of the radiator support. G101 is located next to the battery. The locations may be a little different on a 1991. I am used to and have diagrams for 1992 and up. And yes...they tend to be very different.
Not only inspect, pull the connectors and clean them thoroughly. One time we found an intermittent open ground at G100 causing similar issues, except the symptom was always there. Might as well cover the bases before we spend your money.
Just to be sure I suggest you remove and clean grounds G100 and G101. G100 is located on the drive side of the radiator support. G101 is located next to the battery. The locations may be a little different on a 1991. I am used to and have diagrams for 1992 and up. And yes...they tend to be very different.
Not only inspect, pull the connectors and clean them thoroughly. One time we found an intermittent open ground at G100 causing similar issues, except the symptom was always there. Might as well cover the bases before we spend your money.