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  #1  
Old May 25th, 2010, 17:34
k.allen90 k.allen90 is offline
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fuel pressure when key is on, lose fuel pressure when try and start

Title says it all. I use to have fuel pressure all the time and it wouldn't run and i just had a bad tps sensor and it ran good for about an hour and died but I've replaced the tps sensor again and its still not running. I had a buddy help me and he noticed that when the key is on we have fuel pressure then when i crank it all fuel pressure is lost. My jeep isn't throwing any codes because the person who installed this engine before I bought the jeep didn't wire it right i guess. Thats what I was told by some mechanic at this shop. I did notice when I was just looking around that there are 3 cables not plugged into the ecu but i didn't want to go and just start guessing at what goes where, so i've left it for now. My buddy thinks that something isn't telling the fuel pump the jeep is on so it doesn't start back up, he recommend putting the fuel pump on a toggle switch bc he had the same problem and thats what he did and the truck ran fine. I don't know what exactly is wrong with my jeep so i'm just trying to trouble shoot everything I can fix before taking it in to a shop. Its not my dd but its still agravating. I've had some of the electrical guys at work look at it and they say my best bet would be to just send it to an auto electrician and have them rewire the whole thing because who every did this wiring was horrible at it and stupid as hell. I'm trying to fix it myself first before I pay someone to do it. anything helps. Thanks

I know starting and then staying started is a common problem but i'm reading everything i can find on this forum bout this problem, using the search to.

Last edited by k.allen90; May 25th, 2010 at 17:42.
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Old May 25th, 2010, 20:37
joe_peters joe_peters is offline
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Re: fuel pressure when key is on, lose fuel pressure when try and start

PLEASE start all threads with the basics: year/engine/transmission/transfer case/mods/any other info that might be pertinent.
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Old May 25th, 2010, 20:49
k.allen90 k.allen90 is offline
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Re: fuel pressure when key is on, lose fuel pressure when try and start

I don't know any of that stuff. I bought the jeep from a guy who said his buddy changed out the engine before he bought it from him and he knows nothing about what is on the jeep. The jeep is an 89 with a 4.0 h/o but i'm not sure if its a true h/o but i doubt it. Its the automatic transmission and thats all I know about it. I've looked and looked for data plates so I could try and figure out whats what but i'm not having any luck in figure out anything. I tried to get a hold of the guy who did the engine swap but unfortunetly he is in afgahnastain and i have no way to contact him to find out anything.
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Old May 25th, 2010, 21:10
joe_peters joe_peters is offline
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Re: fuel pressure when key is on, lose fuel pressure when try and start

Ok, its an 89, 4.0, AW4 most likely, transfer case to be determined.

Now, lets try this:

1. When the key is turned to ON, fuel pump is powered with 12 volts from the B Latch relay for 3 seconds, then the ECU cuts the relay.

2. When the key is turned to START, fuel pump is powered with 12 volts from the starter relay.

3. When the key is ON, and the engine is running, from idle to just short of WOT, fuel pump is powered with 12 volts that passes through the ballast resistor and is cut to approximately 6 volts.

4. When the key is ON, engine running at WOT, fuel pump is powered through the oxygen heater relay.

Paying someone to rewire your XJ would cost more than it is worth. You can go to a salvage yard and find an 89-90 with the same equipment and take pictures while you strip the entire harness, then replace the existing harness.
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Old May 25th, 2010, 21:21
k.allen90 k.allen90 is offline
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Re: fuel pressure when key is on, lose fuel pressure when try and start

but since I don't know anything about this motor would it make it any harder? I never thought about going to the jy and trying to figure it out that way
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Old May 25th, 2010, 22:42
joe_peters joe_peters is offline
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Re: fuel pressure when key is on, lose fuel pressure when try and start

Well, normally, but not always, when there is an engine swap the engine management system of the original engine is maintained. Now, lets figure that out--look up under the dash driver's side and see if there is an ECU there. If yes, you are likely still Renix, and all that should have been done was the Renix sensors and manifolds were installed on the HO engine.
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Old May 26th, 2010, 06:00
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cruiser54 cruiser54 is offline
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Re: fuel pressure when key is on, lose fuel pressure when try and start

How many bolts are used to attach the throttle body to the intake manifold? Is the intake tube bolted to the throttle body with three bolts? If so, the Renix injection system was retained.
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Old May 26th, 2010, 16:35
k.allen90 k.allen90 is offline
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Re: fuel pressure when key is on, lose fuel pressure when try and start

it is still the renix system
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Old May 26th, 2010, 17:43
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cruiser54 cruiser54 is offline
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Re: fuel pressure when key is on, lose fuel pressure when try and start

Quote:
Originally Posted by k.allen90 View Post
it is still the renix system
If that's the case and it had a swap, I would refresh all the engine grounds at the dipstick tube stud and rear of head to firewall. Don't just LOOK at the. Remove, scrape paint off, polish, clean and reinstall using dielectric grease.
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