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Discussion Starter · #1 ·
Ok i just finished the engine swap and now i have a problem and dont know where to begin.

The motor will start, but will not stay running for more than three seconds. Second i can keep it running if i pump the gas but it runs rough, if i let off the gas it will momentarly run right for one second then die ... and ideas out there ...
 

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I don't remember any of the details.
So what engine?
what year?
Into what car/truck?
carbed or fuel injected? etc
 

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Discussion Starter · #3 ·
it would be a 89 f-250 with a 90 5.0 and this would be fuel injected
 

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Sounds like a vacuum leak to me. If you hold the pedal partway down, will it run? These older systems had to be taught to idle. There is a procedure for that, but lets make sure that is the problem first. What did you do for the swap? Same size motor? Same processor? Did you change any other components? Give us some info. We can help you better with the most information you can give us.
 

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Discussion Starter · #5 · (Edited)
the truck is an 89, swaped 89 5.0 for a 90 5.0 used my old wiring harness and that was it the rest of the motor was there, put it in. tried to start it and all i got was a rough idol and dies if im not pumping the gas.
 

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Discussion Starter · #6 ·
and another note i cant just hold the accelerator i have to pump it, if i dont the idle drops smooths out for a sec and dies
 

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I'm just gonna throw this out there because it happened to me, but when I did my 5.0 motor swap I had the distributor in 180 degrees off and It wouldn't run. I was sure I had it right but I turned it around anyway and It fired up and ran like a champ.
 

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Discussion Starter · #10 ·
update: i got a fuel pressure tester and checked the fuel pressure im getting 40 psi at the rail. while it running, so i think fuel pump are rulled out, any more suggestion as to why i have to constantly pump the accelerator to keep the motor running
 

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Discussion Starter · #12 ·
Yeah i did check the IAC? and followed the sticky on the cleaning, it wasent able to tell if it made a difference. so i guess the next step is to rent a vacuum gauge and see what happens there.
 

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is the IAC the same as the throttle position sensor?
 

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IAC is Idle Air Control Valve... kinda like the choke on a carbureter.
The throttle position sensor tells the PCM how far you depressed the gas pedal.
 

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Did you swap all of the 89 electronics and sensors to go along with the 89 PCM programing? Check the part numbers of the sensors on the "new" engine against the 89 part numbers. If they are different then swap in the 89 part onto the 90 engine.
 

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Discussion Starter · #16 ·
eww didnt think about that ... have to give that a look at
 

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Suggestion

Did you check the timing yet? If not you should. II did a cam swap in my 89 LTD with a 5.0L and when I went to fire it back up it wouldn't stay idling and I had to hold the gas, but it still didn't run right. As I was in the car controling throttle, my cousin was turning the distributer. As he did this it started to clear up one way, But wouldn't go far enough. Found out I was off one tooth.
 

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Discussion Starter · #19 ·
Well i fixed it, all the trouble was due to a faulty MAP sensor, it runs like a bran new truck ... thank for all the help
 

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Thanks for posting back. Did you pull codes? How did you determine that it was the MAP?
 
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