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Old 07-03-2011, 11:06 AM   #1
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4.0 Conversion wont start

PLEASE HELP

My 1990 YJ with the 4.0 conversion wont start. The new engine and harness are out of a 1992 Cherokee HO. Upon initial crank it fires off once and then just cranks. The fuel pump primes upon first crank and injectors pulse once (checked with noid light) and then nothing. The fuel pump only pumps while cranking (voltage checked at ballast resistor, 12 V while cranking). The only code that comes up is 41, open field circuit (all wiring checks good, and this shouldnt keep me from running.) I assume the 41 pops because I am cranking and cranking without an opportunity for the altenator to send power anywhere.

It seems anytime I read about someone replacing the computer it does not fix anything. However this is all I can think to replace at this point. I have not heard of the auto shutdown relay fixing anything either, will a faulty auto shutdown relay cause this?

I can get the engine to run if I use ether or pour a little fuel into the intake.

I have replaced the Crank Position Sensor, Cam position Sensor, and have installed the proper Fly Wheel for the engine.

Any help is welcomed, I have exhausted myself with this problem.

Please help.

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Old 07-03-2011, 11:22 AM   #2
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What engine was previously in there? Everyone says you must change the computer or else it still thinks you are running a 4-cylinder (or other).

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Old 07-03-2011, 11:26 AM   #3
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Also the 2.5 has a low pressure fuel pump the 4.0 has a high pressure fuel pump.
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Old 07-03-2011, 11:37 AM   #4
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Quote:
Originally Posted by banks316
Also the 2.5 has a low pressure fuel pump the 4.0 has a high pressure fuel pump.
Sounds like you're not getting enough fuel, I would check the fuel pump or just change it out.
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Old 07-03-2011, 01:15 PM   #5
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Thanks for the suggestions.

The previous engine was a carbed 4.2. I am getting 40 PSI to the rail during cranking. If I stop cranking the fuel pump stops pumping.

I also have the 1992 4.0 Cherokee computer installed.
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Old 07-03-2011, 03:48 PM   #6
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The new installed fuel pump is for the 1992 Cherokee and puts out 40 psi to the rail while cranking.
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Old 07-06-2011, 07:55 PM   #7
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Well, it is finailly running!!! Hells Yeah!, Finially. Thank you to everyone for your suggestions and pointers. It came down to the the timing. Yes the timing was off. I guess when we installed the distributor after replacing the Cam Sensor we dropped a gear. We corrected the timing per the Haynes manual and "volia."

I am bouncing off the walls. I was just about to roll the thing off a cliff! Almost time to get muddy!
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Old 07-06-2011, 11:43 PM   #8
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Great. I had the same issue with mine after the swap I did. its tricking when you replace the distro, if you dont get it just right it will throw the timing off. Good stuff.
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Old 07-07-2011, 02:29 AM   #9
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How much was your whole swap? I'm gonna swap mine and I like the fuel injected for smog reasons(ca)!!! Any info would help
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Old 07-08-2011, 09:54 PM   #10
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Where do I start.

I bought a donor 1992 Cherokee 4.0 HO with a new top end for $650.00. My bell housing had the holes to mount the CPS so I lucked out with that and was able to use the donors.

New Fly Wheel...$40.00. The donor had an automatic and my fly wheel on the 1990 YJ is completely different than the donors engine needed.

I had a two pilot bushings milled into one.
big bushing $20.00
small bushing $10.00
labor at the machine shop to machine out the bigger bushing $20.00

K&N Air Intake $206.00 from Amazon

Replaced the faulty Cam Sensor $40.00
Replaced faulty Crank Position Sensor $50.00

Grand Total $1036.

I feel I luck out since I got a great deal on the donor and did not have to get anything from Hesco...ie the CPS relocation kit or wiring harness.

If you have any question at all please feel free to ask. I ran into plenty of problems along the way but I am so close to getting back on the road. Just waiting on the air intake to get here.

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