4.0 idles fine but can't handle load - JeepForum.com
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post #1 of 7 Old 10-30-2008, 12:06 PM Thread Starter
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1991 YJ Wrangler 
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4.0 idles fine but can't handle load

Just had the engine swapped out on my YJ 4.0L. Everything was hooked up and ran it for 30 minutes according to specifications. When they took it for a drive accelerating and decelerating, again according to specs it was running great and just like new then all the sudden it started to jump and sputter and surges rpm's up and down erratically. The jeep started to act like it wasn't getting enough fuel and the last two miles it backfired and bucked like a mule, dying several times before they made it back to the shop. They ran codes Nothing, Replaced the fuel pump, cleaned the tank out and checked the fuel pressure with no luck. Timing checked and it was out 180 degrees but if set right wont even start. Change the O2 sensor, throttle position indicator, rotor and cap, spark plugs and wires, and had the cam sensor adjusted as well. Nothing. Put the timing back to 180 out and it starts, but once its under load it does the same thing. Jeep runs fine under idle, but as soon as you put it in gear and apply load all hell breaks loose. Jeep bucks, snorts, backfires, dies. Borrowed a ECU and still persisted, so they gave up on trouble shooting. Suggestions?

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post #2 of 7 Old 10-30-2008, 01:27 PM
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1996 ZJ 
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Location: Vernon, NJ
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A friend of mine swapped engines in his XJ and had the same trouble. He's still trying to fix it. I can't offer anything other then check every single ground wire. Did the engine you swapped in run? Did you hear it run? Have you triple checked the plug wires are properly installed and in good cond?

96 ZJ.
05' Grand Cherokee Limited, 2"OME lift
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post #3 of 7 Old 10-30-2008, 01:50 PM
1995_4.5L Jeep
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I'm not sure where you got this motor, but you might want to look at the timing chain and make sure it was installed properly. If it's a used motor it might have a stretched chain? Keep us posted

1995 Wrangler 4.5L
8.8 Ford rear
4.11 gears
ax-15 trans
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post #4 of 7 Old 10-30-2008, 02:42 PM
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1992 YJ Wrangler 
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Location: Brooklyn, NY
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you need to index and rotate the dist 180.. but you need to INDEX it

when its trying to advance the timing due to rpm and/or load, its expecting a certain cylinder to fire BTC, if the timing isnt indexed correctly, and/or off 180, its firing the wrong cylinder, it will still start and run, but will freak out when advancing

go here


if anything is off, just make sure you line up the oil pump shaft, by rotating it to align it with the dist...

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post #5 of 7 Old 11-24-2008, 08:22 AM Thread Starter
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1991 YJ Wrangler 
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Still no luck

Tried just about everything out there. Indexing didn't help. At a total loss
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post #6 of 7 Old 01-07-2009, 10:06 AM Thread Starter
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I'm still having issues. Did your buddy ever figure out what the problem was with the jeep after the engine swap?
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post #7 of 7 Old 03-11-2010, 03:51 PM Thread Starter
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YJ idles fine but wont run under load

Well the issue was finally found. The connection at the distributor was loose, tried to pinch the connections tighter and didn't solve the problem. Finally just spliced the together and all is well. Seems the vibration and road bumps caused the short out. There was also a short in the wire going to the fuel pump. Basically, just had to perform the wiggle test on every wire there was while it was running and found the shorts one at a time.
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