swapped 4.0 in YJ no power and backfires - JeepForum.com
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post #1 of 8 Old 09-13-2017, 06:30 AM Thread Starter
4point0YJ
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swapped 4.0 in YJ no power and backfires

Hello,

I have an 89 YJ that i swapped a fuel injected 4.0 with ax15 into. It hesitates to start but will start when i give it some throttle. Then it will idle fine but if you throttle it up or hold the gas around 3000 + RPMS it will back fire out of the exhaust and up through the intake. No power at all when driving..

Here is what i can tell you about the swap.
Block is an 99-01. (all new gaskets)
Intake and heads are from a 96-98.
96 xj ECU
Distributor is from a 96. (is this fine to use with the 01 block?)
Wire harness is a 96.
Changed the idle air contoller
swapped 20gal tank with new fuel pump, lines, filters. Fuel rail has a return line.
New cap, rotor, plugs, and wires.
New coolant sensor
built exhaust with just muffler no cat. has 02 sensor
new coil (good spark)
Fuel pressure seams fine testing tonight.
fire order is good
spliced the wire harness to keep both.
put a 4.2 oil sending unit to use gauge.

I can drive it around the neighborhood but cant get the rpms to high.


I feel that the timing is off. I set it at top dead center and i know you cant move the distributor to set but for some reason feels like timing.

Here are some things i think could be wrong but looking for some help to test/ ideas and get it working.
Map sensor . ?
throttle control sensor?
02 sensor.. never test
ECU from 96 xj believe it was 4wd. (also have 01 tj computer but was only getting spark on 1 and 6 which was prob due to the coil packs? )
distributor ... could be warped

Please let me know any ideas that you might have. Was concerned using a distributor on the 99-01 block.
Could the distributor be no good? heard this happened to another guy..
would the CPS if bad allow the jeep to run?
Could it be any other sensors?

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post #2 of 8 Old 09-13-2017, 06:56 AM Thread Starter
4point0YJ
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FYI this is a new swap. It has not yet or ever ran good since i did the swap a few weeks ago.
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post #3 of 8 Old 09-13-2017, 11:21 AM
mike134
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Timing is definitely off. The only thing that can cause backfire through the intake is the timing.

Could be either the distributor is not on correctly, or the timing chain has skipped a tooth.
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post #4 of 8 Old 09-13-2017, 02:43 PM
NHfireLJ
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96 ECU, needs all 96 or newer sensors correct?

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post #5 of 8 Old 09-14-2017, 09:27 AM Thread Starter
4point0YJ
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i believe so and i do have 96 or newer sensors. i only have 1 02 if that matters.

Do i need to remove anything or drain anything before taking the timing chain cover off? Also do you place motor at TDC before taking it off?
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post #6 of 8 Old 09-14-2017, 09:35 AM
mike134
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Yeah. I'd follow along with the Factory Service Manual.

Also, while you're at TDC, make sure your distributor rotor is in the correct spot.

this can also help http://www.jeepforum.com/forum/f12/t...-style-782349/
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post #7 of 8 Old 09-14-2017, 12:58 PM
Jeepsr4me
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free FSM can be for online to download......

I have swapped a few of the xjs into wranglers... long as you dont mess with the dist 99% +-2% of time its a plug and play deal.
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post #8 of 8 Old 09-15-2017, 05:39 AM Thread Starter
4point0YJ
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Fixed! I appreciate everyone's help. I am 99% sure that i did not have the cam sensor correctly set when i installed the distributor. I decided to get a new distributor and when i did i reset all the timing. fired right up and runs strong. Attached is the picture that i used to set the cam timing inside the distributor which i never read was something that we needed when setting the timing.
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