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RENIX 4.0L Help

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I have an 87 MJ 4.0l 5spd. have been slowly putting it together and trying to chase down a running issue. Its not like any problem I have found when researching.


Here is what it does. It starts and idles great 700rpm. No hesitation when accelerating. As it comes down in RPM is starts to stumble and backfire through the TB. Basically its leaning way out.


Here is what I have done.


-Checked and confirm TPS working properly.

-IAC working properly

-Fuel Pressure 31lbs at idle 39 on acceleration

-cleaned and greased every terminal under the hood

-EGR removed and blocked off

-vacuum leak test

-cleaned every ground

-spark plugs wires and cap rotor all new

-distributor checked and re checked

-Everything above checked and rechecked and rechecked


Any Ideas?

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O2 Sensor looks new. Tested fine when on my scanner and the connection was cleaned.



Map sensor was swapped with a new one we have on the shelf.


I have read thru both the links you have there. Lots of good info but nothing that has helped. I kinda feel like I am missing something stupid obvious.

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You may want to look at your catalytic converter. Listen for a rattle, see if glows at night. New cat ~$125-$175 installed


Backfiring can definitely relate to timing, but you haven't mentioned the cat being clogged, or falling apart. Exhaust back pressure can cause backfiring too, and would flatten the hell out of performance. So, just an idea that I hadn't seen

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Timing and, or a rich mixture.

Rich would backfire through the exhaust. Lean can backfire through the intake.


Timing (distributor and/or cam timing) could be an issue. Also check for vacuum leaks. If you have a leak, the engine pulls the highest vacuum when decelerating, so a leak would have the most effect then and could be creating a lean mixture that the O2 sensor can't adapt for quick enough (or "enough," period).

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If it is an issue with the timing, I would put my money on a loose chain. If it had skipped a tooth, it wouldn't rev that well.


If the chain is loose, it could be pulling just fine when accelerating, then the momentum of the cam changes it's position when you let off the throttle... throwing off the valves, injectors and spark.


Pull the cap off, and turn the motor back and forth with a ratchet to see if there's a lot of play before the dist moves.

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Ignore the above if you are NOT talking about closed throttle coasting down.


If you're just maintaining throttle (small opening, to maintain speed), when the problem happens,

the above wouldn't apply.

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Thanks for the replies. I have a few things to look at during my lunch hour today. I does this primarily when you put the engine under load. I can make it do it by turning the idle way down but as soon as you move it as in let out the clutch and push it back in after being under load is when it will happen every time.


There is no catalytic converter on it at the moment. PO took it off and I have just welded in a straight pipe and muffler so I can hear what the motor is doing.

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Check your intake air temp sensor. Nice Comanche and post some pics of that J-truck!!!


I want one of those bad :cheers:


The J10 is my bosses truck. Its a 72 short box, TBI 350, 700r4, 4.10 gears. Its a fun truck to drive.

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I went out and pulled the distributor cap and have about 3/8 of an inch of a turn in the crank before the distributor turns. I am told I shoudnt be too concerned unless I have about 3/4" I checked out a 2006 4.0l that we use for mock up here and it has just a little less. and it had 17k when pulled. So I am thinking that is good.

I did find that it is missing the vacuum canister that should be behind the front bumper. The lines are plugged and are not leaking. Should this case anything like this. I thought it was there primarily for heater controls?

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