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Need some help, renix MJ idles Ok bogs and backfires at throttle.

Stallacrew

NAXJA Forum User
Location
Cullman, Alabama
Ok, so I've looked over every bit of the Cruiser54 and lunghd.com website sensor problems and I still can't get my AW4 2WD 1989 4.0 Renix MJ Eliminator w/278k miles to run well. I used to be a paid naxja.org member but I've been without an XJ/MJ until recently. I'll re-up on my membership once this new job starts.

I bought it in this poor running condition and I've gotten it to at least idle decently well and crank up when I turn the key without having to fool with the map sensor vac line. Previous owner would crank it and it would then die shortly and then he'd pull the vac line and it'd crank and "lope" hard while it idled. Runs like dogcrap aside from idle.

Things I've checked:
-All the grounds are good.
-New plugs and wires from previous owner.
-Previous owner replaced fuel pump and filter recently so the pressure should be acceptable but I haven't been able to check it yet.
-Previous owner installed a new distributor (which I'm suspicious of)
-I've tried new Map Sensor and still runs the same.
-New coolant temp sensor changed nothing
-TPS was set to 10% and I adjusted it to the recommended 17%
-IAC is working well.
-CPS swapped out and works fine (I did read where one guy used washers to space it out to make it read under load, it was cutting out because the flex plate was bending out of the CPS plane)
-No vacuum leak I can find, previous owner replaced the major lines and tidied it all up. I am missing the vac bulb in the bumper but maybe I'm not supposed to have one?
-EGR is disconnected from the vacuum lines and they are plugged, I'm making an EGR delete plate to get rid of it (no emissions here)
-Battery reads right at 12.02v at the terminals(no cranked) but reads 13.1 on my MT2500 scanner.
-Input voltage to MAP AND TPS are 4.95v each.
-At idle, injector pulse is 6.9 to 7.9 mS, goes up to 27+ as it bogs down under partial and full throttle. It backfires through the exhaust at 50-100% throttle.
-Spark advance at idle is 14 degrees, goes up to 35+ degrees with throttle.
-MAP sensor voltage is 1.8v at idle on the scanner
-O2 sensor is at 4.98v and does not change according to the scanner, I'm sure it's fouled with such rich running recently.
-Idles at 800 rpm, won't go over 1500 and it shoots soot out the tailpipe and backfires and pops in the intake.
-I put a spark tester light on and it cuts out with partial/wot. Replaced the coil with a known good one.
Has tons of blow-by gasses coming through intake boot (I've been running it with everything disconnected and just an open throttle body because the gasses kill the engine if I move the boot back onto the intake)

Ive checked and replaced everything I know to do other than the O2 sensor which I feel is fouled and bad but shouldn't cause this (don't have the cash for another week).

I feel like it might be a tooth off on the distributor that the PO installed, but I haven't tried to check timing yet.

Any thing else to check? I'm out of ideas and I've read over this forum for bog down issues. All the critical sensors seem to be ok unless it's the CPS not reading under throttle because the flexplate is bowing out.


Someone knowledgeable please help, and thank you.
 
try disconnecting the o2 sensor. also did you check the knock sensor?

what does the m52500 show for fuel trims and temperature sensors.
 
Haven't tried anything with the O2 sensor yet. I think I read that it'll run rich with it unplugged since it reads no value of resistance. Even if it's fouled I'd think that a signal is better than no signal, and it shouldn't be causing this anyway from what I've read. Don't want to ruin a $70 part by running a new O2 sensor so rich.

I did read that there is no adverse running condition from unplugging the knock sensor (like mine is) if you aren't detonating. It affects timing and not fuel trims I think. My plug is busted busted but I'll fix it when I get this new job started soon.

Fuel trims are both at 128 and the charge and coolant temp were at 20 and 19 degrees Celsius (didn't change the settings to Fahrenheit).

I'm going to investigate being a tooth off on a potentially worn out distributor gear. If I'm a tooth off I might still be in the spark zone for idle but may not be able to catch a spark for advance spark timing (or maybe I'm reading it wrong and its retarded timing).
 
i have my knock sensor unplugged and its fine. i suppose if the o2 sensor does not switch it will stay in open loop. which is the same as if the o2 is unplugged.

i would check the dist pretty easy to do.

i would check into the vacuum line to the map sensor. sounds like the engine is getting bad data and trying to figure it out. once you have it running you can try unplugging the tps and see if it runs better.
 
What you describe is classic distributor indexing.
 
It worked! It was in fact out of time and I did the Cruiser54 distributor indexing and got it running out pretty well and I'll dial it in tomorrow. Still needs an O2 sensor and evidently a power steering pump, but we are in business. Thanks guys!
 
You can disconnect the O2 sensor and run the engine for diagnostics purposes with no problems. It is great diagnostic step. Same with the TPS. Just do not drive it with out a working connected O2 sensor for very long or it can cook the Cat converter and block the exhaust.

If it starts on Renix, the CPS is good.

Congrats on the index fix!!!!

Haven't tried anything with the O2 sensor yet. I think I read that it'll run rich with it unplugged since it reads no value of resistance. Even if it's fouled I'd think that a signal is better than no signal, and it shouldn't be causing this anyway from what I've read. Don't want to ruin a $70 part by running a new O2 sensor so rich.

I did read that there is no adverse running condition from unplugging the knock sensor (like mine is) if you aren't detonating. It affects timing and not fuel trims I think. My plug is busted busted but I'll fix it when I get this new job started soon.

Fuel trims are both at 128 and the charge and coolant temp were at 20 and 19 degrees Celsius (didn't change the settings to Fahrenheit).

I'm going to investigate being a tooth off on a potentially worn out distributor gear. If I'm a tooth off I might still be in the spark zone for idle but may not be able to catch a spark for advance spark timing (or maybe I'm reading it wrong and its retarded timing).
 
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It worked! It was in fact out of time and I did the Cruiser54 distributor indexing and got it running out pretty well and I'll dial it in tomorrow. Still needs an O2 sensor and evidently a power steering pump, but we are in business. Thanks guys!

Awesome results!! Nicely done (Old_Man too!!)
 
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