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4.0 lifters - flexplate issue?


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So I've been having an issue with my 90mj (95 xj 4.0/renix intake and sensors) this 4.0 happens to have close to 500k mi on it, it got all new gaskets, oil pump and timing gears/chain about 5yrs ago when I swapped it into the mj. 

 

Anyways, a few days back now it backfired and started to run terribly. After sorting through cruisers tips everything checked out so I moved on to pulling the VC and checking the pushrods for straightness and valves for movement, everything checked out fine. 

 

At this point I was betting my lifters were messed up. As a last ditch effort before pulling the head I did an oil change and used a mixture of 10w30 and ATF on the advise of my old man to help clean out the engine internally. This seems to have worked, it didn't at first but after revving the engine up pretty high it now runs perfect even at idle. 

 

Any thoughts on how I can prevent this from happening again/keep my lifters from sticking/clean out the engine better without a rebuild? 

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11 minutes ago, Dando said:

So I've been having an issue with my 90mj (95 xj 4.0/renix intake and sensors) this 4.0 happens to have close to 500k mi on it, it got all new gaskets, oil pump and timing gears/chain about 5yrs ago when I swapped it into the mj. 

 

Anyways, a few days back now it backfired and started to run terribly. After sorting through cruisers tips everything checked out so I moved on to pulling the VC and checking the pushrods for straightness and valves for movement, everything checked out fine. 

 

At this point I was betting my lifters were messed up. As a last ditch effort before pulling the head I did an oil change and used a mixture of 10w30 and ATF on the advise of my old man to help clean out the engine internally. This seems to have worked, it didn't at first but after revving the engine up pretty high it now runs perfect even at idle. 

 

Any thoughts on how I can prevent this from happening again/keep my lifters from sticking/clean out the engine better without a rebuild? 

Use this method:

 

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Thanks I should have said before trying the oil/ATF trick, I split a can of seafoam through the fuel tank and the cranckcase, and another can down the Intake. It didn't seem to help at all. Thats why I tried the ATF mix. 

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1 hour ago, DirtyComanche said:

Use better oil.

 

For about 10yrs now I've been running this engine on valvoline maxlife, if that's not considered a "better" oil, and I know this is opening a can of worms, but what would you suggest otherwise? 

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6 hours ago, Dando said:

 

For about 10yrs now I've been running this engine on valvoline maxlife, if that's not considered a "better" oil, and I know this is opening a can of worms, but what would you suggest otherwise? 

 

Which one, the synthetic or the synthetic blend?

 

What filters are you using?

 

Change interval?

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What was the cost per quart? I don't see the oil you are suing as creating any problem, nor your interval to change it. I trust NAPA or Wix filters. I do run synthetics, still nothing wrong with a good conventional oil. I use Shell Rotella 10W30 in my XJ for a few years. This year I changed to the T6. If you change to a full synthetic, then double your change interval. If you are leaking oil, then I'd stick with conventional oils.

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So large change in the problem yesterday. The truck was running fine, I go to merge onto the highway and the engine seemed to cut out for a second then started running severly underpowered, barely able to get it off the road. 

 

After getting it towed home I tried to start it again and it started and ran like total crap, barely would idle etc. I walked away out of frustration and came back to it a few hours later. 

 

When i I cranked the engine the starter spins but the motor didn't. After looking through a bunch of things I've determined the flexplate is broken (seems like the center is broken and no longer connected to the outer ring and thus not allowing the starter to move the crank) I can spin it by hand with no resistance. Turning the crank pulley with a wrench and it seems the engine itself still has compression etc. 

 

Well obviously i wilI need to source a replacement for a renix motor, which I've read can be a crapshoot but that aside.

 

Could a cracked and broken flexplate have caused my original issues? Throwing the timing off? 

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Yes it is possible 

If the flex plate was rotating off kilter ....the cps might have been getting sporadic readings ...that mighty have been the hesitation you had .....but unfortunately.... You will not know until you get another mounted up......I am surprised it came apart without a lot of noise previous to that....maybe that was not a lifter "ticking" ...

 

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31 minutes ago, yxmj said:

Yes it is possible 

If the flex plate was rotating off kilter ....the cps might have been getting sporadic readings ...that mighty have been the hesitation you had .....but unfortunately.... You will not know until you get another mounted up......I am surprised it came apart without a lot of noise previous to that....maybe that was not a lifter "ticking" ...

 

Absolutely. 

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So I'm mainly asking if the flexplate could have caused my original issues because either way I have to replace the flexplate. Just trying to decide if I should replace the motor as well. I'd rather not replace it as from what I can tell with the valve cover removed the valves etc seem to be working properly. Just don't want to pull the transmission just to fix the flexplate and then find out my engine is junk now too. 

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I'll second the absolutely the flex plate could've caused your problems. 

The crank position sensor reads off the windows around the outer edge of the flex plate. Even if the CPS is working properly, if the thing it's reading off isn't properly attached to the crankshaft the signal it puts out will be garbage, and your ignition timing will be all over the map. 

 

I I also had similar symptoms when my flex plate cracked. It would be fine for days or weeks at a time, but I could hear the flex plate rattling against itself the whole time (I saw it was cracked through the inspection access but it was quite a few months before I had everything together and the time to fix it) and I'm pretty sure the noise actually got better after it had fully separated... But my crack was pretty jagged and it still drove after that point, by magic. And every now and again it would start giving me issues, I'm guessing the flex plate was just slightly out of line or something. She just wouldn't want to start some days, or I'd be driving around and it would start running like crap and eventually die and refuse to restart, then mysteriously be perfectly fine again after it sat for a few hours. 

Fixing the flex plate solved most of my problems. 

A piece of advice on getting the trans out, pick up some replacement quick connectors for the cooler lines, or at least the plastic clips for them. I've yet to take out an aw4 without destroying those clips and/or losing the O-rings or just not getting them to seat properly. I'm at work now but once I get home I can try to find parts numbers for that... None of the parts stores I went to or even the Jeep dealer were particularly helpful there (despite discovering that the parts stores all had everything in stock after I figured out what I needed)

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