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Discussion Starter · #1 ·
91 yj, all stock, bypassed o2 sensor, bypassed mat sensor, changed tps, map, cap, rotor , plugs and wires and ran seafoam thru the carb. this is what started two weeks ago and still does after all ive done for her (as listed above) jeep runs great after its sat overnight. on hot days it will run good fro 20-30 mins then it starts to buck, backfire and lose power. wtf?

im decent mechanically but im ready to run the jeep over a cliff or kill my jeep...frustrated beyond measure :drool:

any help and ill ask my gf to give you a kiss. :kiss:
 

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carbed? check all your vaccum lines on the carb. had a problem similar to this on my 73 mustang recently. I thought it was vapor lock (fuel lines getting too heated causing the gas the evaporate in the lines before it even got to the carb), i got similar results like you did such as backfiring, bucking, popping, half stalling (engine stops for 1-2 seconds then roars back to life without restarting engine). it was as simple as a disconnected vaccuum line, reconnected it and voila, problem fixed. hope this helps
 

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Here, have a cold brew and lay off the coffee some. There's no way your YJ has a carb unless somebody put it there. The 1991's were all EFI far as I know. You really need to hook all the sensors up and clear the codes out of the computer, which is easy to do - just unhook the battery! And then get back to us, relax and grab another cold one, and let us know.

Jeeps actually run a lot better with all of the sensors and EFI hooked up. Compared to carbs.
 

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Discussion Starter · #7 ·
i put a new coil on last year, but, ill test it. why would coil or ecu be heat related? ive had this problem even when its running in open loop mode. is there a way to test the ecu?
 

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Strange things happen with heat, you could have a intake leak when it heats up, your fuel pressure could drop, your cat could cause your problem. You are just going to get down and trouble shoot it and stop throwing random parts in it's general direction.

Dwayne
 

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Discussion Starter · #9 ·
thanks dwayne. i appreciate your insight. ive tested for vac leaks while its running rough. none could be found. as stated earlier, fuel pressure is solid at 31. ive listed the parts ive changed so i could get better feedback. i didn't just start thowing random parts in as a guess. they either coded or tested bad and i still have the underlying problem. the reason i posted was to see what im missing in the trouble shooting.
 

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My apology for being rough, there are many that just throw money at a problem hoping it will go away.
What is the condition of your timing chain?
Does your compression stay the same?
Have you put a vacuum gauge on it to see what that is doing?
Does your distributor pick up read right?
Have you tested your CPS? removed, clean reinstall?
Have you gone through the wiring harness cleaning corrosion grease plugs and reinstall?

Dwayne
 

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Discussion Starter · #13 ·
thanks dwayne. im bypassing sensors to avoid spending the cash on them unless they need replacing. vacuum is solid at 17. when i race engine it drops to 0, i assume that is normal (according to my repair manual) ill check rest of those today and repost. how do i check the timing chain without pulling cover?

reason for frustration is im 48 and my gf is 20. tried taking her to the beach 4 times but this prob keeps stopping us...gotta get it fixed...lol
 

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What is your altitude, a healty engine at sea level will produce 21"hg loosing 1" for every 1000' gain. I am at 5500' and pull 17" hg on a modified engine.
Either you have low compression or your timing is off.
Your CPS or distributor pick-up are prime suspects.
I would go the the JY and grab another dizzy and CPS to rule that out, you can snag them from a Cherokee, they will be the same.

Good luck with the GF hope you get to the beach
 

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Discussion Starter · #15 ·
whole distributor? im 585' above sl. but the hg is good according to chiltons and other repair mans. ill check the compression, then the cps and repost..
the issue that keeps throwing me is this happens when its hot out. if i fire it up tonite it will run perfect. i just dont get what the heat has to do with this.
 

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Any connection will be put under stress when heat is applied, solder joints can be fine when cold and crap out when hot. Pick-ups like in the dizzy can break down with temp as can the cps.
I suggested a JY dizzy and cps to save valuable resources(money)

Dwayne
 

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Discussion Starter · #18 · (Edited)
pretty sure i got it figured out finally but any additional insight would be greatly appreciated (i'll be installing a new o2 sensor upstream, but, i dont get why it ran good at all, its never had one since i bought it): anyway, after compression test showed 106 in no.1 and 120's in all other cylinders, i sprayed brake brake-clean at the leading edge of the head gasket (not a place i check for vac leaks) and tthe motor stalled out. looks like time to replace the head gasket. :eek:

like i said tho, any additional help or things i should check before i proceed would be appreciated, thanks in advance!

suggestions like "while you're at it, you might as well...." are what im looking for.
 

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I would suggest that you get the head skimmed but that is all unless you wish to get it freshened while it is in the machine shop.
Congrats on locating that, I wonder why it didn't just let go all the way.
I noticed #1 was low but just put it down to old age.
Happy wrenching

Dwayne
 

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Discussion Starter · #20 ·
i sorta was too. had an errant shot of brake-clean hit right on the spot. ill take dumb luck, i never would checked there for a leak. will skimming it improve the performance at all? what will it do?
 
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