Bucking erratic rpms, moving or at idle 1991 xj

[my2monkeys]

Re: Bucking erratic rpms, moving or at idle 1991 jx

Try unhooking the o2 sensor and drive it. This will be in open loop mode, might disclose an o2 heater circuit problem.

Hook up the FP pressure gauge as to be able to view it thru the windsheild when driving so you can see pressures when it's bucking and surging.

Test the resistance on the IAT sensor and the ECT sensor. They should be app. 12,000 ohms at 70*f and less than 1000 ohms on a fully warm engine.

Also make sure you"ve done Cruisers Ground Refreshing. DO THIS FIRST

 

[Devilfrog]

I'm on the check fuel pressure wagon ... sounds like the pump is on it's way out to me.

 

[HUSKYnXJnWI]

Thanks guys:

checked fuel pressure regulator- 31psi at smooth idle, disconnected FPR=40 (stock specs/ checked out fine) when idle is Erratic and jumping- the fuel pressure does raise to 36 and down to 28 but that seems related to the idle going up and down- appropriately- like you are reving it up and down: the fuel pressure goes up and down with rpms- I assume that's normal. It stay's within normal range 28-34 when reving with accelerator and when its looping from 200-1300rpms when the problem shows itself.
Thanks

I was also told to test FPressure with the TPS vac line disconnected while it was acting up: it was at 40psi constant.
So Fuel pressure appears normal for a 1991 based on what I have read and understand.

The O2 heater circuit- I had some dificulty understanding testing that, (which wires) and there was talk about a fuse and relay. then I found this thread below which cleared up alot. ASD relay controls the o2 heater, along with a fuse for this year (no specific "o2 relay"). I haven't tested this yet now that I understand...http://www.naxja.org/forum/showthread.php?t=1094877

I will check out the ohms on the IAC and ECT(coolant temp sensor) had tested the old one and I thought it was off- swapped in another one.

I will check all engine grounds- I don't think I ever really put much emphasis on that since I never knew to have a problem- but I get it now- all the grounds are vital and it could be oil/dirt.

***All that said- I have to now consider whether to continue with this XJ. I have kept up the steering, drivetrain, and suspension, basic motor maintenance. But the body has slowly degraded- I haven't had floorboards for years- fixed them once- and they slowly rusted away. Being that I have been under my jeep alot- I noticed the frame rails are now getting bad. The Drivers side frame rail- around the leaf spring eye is gone. I FEAR that if I now jacked the rear end and hit the leaf with a 5 pound hammer that it would drop from the frame. I can weld and could fix that- but I have to assess whether I should continue investing in this body- I think I should start over even though I am attached to this Jeep. I may benefit from finding a 1999 with a good body- and swapping some of my parts to it as I have time and as needed. Any advice here would be appreciated as well. I am sure many of you have reached this point. Wisconsin winters are not kind to steel.***

Thanks for any and all feedback- still kinda want to solve the puzzle- but its probably smart to assess the long run.