1990 XJ 4.0 Manual (AX-15) with 280k

First thing I am have been driving stick for multiple years, so I am quite confident that its not that my driving is causing it to stall. (While I might not drive like a expert but im sure that it isn't the problem.)

When driving my Jeep home from work one day I was slowing down for an intersection and then all the sudden my RPMs dropped and it stalled when downshifting. I'm not 100% sure if it will stall when i'm in gear an slowing dow, but I know it will for sure happen when I coast in neutral.

Also I think when I'm cruising in 4th (not specifically 4th) and not accelerating the engine seem to jump and want to stall (almost like I tired to go from 2nd to 5th) and only when I give it more gas will it seem to go away.Also I think when I'm cruising in 4th (not specifically 4th) and not accelerating the engine seem to jump and want to stall (almost like I tired to go from 2nd to 5th) and only when I give it more gas will it seem to go away.

So when this all happened I looked things up and thought it was the Idle Air control valve. So I got throttle body cleaner and cleaned the IAC, but that didn't seem to work and the problem persisted. So I don't know what it could be, is my IAC just bad, something with fuel or spark or my transmission, I have no idea.

 

Get a shop manual - testing procedures and a list of codes are contained in a manual. A simple VOM can test the sensors BEFORE you buy them. This is my drill, because I'm bad at guessing; open the shop manual, take the VOM out and a cold one. You'll need jumpers to test a MAP. Assuming your air filter is clean, spark plugs, and wires are good, check for dangling wires grounding. The wires coming up from the O2 to the fuel rail wiring harness are a good place to start. Craw under your jeep and survey the wiring and the evap line. Then test your sensors: the temp sensor, idle speed stepper motor, O2, MAP, MAT, CPS and TPS. In my experience, the crank position sensor is good - or you stranded by the side of the road - but test it anyway. If they are good and your throttle body is clean, move to the injectors: clean them and test the resistance for each one. You can pull them and use a 9v battery to produce a click unless it's defective.

 

Start with the easy stuff first...Check the condition of the spark plugs...High mileage engines will throw a bunch of deposits on them in a shorter duration than a fresh engine... Next would be the distributor cap and rotor and wire condition...Take a peek at the air cleaner element....If all that checks out you're going deeper as stated...

 

Uploaded a few photos of spark plug and distributor. I'm guessing it would be best to replace the plugs, I think I did them maybe 5 years ago. Looking at them I'm not sure if they my engine is running rich, If i have oil blowby, if they are just old or anything else so maybe some help with that if anyone knows. Also I looked it up and these are copper core plugs. If I need to replace them should I get better ones like platinum or iridium or does copper have its benefits in older engines?

As far as the distributor goes the cap and wires seem fine as far as I can tell but the guts of it looks quite rusty and stuff. I don't know if that could be the source of the problem or not. If so is there a way to easily clean them or would i be best in just buying a new one? I appreciate the help!

 

Your right, the plugs should be tan - not black. A defective MAP and/or a MAT sensor can cause it to run rich.I'd test them. However, the electrodes look pretty good.Better plugs would last longer - but only if it's not running rich. How many miles did you dive your XJ in 5 years? The rotor and cap looks like replacement is due. When did you last perform a compression test on the cylinders? Low compression could also darken the plugs. Usually miss-firing plugs can be felt under acceleration which isn't the problem you cited in your post. Often, multiple issues come up - just be patient

 

Bought my Xj as a fun car so in 5 years I'd guess less than 30,000 but I have no clue.
As far as the MAP/MAT sensor I have no clue how to test them so if you have resources I be grateful.
Also you say the distributor cap looks bad, is that just the cap that looks bad or does the rotor/whole thing look like it needs to be replaced?
I've also never done a compression test, but I do have some oil in the air filter so I know I've got some blowby. Also I don't know if its related or helpful, but the Oil pressure seems to be up in the 70-80s when under load but when it warms up and not under load it dips 30-40s.

 

Get a shop manual - test prosecutors are spelled out for all of your sensors - this is the best investment you can make if you intend to do the work on it yourself. You will need a $20 VOM, test leads, jumper leads, and a vacuum pump.This is the manual I have:
A compression test is the best way to determine the health of your piston rings, compression should be between 120-150lbs.How to do a compression test is outlined is Chapter 2 part D of the manual. If the compression numbers are inconsistent, or low you'll need to rebuild your engine sometime in the near future. The fluctuating oil pressure could be worn main bearings. Rebuilt engines are available at reasonable prices for the 4.0.

YES, REPLACE BOTH ROTOR AND CAP - 30k is the limit for a set of plugs too

 

Go to Crusier54's website. https://cruiser54.com/?page_id=75 and work through 1,3-5, 8, and 11.

You can find some addition material here: https://web.archive.org/web/20110706....com/tech.html