Engine missing at high RPM!!

90 XJ 4.0 5 Speed

I need a little help guys. A while back I was messing with my distributor and got it all out of whack. I tried starting it several times when I thought the distributor was indexed correctly, but it wasnt. I dont think I caused any harm to the motor. I finally figured out what I was doing wrong and got it started. I lined up the notch on the harmonic balancer with the site to use for timing and got the rotor to point to the #1 plug on the cap. It now starts and runs, but not smoothly. It idles a little rough, and when its cold it misses really badly until it warms up. Is it possible that I am still off with the distributor, or maybe I dirtied up the injectors and spark plugs. I have just put in some injector cleaner with a full tank of gas and it seems to be getting a little better, but the idle is still rough.

I have also cleaned the throttle body, new TB gasket, new TPS, new ISA (I think that's what its called, next to the TPS on throttle body), air tube w/cone filter. When I cleaned the TB I also adjusted the screw on the TB for the idle. When I cleaned the TB the butterfly wouldnt close all the way, so I adjusted the screw to get it to where it would idle lower. Before that adjustment it idled really high, not sure how high since I dont have a tach, but high enough too cause it to overheat sitting at a light.

I appreciate any and all suggestions.

Thanks guys. RULES!!!!

90red

This happened to me once after I put a new 2.5L in an 88. I found the problem to be the on the flex plate. Dont ask me how or why, but the sensor and or magnet on the bell housing and flex plate wasn't working properly any more. The Jeep would start and run fine, but an higher RPM, any thing higher than idle, it would drop the #1 cy. I found this out by accident when I hooked the timimg light to the wrong plug wire.

Try putting the timing light on the #1 wire, rev it up and see if it fires all the time. If it dosn't, put the light on the #2 wire and see if it fires constantly.

If you need more details PM me.

If it isnt firing every time, how do I fix it?

90red

There is a procedure to "index" the distributor.

First, get the #1 piston to top dead center (TDC) on the compression stroke. (Both valves will be closed, and you'll have compression at #1).

For this you need a sacrificial distributor cap. You're going to need a Dremel or something similar, to cut away the sides of the cap to expose the #1 contact inside the cap. Leave the top, so the turret remains in position, but cut away enough on the side to allow access to the contact tip.

Install that cap and look at the position of the rotor tip relative to the #1 contact in the cap. Remember the distributor rotates clockwise. The trailing edhe of the rotor tip should be BEYOND the contact, with an air gap of 0.020"

Thanks Eagle, I will try that.

90red

Liek Eagle posted... with a little clarification based on what was in the original post:

90red said:

90 XJ 4.0 5 Speed
... I lined up the notch on the harmonic balancer with the site to use for timing ...
90red

Click to expand...

Just so we're all on the same page. Use the Zero degree mark on the timing "comb" --

The reason I mention it is that the reference to "the site to use for timing" is a bit vague and I recently had to rescue a fellow 4.0 XJer from a shop that had lined-up the crank-pully's line with the sensor (round tube) sight on the timing comb - this was indexing hhis dist to like 35*BTDC... After asking the shop-boss to let the tech go home for the day (after hours anyway) I reached in and rotated the engine to TDC by lining-up the crank-pulley's mark with the Zero-degree label printed on the other end of the timing comb, then stabbed the distributor... Everyone was able to drive home!

kinda FYI that round tube is for a sensor that some diagnostic tools have on em - not too different from a magnetic trigger, but only set for DTC indication (and off-set from the crank-pulley's mark)

Well that makes sense. The round tube looking thing is what I used to line up the distributor. I did not see a timing comb to line up the notch. Is it anywhere near that tube, or is it located somewhere else? Is it possible I dont have one? If so, can I put one on and where do I install it.

Thanks for all your help guys. I really appreciate it.

90red