'89 4.0 Missing Under Load

Ok, so as the title says, '89 XJ with a 4.0 and an AW4. At idle it runs ok (idle fluctuates some, but no missing), rev it up in neutral and it seems fine. Great throttle response, no discernible missing.

However, put it in gear and give it some gas (while on the brakes, or go drive it) and soon as there is load on the engine, it starts missing. This started as a now and then thing, and now its to the point that its really pretty bad.

I have done some searches on here to try and find a common cause, but I havent found any post with this exact issue.

I should note that I gave it a tune up a year ago (Cap, Rotor, Wires, Plugs, coil was done about 2 years ago by PO).

Here is what I have found:

1: I check the TPS (it was replaced 2 years ago by PO), idle voltage is at 0.288, however WOT is only at 3.9V. The source voltage is 5.0 with the connector unplugged. With it hooked up, the Voltage drops to 4.9V (acceptable?) I did a check of the grounds, they all look clean (California, zero rust anywhere) and OHM out ok.

2: I checked the EGR, and the line going from the solenoid to the EGR is cracked, so the EGR never opens. I will fix this tomorrow, but I highly doubt its my issue. I check the source vacuum going to the solinoid as well, and its right at 15 Hg.

So, I am kind of stuck, I am not an expert on these Renix engines. So I don't know all their quirks or common issues.

Any help would be greatly appreciated

EDIT: Forgot to mention the engine has 182K miles on it.

IF your reference voltage--four-wire connector disconnected from TPS, key ON, "A" to "D", is 5.0 volts (too perfect), then the output voltage--four-wire connector connected to TPS, key ON, back probing "B" to "D", output voltage should be 83 percent of reference voltage, or 5.0 x .83 = 4.15 volts. Adjust the TPS to achieve this, if you can't, replace the TPS and then adjust the new one.

This sounds more like a "spark going to ground under load" issue or fuel issue.

Put a fuel pressure gauge on the Schrader valve on the fuel rail--31 psi engine running with vacuum applied to the fuel pressure regulator; 39 psi with vacuum disconnected from the FPR.

Check your plug gaps, 0.035, examine the plug bodies for carbon tracks. Check the condition of the cap contacts and rotor, examine them both for carbon tracks--if the cap has aluminum contacts throw it in the trash and purchase a new one with brass contacts. Engine running use a kitchen spray bottle and "mist" the spark plug wires and watch for any spark running to ground.

Post up what you find.

Ok, I do believe it is fixed! Will wait and see if my wife reports anything on her way to work tomorrow. But my test drive resulted in zero misses.

So, here is what I did. The spark to ground issue made sense, that or weak spark over all resulting in a drowned spark when more throttle was applied.

So I replaced all the plugs and wires. The plugs actually looked fine, and the gaps on all of them were fine. One showed a bit more wear than others. However, when inspecting the plug wires, one of them was severely frayed on the plug side. The outer carbon was almost entirely missing, and only the inner strands were visible. I believe this wire may have been the culprit.

I also fixed the vacuum leading to the EGR, so it is functioning again.

So thanks Joe for the idea. It turned out (I think) to be the issue

Hope you got it!

Post up when you are sure (or not).

Good luck.