I have searched this issue and found similar problems, but have not seen anything exactly like it. Apologies if this has been brought up in the past, as I’m new here.
I recently upgraded my headlight wiring harness which fixed an intermittent failure issue I had. Great!
Or so I thought. Lights work great now, but right after the installation…hard start and check engine light. I had to pump the gas pretty hard to get it going. I let it run for a few, drove around a bit, started it again several times and same problem. In addition, the temp gauge now doesn’t seem to be moving much above 100.
The battery is like a week old, brand new. Although i did have a hard start after replacing it (the old battery must have been really dead). But after letting it run for a little while, it was starting right up just fine, consistently. Computer had just forgotten how to Jeep…

This XJ does have a few little mods on it but I don’t think any are related to this problem. The only thing that’s (maybe?) related is that my electric fan has been wired directly to a switch for a year or so now. Pretty common as far as I know. Electrical issues are super annoying for me as I am embarrassingly bad at diagnosing and fixing them.

I haven’t gotten the code(s) it’s throwing yet, which is obviously the next priority. My scanner is broken and I’m waiting on a friend to come through with his.

I don’t want to just start replacing parts if it’s just a ground issue or a broken wire. My guess would be something like that. Perhaps something got knocked loose in the process of installing the headlight harness? I see in other posts where the sensor in question is, but not exactly sure where specifically to look for grounds or other connection points that could be the culprit.

Has anyone ever had a similar issue? It’s a strange combination of things happening at the same time, as these XJs seem to be famous for.
If anyone more knowledgeable than myself has any idea where to start, I’d be very grateful!

 

Well, does it start right up when the engine is hot? Did you try cycling the key from OFF to RUN (but not cranking it) a couple times to build up fuel pressure? Try pulling out the fuel pump relay and re-inserting it. If there is any obvious corrosion around your ground wires (at battery terminal or the short ground wire that is bolted to the fender next to the battery) then that might be your culprit. You could have a failing starter and/or fuel pump and/or fuel check valve (at the fuel pump). I would seriously consider "renting" a fuel pressure gauge kit at your local big-box auto parts store. Takes seconds to unscrew the schrader valve on the fuel rail. I would be curious to what the fuel pressure is when your truck sits for a while.
Your issue could be a number of things and I wouldn't just tie the issue to your recent headlight work. Many coincidences happen when dealing with these vintage rides.

 

User Unknown- yes I agree that nothing about what was done with the headlights seems like it could cause these symptoms. Just strange that it happened directly after. And i do mean immediately.
Hot start is the same as cold. I did not try cycling the key as you suggest, i will give that a go tomorrow. Thank you for all the ideas, some of them i hadn’t considered.
But do any of these possibilities tie into the temp gauge staying suspiciously low? That’s what is confusing me most. I do have a check engine light, but no check gauges light. Granted, it’s pretty cold here in CT but it still should have been reading hotter.
Again, thanks for taking time and giving me some things to look into. Cheers!

 

Cold usually adds fuel so it should start easier. Disconnect the temp sensor in t-stat and see if it improves...you will throw a code

 

Update: cold start after it sat for a day, started right up normally.

I realize now I misspoke in my original post—all those starts were technically warm, if we agree that a cold start is 24+ hours not running. Today will be the true cold start test. I’ll see if the start is better or not, and I will try your sensor idea. I know codes always get thrown when sensors get pulled, but I assume the moral of the story is that if start-up improves with no sensor, that could be where at least part of the issue lies.

 

Thats a bit extreme, in my book...cold start is overnight in cool or cold temps...Miami or LA in August doesn't count as cold

 

I suppose my school of thought on that comes from buying used vehicles. A much more experienced professional than myself always advised me to request a cold start, and I suppose overnight is plenty of time. I’d never thought to take that into consideration when buying before then.
I’m a **** mechanic and this is my first XJ, but I’ve been loving it and learning as much as I can.

Anyway, the point is the Jeep sat overnight and started beautifully. Still shining a check engine light in my face, but at least that narrows the possibilities down a bit. Any other advice is welcome and much appreciated. 🙏