Cooling System Help

I'm trying to solve an overheating issue on a stock daily driver XJ and could use some help. It's a 99 with a 4.0L and an AW4.

As long as the jeep is moving it runs just under the 210 mark. In traffic or after a bit of idling, the temperature will come up to ~220 (which isn't concerning) and then jump very rapidly to ~250. I'm told that running the heater or avoiding traffic keeps it cool. The one time I've personally seen this happen, the gauge was at 220, I looked up to make sure the the electric fan was coming on, and when I looked back down the temp was high and I had a 'check gauges' light. I shut the jeep down and turned the ignition to 'run' to watch the gauge. After about a minute the gauge jumped back down to 220. The gauge moves so fast I would believe it's an electrical issue, except that the coolant boils.

I don't usually like to throw parts at problems, but that's what I'm doing. So far I've replaced
- upper and lower radiator hoses
- radiator
- radiator cap
- thermostat
- water pump
- fan clutch (not new, taken off my XJ a while back, functioned adequately at the time)

And of course I've flushed the system twice. Coolant looks fine.

I think tomorrow I am going to buy or rent a pressure tester and see if the system can hold pressure. I could really use some more suggestions or ideas. Thanks.

The late 97+ models DO NOT have a sending unit to the gauge.

The temp sending unit and CTS were combined, and the info the gauge presents is coming from the PCM.

1. Pull the CTS, clean the body, and reinstall.

2. Did this problem present BEFORE or after the thermostat change? What brand thermostat?

3. When the engine is HOT, if you shut it down and move the clutch fan by hand, how stiff is the resistance?

1. I picked up a new sensor. Hopefully I'll get that in tonight.

2. Problem existed before thermostat change. The one I put in is a Stant Superstat from NAPA.

3. I didn't have the chance to try this immediately after shutdown. An estimated three minutes after shutdown the clutch felt pretty loose. I restarted the Jeep and idled it for a minutes or so, then felt the clutch again. It was still pretty loose.

This clutch was previously on my jeep, and I didn't have overheating problems, but I replaced it because my electric fan was having to kick on more than I liked.

I know the symptoms are pretty typical for a bad fan clutch, but like I said, my jeep never overheated with this clutch. Anyway, I'm getting another clutch from a co-worker, and they're so easy to change I'm going to throw that in.

What kind of outside air temps were you having that day in MI?
100*+ days are tough.

A rear hood lift really helps allow hot under hood air to escape in stop and go driving. Simple experiment is get a box of 100 1/4" washers and 4-1" longer than stock IIRC 10mm bolts and raise the rear of the hood 1-1.2" with about #20-22 washers as spacers on each bolt. $10 at Lowes.

CobraMarty: The issue first popped up during a couple unusually warm days, not 100+, but hot with high humidity. Since then it's also shown up during cooler morning commutes though.

I haven't gotten the fan clutch or coolant temp sensor in yet, but I've done a little testing think and learned a few things:

1. I spun the mechanical fan immediately after shutdown. Then I warmed my jeep up and did the same thing. Mine spins with more resistance.

2. I used a tool to monitor temp on the PCM. It climbs and cools gradually like it should. The 'spikes' and 'jumps' I saw were just the non-linearity of the gauge (or maybe a sticky gauge).

3. The electric fan is not working properly. Sometimes it runs at full speed, sometimes it barely turns, and sometimes it doesn't run at all. I saw all this while ECT was up around 140. I appear to be getting a constant 13.8V to the fan at elevated temps. So I'm concluding there's a problem with the fan motor. I think I never noticed this as an issue because the temp gauge tends to 'hang' just over 210. The fan turning on and off in that temp range didn't grab my attention.

So neither fan appears to be working at full capacity. This fits with the symptoms which indicate an airflow problem. I'm going to throw in the fan clutch from my co-worker and grab an electric fan from the junk yard or classifieds. I'm hoping that will solve the issue.

Changing the fan clutch and different JY electric fan sounds like a great start. Let us know how it turns out.

Thanks to both of you for the help. Apparently the gauge sits just under 210 now, even in traffic. I sat in it and let it idle for a while yesterday and the temp stayed in the 201-208 neighborhood. So at this point everything looks good. Thanks again.

Upgrade Alert!!

ZJ HD fan clutch p/n 272310 at NAPA

NOT MAX COOL ... Insist on the p/n ... Did wonders on my 90 XJ.

I appreciate the suggestion, but I didn't go that route for a couple reasons. First, when the factory cooling system is working properly, it's perfectly adequate for a stock daily driver. Second, this jeep is a cummuter, so i didn't want to make unnecessary compromises on fuel economy or fan noise. Lastly, the reduced clearance to the radiator doesn't seem like a smart gamble for a jeep with stock style engine mounts.

The zj clutch might make more sense in a built jeep with brown dog mounts.