Overheat at Idle

[TheSloMoShow]

Hi all.

Yes, I picked probably the most widely used subject title for this post.
And yes, I know there is a wide variety of discussion already ongoing about system overheating.
But I think my case may be a bit more unique.

I've had my XJ for almost a year now, and for the most part no overheating problems. But this summer is reaching the 90's here in OH, and I'm recognizing some overheating issues now.

While driving (no stop-n-go), coolant stays 203-207 in the morning, and usually pegged at 210 in afternoon or early night.
But as soon as I stop, any time of day, automatically the coolant temp rises within just a minute or two.
If I've been driving for quite some time and many stop-n-go, it'll hit 225 at which point the e-fan kicks on but usually it just remains at 223-225. The e-fan doesn't help reduce the temps, just keeps it from rising ever more.

Within the past month or two, I've done the following to try and help aid the cooling system.

- Back flush (heater lines, block, radiator).
- Upgrade to ZJ fan clutch
- New T-stat (mopar) and sender unit (can't remember brand).
- 60/40 Water/glycol mix
- Added two new 10" electrical fans to front of oil cooler, behind grill. These run at all times ignition is on.
- Stant Radiator Cap (replaced about 6 months ago)

Today, while sitting in really heavy traffic for over an hour after work, I slowly watched the system rise to 237F....made me cringe.
So I pulled over as soon as I could, popped the hood to check out the condition of the system. Surprisingly, the upper radiator hose was hot but not stiff. Also, even after hitting 237 F, the overflow tank was not boiling or overflowing. As a matter of fact, it's been at the same fill mark for the past month with no movement.

So, my thinking is that either the T-stat is stuck closed or my water pump is done for....maybe the fins have eroded away. When I had the belt of last week, I played around with the pump and felt no restriction in movement so I believe the bearings to be OK, but maybe somehow still the fins have seen their end. I can't think of any other reason why it would only overheat at idle, and yet the overflow tank does not spill out any liquid. The only two system components which I haven't yet upgraded/replaced are the water pump and radiator. When I flushed the radiator out a month or so ago the discharge was crystal clear and no restriction in flow. So I'm nearly positive this isn't an issue with the radiator, but maybe is with water pump.

I was planning to buy new water pump, t-stat, lower radiator hose, and even radiator cap and replace all, but wanted your thoughts first.

2001 XJ, AW4, 170K

[Tim_MN]

At 170K, needing a water pump and/or radiator would be routine maintenance. Replacing one or both before you crack the head or blow the head gasket would be wise preventative maintenance.

[Patrick S.]

I agree 170K is a lot for a water pump. Maybe it was changed once? I would replace that for sure if its the original.

You could check temperature with an infrared thermometer. Maybe new aftermarket sensor is giving a high Reading?

Regardless the electric fan should lower temperature.

[TheSloMoShow]

Originally Posted by Tim_MN

At 170K, needing a water pump and/or radiator would be routine maintenance. Replacing one or both before you crack the head or blow the head gasket would be wise preventative maintenance.

You hit the nail on the head. This is my main worry. I've spent the last 9 months of owning this Jeep, fixing all the many things I could or had to in order to make it survive for another couple years (hopefully). The only thing I haven't yet seen fail is the 331 head, so I'm trying to get to the root cause of this overheating issue before it cracks. So far, I'm not losing coolant level and through the valve cover the oil is not milky. Exhaust also isn't smoking white. At next oil change I'm sending in an oil sample for test confirmation of any present coolant, but until then I think I'm OK.

Originally Posted by Patrick S.

I agree 170K is a lot for a water pump. Maybe it was changed once? I would replace that for sure if its the original.

You could check temperature with an infrared thermometer. Maybe new aftermarket sensor is giving a high Reading?

Regardless the electric fan should lower temperature.

I actually have an IR thermometer that I'll be using later today. I'll post up the results, at least this will narrow down the scope a tad. I also doubt it's the sender, but worth a quick check.

[TheSloMoShow]

Tested out the IR thermometer on the XJ today.
Unfortunately, the issue isn't my temp sensor - it's reading temps at the correct spot.
So back to drawing board it is.

I think I'm going to go with new radiator (steel molding), new water pump (Flowkooler), new thermo housing, new upper & lower radiator hoses and of course new gaskets.
If it still heats after I replace all these then I'm baffled.

Hopefully when I pull the water pump or radiator I find the smoking gun to my issues...

I'll post up later.

[Ecomike]

Check the overflow fitting on the radiator neck and the hose to the overflow bottle and the botttle very closely for vacuum leaks. The Cap can be leaking top side but holding pressure on the bottom side and not fill the bottle. The level in the bottle should be changing. Sounds like trapped air in the cooling system or radiator to me.

Check the O2 sensor data to see if the engine is running too lean and not throwing a code yet.

If those are OK, then go for a new radiator and pump.

[TheSloMoShow]

Originally Posted by Ecomike

Check the O2 sensor data to see if the engine is running too lean and not throwing a code yet.

I can check this tomorrow using the OBDII reader - what's your reasoning behind this though?

[TheSloMoShow]

Originally Posted by Ecomike

...The Cap can be leaking top side but holding pressure on the bottom side and not fill the bottle...

I guess I'm not really understanding this either.

Are you referring to the top and bottom sides of the over flow tank?

[Ecomike]

Originally Posted by TheSloMoShow

I can check this tomorrow using the OBDII reader - what's your reasoning behind this though?

To see if the engine is running lean and causing excess heat beyond design specs. It should lock into a tight range near 0.45 Volts at 2000 rpm, and vary wildly at idle from as much as .1 to .9

Low fuel pressure can make it run lean at idle for instance.

[Ecomike]

Originally Posted by TheSloMoShow

I guess I'm not really understanding this either.

Are you referring to the top and bottom sides of the over flow tank?

the radiator cap has two seals, one at the bottom that holds pressure in the radiator. There is a second seal at the top of neck to the Rad cap that seals the inner section between the 2 cap seals and the overflow hose and the bottle. It keeps air out of there so that coolant only flows from the radiator to over flow bottle in both directions as the Rad heats (fills the bottle) and cools (empties the bottle back into the radiator). If the vacuum is not sealed, if air gets in, the bottle level does not change and air gets into the radiator, thus the soft hose on the top rad hose and the unchanged level on the overflow bottle.

I had your problem once for 3-4 years, drove me nuts, turned out to be a tiny leak on the water pump gasket on the suction side that let air in while the engine cooled.

[TheSloMoShow]

I checked my O2 level while driving to the gym this morning. Unless the sensors aren't going bad, it seems that my engine is running a little rich after 2k rpms. It fluctuated a bit between .35 and .75, but for the majority of the time it was on the rich side .55-.75. It would make sense since my long term Fuel cuts are about -3%.

Sent from my SM-G900V using Tapatalk

[Ecomike]

Originally Posted by TheSloMoShow

I checked my O2 level while driving to the gym this morning. Unless the sensors aren't going bad, it seems that my engine is running a little rich after 2k rpms. It fluctuated a bit between .35 and .75, but for the majority of the time it was on the rich side .55-.75. It would make sense since my long term Fuel cuts are about -3%.

Sent from my SM-G900V using Tapatalk

Ok, so here is the deal, if the O2 sensor data the PCM is getting is defective and the PCM thinks the engine is running rich, the PCM will try to compensate and run the engine lean, which makes it run way too hot, producing excess heat.

Does the engine run smooth all the time? If Not, if it has a stumble issue near idle, the O2 sensor wires or heater in the sensor may be bad.

Is it in fact running in closed loop? Check the O2 sensor wiring with an ohm/volt meter, is the sensor data matching the PCM data????? Test the heater in the O2 sensor (should read about 8 ohms, with the wiring disconnected).

Look for any damaged wires on the sensor to harness run, and check the connector contacts to see if they are clean

Also look for exhaust manifold leaks near the O2 sensor, they can adversely affect the O2 data to the PCM.

The next possible issue is partly clogged injectors and injector wiring-contacts if I am right about running lean issue. Engines run cooler when running rich, but some cylinder could be running rich, other leans....

Check the spark plugs to see if any one or two look like they are running too lean or too rich.

[Ecomike]

FWIW, O2 sensors have a porous ceramic core that breaths and compares the O2 concentration in the exhaust to the outside O2 conc. Exhaust leaks, cracked manifolds can cause measurement errors and the computer acts on the data it gets, right or wrong.

[TheSloMoShow]

Thanks for the tips. I have to check all 4 out. I know that at least 2 of them were replaced last fall. But I can't remember if it's those that are pre or post cat.

I do have an ohm/voltmeter so I'll give that a shot just to be sure the PCM data matches.....if that's the issue I'm not sure how all of a sudden they went bad though :/

Sent from my SM-G900V using Tapatalk

[Ecomike]

Originally Posted by TheSloMoShow

Thanks for the tips. I have to check all 4 out. I know that at least 2 of them were replaced last fall. But I can't remember if it's those that are pre or post cat.

I do have an ohm/voltmeter so I'll give that a shot just to be sure the PCM data matches.....if that's the issue I'm not sure how all of a sudden they went bad though :/

Sent from my SM-G900V using Tapatalk

You only need to test the two closest to the engine. The other two after the cats have nothing to do with engine fuel control. They just confirm EPA limits are being met with a surrogate test of final O2 levels.

What brand O2 sensors did you use? 4 wire?

Analog ohm meters are best suited for the O2 sensor tests. Digitals are hard to read on O2 sensors unless they are fancy and graph the highs and lows, as the O2 output looks like a digital SIN wave. At idle the SIN wave swaps about once per second, side to side, round trip, at 2000 rpm stead the signal is almost DC at .45 Volts (+/- about .05 V) if everything is working right. If you use an analog meter it needs to be high empedance at least 20,000 ohms per volt or it affects the readings.