OK, folks. Here's a strange one. On my 2005 XJ8 (normal aspiration & 51,000 miles) when you are cruising at less than 60 mph, the temperature gauge is right square on half-way up. At over 60, it reads about one notch down. After about 15 minutes over 60, I get the Restricted Performance annunciation and the temp climbs to the half-way point. Then, after about another minute, the temp climbs up rapidly until it pegs out at hot and I get the red light.

We are on a short road trip, and it happened for the first time yesterday. I pulled off and popped the bonnet and there was no evidence of overheating. Things looked and felt normal. I got back in, started up and it was like it never happened: everything normal. We hit the road, and in about 15 minutes it happened again. This time I just shifted into neutral, turned off the ignition, coasted for about 20 seconds, fired her back up and all was fine. 15 minutes later, same drill. And again and again until we got into some slow, urban traffic. I didn't get over 60 for about an hour and did not get a recurrence. Back on the open road, every 15-17 minutes, Restricted Perf. with Overheat. It's manageable, but a pain in the neck.

I read one thread where a messed up air inlet duct caused restricted performance, so I need to mention that the night before we left, I checked the air filter, which means disconnecting and twisting the induction inlet duct around a bit. The filter needs replacing, but it wasn't the same size as the XK8 filter I had on hand, so I just put it back together. I don't have my scanner with me, so I don't have any codes to offer at this point.

So, there you go, my brilliant Jag-savvy buddies. What do y'all think??

Thanks in advance,

 

Either the gauge is telling lies or the engine really is overheating, but as you can easily repeat the fault, it should be easy to solve. My thought is maybe a faulty coolant temperature transmitter the only way you can tell is by testing it. Other thing is it really is getting too hot, and the thermostat is faulty. Or maybe even a blocked radiator

 

Ok, so I'm just getting my feet wet with these things, but this sounds very much like what I just went through with the 2006 4.2 XJ8L. You mentioned that you have a scanner. Does it only read codes, or does it read LIVE DATA also? The dash gauge on "mine" was doing almost the identical thing that yours is doing, but the actual coolant temp sender for the ECU was reading 240+ deg F. This can only be seen by accessing the ECU's temp sender. I would have never known it on this one except the dash gauge wasn't making sense so I hooked up the scanner and checked.

I happened on the answer to my problem earlier this week. I posted some pic's under my post 2006 XJ8L 4.2L Running hot... or is it? That will show you what my problem was.

Like I said, I'm by no means an expert on these animals, but it does sound like the same growl! I'll follow yours along. Please post what you eventually find. I do have LOTS more pictures, and if you find the same thing, I can maybe offer some quicker steps to replacement.

GOOD LUCK!

 

Hey Jeff - Thanks for weighing in on this. Yesterday I read your thread with great interest and could certainly see the similarities. You had a really odd deal there and I commend you on solving it. I remember reading about plastic thermostat housing problems on early XK8s over on the XK8 & XKR forum, but that was fixed in the later years. Did they not address this on X-350s??

My scanner does have the ability to look at the parameters in real time, so I sure do wish I had brought it along. I will certainly do that when I get home next week.

Your situation did make me think about the possibility of the thermostat screwing up, but I am pretty much convinced that I am not actually overheating, because turning the ignition off for 20 seconds fixes it. There's no way an engine could cool off that much in just 20 seconds.

I've driven the car about 100 miles in the last 2 days with no problems at all, but I haven't been over 60 mph for any appreciable time. We'll be back on the freeway on Monday, so we'll see what happens then. I'll keep y'all posted, and any additional theories are appreciated.

 

Hi Larry,
I hate to say "exactly." I hate that on bulletin boards! However... with my scanner reading ECT, my car was at 242 deg. I'd turn it off, turn the ignition back on (but not start the engine) and watch the ECT reading drop off to 215 in under 10 seconds. I too was questioning if I even had a mechanical failure, or if it was electrical. The dash gauge would only start to fluctuate after about 25 minutes on the freeway at 70+, but once it started to go up, it would shoot up! Same thing though, wait a minute and turn it back on, and she'd be normal again.

My thermostat wasn't actually the problem. What was happening was hot coolant from the engine was being circulated directly back into the engine, and cold (well, cooled) water from the radiator was rarely being introduced into the engine. Best I can figure is when I'd shut down the car, thermal convection, or maybe just the pressure difference between 240 deg coolant and 150 deg coolant, would take over and move just enough coolant so that either the dash gauge's sender unit, or the ECU's sender was no longer being hit with 240 deg water. I'm not an engineer, just trying to rationalize the why behind someone else's engineering!

As to whether mine is the same as yours, I can't say. I did find an aluminum housing listed for a 2006 on a website, but it listed with a conditional "up to vin number...." This engine was newer, so I was stuck with another OEM plastic one. I know the dealer tells me that mid 2006 there was a design change, not just on this part either. I will say that this setup is the strangest thermostat manifold I've ever seen.

I'd love to hear what your scanner shows in comparison to your dash gauge.
Oldmots said that sending unit failure is common, so if your ECU thinks the coolant is fine, then the gauge sender would be my suspect.

 

OK, chew on this a while. By the time we left to head home, the "Check Engine" light had gone away. I guess we had enough starts with no falults so it cleared. I drove the 300 miles home (Oregon Coast to Bellevue, Wa) with absolutely no glitches. I spent most of the time over 60mph and had a 1 hour stretch with the cruise control locked on 72mph. Nothing. Nada. The car couldn't have run better.

The only thing I did to it while down in Oregon was to feel around the air induction inlet duct. In the process I wiggled the MAF sensor conector just to be sure it was tight.

This is a total mystery. I haven't hooked up Mr. OBDII yet to see how the readings look, but I'm hoping the previous codes are still in there even though the light is out. Might give some clue.

Cheers,

 

Hey Larry,

My guess is the thermostat is the trouble. There is a possibility the outlet pipe [what some folks call the crossover pipe] has suffered an internal fracture or breakage. It will be interesting to see what if any faults you logged during your trip. That may tell us something, or confirm a suspicion. Let's see if a P0128, P0116, P0117 or P0118 shows up. A P0118 for instance would confirm a true engine overheat, for instance. So let's see what's there and then assess.

That thermostat is an odd jigsaw puzzle affair that has been known to be somewhat flaky at times, so I'd prepare to get myself one to replace the current one.

Standing by..........

 

Thanks, Steve. Your input seems to lead down the same path that Jeff has been illuminating for me. I'll pull the codes as soon as I get done with my Income Taxes.
Should I plan on replacing the thermostat housing as well? Is that where the outlet pipe is located??

Thanks again.

 

Considering the material they are made out of, that may not be a bad course of action. Here is a picture of the exploded outlet pipe setup. It's a little misleading in that the thermostat, [puzzle, as I called it] fits into the cover [#2], not the pipe itself as the graphic seems to show. If you get the pipe, you get everything you see in the graphic, pipes, thermostat, sensor, all of it, seals and all. I don't know your VIN# so I can't give you a part number, but there you have it.

Still, it will be interesting to see if any faults were recorded or not, and what condition the interior of the pipe is in. If you see much of the material turning from black to a brownish color, junk it.

Good luck.

 

I do agree with the thermostat being part of the problem and replacing it is the right move but I would replace the hoses at the same time, you could have one that is collapsed causing a restriction. I would also look at the possibility of the water pump being part of the problem. When you remove the thermostat test the old one. You can do this by placing it in a pan of hot water and monitor the temperature and see that it operates improperly. You may ask why test it, If the thermostat test good look at the pump. The advantage to this is that you are halfway there with the thermostat removed. I would be remiss in not mentioning the replacement of the housing if it is plastic. I am not sure if your car has that problem and flushing the system is not a bad idea. I hope this helps!

As for the restricted performance get the codes and keep in mind not all readers can read all the codes. You may need to take the car to a upscale garage that has the OBD reader that reads all the codes.

 

Don't sweat over the taxes. I'm sure someone will call if they want the money

 

Still haven't had a chance to check codes.

Steve: Sending you a PM with VIN#. Thx.

Gus: Are you talking about the thermostat housing or the water pump housing? (cc'd you on the VIN# PM in case that helps identify what I might have.)

Jeff: If your problem diagnosis is as accurate as that statement, we've found our problem.

Thanks, guys,

 

I've been following this thread with interest. Wouldn't an infrared themometer be useful in chasing down this problem?

 

Here is the whole assembly, for a hundred bucks, I wouldn't even mess with just the thermostat. Part number AJ89485 for your VIN#.

The little yellow clip is only there to hold things together 'til it's installed, then discard.

Cheers,

 

Man, I hear you on that one. So, if you buy the assembly, all the puzzle pieces are already assembled. Sounds like a sure winner to me.

It does look like it's not made of metal, though. Is that asking for trouble??

Thanks.

 

I haven't heard of anyone making one of these contraptions out of metal like they have the small one on the 4.0L. So I don't think it's an issue.

I forgot to mention too, that all the seals necessary to install the thing are already on the part. Huh, forethought, can you believe it??

Cheers,

 

Sounds like a guy should stock up on them before Jaguar figures out they are under-priced.

 

I was speaking of the thermostat housing.

 

OK. I finally got time to to hook up the scanner (which also means I just wrote a very painful check to the IRS) and here is what I saw:
Stored Code: P0116 - Engine Coolant Temp Range/Performance
Pending Code: P1111 - System Tests Passed
That's it.

I then started the engine and looked at real-time data. I drove around a bit and watched what was happening, then just parked and idled for a while. As the temperature was coming up, the rise of the dash gauge and the "Engine Coolant Temperature" on the scanner seemed to be tracking each other up to 185F, where the gauge stopped at the half-way point. The reading on the scanner kept rising to 198F, but the gauge never moved off of the center mark. The scanner reading settled into a very regular 194F to 198F cycle with a period of a minute or two.

I don't know if there are any clues there, but thats what's happening, at least off of the freeway. I cleared the codes and my wiife is out driving around town now, so I'll check them again later, but I don't expect to see anything new.

That's all for now.

Cheers,

 

Any news? Those temp readings were different from mine, so failure of the housing might or might not be your issue. But a sticking thermostat could still be a culprit. We paid $56 for the new stat, which came included with the new housing that ran $180. If you can get the assembly out there for $100... I'd do it as routine maintenance!