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Thread: Cooling system obscurity thermostat or sender? So confused...

  1. #1
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    Cooling system obscurity thermostat or sender? So confused...

    Hi, I'm new to the forum but I have a baffling problem...

    I have been working on cars of various types since I can remember with my dad (ex tool fitter/engineer/fabrication factory owner with a love for restoring pre '70s classics) so I'm not a mechanic by any means but I know enough to be dangerous.

    My problem is I live in ibiza and my dad is in Worcester so he can only be so much use.

    I don't trust anyone here it's the Wild West to put it in context I got hit by a dustbin truck (who fled no trace) and dismounted back bumper, the locals in the body shop power drilled it back on with stainless steel wood screws, the guy kissed his teeth as he proudly presented his work... anyway.

    Car is a xj308 sovereign 4.0 (48000 miles)

    I have a list of niggles which I will go though at a later date but the main problem I have is underheating. The car is behaving exactly like it has a thermostat stuck open or not present, except it has a brand new thermostat. Starts on the button and if left sat will come up to temperature (needle sits on the centre white line of the gauge just a hairswidth above centre). However when I start and drive as I exit town (about 3miles) temperature starts to come up will reach about 1/8 maybe just tickle 1/4 if traffic. When I reach the highway it stays neatly around 1/8 for about 10minutes and then the needle hits the floor and the car goes to restricted performance. Slow down or stop for any length of time and the needle will Ping up to a 1/4 and restriction goes away without fuss. All the time the heater seems to blow a temperature that represents the gauge hot when up to temp and much more moderate when on highway.

    Troubleshooting:

    •Coolant level stable no signs of leaks
    •Coolant circulates freely back to filler tank no bubbles when squeezing the radiator pipes
    •fans come on and off when due (with ac on at standstill for e.g)
    •crap fuel economy 18mpg

    The complication is that I had a guy change the thermostat for me here (who hadn't tightened a hose back on properly causing me to lose coolant only noticed after a drive I saw steam and investigated to find pipe popped off! (Checked oil no white, normal pressure in hoses blah blah the guy just didn't tighten it, that's fine easy mistake I guess...)) the point is, he went to the parts shop with chassis number and they gave him a 92* thermos or a 96* can't remember what he said and he said no that's too hot and took one that closes at 87* which means it probably opens at 80? But I think the optimum temp for my car is 90, dad said 88 which is the generic but I read a lot that it's 90.

    So is my continued problem because I have a cold thermostat that's opening too soon baring in mind we are talking 10* and the car doesn't reach even 1/4 at even low speed (but I guess that the gauge probably starts at 50* it doesn't read down to 0 right? so 1/4 could be 75*?), or is it a connection, a sensor, the gauge? What the hell? Help me please?!

    The mechanic has offered to put the hotter thermostat in for free and change all the sensors involved in the cooling system for 90euros Inc labour but I'm hesitant as Tbf 96* does sound hot

    HELP!!

  2. #2
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    Oh additionally, I bought the car a year ago and have driven 10000 miles. Only the last 2 months has it been doing the restricted performance thing consistently but the temp was always low, sort of hovered between 1/4 and 1/2 most of the time with some rare occasions doing restricted performance I just assumed it was the gauge acting dodgy. That was summer though 35*C now it's freezing!

  3. #3
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    Sounds like a sensor to me, but I'd be looking at doing 2 things initially:

    1. Read the codes to see if anything else is happening.

    2. Change the thermostat with a genuine Jaguar or OEM part. I think that it's part number AJ86484 - but check.

    According to the workshop manual "the thermostat starts to open at 80 ° to 84 °C and is fully open at 96 °C."
    Last edited by Bob1198; 10-01-17 at 09:58.
    Present car: 2000 XJR Platinum Silver 200,000+ miles

    Previous Jaguars: 1977 XJ6L 4.2
    1985 Daimler Double Six V12
    1990 Jaguarsport XJR 4.0

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    First, the temperature gauge is an idiot gauge. That is, its readings are 'dead cold', 'normal' or 'you're buggered'. There is simply no finesse in there whatsoever.

    Is it just heat in the cabin you're not getting? If so, I'd bet my last pound your problem is the auxiliary water pump. Ask me how I know...

    However, from what you've said, with the temperature going up and down, it sounds like you have broken impeller(s) in your water pump. If you've never changed it or don't know if it's been changed in the relatively recent past, do it now before you end up with no coolant circulating and an overheated engine. Ask me how I know that, too!
    JagFab
    98MY Madeira Pearl Daimler Super V8 150K miles (RHD)

  5. #5
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    Thanks for your help! but if the impellers were broken there would be little circulation meaning the coolant wouldn't be able to do its job and I would have overheating rather than a problem coming up to temp and staying there? The car has only done 48000 miles seems shy of the sort of mileage for things like that to be occurring?

  6. #6
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    Hey thanks for the response! Yes my dad says just get the codes read by a pro but that sounds expensive, is there a unit I can buy to diy? How do I know if the parts are genuine if they are being installed by another guy? I'm so not used to using mechanics, I don't know if it's normal to ask to see the old parts or the new ones before they go in? If I was doing it with dad we would have had the thermostat out and then dropped it in boiling water just as a matter of interest to see if it really was that failing, i don't know if they guy did that?

    My feeling is that the guy put an incorrect thermostat in there and it doesnt come into range of 'normal' temperature in the eyes of the on board computer within the specified time scale so it goes into restricted performance. Then at low speed while the engine is able to come up to temp once the engine comes into range the computer says ok your good to go and comes out of the restricted performance mode

    I have thought a lot about it being a sensor but I can't figure out why the temperature in the engine would physically track the temperature read by the gauge if it's an electrical fault when the temp of the engine is controlled mechanically by the thermostat unless there is a digital control aspect that when it fails or detects a fault automatically bypasses the thermostat? Idk?

  7. #7
    Senior Member Lancealot1969's Avatar
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    Dont know much about the coolant but first thing id check is for a wrong thermostat, that would be my first concern. Then id look at the water pump. Dont know if its possible but could the mechanic who fitted it have put it in the wrong way round ???
    Also there is a temp sensor just under the header tank its only a push fit sensor with a Brown plug to the left under the tank.
    If the thermostat could be inserted the wrong way round wouldnt the back preassure blow of the pipe ??? But if I was you I wouldnt use that garage/mechanic again. Hope you find the problem soon.

    Lance.
    1998 Jaguar XJ8 4L LWB in Jaguar Tudor White, Huggy
    1999 Jaguar XJ8 4L LWB in Jaguar Tudor White, Celeste
    Above all, Have Fun

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    Quote Originally Posted by Morgan lewis View Post
    if the impellers were broken there would be little circulation meaning the coolant wouldn't be able to do its job
    IOn re-reading your post, I remembered mine did a very similar thing a couple of times on the motorway/highway. Because the needle settled down almost immediately - and not then knowing about the famed water pump failure - I dismissed it as an electrical glitch, something to look into "when I have time." The third time it happened, the needle went right up to 'H' and stayed there, apart from a few minutes once I'd slowed down. Anyway, that led to a new head gasket, renewing pretty much everything in the cooling system bar the radiator. I took the opportunity to have the timing chain, etc. done, too.
    JagFab
    98MY Madeira Pearl Daimler Super V8 150K miles (RHD)

  9. #9
    Senior Member jagman98's Avatar
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    ....>>>>I have a list of niggles which I will go though at a later date but the main problem I have is underheating. The car is behaving exactly like it has a thermostat stuck open or not present, except it has a brand new thermostat. SHello Morgan,.....>>>>

    Sounds to me like you have the wrong thermo installed, it should be a 96 for a 4.0 litre v8
    When you have covered a few klm on the highway stop, open bonnet and feel by hand he following:
    The coolant hose leading to radiator from the head collector pipe, this should be good and hot.
    Then feel the coolant hose leading from the rad. to thermo housing and if you have the wrong thermo this will feel cooler.
    I would also check your coolant pump (easy enough job to do) for any damage or broken impeller.
    But I think it's basically the wrong thermostat.
    If it was fitted backwards (has been known) it would not allow coolant flow and you would be getting an overheat situation.
    The coolant hose 'blow off' is well known and to prevent this you should use two narrow band worm clips where the hose attaches to thermostat bib.
    I doubt very much it's anything electrical.....check and get back to us. ;o)
    Cheers,
    Jagman98

    http://www.pbase.com/padme002/jj_jag_1

    '98 XJ8 Sovereign 4.0 ltr. Sapphire Metallic, Oatmeal/Antelope Int.
    '03 Peugeot 307 XSEA 2.0 ltr 5 door hatch. China Blue
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  10. #10
    Senior Member Lancealot1969's Avatar
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    Pbase is very good, I found it by accident and I have it in my favorites. All credit to the guy who did it, there is a very good instilation of the alloy thermostat tower install. Along with other great projects.

    All the best with it.

    Lance
    1998 Jaguar XJ8 4L LWB in Jaguar Tudor White, Huggy
    1999 Jaguar XJ8 4L LWB in Jaguar Tudor White, Celeste
    Above all, Have Fun

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