One year overheating drama. Please help

XJ-S
1

Hey, this is going to be a long read, as its 1 year story with my jag, so sorry for that, but to avoid most common suggestions, i need to write everything down

Car: Jaguar XJS 1992, 5.3

In short: Car overheats, after warm up looses any power after 3k RPM (not always, one rev it goes higher, next one, doesnt want to, popping sound from exhaust sometimes) , slight vibration when seating inside. Not sure if those problems connected, moreover loss of power came somewhere in between and was not present at the beginning of that saga.

Also, note that car overheats more when driving fast, at idle it doesnt, at least for the first 30 mins, then it cant get back to close thermostats even with efans on, they stay on forever, but when you driving it, no matter of speed it climbs up.

I bought the car more then a year ago and from day one it was overheating between N-H.
No big deal, i thought. So i got into forums, found sticky document explaining how to deal with overheating and started complete overhaul, bellow is the list of things done so far withing a year

all hoses
new thermostats (checked in boiling water for proper length)
all new seals and gaskets including rail pipes
radiator recored (twice)
new water pump
new banjo bolt from ebay (modified)
tried 2 fan clutches, then moved to efan controlled by separate thermoswitch
all air gaps closed
bottom spoiler in place
installed glass coolant filters, now i can see that flow is good and equal on both sides
both tanks (metal and plastic) cleaned
tried to fill the coolant in 5 different ways, front up, down, left, right, all possible ways i could find
all spark plugs and wires
injectors removed resealed and cleaned
bottom of V cleaned,
ALL vacuum hoses replaced
fuel lines replaced
fuel rail cleaned,
valve cover gaskets replaced
moon seal metallic one
both new crank speed sensors
new CTS
injector harness rebuilt, rest of harness checked
MAP vaccum line fixed(was leaking)
new fuel filter
both new coils
all injectors checked with test light
check for exhaust fumes in cooling system using special liquid - nothing
TPS voltage adjusted
throttle bodies adjusted
new dist cap
every sensor checked, resistance, resistance at ECU.

vacuum - 16 at idle
compression within normal range on all cylinders with less then 10% difference between each other

Also, it feels like B bank is hotter a bit. That slight vibration no idea where is coming from.

Now, last thing to check is clogged cats, BUT i dont hear any noise, exhaust flow feels steady and strong and no smell.
Other then that, i have no idea what it could be

P.S maybe it doesnt overheat, gauge not working properly? i dont think so!
here is my logic
oem small fan turns on somewhere at 92, it does
my new thermoswitch for efan goes on at 95 and switches off at 85 (dont remember exactly, but it doesnt matter)

When interior gauge shows a hair past N, both fans turning on, when it goes towards mid to H, both fans are on and not switching off even driving hghway. But at idle they both goes off when gauge drops bellow N

Honestly, now i wish it would be blown head gaskets, at least i would know for sure the reason and deal with it accordingly. One year of work, testing, checking, cleaning, praying is devastating

BTW! Problems i found and fixed and hoped THIS IS IT

  • B bank thermostat was incorrect, shorter, no hole at the top
  • MAP vacuum line was leaking badly
  • Old radiator was completely gone
  • TPS was out of range
  • old plastic fan was cracked
  • spark plug wires was eaten by rat

and after fixing and replacing half of the car its even worse then it was.

1 Like
2

What year is your Jaguar and what engine does it have?

Paul

3

@Paul_M_Novak avatar says V12 1992

4

IF not done already: Confirm overheating with alternate gauge. Do a somewhat crude but effective Cat Test with your vac gauge.

5

i bet there is no need in that, as i stated at the end of my post, two thermoswitches working in line with interior gauge, they do switch off when gauge shows bellow N and start right at top of N. Driving on the highway shows temperature raising and both efans working non stop. im now in the proccess of removing cats, so i better check it visually.

6

Ignition advance issue? Lack of spark advance will result in lack of power along with a lot of additional heat for the cooling system to deal with. That you have 16” of vacuum at idle indicates base timing is close, but does the ignition advance from there?

7

its a Marelli system, you cant adjust timing, not sure if i can do any additional check on that system, at least i havent seen such. Sure marelli ecu could be bad, but again, not a common issue and i dont know how to test it

8

Robert,
Timur has now identified his car as a 1992 5.3L V12 and unless it has been modified I believe that it has a Marelli digital ignition system which controls the ignition timing digitally and there is no vacuum advance capsule like on the earlier Lucas ignition cars. I know that this is true of our 1990 XJ-S convertible but someone would have to confirm this is true with a 1992 as I know little to nothing about the facelift cars which are different is so many ways than the earlier cars like ours.

Paul

9

Thats very right, its mareli, but to be honest apart from that there is not much difference in facelift vs pre-facelift, ignition, slightly different fuel line arrangement, updated fuel rail, no major differences i would say

10

That’s a tough one Timur.

This indicates a coolant flow issue or air in the system.
Add a purge valve at the heater hose, it will make the purging process much easier.
Details at Kirby’s book.

Maybe flow test the engine?

Well, it could be the issue.
I had bad head gaskets twice:
One with a BMW 318i, no coolant in oil, no oil in coolant but car was overheating after 10 mn driving.
And one with a Mercedes 190E 1.8, loosing coolant, mucky oil, but temperature steady as a rock… go figure.

As you are running out of options, I would investigate this further.
Had a clogged exhaust on my motorbike and it was a dog above 3K rpm.

I’m sure.
Bon courage.

11

i have it, bought it after kirby’s book, just need to install it

Maybe flow test the engine?

can you please tell how?

One with a BMW 318i, no coolant in oil, no oil in coolant but car was overheating after 10 mn driving.

So how did you figure it out?

i hae no white smoke from tailpipes , except first 1 min (guess its just condensation)
clean oil
no co2 in coolant

i mean literally no signs of it

12

I didn’t see a fan clutch on your list of bits? Also, O2 sensors, temp sensors?

Did you pull the radiator and flush or send out / clean the lower fins below the bumper?

13

The small cat inserts at the entrances to the downpipes can also fall down and block or partially block the flow. The downpipes need to be remove from the heads to check.

14

This picture shows the entrances to the downpipe. The top inlet shows the cat in the proper position. The cat fir the bottom inlet has fallen into the downpipe. You can’t see it in the picture, but it is sitting sideways and partially blocking the pipe.

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15

What a pickle!

In my view.

  1. You have two issues. Over heating asnd intermittant power loss. Possibly related.
  2. Teh partly blocked down pipe is a possible cause of both. Is it fixed, Were it my car i would remove the 'filter". It has to restrict the flow.
  3. I may have missed the use of a separate temperature guage to verify or negate thje dash guage numbers. A mechanical guage if avalable.
  4. heat up under speed! Well, At what RPM’s?
    My lump crukies at 70, the engine at about 2K.
    It is not teh speedo reading that counts, it is hw many BTU’s are created that over whelm the cooling system.

But, unfortunately, no over drive on the TH400.

One more. Some cores for radiators are better than others.

CHJ

16

Your temp thermoswitches “inline” implies they use stock sender. If a grammar oversight- disregard. If not, get a true second opinion (gauge) on your temp. Faster and cheaper than continued parts chasing. Did you see vac drop off post start? A trashed cat or muffler can talk to you, perhaps not with a computer.

17

Knowing what I learned this year, I would cut those off and toss them over the hedge. I had a set of Euro stainless steel downpipes installed.
Two things happened, the V-12 could breathe and the heat seems to exit the engine way better. If by local laws you require Cat Convertors find a less restrictive ones and install further downstream.
I can’t seem to get the engine to overheat if I tried. The gauge won’t budge under the N as long as the air is out of the system. The original poster seems to have covered many of the recommend steps.
I almost feel like we’re overlooking something here. If cooling system is fully functional, I keep wanting to go back to exhaust flow and distributor.
My two cents.

18

i tried 3 fan clutches, even 1 heavy duty GM one, however it blow off the plastic fan which destroyed the rad and then i switched to efan

19

thas what im hoping for now! cat nuts in wd40 now, then will come off (hopefull) so i will be able to see whats happening there. But as i said, honestly not much hope, reason is

  1. flow at the tailpipes is strong and equal on both sides

Also, i have read somewhere that US cars head those cat inserts in downpipes, not EU cars, my car is UK, so no idea if it has it or not, but this is what i see from another side

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20

can you please explain this further? additional gauge that would go into the rail? replacing what?
havent check how vac acts on load, will certainly do!

If my car not overheating, then why it looses power when its hot (above N) and also i can feel by hand that B bank is hotter.

Dont get me wrong, i would be happy to know that my car just doesnt overheat and i can ignore the gauge.