Classic 3.9 V8 - Over heating - still

Hi people, having read numerous threads on here about this I've tried to do most of what's been suggested, things I've done so far are; compression test, all good / checked thermostat with snap-on diagnostic, full flow at 98 degrees / replaced viscous fan / replaced water pump (old one seemed ok but .....) / tried to do a sniff test but as there are no signs of bubbles, nothing happened / replaced expansion tank cap. But, it will only run for about 6 miles before temp starts to rise, the air con fans kick in but make little difference, it's only when I put heater on full I can drive it but it's bloody hot. Lifting the bonnet after a short run I can hear water either bubbling in the system or flowing around it, wearing hearing aids means I can't pin point where the noise is coming from, it seems to be the mass of pipes to the left of the plenum chamber. I haven't got much hair left so any help would be very much appreciated

Sounds like you have an air lock,park the front on ramps or uphill on a steep incline when you "burp" the system when filling,do you have flow into the expansion tank from the small pipe from the top of the radiator? If you do a search I'm sure someone posted their method not so long ago.good luck.

If it's not raining tomorrow, that's the morning sorted, much appreciated

Try UV dye in the coolant and then check the dipstick with a UV torch each night until it glows or you've done a hundred miles or so.

Slight problem, on one hand I daren't do a 100 miles as it is at present, the cab gets a tad worm with the heater on full blast, on the other hand, I don't want to see UV any sooner if you get my drift.

I would have to say that the thermostat seems to be a very hot one. Try fitting an 82° thermostat. These engines were designed (like most US engines) to run at 180°F. In later years they kept bumping up the temperature to try to meet emissions regulations etc.

Make sure any thermostat you fit has a jiggle pin in the plate, and fit it at 12 o’clock, otherwise you will get air locks in the system. If it doesn’t have a jiggle pin, drill a 2 or 3mm hole in the plate prior to fitting.

I just went by the 1994 manual (and manufacturer's site) which said an 88 opens fully at 98 so thought that was ok, I did buy another thermostat (88, and no jiggle pin) before I did the test so if 'burping' doesn't work, 82 it is, thanks

peanuts01 said:

I just went by the 1994 manual (and manufacturer's site) which said an 88 opens fully at 98 so thought that was ok, I did buy another thermostat (88, and no jiggle pin) before I did the test so if 'burping' doesn't work, 82 it is, thanks

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Test the thermostat in a pan of water and bring to the boil before you fit it to make sure it opens. If there is no pin and you haven't drilled a hole then there's no way to clear the trapped air. As @Marshall8hp said make sure it's fitted in the 12 o'clock position.

peanuts01 said:

I just went by the 1994 manual (and manufacturer's site) which said an 88 opens fully at 98 so thought that was ok, I did buy another thermostat (88, and no jiggle pin) before I did the test so if 'burping' doesn't work, 82 it is, thanks

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Both EFI engines for the so called Classic are fitted with the 88C thermostat when they left the factory, so thats the 3.5 and the later 3.9.
My 1987 3.5 had the original thermostat when i sold it 10 years later and my 3.9 disco still has the original 88C thermostat with out any issues. Why I dont know, but the 3.5 EFI disco was fitted with a 82C thermostat.

Have u tried driving with out a thermostat.?

Always use a thermostat. If you remove it completely you run the risk of the water circulating too fast and not cooling sufficiently in the radiator. If you decide to remove it, put a restrictor or plate in with three holes at 5/16”. I have had to sort this issue out many times over the years, people bringing in a vehicle that is overheating and saying they had removed the thermostat. Put one in, problem goes away.

It might sound counter intuitive, but trust me on this.

By the way, the non emission 3.5l had an 82 degree thermostat.

Thanks for that. Mine hasn't got Cats so it seems an 82 degree (with jiggle in or hole) is the way forward

peanuts01 said:

Thanks for that. Mine hasn't got Cats so it seems an 82 degree (with jiggle in or hole) is the way forward

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Without either you'll never get rid of the air

Just ordered one with a jiggle pin

peanuts01 said:

Just ordered one with a jiggle pin

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Remember the 12 o'clock position and I'm not being suggestive.

Is 12 o clock facing the back or facing the radiator?

oakey said:

Is 12 o clock facing the back or facing the radiator?

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Doesn't make any difference. The stat on all the older V8s is mounted vertically.
Lrcat Item 9

oh right lol ta

oakey said:

oh right lol ta

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Someone had to fall for it.lol

I can be relied on lol