Temperature guage fluctuating

[courier118]

ZT 190.....temperature guage has always sat at a fraction under the halfway mark.Had the car serviced 4 weeks ago by my usual Bentley specialist,replaced the coolant and OAT antifreeze,vacuum refilled.Ive noticed this last 2 weeks that the needle when driving,lets say a bit quicker,drops halfway between the halfway mark and the next lower mark,then when you drive slower it goes back to the normal position.Its usually only at the lower position for a minute or two.Checked the coolant level its fine,the heater/aircon/climate control works as always,plenty of heat.I know the outside temperature has dropped but last winter we had -6 and the temperature guage never dropped lower than normal position.Just wondering if anyone else has had this problem.Maybe a faulty temperature sensor? temperature guage? thermostat?........At least its not heading in the opposite direction,getting warmer! Oh and BTW plenum drain holes are not blocked

[GreyGhost]

ZT 190.....temperature guage has always sat at a fraction under the halfway mark.Had the car serviced 4 weeks ago by my usual Bentley specialist,replaced the coolant and OAT antifreeze,vacuum refilled.Ive noticed this last 2 weeks that the needle when driving,lets say a bit quicker,drops halfway between the halfway mark and the next lower mark,then when you drive slower it goes back to the normal position.Its usually only at the lower position for a minute or two.Checked the coolant level its fine,the heater/aircon/climate control works as always,plenty of heat.I know the outside temperature has dropped but last winter we had -6 and the temperature guage never dropped lower than normal position.Just wondering if anyone else has had this problem.Maybe a faulty temperature sensor? temperature guage? thermostat?........At least its not heading in the opposite direction,getting warmer! Oh and BTW plenum drain holes are not blocked

Could be the thermostat failing to operate correctly under higher pressure as speed increases. Having said that it looks like this has only occured since coolant replaced. It may simply be an airlock somewhere around the heater matrix, even though vacuum refill.
Unlikely to be sensor or gauge, if that were the case I would expect the fault to be more erratic.
Perhaps a complete flush through and another refill would be the cheapest option to start with.

Others more knowledgeable than myself will be along to help. :)

[courier118]

Ive thought the same myself TBH...........booked in for next friday for another coolant change........if that doesnt solve it,going to change the thermostat.
Thanks for the reply

[MartinW]

You say vacuum fill? I had my car serviced last year and it overheated within 3 miles of the garage. They claimed they had not touched the coolant, but then admitted removing the cap to check the level (I had spotted the loose cap when I stopped to investigate).

When the AA came out, he refilled, and used a bleed valve on the top hose (built in the rubber hose). I drove it back to the garage with him following, and only once did I see the temp needle shoot up to halfway between normal and hot, and immediately drop again to normal - an air lock perhaps? However, the garage were very good and changed teh coolant accordingly, but they did tell me that they did not use a vacuum fill, prefering to let the engine idle and add as necessary with any air-locks being removed from the top hose. They also said it needed to be checked for a few weeks just in case it hadn't got all the air out. Suffice to say 20k miles later inc a mid-year blast on German autobahns, and it's been ok.

I'd say check the bleeder.

[M47Rman]

ZT 190.....temperature guage has always sat at a fraction under the halfway mark.Had the car serviced 4 weeks ago by my usual Bentley specialist,replaced the coolant and OAT antifreeze,vacuum refilled.Ive noticed this last 2 weeks that the needle when driving,lets say a bit quicker,drops halfway between the halfway mark and the next lower mark,then when you drive slower it goes back to the normal position.Its usually only at the lower position for a minute or two.Checked the coolant level its fine,the heater/aircon/climate control works as always,plenty of heat.I know the outside temperature has dropped but last winter we had -6 and the temperature guage never dropped lower than normal position.Just wondering if anyone else has had this problem.Maybe a faulty temperature sensor? temperature guage? thermostat?........At least its not heading in the opposite direction,getting warmer! Oh and BTW plenum drain holes are not blocked

I had this problem on a 4 cylinder K series some years ago, and it was caused by a huge air lock. The temperature reading drops each time the 'air bubble' is around the temperature sensor, and then returns to normal when the sensor is surrounded by water again. Equally, a low temperature reading can also be BAD news, because in the instance of a loss of coolant, the gauge can drop rapidly, before then ascending equally rapidly. Again I speak from experience, in a Rover 200, which lost a great deal of coolant - however I was extremely lucky and had no HGF. You definitely need to get the cooling system bled at the very least.
Andy

[courier118]

Temperature guage still acting strange............one minute at normal,the next,dropping to the next mark down.............vacuum refilled 7 times in a row.............Any ideas?

[Simon]

Have you (or the garage) checked all other points such as temperature sensor itself and checked there isn't a kink in a pipe anywhere (unlikely to have happened, but always worth a check).

Have you replaced the cap on the coolant tank? The system is pressurized and if the seal is on the way out it could cause erratic behaviour. Worth fitting a new one to see if this cures the problem.

[courier118]

Cap was replaced last year as a precaution as i had read about cap failures,so its fine.No kinks in pipes and no air being released from bleed screw in lower pipe.So ive reverted to old style to see if i can get air out........basically leaving the cap off and holding it 2000rpm let out any air and let the water overflow,back off the revs to idle and refill.Quite alot of air came out and im now in the process of checking the water level and topping up as neccesary.Fan kicking in as usual.Guage is at normal temperature whilst doing this,so fingers crossed.

[Simon]

Cap was replaced last year as a precaution as i had read about cap failures,so its fine.No kinks in pipes and no air being released from bleed screw in lower pipe.So ive reverted to old style to see if i can get air out........basically leaving the cap off and holding it 2000rpm let out any air and let the water overflow,back off the revs to idle and refill.Quite alot of air came out and im now in the process of checking the water level and topping up as neccesary.Fan kicking in as usual.Guage is at normal temperature whilst doing this,so fingers crossed.

Well, if you got some air out then this is a good thing. Hopefully this will solve your problem! :lol:

[patch1]

Hi courier118, Have the same problem as yourself. Had it for a month now, Temp quickly rises to a quarter on start up in the morning, but goes no further until I stop at lights, then the gauge quickly rises to a half. I pull away from the lights and get up to about 2500 revs and the gauge plumits to a quater again, where it stays until I stop again. Luckily I am not motorway driving and only travel 11 miles to work. I tried adding a little more antifreeze to the header tank, my thinking was to increase the level to aid the removal of any trapped air. This seemed to make the gauge worse, so this weekend I gave drained and flushed the system, and thats when I saw the Max Min marking in the bottom of the expansion tank. Until now, the level had been a good 30mm above this, so I had not seen them. Today I went to work and all seemed ok, although I am still not convinced, I will continue to monitor the level after each journey and continue to gently squeeze any hose you can get your hands too to aid air removal. I will let you know how I get on later this week.
Patch

[Simon]

Hi Patch, and welcome to the club!

Don't know how long it'll be until some one more important than me gives you an official welcome (you've hidden yourself well in this thread on this area of the forum ;) ), but you can consider yourself welcomed from me!

[GreyGhost]

Hi Patch, and welcome to the club!

Don't know how long it'll be until some one more important than me gives you an official welcome (you've hidden yourself well in this thread on this area of the forum ;) ), but you can consider yourself welcomed from me!

There's no one more important than anybody else here, we are all members of the same club. Some of us just help out a bit and purport to know where everything is. :D
Welcome to the club Patch enjoy your stay

[courier118]

At last someone with the same problem.........sorry.........I knew the maximum and minimum marks were there but thanks.Ive vacuum filled till im blue in the face............had it running with no cap on to let air draw out.........checked levels last night and today..............sitting bang on the maximum mark..............guage still all over the place between halfway and the next mark down...............i dont know whether theres air in it causing hot spots in the engine.......heater works fine...........whether its the temp sensor,faulty thermostat,faulty guage.When the cars been idling for a while the guage is at the halfway mark and the cooling fan cuts in..............im going to look for my axe soon and chop the damn thing up.......or may just go out and buy that 6.0ltr V8 vauxhall/holden monaro i like

[DeeCee]

What about trying the old fashioned way of bleeding the system.

The cooling system runs at pressure to increase the boiling point of the water(antifreeze mixture)
The pressure is about 5 to 10PSI, normally controlled by the radiator cap, sometimes the pressure cap is on the expansion tank.
If you look inside the pressure cap you should see a spring and sealing washer.

With Radiator system sealed.... all caps on, start ending from cold.... first start of the day.
Start engine and wait till it warms up, you do not need to wait until the engine reaches normal operating temperature... probably safer if you do not.

Switch off engine, the cooling system will be pressurized now. Go to the highest connection point in cooling system.
Sometimes you will find a bleed point there, else loosen a hose or fitting a little bit and flex it open, looking for air or water appearing, if no air/ bubbles tighten up and go to next highest point.

You should complete the bleeding within a few minutes before the system cools and causes a vacuum.

You can even try raising the front or rear of the car..... park it on a big hill!

In some of the old cars this was the only way you could get the heater to work.... the heater core was higher than the top of the engine and would airlock when refilling the system.

[apywell]

I have had my 75 for 5 months now and the temp guage rarely stays on the half way mark. It usually resides mid-way between quater and half way. Though recently I have noticed it going as low as about 2mm above the quater mark. Never thought any more of it because the car runs fine.

[courier118]

Ive had my car for 3 yrs and the temperature guage has always sat at a fraction below the halfway mark whatever the conditions.........would be nice to know what others temperature guages sit at

[Simon]

Ive had my car for 3 yrs and the temperature guage has always sat at a fraction below the halfway mark whatever the conditions.........would be nice to know what others temperature guages sit at

For reference, my 5 year old ZT160 has always registered it's temp gauge at half-way. It gets to that temp after only a few minutes, even during the recent frosty spells a few weeks back. It doesn't seem to move about. Not had the chance to test it in very hot weather though as I've only had the car since September this year.

I believe the temperature gauge in modern cars is designed to 'point at halfway-ish' so long as the engine temperature is within a pre-determined range. If it starts to move then it would be because of rather significant temperature changes or because of a fault somewhere (the sensor itself or the electronics that move the needle on the display, for example).