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Author Topic: Poor MPG and Low Oil Temperature  (Read 1260 times)

Offline wellfan

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Poor MPG and Low Oil Temperature
« on: 06 February 2005, 19:19 »
My 1989 model 8 valve Gti is giving fairly poor MPG figures. I think it may be related to the engine running too cool. The coolant gauge needle barely moves from its resting position and my oil temps average around 78 - 86. I have just returned from a 70 mile round trip doing speeds up to 90 and the oil temp never got above 86. Around town it never goes above 78. My previous 2 Gti's showed higher oil temps. The car is otherwise great, it runs very well.The thermostat appears to be functioning; however, it may not be fully closing. What do you think. Could a cool running engine cause low MPG figures?
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Offline rubjonny

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Re: Poor MPG and Low Oil Temperature
« Reply #1 on: 07 February 2005, 11:55 »
Start it up from cold and let it idle. The lower radiator hose should stay stone cold until the temperature gets up to half way, if it gets lukewarm before this then the stat is stuck open.

I would be inclined to say the temp sensor might be broken, but since the oil temp also reads low I doubt that is the issue.
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Hello my name is John and I'm a dub addict.

Offline Mk3GTI_Spain

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Re: Poor MPG and Low Oil Temperature
« Reply #2 on: 03 February 2006, 10:39 »
I had exactly the same problem with an MK3 GTi and replaced the thermostat yesterday, water and oil temperature rises much faster and mileage has improved as the engine does not take so long to reach its optimum temperature. Water gauge sticks in the middle after 5 minutes of city driving and oil stays on 96 C when hot. That tells the ECU to use lean air-gas mixture and therefore improve mpg.

Hope it helps  :wink:



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Offline baz_the_man62

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Re: Poor MPG and Low Oil Temperature
« Reply #3 on: 03 February 2006, 10:53 »
sounds like the thermostat to me, if its running cold its gonna be running a richer fuel mix which will guzzle ur petrol. think halfrauds do em for around a tenner.
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Offline Neaty

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Re: Poor MPG and Low Oil Temperature
« Reply #4 on: 03 February 2006, 13:29 »
Quote from: Mk3GTI_Spain on 03 February 2006, 10:39
I had exactly the same problem with an MK3 GTi and replaced the thermostat yesterday, water and oil temperature rises much faster and mileage has improved as the engine does not take so long to reach its optimum temperature. Water gauge sticks in the middle after 5 minutes of city driving and oil stays on 96 C when hot. That tells the ECU to use lean air-gas mixture and therefore improve mpg.

Hope it helps  :wink:





ah, i have this problem with my mark 3 8 valve! the list of bits is getting longer and longer! damn!
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Offline bulk66

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Re: Poor MPG and Low Oil Temperature
« Reply #5 on: 03 February 2006, 21:26 »
I have nearly an identical problem as you wellfan. Dose your temp gauge go up if you are sat still for a couple of minuets then go back down when you start driving again. Ive got a thermostat in the garage i might get it changed just to see if it dose anything.......... :smiley:
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Offline wellfan

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Re: Poor MPG and Low Oil Temperature
« Reply #6 on: 03 February 2006, 21:42 »
Sorry, I should have replied ages ago. I changed the thermostat some 10 months ago and it solved the problem. On testing the old thermostat, it was not fully closing, hence the problem.
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