A new thought on overheating

[CORKY]

I have noticed that as my fuel gets used up a vacuum builds up in the tank, there is often a swish of air being sucked in when I open the filler cap. I have also noticed that my 2001 MGF tends to overheat after a long, fast run when I am sitting in much slower moving traffic. It does not overheat after short 40-50mph runs and I am sitting in traffic. In other words it is an intermittent problem, sometimes it happens, sometimes it doesn't. Could it be that the long run at speed uses a lot of petrol, creating a strong vacuum in the tank leading to fuel starvation? If the engine has been running lean at high speed for quite a while it will be very hot. It remains very hot in the slow traffic but has now lost any air-flow cooling it had. And boils over.

Answer: Find a way of allowing air to get into the tank but no fuel to escape.

Views/Suggestions?

[CORKY]

The answer is so simple it is going to really annoy people - some of whom have spent years and lots of money trying to solve this problem. REMOVE THE (GREEN) INNER SEAL INSIDE THE PETROL FILLER CAP. Although the cap feels a little loose after this, it still locks - and removing this seal allows air into the tank when the fuel level drops. Problem solved. Obviously it is still very important to have the whole cooling system in tip-top condition. This morning I did a 150 mile fast motorway run and then parked up with the engine running. Both fans came on, coolant level rose a bit, exactly what you might expect - but NO overheating.

[nigelandjo]

Sounds as though the vent hole in your fuel filler cap is blocked and not doing it's job. You should not have a vacuum when removing teh filler cap.

[John SS]

I've never been aware of releasing a vaccum when removing the filler cap.

[CORKY]

Hi John SS. Does your car overheat?

[Bigfootmgf]

Hi my car doesn't overheat but does release pressure when removing the fuel cap and has been like it for the 20years I have owned it! They are designed to be under pressure as far as I know and have a release valve.

[CORKY]

I have two MGFs, both 20 years old, both have had the head gasket replaced. My belief is that in both cases the head gasket blew because of overheating. Neither of these cars overheat now - and I think that is because I have made sure that I have a flow of air into the fuel tank in both. Some guys I know have replaced the the head gasket, the water pipes, the thermostat, the water pump, the radiator, the header tank and its screw cap - and bled the cooling system of air several times. They have flushed the system under pressure and treated it with all sorts of chemical concoctions - they have no oil in the water and no water in the oil, they have no gases in the coolant - but they STILL have overheating. All I know is that running lean means an engine gets very hot. Running lean while standing in traffic makes this worse. My solution fits all the symptoms, the cause is a vacuum build-up in the fuel tank. Of course there is an element of luck involved here - some cars don't have the problem - those are lucky owners. But lots of cars do. If you have ruled out everything else then try what I have suggested, what have you got to lose? I'd be interested to know if it works or not.

[Bigfootmgf]

In relation to your idea I have owned my car for over 20 years and 105000miles. It has never had overheating issues even when used regularly for track days. It's cooling system is standard. The head gasket has only gone once at 80000!
I would suggest there is something else wrong with car if it overheats.
If I remember right I think that fuel tanks aren't aloud to be vented to the atmosphere because it's a fire risk!

[CORKY]

Hi Bigfootmgf. Do you know why your head gasket went at 80,000?

[CORKY]

Hi nigelandjo. I can't immediately identify a vent hole in the filler cap. Can you post a picture showing exact location?

[Bigfootmgf]

Hi Bigfootmgf. Do you know why your head gasket went at 80,000?

Yes the gasket went because It had been on the car for 15 years and I think it had something to do with a air lock after a coolants change but not certain! But at 15years I couldn't complain as that is along time compared to some and including the amount of track day thrashing it had!

[Charless]

The tank is always supposed to have a slight vacuum, caused by the fuel rail being pressurised to ~3 bar in normal operation (way more than the vacuum), so 'most unlikely' that fuel starvation is involved with this overheating.

[churchsquare]

Cracked piston linner, had one a few years ago, David

[talkingcars]

The main Lambda probe should detect when the car is running lean and the ECU will increase injector opening times to compensate.

IIRC the main reason for the vacuum system is environmental, it is to prevent petrol vapour being released to the atmosphere.

[PeteT]

I have had that slight fuel tank vacuum for 20 years and the MOT man wants to hear hiss that too. Older cars with breathing fuel caps suffer nowadays with ethanol causing the ingress of moisture and tank corrosion when not used over winter. I will stay with the sealed slight vacuum thank you.
PeteT

[paulg]

My regular MoT guy always insists on checking the fuel cap by listening for the hiss as he removes it.