Topic

This is so simple it is going to annoy a LOT of people. Especially those who have spent years - and a fortune - trying to find the cause of MGF/TF overheating. My MGF was overheating after a longish run at speed (say on a motorway) when I left the motorway and sat in slow moving traffic. Steam from the rear vents, bubbling coolant, temp gauge going through the roof! The problem has nothing to do with air-locks, HGF, thermostat failure, blocked radiator, fans failing to come on - or any other obvious answer. The answer is this: If you petrol tank is, let's say, half full at the start of your long journey, then, as you travel you will use a fair amount of fuel. This leaves quite a powerful VACUUM in the fuel tank (have you ever noticed the rush of inward air sometimes when you remove the filler cap?). This vacuum build-up leads to fuel starvation - running lean in other words - and this means the engine, while quite lively in performance, has been running HOT, possibly very HOT if your coolant level is a bit low. This is fine on a motorway when you have loads of airflow and the temp gauge looks fine - but the moment you stop, or join slow moving traffic you have no airflow. The engine has got very hot, and it very quickly gets hotter - because you are STILL RUNNING LEAN. The answer is ultra-simple and of no cost. REMOVE THE (GREEN) INNER SEALANT RING IN THE LOCKABLE FUEL CAP. While this might make the fuel cap very slightly loose, it still locks, and AIR CAN GET IN. But what is better a slightly rattling fuel filler cap or the risk of HGF!!!!!

I'm not at all convinced by this.

It might have been true in the days of carbs and SU fuel pumps, but I find it hard to believe modern high pressure fuel pumps could't overcome a slight vacuum in the fuel tank.

If the mixture was running lean because of a slight vacuum, wouldn't the ECU detect it and compensate?

Someone more expert than me may care to comment!

The way the fuel system works is the pump provides more than adequate fuel flow for an engine at full power. Excess fuel is returned to the tank. Tank breathing is provided by a small pipe venting at the filler cap when the cap is removed and the tank is being filled. When the fuel cap is correctly installed it seals the tank to eliminate vapours leaking to the atmosphere. The expanding vapours are collected and trapped by the evaporation canister which has an amount of charcoal inside to collect the vapour. When the engine is at 70Deg C and above 1800rpm the purge valve opens and allows the manifold pressure to pull the vapour out of the charcoal canister via a pipe between the canister and inlet manifold. Operation of this purge valve is not allowed on a cold engine to prevent excessive fuel entering the cylinders and messing up the air fuel ratio at idle. There is a 2 way valve between the evaporation canister and tank. I assume this is to prevent liquids moving yet allowing vapour to pass between tank and canister. I suspect this is where you have a fault. the 2 way breather valve may not be allowing air back into the tank once purging has been done, blockage of the canister pipes, one of which is open to the atmosphere to allow the tank to breath, or it could be the purge valve on the canister of wiring at fault.
Hope this helps.
Nigel

The fuel tank is sealed and maintained at a slight vacuum.

This is normal and correct.

Removing the tank seal is foolish and most definatly not recomended, and could result in a fuel spillage in the event of an accident.

Hi and welcome to the forum

Nice idea but I don't think it works.

A modern fuel injected engine is designed so that if the ECU senses that the exhaust is lean it will inject more fuel to remedy this. If it doesn't get any signal from the exhaust monitor, the lambda probe it will run an open loop fuelling system which is running rich.

James

All I can tell you is that it works. I made sure air could get into my fuel tank over a year ago and have had no overheating problems since. I have done hot weather, fast runs with a half-full tank and then slowed right down, even come to a complete stop in traffic - engine still running. Not the slightest sign of overheating. I'm pretty certain this is the answer despite the comments above. Try it, as long as you don't have plans to roll your car, what have you got to lose?

No. What ever caused your overheating, this was not the cause and this is not the cure.

Wrong. It IS the cause and I have found the solution. I said right at the start that a lot of people wouldn't like this solution. That is their problem.

guys please, no beef here for no reason.
Whatever the reason and fix for the issues may have been, it's great for Corky to have a runaround for the sunny days.
But please have a look at the temp gauge every now and then