MK5 Golf 1.9 tdi - Overheating

[Adam95]

Hi

I’ve got 07 plate Golf 1.9tdi (BXE engine). I got a service and timing belt/water pump replacement last week. It was running perfect for five days but two days ago whilst on the motorway it start overheating and the heaters went cold. I wasn't far from home so took it easy and drove it home thinking it must be a leak in the coolant. When I got home and checked the coolant was full. I then let the engine cool down, opened the cap, put the heaters on full and started the engine just in case it had trapped air. This did nothing so I took it to the garage and the mechanic advised me to replace the thermostat being the cheapest option. I replaced the thermostat and that did nothing as well. The heaters blow out cold air when idling or low rpm (1000-2200) but when driving at around 2300-3000 rpm the heaters blow out warm air and the temp gauge stays at 90. If i’m driving at a constant 60mph in 4th gear and around 2500 rpm the heaters will blow out hot air and the temp gauge will stay at 90, if I shift up to 5th and the rpm drops to 2000 the heaters go cold and the temp starts rising. I’m thinking it could be a faulty water pump or a head gasket but is there any way I could test this before as my mechanic is going to charge me labour regardless. I’d appreciate any help or recommendations.

Thanks

[Crasher]

I would hope that is a faulty water pump as the heat (i.e. flow) did not seem to come back and it does not appear to have blown out its coolant which is the classic sign of PD head gasket failure. If you do pull the head, fit a new aftermarket unit by KS, AMC or even a Chinese copy as the job will last longer than a rebuilt unit.

[philipharmes]

Have you got a flow of coolant via the small pipe at the top of the coolant reservoir when the engine is idling.

If not, it's probably blocked & you have an air lock in the system.

[Crasher]

I have only ever seen that once, many decades ago on a Golf 2; it had me scratching my head for a while as in those days I was a new boy to water cooled rubbish... an air head.

[Adam95]

I am hoping for the same or a coolant pipe that hasn’t been fastened properly. The cars not losing coolant but no coolant comes through the return pipe unless I rev the engine over 2500rpm, even then it comes through in spurts. I got the mechanic to drain out and replace the coolant I then left it started for about half n hour with the cap off and heaters on full, I put my finger in the coolant reservoir and it was still ice cold so it’s definitely not circulating on idle.

[Adam95]

No there’s no flow via the small pipe at the top of the coolant reservoir whilst idling. The small rubber pipe connects to one of the three thin metal pipes that go along the front of the engine. I disconnected the rubber pipe at the other end of the metal pipe (above the gearbox side of the engine) and even then the coolant wasn’t flowing, though it has more flow than the small pipe at the top of the coolant reservoir especially when you give it a rev. My mechanics told me to bring the car tomorrow (Tuesday) I’m going to ask him to check all the coolant pipes and fasteners and replace the thin metal pipe and small rubber pipe at the top of the coolant reservoir. I’m also going to replace the cap but I doubt that’s going to do anything. Would you know the best way to bleed air out of these engines? Or is it just the usual heaters on full and coolant cap off and are there any tests I could do to pinpoint where the blockage/leak or air lock is.

[philipharmes]

You need to trace that thin pipe right to the end where it joins a thick pipe, it sometimes splits & goes to several thick pipes, depends on engine type.
Remove each thin pipe from the far end one at a time & blow through them from the reservoir end.

The thin pipe is how the air bleeds back to the reservoir.

If still no flow after they test clear, it could be that the water pump impellor isn't secure on the shaft so not pumping properly.

As you stated originally that the heater was intermittently working at higher revs, my money is on the water pump being faulty.
Hope i'm wrong.

[Crasher]

These systems self bleed and the heater matrix is constant flow so you don't have to worry about that, just fast idle until the fans come on but watch for it overflowing so you can get the cap back on in a hurry. Usually overnight the system will burp and need a top up and that's it.

[Adam95]

Thanks for the advice, I’m going to take it to the garage tomorrow and get all the pipes looked at. I just want to try the cheaper and easier options before changing the water pump or head/head gasket.

[Adam95]

Thanks I’ll try that now. The cap is sometimes stiff and gets impossible to tighten/loosen, though this rarely happens. Would you recommend getting a new cap?