High Pitch Humming Sound

[Revant01 0]

I have a 93 205 GTI 1.9 with the DKZ engine. I bought the car apparently with a blown head gasket, but after a bit of poking around i think it is fine. There is no water in the oil and the engine runs fine. However, after warming up the coolant would start to bubble gas (steam?) through the expansion tank. I measure the temps of everything and found that the radiator was not getting any flow through it. Found the thermostat stuck closed. Fixed this but the thermo switch is not turning on the fans to the engine still overheats if it is sitting still without any air flow, at least the water is flowing through the radiator now. The heater on full with the windows open was enough to stop the engine overheating when idling in the driveway.

 

I am hoping that there hasn't been any damage to the engine with the overheating. There is a high pitched humming sound coming from the engine. By ear i have localized the source to be somewhere behind and below the right of the inlet manifold. Does anyone have any idea as to what would cause this sound? Could it be something that has been overheated and lost its oil? I am not sure what is in this are of the engine. As it is such a high pitched sound it is easy to pinpoint it's source.

 

I am going to replace the gearbox, but would rather not have to strip the rest of the engine apart to search for something without any real ideas.

 

Thanks

 

Alex.

[DamirGTI 342]

I'm having very similar kinda problem/sound on my Ph2 DKZ car as well , however i don't drive/use this car as the MOT is expired so i didn't really need to chase the source of the sound .. i just start it up occasionally and this sound appears similarly as you described - humming when the engine si fully warmed up to the operating temp. and it seems that it comes form the cambelt area and this engine also had blown head gasket which i've fixed but i dunno if the "sound problem" was present before the HG blew as i bought the car with blown HG

 

Anyway id try with new cambelt tensioner as me thinks that it might be the tensioner

 

Damir

Edited February 25, 2009 by DamirGTI

[Rob_the_Sparky 9]

How soon after start-up does it "bubble"? This could be an indication of a bust HG as they pretty much always go between pistons and water jacket. The engine then pumps air into the water making bubbles...I think a compression test or leak down test is in order.

 

Also a bust thermo switch is a VERY common reason for a bust HG, fan fails, engine overheats, HG breaks.

 

Rob

[Revant01 0]

How soon after start-up does it "bubble"? This could be an indication of a bust HG as they pretty much always go between pistons and water jacket. The engine then pumps air into the water making bubbles...I think a compression test or leak down test is in order.

 

Also a bust thermo switch is a VERY common reason for a bust HG, fan fails, engine overheats, HG breaks.

 

Rob

 

Yes this is what i expected when I bought the car, as you describe the same as what the previous owner thought was happening. However, the bubbles only appear in the expansion tank after the coolant has heated up. I have an infra red thermometer and monitored the temps as the engine heats up. With the heater on full inside the car I could keep the engine idling around 90 degrees C for a while measured on the coolant expansion tank hoses. As soon as i turned off the heater the temp would shoot up to 100 deg and the coolant would boil somewhere, creating the bubbles.

 

As there is no bubbles until it gets to 100 deg i don't think it is coming from the combustion gases, as they would be obvious as soon as the engine was started? Also the oil is still looking normal.

 

I am borrowing a compression test gauge this weekend to double check this over the weekend as well.

 

As for the fans, as there is two it is unlikely they have both broken together, so i started tracing possible errors. The most likely culprit i think is the relay in the plastic box behind the LH headlight, as this box had been repositioned under the front bumper to fit in a short induction kit and is consequently full of mud I have also got a spare thermo switch to try out.

 

I will report back if i can get the fans to work and stop the engine overheating to see what effect this has on the noise after it is run for a while.

 

DamirGTI, the source of the noise is on the other side of the inlet manifold from the cam belt so i think our problems are different, even tho they sound the same, unless it is somewhere in the block affecting different pistons or something? (Just a guess)

[DamirGTI 342]

As soon as i turned off the heater the temp would shoot up to 100 deg and the coolant would boil somewhere, creating the bubbles.

 

Might be faulty expansion bottle cap if it's leaky the system will not be pressurized and that will lead to the coolant boiling and thus creating air pockets/bubbles inside the system .. but mostly when the engine is at idle/low revs or after is switched off .. worst case scenario will be cracking of the metal .

 

I had this once and didn't really notice it until my engine started to behave "funny" .. removing the head revealed cracked cylinder liner

 

 

Damir

 

EDIT : but do fix the fans asap ! as they should switch on way before 100 C mark , better still , buy low temp termo switch for the fans of say - 87/77C rating .. and in the pair together with low temp thermostat of 83 C rating that would do nicely for your hot climate down there !

Edited February 26, 2009 by DamirGTI

[Henry Yorke 269 3 Cars]

I have skim read this post so apologies if this has already been said, but does the fan switch cut in at when it gets hot - you will need a test lamp or a multimeter for this. A high pitched buzzing noise may be a stuck electrical fan if it only happens when hot. Be very careful if poling the fan whilst buzzing as it may suddenly start!