If there is sudden and gradual loss of power - BEWARE! - The Unofficial BMW M5 Messageboard (m5board.com)

DavidS · 4th October 2006, 00:34

A word of caution regarding coolant leaks on all BMWs:

The coolant temperature sensor is located high up on the engine. If there is a massive loss of coolant, and the sensor gets dry, it will happily keep reporting a nice low temperature, while the cylinder heads will melt at the valve seats from the heat in the exhaust gases. In that case there will be no warning lights or anything, just a destroyed cylinder head.

Any sign of sudden gradual power loss, immediately stop the engine.
Depending on how much power the engine is running at, you have between 5-30 s before there is insanely expensive damage.

The situation is far more immediately dangerous to the engine than a complete loss of oil pressure would be.

That power loss likely comes from overheated cylinders causing pre-ignition and the DME pulling ignition timing to compensate.

There are sensors for pretty much everything, but not for the temperature of the cylinder heads. (Apart from the coolant temperature sensor, which stops working if it gets dry) Forget about noticing the problem on the oil temp gauge, it is a very slowly reacting temp sensor, and it does not start to move until it is far too late.

David

(I just posted this in a thread about coolant leaks, and then it occured to me that it might be of value as a thread of its own.)

FallbrookM5 · 4th October 2006, 00:41

not to mince words, as this sounds serious....but is sudden and gradual not mutually exclusive ?

DavidS · 4th October 2006, 01:02

Quote:

Originally Posted by FallbrookM5

not to mince words, as this sounds serious....but is sudden and gradual not mutually exclusive ?

Not mutually exclusive in this context. Suddenly starting powerloss, that gets gradually worse by the second as the cylinder heads overheat.

I´ve seen perhaps 3 or 4 such heads in BMW dealer workshops, and it is not a pretty sight. Molten aluminium and holes straight through the valve seats.

David

FallbrookM5 · 4th October 2006, 01:04

My old man fried his head on a 1986 325i in germany.....I think it was the same issue. Scary to think about what thecost that would be on the M5. I agree with your explanation though ! I will be very aware.

DavidS · 4th October 2006, 01:15

Quote:

Originally Posted by FallbrookM5

My old man fried his head on a 1986 325i in germany.....I think it was the same issue.

Sorry the hear that. I have no first hand info from anyone who has driven a car it has happened to, just good advice from mechanics.

Perhaps you could ask him what happened, and what, if any, warning signs there was immediately before the damage.

David

psunitlions1 · 6th October 2006, 05:43

im experiencing some lugging of the engine around 3500 rpms like it kinds wants to go but make a deep gutteral sound and you can feel it in the seats but once it gets what it want at higher rpms then it seems fine

is this lack of oil, cps or the dreaded cylinder heads?

veilside02 · 6th October 2006, 08:45

thanks for the info

racerock · 6th October 2006, 14:28

My brother in laws dads car was a few miles from this condition. Exactly as you describe. Pinhole leak in the radiator, overflow tank sensor stuck. No indicators that it was low on coolant.