M40 Mayonnaise (head gasket?)

Post Sat Mar 13, 2021 8:08 pm

Hi guys,

Just as I was getting onto the fun interior jobs I had this thrown up.

Took it for a drive in the morning where it ran fine. Parked it up for 10 or so minutes with the engine off and at the next startup the idle was almost stalling and jumping. I put the idle up on the throttle body and it felt fine as before. Went for a 20 min drive yesterday and had the idle jumping all over the place, between 600 and 1200. Checked the rocker cover cap which had steam coming out and a small amount of emulsified oil on the underside of the cap. Checked the dipstick and found a tiny amount on the tip.

I'm assuming this means the head gasket is gone. Any advice on how to confirm would be appreciated. Haven't got around to a compression test yet.

Also, I haven't had the car overheated in my ownership.

Any advice is appreciated. Cheers

Post Sun Mar 14, 2021 2:27 pm

Check for air leaks in the induction side. Much more likely to be the cause of the dodgy idle.

Mayo could just be due to short journeys/lack of use. Drive it and keep an eye on the temperature gauge.

Ben

Post Sun Mar 14, 2021 8:44 pm

BenHar wrote: ↑
Sun Mar 14, 2021 2:27 pm
Check for air leaks in the induction side. Much more likely to be the cause of the dodgy idle.

Mayo could just be due to short journeys/lack of use. Drive it and keep an eye on the temperature gauge.

Ben

Cheers for the reply. Temp has always been steady which is the one thing making me question whether it's the head gasket.

My other theory with the idle is if it's the blue temp messing things up while the coolant is escaping? Probably going to have a look over the intake first since it's in the way of the head anyway.

The idle is confusing since it was idling perfectly steady and now it's all over the place.

Post Mon Mar 15, 2021 10:46 am

Is the coolant level in the bottle decreasing? If not, then there's nothing wrong with the head gasket.
Mayo on the cap is normal in winter, especially if not doing long runs.
Whatever is wrong with the idle is nothing to do with any possible head gasket problem.

Post Mon Mar 15, 2021 2:18 pm

Brianmoooore wrote: ↑
Mon Mar 15, 2021 10:46 am
Is the coolant level in the bottle decreasing? If not, then there's nothing wrong with the head gasket.
Mayo on the cap is normal in winter, especially if not doing long runs.
Whatever is wrong with the idle is nothing to do with any possible head gasket problem.

Cheers for the reply Brian.

Just had a look now and the expansion tank is pretty much empty. Had the coolant emptied and refilled when I did a load of work around 1300 miles ago. Probably should've been keeping a closer eye on it but I haven't seen anything strange going on with the temp gauge so it didn't occur to me to check.

It's booked into the garage on Wednesday so hopefully they'll be able to point the finger at something. Obviously it's a small amount of mayo but I'm just confused as to why it would be at the bottom of the sump as well.

Appreciate the help.

Post Mon Mar 15, 2021 8:33 pm

Near empty coolant bottle is not a good sign, but if you've done other work, it could simply be slow leak or the system not bled properly. Fill the coolant bottle and monitor the level and the oil condition.
Note that as long as there was still some coolant in the bottle, no harm should have occurred.