99 E39 M5

[scerbera]

My M5 seems to lose power when it gets hot or if the air temp is hot, it runs much better first start up or when air temp is below around 15 degrees c.

I thought it was the fuel pump failing, and it did fail, I have replaced it and thought that the issue was gone but it is still there intermittently, I have a fuel filter to go on this weekend so that will rule out fuel delivery.

I'm thinking ht leads possibly? Or there the air temp sensor is sited in the MAF and I know this can be moved, could it be that? When it happens it sounds awful and like it's running lean. I'd say it's a good 100bhp down when it does it.

Any ideas?

[Baz Tench]

I've never had this problem, but by your description, I would be looking at the MAFs (I think there is one on each bank?).

If HT leads are failing, it's normally more noticeable when the engine is cold (IME).

I think it would be best it get it plugged in at a BMW specialist/dealership. Could also be a coilpack failing?

Edited by Baz Tench on Friday 3rd October 05:17

[tjlazer]

Check the fan clutch, mine had a similar issue causing it to trim the fuel mix once hot.

[Pip1968]

Mine was a 1999 and had some power loss which I think was down to the oxygen sensors. If I still had it I would have had a look through my wad of receipts (8 years ownership) as my memory is none too brilliant.

Pip
Ps still miss it.

[scerbera]

Something else to add is that I also get an intermittent misfire? between 1000 and 2000 rpm when going on the throttle quickly or holding it flat open in a higher gear for example it shudders sometimes, it did it today on first 2 drives, then 3rd it wouldn't do it again.

[scerbera]

Changed fuel filter and seems better. Will see if that has cured it or not?!