91 520i overhating probs

[darkz]

hi peepz, need alittle help.. just aquired a 91 520i for peanuts.. has the m50 6cyl 24v engine.. got it cheap cos had suspected head gasket probs.. the car heats up but doesnt overheat.. only starts to overheat when u put any strain on engine.. thort it culd maybe be airlock being as it had just had brand new rad installed. after many bleeds have finally got car running ok.. will drive fine without going past half on temp guage. after stopping then restarting engine or occassionally whilst driving.. heat stops coming from matrix thru heaters and the coolant temp rises up to red.. sometimes heat comes back on b4 reaches red and then temp guage drops but sometimes gotta stop car.. now i think there may still be air in system but does anybody out there have any ideas or had any probs along these lines?? could this be the head gasket gone?? air lock still?? any help would be fantastic
cheers
darkz

[4-Door Flunky]

Hey, Darkz-
You are probably right. This sounds like a head gasket problem.
The best way to check is to do a compression test.
You'll need a compression tester. It's a gauge that measures air pressure in pounds per square inch.
Here's how it works: take out a spark plug. There's an adapter that will fit perfectly into the spark plug hole. Screw the adapter fitting into the plug hole, and then attach the gauge.
Take the coil wire off the ignition, so the motor won't start, and then twist the key for a few seconds. The engine will spin around a few times, and then you can get out and read the guage. It'll tell you how many pounds per square inch of pressure the cylinder pumped up to.
A shop manual or other resource on the internet will tell the correct reading for a cylinder in good shape. I'll look for that number.
Anyway, if the first one is fine, you go to the next plug. If the head gasket is toast you will find one...or two, that are wayyyyy lower than the others.
Then you'll know for sure. If pressure is leaking out of a cylinder through a bad spot in the head gasket, you'll find foam on the dip stick: a sign that water is in the oil. Check your dipstick and if you see that foam, quit driving.
Changing a head gasket may not be that bad: fixing the damage from overheating can be quite costly, especially since you have the hot-rod 24v engine.
You can buy a cheap compression test kit from Autozone for about $40. You can get a good one from Sears for about $50.
The compression test will also show you what kind of shape the rings are in. (You do the first test, write down the reading, and then squirt a little oil in the plug hole. If the reading goes up a lot, then the rings are worn.)
Another possibility is that your water pump isn't working right. That would explain the heater blowing cold air after the motor has warmed up(although a malfunctioning valve controlling the flow to the heater core will do the same thing), and it would explain the motor getting hot.
If you absolutely HAVE to drive it, you can get it to cool down a little bit by turning the heater on full blast...but that only helps if you have water flowing properly through the motor and the heater.
I haven't played with these cars enough to know how to check to see if the water pump is working. If I find out, I'll let you know.
Flunky
PS-I see you're from the United Kingdom...guess you won't be going to Autozone or Sears, but I'm sure a shop near you will sell the item I mentioned. It's pretty common.

[4-Door Flunky]

Also, hondas have a little nipple to bleed off air from the coolant system. I don't have a shop manual, so I don't know if BMWs have that same feature...
Flunky
PS-What if you had two problems? Head gasket AND a cooling problem? Ok, so that's not comforting information...

[darkz]

ok heres the deal.. i got this car after rebulding engine on peugot for someone... so far as i can tel there is no obv signs of gasket probs i.e .. oil in water ro water in iol - mayo.. which is why i tried bleeding in first place.. b4 it wouold get to 3/4s on temp guage after 2 mins now after numerous bleding attemts have got it too run fairly ok.. prob is the bleed nipple is only on rad. is type with expansion tank on rad.. now this nipple isnt at highest point, the electronic valve for heater matrix is but this has no bleed valve, also found out that water pump has plastic impeller fins which arnt very good..., erm thats about it for now.. havnt gotten around to compression test yet but will prob do soon if prob persists after another attemt to bleed system
tnxs 4 advice enways and if any other ideas pop up just drop em down
l8a darkz