Random overheating

[Jay 735i 0]

I only seem to come back on here when I have a problem...

Anyhoo, the 735 overheated randomly yesterday while in traffic (car had been driven a short distance, parked up for 15 minutes, then driven a short distance again in slow traffic). The warning bell alerted me the temp was on the way up, so I immediately pulled over and turned the car off. After 5 minutes, I started the car and the temp gauge quickly dropped back to operating temp. I needed to get to work, so risked the drive and spent another 20 minutes in crawling traffic, temps normal, but with the heater and blower on full (luke-warm air only). To test, I turned off the heater and blower and it overheated again almost instantly. Turned the car off again, turned the heater and blower on full and restarted. Again temp gauge quickly returned to operating temp. The rest of the drive was all good, 10 minutes out from the end of the journey, I turned off the blower and put the heater on cool, but no overheating. Wound down the window and could hear the fan engaging, as it should.

On journey home, car slowly got up to temp, but when waiting to get onto the motorway it started overheating again. The lights were green, so just drove onto the motorway and watched to see if the temp would drop with the extra air flow. It did, back to operating temp. All was good for the rest of the journey, even when siting in my driveway with the blower and heater off and reving the engine at standstill.

Done some quick searching on here, there's a few posts that are very helpful, but nothing specific to my problem. I will attack the issue on the weekend, will replace the thermostat, check the fan clutch, check the return hose to the expansion tank and bleed the system fully.

Any other pointers that anyone can share would be greatly appreciated!

[nzhaystack 0]

I only seem to come back on here when I have a problem...

Anyhoo, the 735 overheated randomly yesterday while in traffic (car had been driven a short distance, parked up for 15 minutes, then driven a short distance again in slow traffic). The warning bell alerted me the temp was on the way up, so I immediately pulled over and turned the car off. After 5 minutes, I started the car and the temp gauge quickly dropped back to operating temp. I needed to get to work, so risked the drive and spent another 20 minutes in crawling traffic, temps normal, but with the heater and blower on full (luke-warm air only). To test, I turned off the heater and blower and it overheated again almost instantly. Turned the car off again, turned the heater and blower on full and restarted. Again temp gauge quickly returned to operating temp. The rest of the drive was all good, 10 minutes out from the end of the journey, I turned off the blower and put the heater on cool, but no overheating. Wound down the window and could hear the fan engaging, as it should.

On journey home, car slowly got up to temp, but when waiting to get onto the motorway it started overheating again. The lights were green, so just drove onto the motorway and watched to see if the temp would drop with the extra air flow. It did, back to operating temp. All was good for the rest of the journey, even when siting in my driveway with the blower and heater off and reving the engine at standstill.

Done some quick searching on here, there's a few posts that are very helpful, but nothing specific to my problem. I will attack the issue on the weekend, will replace the thermostat, check the fan clutch, check the return hose to the expansion tank and bleed the system fully.

Any other pointers that anyone can share would be greatly appreciated!

Hey,

I had the exact same problems with my e30, turned out my thermostat was shot. took about 20m to replace on mine.

[Eagle 1662]

May have an air pocket. It sounds like you got it sussed what to do

[Jay 735i 0]

Cool, cheers for the replies... yeah am hoping it's an air pocket or thermostat and from searching on here I should have everything covered, but just wanted to see if anyone had similar problems and could add a "don't forget to check...".

All good (hopefully)

[Wom 0]

yea I think you've got a good list to work from

only other suggestion would be to get it scanned to see if there's any fault codes

a couple of years ago my e36 was overheating when the car was stationary, but when moving it was fine

ended up the viscous fan coupling was the culprit

before that, I had thermostat issues which resulted in my engine taking about 20 minutes to get to proper running temperature

[w_elliot 7]

try giving the cooling system a good bleed, and then giving it a bleed again. doesn't cost anything and worked for me when I had the same issue in my e30. lack of a working fan also causes stationary overheating, as will a waterpump with a damaged impeller that can't circulate enough coolant at low rpm.

[BM WORLD 1283]

sounds more like the visvous clutch for the fan is stuffed .

over heats when sitting then cools back down when moving faster.

also the other thing on these bmw's is the electric aux ac fan on the front should also cut in at a certain temp

there are 2 speeds and double temperature sensor on the side of the radiator.

the slower speed works when the ac is switched on or if the temp goes over 91deg C then the higher speed cuts in when it goes over 99deg C , if it doesn't cut in at the 91deg C or when the ac is turned on the resistor thats mounted in front of the electric is probably no good and will need replacing or bypassing.

but i think your main problem is the mech viscous clutch.

but if you keep using it like this you might do damage to the engine

"Wound down the window and could hear the fan engaging, as it should" is this electric fan or the viscous fan cutting in???.

Double temperature switch part No.5

fan low speed resistor part no.6

viscous clutch part no.2

Edited April 6, 2011 by Brent HARTGE535i

[Silver Fox 43]

Agree with Brent, I had exactly the same problem with the E32, and changed the viscous, I can now sit in traffic all day and the needle won't move over halfway.

[Jay 735i 0]

Thanks for the further help guys, much appreciated.

I do suspect it might be the viscous fan (which is what I thought I could hear when I listened out for it, but could have been the electric fan, now you've mentioned it).

To clarify, it doesn't overheat in stationary traffic, in fact when I got home after the evening drive, I sat on my driveway for a good 5 minutes trying to get it to overheat, which it didn't. I'm figuring it's a thermostat that is taking a lot longer to open fully than it should, hence why it overheated first, then returned to normal operating temps after it's got really hot. The second time it overheated in the morning goes against this theory though, which is why I'm going through the other possibilities, but after each overheat, I still ended up stuck in crawling traffic and the temp stayed at normal (with or without the heater on full).

Car won't be driven for any more than test drives until it's sorted, I have heard the viscous fan can cause all sorts of damage if it fails, so will check that first.

[Jay 735i 0]

Swapped out the thermostat, seemed to work on the test drive, but just been for a long drive and it's still overheating occasionally.

What seems to be happening is the viscous fan is engaging some of the time, but when it starts overheating, it's quite obvious it's not engaging.

What actually causes it to engage/disengage? Is it linked to a sensor? Will track down a good one and swap, see if that helps. Anyone got a good one lying around?

[BM WORLD 1283]

Swapped out the thermostat, seemed to work on the test drive, but just been for a long drive and it's still overheating occasionally.

What seems to be happening is the viscous fan is engaging some of the time, but when it starts overheating, it's quite obvious it's not engaging.

What actually causes it to engage/disengage? Is it linked to a sensor? Will track down a good one and swap, see if that helps. Anyone got a good one lying around?

viscous cuts in and out by the amount of heat that is transfered through the water pump shaft, heats up the fluid/oil inside .

and what happens is over time the fluild/oil leaks out then it slowly becomes useless

[Jay 735i 0]

Ah sweet, thanks for that. Was looking at it and wondering how it worked, couldn't see any wires or anything that would point to electronic control.

Just got one off tardme, so will swap out and see if she's all good.

Edited April 9, 2011 by Jay 735i

[kiwi535 538]

when you say you are going to check the line back to the coolant overflow you are gonna check to see if its clear right?If you drive with the window down there is no mistaking if the viscous fan is engaged.When you take off from even the shortest stop the fan will have engaged and the ol m30 sounds like a truck...the fan ussually cuts out as you change into third..so it reacts very quickly to the cooling air flow

Edited April 9, 2011 by kiwi535

[Jay 735i 0]

when you say you are going to check the line back to the coolant overflow you are gonna check to see if its clear right?If you drive with the window down there is no mistaking if the viscous fan is engaged.When you take off from even the shortest stop the fan will have engaged and the ol m30 sounds like a truck...the fan ussually cuts out as you change into third..so it reacts very quickly to the cooling air flow

Yeah I checked that and it's clear, in fact the coolant system seems pretty healthy, no signs of gunge or sludge.

It is confusing with the fan, yesterday I sat on my driveway after the first test drive and I could hear it, but on the longer run I was sat at the lights and it started getting hot, when the lights went green I pulled away and there was definitely no fan. Will swap the clutch out when it arrives and see what happens...

[39KiwiTouring 2]

I reackon it will be air in the system, youre op said you could hear the fan clutch engaging like it should and the temperature guage suddenly shooting to the red means, thats the air pocket passing the sensor, more throttle pushes the water pump harder, pockets passes the sensor, they are a mere to bleed the E39's are anyway.

[39KiwiTouring 2]

I was also going to ask if you have done any coolant system maintenance to back up my claims, new coolant or a big top up can add air into the system.

[Lubed 55]

Another thing to look at would be checking for a blowen head gasket given the symtoms you have discribed.If you have access to a cooling system pressure tester then you can carry out a accurate test for a leaking gasket by removing the spark plugs then apply pressure overnight then checking for coolant in the cylinders the following day, in particular cylinder No 6.

[Jay 735i 0]

I reackon it will be air in the system, youre op said you could hear the fan clutch engaging like it should and the temperature guage suddenly shooting to the red means, thats the air pocket passing the sensor, more throttle pushes the water pump harder, pockets passes the sensor, they are a mere to bleed the E39's are anyway.

Reackon you may well be onto something, I bled it again last night, just more out of curiosity than anything, and there was plenty of air back in the system. Will bleed it a few more times as, like you say, it's really simple and takes about 30 seconds (assume E39s and E32s are a similar set-up). I haven't done any maintenance or anything that would introduce air into the system, so no clue as to how it would have got there in the first place... unless... (see below)

Another thing to look at would be checking for a blowen head gasket given the symtoms you have discribed.If you have access to a cooling system pressure tester then you can carry out a accurate test for a leaking gasket by removing the spark plugs then apply pressure overnight then checking for coolant in the cylinders the following day, in particular cylinder No 6.

That's worst case, really hoping that isn't it, but I have to consider the possibility. However, there's no sign of coolant loss, unless its the gasket failure that's somehow letting air into the system. No signs of condensation on start-up, no mayo in the oil, runs same as ever, so fingers crossed I'm not looking at a head gasket. Has done 250ks though, so wouldn't be that surprised if the gasket has gone.

[39KiwiTouring 2]

keep bleeding it it took me 4 hours to bleed my one, it's bleed once water only runs out the bleed screw any bubles or hissing still air in there.

[Jay 735i 0]

yeah will keep bleeding it over the next week. Not driving it at the moment, but will come home from work, bleed, go for a drive, let it cool, bleed again, then do the same the next day. I've also read that it can help if you have the car on a slight incline, so as the driveway is a slight incline, I'll reverse in and try that.

Thanks for the help!

[Eagle 1662]

Way ive always bleed my M20 and M30's

Block the overflow tube with finger

Put mouth over the expansion tank( basically using your mouth to keep system pressurized and push air out )

Uncrew the bleeder and blow

Helps if someone to check that no air coming out the bleeder but i find doesnt take many blows and you can always close the bleeder if you need a rest

Bit weird but worked every time for me

[Jay 735i 0]

Cool, will go home tonight and... um... give my car a... erm.... blowjob...

But seriously, can see the logic behind it.

Edited April 11, 2011 by Jay 735i

[MGL 4]

See if you're new thermostat has the little bleed hole thing at the top of it. I brought a cheapy one recent for my old e30, and it didnt have the little bleed hole in it. Made it a massive pain to bleed, I gave up and drilled a 2mm hole in the top of the thermostate plate. Then used the bong method to bleed it, took about a minute and hasnt given any problems since (well, until I sold it at least!)

[Jay 735i 0]

New thermostat is exactly the same as the old one, so should do.

Got the new fan clutch installed on Weds night, bled system yet again and finally braved driving the car to work this morning (so plenty of crawling traffic). All good, no overheating, fan engaging nicely, way earlier than it used to even before the overheating started, so fingers crossed all is ok...