Engine goes hot>cold>hot, heater goes cold>hot>cold

paul

paul

XjunkieNL
Been puzzling with the engine temp for some time.
Can't find the real problem. Anyone had similar issues or tips?

On the highway with 4500 revs I have this curious engine temp cycle.
The engine heats up from 90C/195F to 100C/210F and goes back 90C/195F in a timeframe of 5 minutes.
This cycle repeats all the time, the temp never stays constant.
At the same time the heater goes from hot (engine at 90C/195F) to cold (engine at 100C/210F) and back to hot.
It also happens at lower revs only the difference between hot and cold is smaller.

Things I tried: getting air out, changing thermostat, filling anti freeze up to the max.
Tubing and radiator are 5 year old. Fan is working and comes in at approx 100C.

At the moment I'm considering ordering a new waterpump.
Although I don't really know if it's possible a waterpump can work intermittent.

/Paul
 
Sounds like there is still some air in the system or lots of crud moving in the system causing blockage. I lean towards Akron in the system as it is more common.

If the heat is intermittent, that is the clue for me. Probably air moving thru the system.

I start my bleed process by filling the res pretty full. I open the heater valve and then crack the radiator bleed valve to see if I hear any air come out prior to starting the engine. Close the rad bleed valve. Then I bleed it with the engine running, heater valve still full open and nose up if up if possible. Rev the engine and crack the rad bleed occasionally till coolant flows out. Don't forget to keep an eye on the res level as you release coolant up front. The last thing you want is to suck more air in.
 
Also if you have any seeping fittings anywhere in the entire system(hose clamps, gaskets, etc) it will suck in air and you will never bleed it properly. Doesn't hurt to recheck all the hose clamps, mating surfaces (t/stat housing, w/pump housing, etc) for ANY signs of crud or seepage when you repeat the bleed process.

Other question would be what brand & model T/stat you used, as many will fit but don't provide the proper cycling, which could also lead to temp fluctuation, IMO.
 
Yes, does sound like air in the system is the main issue. But in regard to the thermostat, I will just say it again: For me the Stant Superstat was the best thing I have ever done for the cooling system. The temperature gauge use to move back and forth. But with the Superstat the needle moves up to 190 and nails itself there for the duration of even hot weather driving. Amazing result for so little effort.
 
Thanks, happy to see it's most probably air pockets.
Although I don't fully understand, how can they move so freely and quick through the system?

I can't spot any leakage under the car, but will check all fittings.
The thermostat is an after market type purchased from a UK X19 vendor.

Bleeding I have always done, nose down with the expansion cap off.
After fifteen minutes all was done. Maybe I have to review this method.
It has always worked well, I never installed the bleeding valve on Bob G radiator many years ago.
 
paul said:
It has always worked well, I never installed the bleeding valve on Bob G radiator many years ago.
Click to expand...

The bleed valve is there to allow you to release the air pocket that forms at the top of the radiator. IMHO you haven't properly bled the system since you deleted the radiator bleed valve. Coolant enters the rad thru the highest tube and leaves from the lowest tube. Without the bleed valve, you are going to have a difficult task getting the air out of the top of the radiator. Even with the nose down and the res cap off, the exit port from the rad is well below the top of the rad.

I believe the fluctuations are from the coolant moving thru the system inconsistently. As you accelerate, stop, go up a hill, go down a hill, turn left or right, you are exerting forces on the coolant and air in the system. These forces vary the location of the coolant and air in the system causing the temp fluctuations you are seeing. In effect, you are pushing an air bubble around your cooling loop. As air builds in the system, the fluctuations will become more apparent.

I know messing with the rad is no fun, but you probably need to put the bleed valve back in the top of the rad. You could try unclamping the top rad hose to see if you might coax the air out of the loose hose connection. The original valve is the best solution in the long run. I open that valve a few times each driving season just to check how much air is in my system.
 
A brief pictorial explanation of why the bleed screw is important even if you tilt the car up in the back.

radiator bleeding.jpg
 
I would have to agree. No way you are removing all the air pockets without a way to bleed the top of the rad.
 
I recommend that when you open the bleed screw, just open it several turns in order to unseal the bleed hole. Do not remove the bolt all the way unless you are paying close attention to the fiber washer gasket. It likes to fall off and get stuck somewhere inaccessible (Don't ask how I know.).
 
Great input! Much appreciated.

First thing I will do, before looking at other things, is putting a bleeder valve in.
As I have thrown away the old one together with the rad, are there any inventive new ways?
Or is the old bleeding screw the best way.
I heard for central heating there are some that automaticauto bleed the radiator.
 
If you still have / can find an original bleed screw, by all means reuse it. Remember to check that the fiber washer is in good shape; replace it if it isn't. There are options if you need a new one, I got one of these for my car. It is a lot shorter than the original, so it is less critical how the radiator bung lines up with the hole in the frunk.

Edit: I believe the BMW part number for the original plastic version of that bleed screw is 17111712788, but definitely get one of the brass or aluminum after-market replacements.
 
Last edited:
BBE0907B-3E85-4093-9530-69A589483346.jpeg In the light cavity is the automatic bleeder. It is possible to use a standard bleeder from central heating. With the small key on an extension it is very easy to bleed. Less change to screw up the weak copper tread in the rad.
paul said:
Great input! Much appreciated.

Or is the old bleeding screw the best way.
I heard for central heating there are some that automaticauto bleed the radiator.
Click to expand...
 
An automatic bleeder is a fantastic idea, Max.
I should have check it last Saturday.
Can you still store the roof in the front?
 
Thanks for the alternative, Bjorn.
I can see how it makes it easier even it the radiator itself is a little of.

Is it easy to operate when installed?
I probably have to go in this direction as I threw the original bleed screw.
 
paul said:
Thanks for the alternative, Bjorn.
I can see how it makes it easier even it the radiator itself is a little of.

Is it easy to operate when installed?
I probably have to go in this direction as I threw the original bleed screw.
Click to expand...
It is easy enough to operate from the access hole in the frunk using a short, flat blade screw driver. Perhaps a tad more fiddly than the OEM bleeder with the internal hex head but not a big deal.
 
I'd love to know what procedure they used on the assembly line!


I tilt my car up from the rear and use the bleeder up front, I do it cold and then start/run the engine for a few fan cycles - then bleed again (with the heater valve open). So far I've managed to get it to work well.
 
I fitted a flexi brake hose in place of the original bleed nipple. It's probably the best mod I've made to my car so far in 28 years.
Use a standard banjo bolt at the top of the radiator and screw in a bleed nipple where the copper brake pipe would have attached.
I made this mod over 10 years ago, I think the bleed nipple was a different thread to the ones fitted to the calipers as standard.
Bleed fitting 2.jpg
Bleed fitting.jpg
 
Nice modification, Steve. I think I understand what you did.
Where do you go with the hose? In the frunk, or somewhere else?
 
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