Replaced my thermostat and now the car overheats

[urnvs]

I have a 2001 740i that has for as long as Ive owned it had an error in the computer about the thermostat not responding as expected (this was not an actual 'check engine' type of error). This has never caused any sort of issue, and was more of a nuisance. I was doing some work to the car a few days ago, and decided to at that time finally swap out the thermostat for a new one. When I remove the housing, I noticed that there wasnt a thermostat present inside! I installed the new unit, and now the car runs hot.

How can I test the electronic thermostat and/or the signal from the car to open it? Is the signal just supposed to be increasing voltage, i.e. the connector is at +12v when the computer wants maximum cooling?

[dburt86]

The electronic part of the thermostat can only make it open earlier, it cannot prevent the thermostat from opening.

What makes you think that ur car is running hot?

[Nyc Dito]

I have a 2001 740i that has for as long as Ive owned it had an error in the computer about the thermostat not responding as expected (this was not an actual 'check engine' type of error). This has never caused any sort of issue, and was more of a nuisance. I was doing some work to the car a few days ago, and decided to at that time finally swap out the thermostat for a new one. When I remove the housing, I noticed that there wasnt a thermostat present inside! I installed the new unit, and now the car runs hot.

How can I test the electronic thermostat and/or the signal from the car to open it? Is the signal just supposed to be increasing voltage, i.e. the connector is at +12v when the computer wants maximum cooling?

sounds like the PO covered up a issue he didnt want to fix...

did you check the temp via the cluster?

[urnvs]

The temperature gauge goes to red, the coolant cap starts hissing :P

[kouks]

Not sure if this can be done but you did not install the t-stat backwards did you?

[iDreadnaught]

That's interesting. The mechanical t-stat should open regardless of if the electronic element is plugged in or not. I mean, you can test it if you put it in a pot of boiling water and it will open up once it gets to it's opening temperature. Do you know what temperature t-stat you purchased? OEM opens at 105C.

My guess is that you just need to bleed air out of the system.

[urnvs]

No the thermostat is attached to the housing so it can only go in one direction. I purchased whatever the OEM one was.

I guess Ill start by pulling it again to see if the new unit is stuck closed.

[psjr]

Are you sure the car is fillied with 50/50 coolant?
A moderate overheat shoudln't generate enough pressure to blow the cap.
It does sound like something fishy with the previous thermostat being gutted.

[urnvs]

Just to clarify, I thought the point of the electronic thermostat was to keep it closed so the engine would hear up faster at cold start...?

[psjr]

Just to clarify, I thought the point of the electronic thermostat was to keep it closed so the engine would hear up faster at cold start...?

No, the DME can open it early when it anticipates high load events.
It is like a normal thermostat, but contains an electric heater the DME can send current through to open it sooner.

[urnvs]

No, the DME can open it early when it anticipates high load events.
It is like a normal thermostat, but contains an electric heater the DME can send current through to open it sooner.

Well as a test I disconnected the thermostat, and my overheating issue went away so the computer is definitely closing it.

Can someone possibly check the thermostat connector on their car? Id like to find out (right/left if you have the connector clip facing you);
1. What voltage does the thermostat get cold, and which side is positive/negative?
2. What voltage does the thermostat get with the engine hot, and which side is positive/negative?

[Kurt C]

Everything you need to know about the characteristic Map cooling thermostat, courtesy of Timm's 7 series page at meek net

http://www.meeknet.co.uk/E38/E38_Map_Thermostat.pdf

[locknload]

Obviously, the po went through several attempts to remedy this before throwing up his hands and just removing the t-stat. Too bad you can't track him down to see what he tried before giving up on it.

Edit: Just had a thought. I think these cars have 2 temp sensors. One feeds only the temp gauge (which seems to be working properly here), the other feeds the DME. If that one has failed, it could be telling the DME the engine is ice cold, which would force the t-stat closed until it warmed up, which, in this case, will never happen. Just a theory.

[urnvs]

So...I had a chance to mess around with it a bit more last night. The results are the same with the plug on the thermostat disconnected.

Here is the really odd part, the car will start and idle for 30+ minutes without any indication of getting too warm, but the instant I start to drive it anywhere it quickly heats up to the red line?!?

[DrCharles]

As dburt86 already pointed out, the heating element in the t-stat cannot "force it closed", it can only make it open sooner.

Make sure the system is fully bled of air. After that, I'd be worried that you have a head gasket leak (from a cylinder into a cooling passage). I had a M106 (turbo M30 engine) that would run fine all day long, unless I floored it into boost. Within seconds it would overheat and puke all the coolant. Thought it was a t-stat problem at first but changing it didn't help. Finally tore it down and found the expected head gasket leak.

[Timm]

From the first post it seems there are two possibilities:

When you fitted the thermostat you failed to bleed the coolant system correctly, follow this:

http://www.meeknet.co.uk/E38/E38_Bleeding_The_Cooling_System.htm

Or, the coolant system cannot cope with the amount of heat generated - now that could be because the head gasket has failed, and it will fail if you keep running the engine into the red, or it could be that the radiator is blocked with debris internally or externally, or it could be that you have a water pump with a plastic impeller that has disintegrated. A leak-down test will find a head gasket leak, as will a block tester kit (probably cheaper). Check if the radiator has any cool spots showing that it is blocked internally - and check for any obstructions, there can be a huge build-up of debris between the radiators.