Thermostat Soft-Failed at 85k - Do I need to replace other parts?
 

Hi,

Somehow it's taken until Dec for it to start getting consistently cold here. I've been noticing my temp gauge not getting up to the center line every now and then (weather would randomly get hot and cold so it wasn't consistently below temp). Today I finally opened the test menu and went on my morning commute (about 15 min - 2-3 min street, 7-8 min highway, 3-4 min in traffic on a congested 2 lane road). Temp only barely got to 62 C and that was after I parked! Planning to replace the tstat since most threads here say it's not too bad of a job, but at 85k miles is it worth doing the other "while I'm in there" items or could I get away with just the tstat? It recently became my daily so trying to shorten the amount of time it spends taken apart.

Thanks!

I'd do the water pump if original...and I would squeeze upper and lower rad hoses..if they are pillow-ey soft, I would replace them...

What is the age of your cooling system generally?

2004 was a long time ago and rubber and plastic wear out.

That's an interesting way to diagnose. Depends a lot on temperature.

I'd be more concerned about the hoses (good point about the plastic hose ends being a concern as well as the rubber - on my 3.0i's upper radiator hose, it was the plastic end near the radiator that failed first, at around 180k miles) than the thermostat. And it sounds like if anything, the thermostat failure mode is that it sticks open, which is not going to blow anything up.

But on the thermostat, does anyone have a good number on what the resistance of the heating element should be? My 2001 3.0i has a 50k-mile old, dealer-installed Genuine BMW thermostat, and I don't have an OBD code or engine light, but ...

Recently I've been getting the following code on my Foxwell:
1-DME, 7B DME: Activation, map cooling
Looking that up gives reports of it being the thermostat heating element. I measured the resistance of my heating element as 3.7 Ohms, which seems in the right ballpark, but am wondering if it is in spec.

OP, related to this, have you measured your thermostat heater resistance? Scanner info (note that I have no issues I can see other than that code which only appears in the Foxwell BMW diagnostics)? Have you tried leaving the connector unplugged to see if your engine runs hotter? Suggested for debugging, not as the solution.

I'm aiming to replace mine tonight. I'll get an Ω reading if I get a chance.


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Thanks for the info everyone. AFAIK my cooling system is fully original. The CEL hasn’t turned on related to the temp but did turn on a few weeks ago with lean codes so I replaced the intake boot near the throttle body. Interestingly there was no noticeable problem with the boot other than the rubber looking old as expected. After replacement I haven’t had a CEL.
Yesterday I unplugged the thermostat heater. The car warmed up to 78c but dropped to 70 when I got on the highway. Ambient temp was 10f higher though so not sure if it’s a coincidence.

Definitely going to order a thermostat but while I’m in there anything else I should replace?

Thanks!

I'd take a good look at the two hard plastic cooling pipes that enter just behind the oil filter housing.
I saw some residue there during other repairs and for a long time was trying to find a slow leak in the cooling system.
When I did the head gasket, I changed those two and the ends had degraded to the point they were weeping coolant just fast enough to evaporate but not drip.

For M54 engine, the 2 plastic hard pipes are good until 20y/200K or so, as long as you don't touch them. They don't burst but rather leak a bit here and there, if it happens.

The tstat failure is a good wakeup call.
I posted a DIY "Partial Cooling Overhaul" if you have around 100K-130K or so...
It is in this forum.

Thanks. I’m at 85k. I wouldn’t have to mess with the plastic pipes or even push them out of the way at all to replace the tstat would I? If not I may just do the tstat and save the cooling overhaul for around 15-20k miles or so. Currently no coolant leaks *knock on wood*