[Dazza335iC]

My SES popped up today. Torque Pro says P0128 Low Coolant Temp. I monitored the temp on the freeway and it dipped as low as 60C under light load, much too cold! After I got home and parked, I let it idle for 15 minutes and the temps got as high as 97C... Also not good. My plan is to replace both the coolant and EGR thermos, anything else I should check or replace while I'm in there?

[locolobo]

Replace thermostat - easy job. I have done it. There is a DIY somewhere. I wouldn't mess with the EGR thermostat and I'm not sure US cars even have one. I replaced mine due to temps always running 82C when it's supposed to be 88C I think. Even afterward it almost never gets up to 88C.

[335dwanted]

I replaced my thermostat last fall. Posted a poor man's DIY on this forum, you should be able to find it. Not hard.

North American cars do not have an EGR Cooler Thermostat, just the main one.

[Dazza335iC]

Looks like the OEM part is about $55 so that part is easy.

But I'm wondering if the thermostat is the root cause. I see temps too low on the highway (60C) and too high if idling for an extended period (97C). Is the thermostat electronically controlled? Or is it the old-fashioned metal spring type? Also are m57 water pumps mechanical (constant volume) or electrical (variable volume)?

Is there something else that could cause coolant temp to be too low?

[335dwanted]

Originally Posted by Dazza335iC

Looks like the OEM part is about $55 so that part is easy.

But I'm wondering if the thermostat is the root cause. I see temps too low on the highway (60C) and too high if idling for an extended period (97C). Is the thermostat electronically controlled? Or is it the old-fashioned metal spring type? Also are m57 water pumps mechanical (constant volume) or electrical (variable volume)?

Is there something else that could cause coolant temp to be too low?

Thermostat is the old spring type, not electronically controlled. Similarly, the water pump is a traditional mechanical/constant volume pump.

I guess it is possible that the thermostat is stuck in a partially open position which results in lower temps when air is going across the radiator (highway) but does not flow enough when stationary. This latter part has me a bit surprised; did the radiator fan come on when the temp spiked while stationary?

[Dazza335iC]

Oddly enough the fan didn't come on when I was monitoring the temp, but I've heard it come on other times, especially if I park the car soon after some hard driving. So it does work, but I was surprised it didn't come on when the temps got in to the upper 90s.

I ordered a thermostat assembly, hopefully I can install it this week before I take a nice long roadtrip over the weekend. I'm guessing this is the primary reason my MPGs are so terrible!

[335dwanted]

After some more thought (and converting to Fahrenheit) I realize my car (with new thermostat) will get to 97c (206f) fairly readily when going up a hill or spirited romp and I can't recall the fan coming on.

I guess your coolant temp sensor could also be shot, you'll see soon enough.

If you are really running at 60c I suspect your mileage should improve quite a bit.

[Dazza335iC]

After 2 miles of street driving and 3 miles of highway, this is where the temp settled at a steady cruise. Outside temp was somewhere around 45-50F:


I saw it peak at 54C when going up a long hill, then drop down to 45C on the way down the other side!

[Dazza335iC]

Picked a thermostat/housing assembly along with 2 gallons of BMW coolant, I'll swap it out tomorrow and report back!

[Dazza335iC]

Originally Posted by 335dwanted

After some more thought (and converting to Fahrenheit) I realize my car (with new thermostat) will get to 97c (206f) fairly readily when going up a hill or spirited romp and I can't recall the fan coming on.

Piggybacking on this, I noticed today that when I parked my car it was at 89C, fan not running. As soon as I switched the engine off, the fan came on. Switched engine on, fan off. Engine off again, fan on. Is it normal for the fan activation temp/condition to be lower when the engine is not running as opposed to running?

[Yozh]

You were probably on the middle of a DPF regen. Would be a good idea to keep on driving or even better get on a freeway for 10-15 minutes to complete a regen.

[Dazza335iC]

Forgive my ignorance, but what's the connection between regen and the fan activating while the car is off?

[charlie fairmont]

I believe it's simply that a regen really heats up the engine bay.

[Dazza335iC]

Right, got around to changing out the thermo last weekend.

New vs old, notice a difference?


A closer look. The tabs that hold the thermo in place were broken off and the thermo was just rattling around inside the housing!


So along with the broken thermo tabs, these plastic pieces in the cooling system were either already broken when I started in on the job, or broke during removal. Oh and of course teh coolant bleed screw was broken too


This is my first time working on an e90 cooling system and it looks like BMWs quality control on their plastic is getting worse and worse compared to earlier chassis. What a fucking nightmare! I'm just lucky the local dealership had that EGR coolant hose in stock or I would have been f**ked. These flimsy pieces make e36/e46 cooling systems seem bulletproof

[335dwanted]

Dazza, I feel your pain brother! Having recently replaced my thermostat I will agree that what should be a simple/trivial task should not be this difficult. Unfortunately this is not just a BMW problem, the thermostat on my Mazda RX-8 was even more involved than the 335d.

Sorry to see the additional breakage beyond the thermostat itself; I am not totally surprised you had complications after just doing this myself.

Please let us know if you economy improves once you burn through a couple of tanks.

[Dazza335iC]

I plan to replace both the radiator drain screw and the coolant bleed screw with brass units from Turner, I just don't trust these plastic ones at all after seeing how easily they both failed.

I'll report back with mileage for sure, but I did manage to hit 32mpg on my trip to Bend and back (~350 miles, all highway) after the thermo replacement. It's still not close to what I would expect for a 100% highway trip, but I know I ned a CBU cleaning too so hopefully it will continue to improve once that's done.

[remlemasi]

I broke the same EGR coolant hose last week. Also PITA getting the FMIC back in and two boost hoses back on the FMIC ends!

[Adieu]

those hoses get ridiculously brittle

[Dazza335iC]

Yes they do.

Actually thanks for bumping this, I meant to report back on any MPG changes after the thermo replacement. In short, I noticed very little if any difference. Maybe +1mpg. My CBU is definitely the main culprit of my poor mpg.