New Engine + Thermostat, Gauge Reading Cold (FIXED)

[EatsHondas]

My B25 started knocking on the #3 cylinder after ~180k miles. I just put a new (to me) bottom-end underneath my existing head and it runs great. I assumed since the head that came on the motor is an 885, that the block is also from a 2.5. However, the serial number on my new block does not match either of my two M20 blocks in the garage. Did I just install an eta stroker motor without even knowing? I know the block must be an early model because it had the stamped intermediate gear (BAD), no oil cooler fittings (filter sticking out of the block), and hex head bolts. It's running on a 173 DME with SssQuid's 91 octane chip (for a B25 motor).

I installed a Stant/Advance Auto Parts 180 degree thermostat and now the gauge sits a needle's width past the 1/4 mark. Is this normal? How much actual temp difference is there between the quarter mark and halfway mark? I guess I'll just watch my gas mileage in the meantime and see if it gets lower all of a sudden.

The original motor and thermostat combo made the gauge sit at exactly 1/2 (except when it used to run hot , hence the new one thermostat). I thought Stant was okay... Do I really need to buy an OEM BMW thermostat?!

[Kozworth]

I know all the e30s Ive had/ridden in are usually a hair below the middle mark, while my e36s/moms e34 has been dead in the middle.

I would double check your thermistors for the coolant temp sensor and for the sending unit. I am sure you can somehow find a chart that dennotes x voltage= y engine temp. You could try that and see if the engine is just transmitting the wrong signal to the gauge, or if the pcm is getting the wrong reading. Recheck all coolant hoses with a thermal gun to make sure its properly bled aswell.

[EatsHondas]

The gauge was reading fine before I had to swap the motor. The problem has to be caused by something I just did. I haven't replaced the sending unit to the gauge ever. I'll probably go ahead and do it, but I wouldn't expect it to fix the problem. I thought an improperly bled system will always run hot. Pretty sure it's perfectly bled - had a lot of bubbles come out of the overflow tank while revving conservatively. Cracked the bleeder after that, and idled a few more minutes just be sure.

I've seen a couple of jlevie's posts that say about halfway between the 1/4 and 1/2 mark is normal, but I'm just not sure because my gauge used to stabilize in the middle.

[oldmann]

OEM temp is 192 degrees, CK temp with thermometer, guage is probably correct for 180 thermostat.

[jlevie]

Before replacing any parts use an IR or contact thermometer to determine what temperature the engine stabilizes at by measuring the head and thermostat housing temperature. If the thermostat is is working properly you should see right at 80C.

[dougie30]

My m20 sits a hair past the 1/4 mark. I don't see why this would be a bad thing?

also put in new refreshed engine/thermostat

[dnguyen1963]

If you are running too cold, your mpg will suffer. Plus, in colder climate, it takes forever to warmup the car in the winter.

[EatsHondas]

OEM temp is 192 degrees, CK temp with thermometer, guage is probably correct for 180 thermostat.

Where'd you get 192F? Thought it was 80C/176F...

[dnguyen1963]

OEM is 80C for the US continental market. Not sure about Alaska or other regions in the world.

[EatsHondas]

OEM is 80C for the US continental market. Not sure about Alaska or other regions in the world.

That's what I was thinking, 'ppreciate the confirmation.

Got a chance to hit the motor and t-stat housing with an IR gun today... Valve cover was sitting at exactly 80C while the housing sat at 76-77. Exact same temps for needle in between the 1/4 and 1/2 marks as it was at the 1/2 mark (took a min or two of idling to get there). Never replaced the sender when I did the temp sensor, so I'm just going to replace it anyway.

Should the housing read 80C or am I being too cautious?

[Snou52]

can someone tell me where the temp sender for the guage is located? I replaced the one on the radiator.

also, did you resolve the issue?

[vpilarrt]

Gauge sender is the brown one in the thermostat housing. The one on the radiator is the temp switch for the aux fan.

[Snou52]

^ thanks! that's right, forgot that's what I was trying to fix previously =)

[Snou52]

Before replacing any parts use an IR or contact thermometer to determine what temperature the engine stabilizes at by measuring the head and thermostat housing temperature. If the thermostat is is working properly you should see right at 80C.

was wondering, what part of the head would we measure the temp at? I measured the head on the exhaust side and it was reading 300+ degrees f.

measuring the thermostat housing gave me a good reading ~168 degrees f.

also do anyone know what the operating temp should be? or what the overheating temp for the m20 is? I've read that 210 is when the motor is considered to be overheating...is that accurate?


looks like my temp gauge is wigging out. i'm going to replace the sending unit and if that doesn't work, i'll replace the SI Board batteries. I've read that could make the gauge do some wacky things. or the gauge could have a bad ground.

i'll let you guys know how it goes...wish me luck!

[EatsHondas]

No, my gauge still won't pass the 1/4 mark much. I shot it with a temp gun and got okay readings, but now that's starting to get colder out, it'll settle a needle's-width under 1/4. I replaced the sender just because, no difference. I'll try getting another thermostat, but the one that's in there is brand new. Has anyone ever had problems with Stant thermostats? It's an Advance Auto Parts brand, so it may be crap.