Intermittent P0128 After Switching to 88-Degree Thermostat

[peter540]

'03 540i Sport. I switched to the 88-degree thermostat as a proactive measure. Now, however, I get the occasional P0128 code (Thermostat Low Coolant Temperature), mainly when the engine is, unsurprisingly, cold. I never got this code before the thermostat switch. I should also note that I'm running the Zionsville radiator and electric fan, and my water pump is fairly young (maybe 20k miles on it).

My main problem is that I need to get the car through its emissions inspection this month so I can register it. I get CLOSE to all the monitors being ready, but the SES trips before I can actually get the inspection done.

Some folks have put a resistor in place, splicing it to the old thermostat's harness/wiring connector (there's no electrical anything on the 88-degree thermostat). I'm not sure whether this would solve my problem; I assume it might, since the original thermostat is, if I understand things right, supposed to heat the coolant to improve emissions.

I need some advice about what to do. Thoughts?

Thanks, as always, for your help!

--Peter

[edjack]

Install an OEM t'stat.

If you were having overheating problems, fix the source.

[jclausen]

Also if the thermostat is not allowing the engine to get to,the correct temp, the coolant temp is sending a signal to the dme that the engine is cooler, I turn using more fuel over time and cats hate rich mixtures.

[peter540]

I'm not the only one who's done this retrofit. I am among a number of forum members who have installed this tstat as a proactive measure to lessen damage to the cooling system and TCGs, among other things. This is a BMW OEM thermostat, albeit from their diesel engine.

Geargrinder, any thoughts? Philly?

--P

[BMW540san]

I've been texting back and forth with Peter since I sold the thermostat and you could call it after sale customer support.
Few thoughts:
As I told Peter already it's rare issue and I only had one other complaint out of almost 2 dozen sold.
I strongly suspect that there is more in play than just thermostat.
Here is why:
I have 95 C thermostat on my X5 (4.6 M62TU but same DME) and I had the same code on and off even though I have resistor.
I recently posted where I replaced temp sensor and I thought that it solved it since there was different resistance reading between new and old temp sensor. Well, after a while same symptoms came back:
Temp fluctuating down to around 80 c.
Finaly I got coolant leak at WP weep hole and after I replaced it, temp is rock steady at 95 +/-2 c.
While strange symptom of failing WP, that was the cause of my code.
In Peter's case I'd check the temp sensor as it could definitely be the cause.
My final point is that if everything else is functioning correctly, 88 C thermostat is usually not an issue and I'm commited to help Peter out not just because I sold it but out of curiosity too.

[peter540]

Thanks, Z. It's been great chatting with you about this, and you've been terrific to help me sort through the possible issues and solutions.

I don't suspect the water pump in my instance. It's BMW OEM with the metal impeller and was just replaced in the last 20k miles. Also seems unlikely to be the culprit, as I texted Z, because this code never occurred before swapping in the 88° thermostat.

I need to replace the temp sensor's o-ring (forgot to replace it when I replaced all the coolant hoses recently, and it has a tiny leak). I wonder if the leaking coolant could be compromising the connector/connection.

I thoroughly bled the coolant after replacing the coolant hoses, and all seems normal, but I'll ask anyway: could the P0128 code pop up if there's air in the system? I'd think in that case, though, that I'd have an overheating issue.

I'll also double-check all the wiring splices for my Zionsville e-fan in case something's not quite right there, but again, I don't suspect that to be the cause; I've been running the Zionsville setup for six or seven years without issue.

--P

[XAlt]

Get rid of the fan.

Colder does not = better. Big wave of alarmism and slander against the MAP t-stat and how it was supposedly responsible for TCG failure, is only there for MPG and because "muh M5 runs cold, must be great for your car".

It's great for Evans Coolant
It's great for a supercharger
It's great for a fan delete.
Having the car actually run closer to 80C is not great.

[JimLev]

Peter, have you looked at the KTMP in the cluster to see what it was reporting?
Last week my temp was reading in the low 80's, it was just a bad connection at the dual temp sensor.
I do have a 10 ohm resistor connected to the wires that use to plug into the t-stat heater.

[peter540]

Peter, have you looked at the KTMP in the cluster to see what it was reporting?
Last week my temp was reading in the low 80's, it was just a bad connection at the dual temp sensor.
I do have a 10 ohm resistor connected to the wires that use to plug into the t-stat heater.

Thanks for the ideas, Jim!

I haven't. I'll have to remind myself how to get into the cluster. Pretty sure I can remember.

As for the dual-temp sensor, that's not the one on the lower radiator hose, right? It's the one up higher that comes out of the passenger-side radiator chamber, right? In any event, I'll double-check to make sure everything's fully plugged in/connected. The dual-temp sensor is a tough one to reach for my fat fingers, so it's possible I didn't get it fully seated.

--P

[philly98540]

+1 to Jims call. Check your KTEMP to verify your temp sensor is working correctly. The 88 degree stat should give you a KTEMP in the low to mid 90's even when its very cold outside. If your KTEMP is low, it may be the dual temp sensor located on the water pump or it may be the stat (less likely).

[JimLev]

The ZF fan shouldn't be running if he is really that cold.
Doesn't the ZF rely on its own temp switch to control its fan?

[philly98540]

Fan should be controlling radiator outlet. Tstat controls engine temp.

[edjack]

What evidence do we have that lowering the coolant temp by 10-15 deg forestalls cooling system failure?

Or is this a shade-tree mechanic's solution?

THe dual-temp sensor, which signals the ECU and the temp gauge, is in the pump housing.

[XAlt]

What evidence do we have that lowering the coolant temp by 10-15 deg forestalls cooling system failure?

Or is this a shade-tree mechanic's solution?

THe dual-temp sensor, which signals the ECU and the temp gauge, is in the pump housing.

It's all about the TCG alarmism and how the M5 uses a cold stat.

[philly98540]

What evidence do we have that lowering the coolant temp by 10-15 deg forestalls cooling system failure?

Or is this a shade-tree mechanic's solution?

THe dual-temp sensor, which signals the ECU and the temp gauge, is in the pump housing.

No evidence! You must do as your German car manual says you fools

- - - Updated - - -

It's all about the TCG alarmism and how the M5 uses a cold stat.

Yup, no TCG failures on the 540's. Hottest engine temp is the states FTW!

[JimLev]

Higher temps = higher cooling system pressure. All of that nice recycled plastic that Europe uses can surely withstand the pressure in our feeble cooling system.....right?
Radiators and expansion tanks never fail....LOL

[BMW540san]

It's funny how some think that everything BMW does must be correct. It's a simple fact that 105 C thermostat in conjunction with dumb (buffered) temp gauge doesn't leave you much time to react when shit is hitting fan.
Me and some others are not advocating 79 C M5 thermostat which is way to cool for M62TU, but more reasonable 90-95 C. Unfortunately there is only 88 C available for M62TU while 85 C and 95 C are available for M62. Even with 88 C, you're looking at 90+ C average temp and that's hot enough for to keep combustion chamber free of contaminants.
Also, there is considerably more pressure with higher heat and BMW could care less when your plastic coolant components fail in relatively short period of time.

[peter540]

The ZF fan shouldn't be running if he is really that cold.
Doesn't the ZF rely on its own temp switch to control its fan?

Jim, I believe you're right that the fan essentially runs on its own circuit. In theory it should come on LESS if the coolant temp is lower.

[ninetyseven1]

'03 540i Sport. I switched to the 88-degree thermostat as a proactive measure. Now, however, I get the occasional P0128 code (Thermostat Low Coolant Temperature), mainly when the engine is, unsurprisingly, cold. I never got this code before the thermostat switch. I should also note that I'm running the Zionsville radiator and electric fan, and my water pump is fairly young (maybe 20k miles on it).

My main problem is that I need to get the car through its emissions inspection this month so I can register it. I get CLOSE to all the monitors being ready, but the SES trips before I can actually get the inspection done.

Some folks have put a resistor in place, splicing it to the old thermostat's harness/wiring connector (there's no electrical anything on the 88-degree thermostat). I'm not sure whether this would solve my problem; I assume it might, since the original thermostat is, if I understand things right, supposed to heat the coolant to improve emissions.

I need some advice about what to do. Thoughts?

Thanks, as always, for your help!

--Peter

Put the resistor on the old wires that went to the factory thermostat , use a 12
Ohm 100 watt resistor.

[mandd]

Or just put back the original thermostat set your monitors and run the e test, then refit the 88 deg Thermostat after if you really need it.

Mark.

[JimLev]

Peter, I'm using a 10 ohm 20 watt resistor.
Your t-stat heater is ~10-11 ohms and with full power going to it (hardly ever) is only would be dissipating a little over 12 watts.

[peter540]

Peter, I'm using a 10 ohm 20 watt resistor.
Your t-stat heater is ~10-11 ohms and with full power going to it (hardly ever) is only would be dissipating a little over 12 watts.

Thanks again, Jim and everyone. I'll install a 10-ohm, 20-watt resistor and see what happens...

[JimLev]

Jaun, the resistor was just to replace the no heater in the t-stat.
The lower reported temp is a different problem.

[peter540]

Update: after the last time the same code popped up, I cleared it and used contact cleaner on the dual temp sensor in the water pump, also verifying that there were no leaks from the tstat and water pump. Drive the car with the hope that I could get all monitors to a "ready"state so I could get the car through emissions. As with the last time, I drove until 7 of the 8 monitors were ready, and then, just today, back came the P0128. Cleared the code again and started monitoring the KTemp, and the numbers look fine -- 91° - 95°, but no lower than 91°.

I'm not sure how the KTemps looked when the SES light popped up, unfortunately, but I'll keep the KTemp display up and continually monitor it.

Could it be that I need to bleed/burp the system more? I've done it 1000 times with this Zionsville system without any problems.

Here's a question: how will I know when I've successfully burped the system?

A second question: how many miles over how many days must I drive the car in order for monitor to achieve readiness?

Third question. If I give up and decide to reinstall the old tstat, how much coolant do I need to drain, if any?

Thanks for your help!

--Peter

[ninetyseven1]

What monitors ? The car has no idea or knowledge of what temp themostat your using , it's not that smart. Just replace thermo with an 88c and install a resistor on the wires that go
To the dme.