Temp gauge very sluggish after monovalve replacement... - PeachParts Mercedes-Benz Forum

MercFan · #1 11-23-2009, 07:08 PM

Ever since I replaced the monovalve in my 300D the temp gauge has been very very sluggish. When I went for a ride after replacement I thought it was broken - after 10 minutes of driving it was still nudging upwards to where the normal temp normally is...

What's the relationship between the temp gauge and the monovalve - I know the monovalve modulates the coolant flow, but the default position is OPEN and the heat that I'm getting in the cabin is the testimony that indeed the valve is open, so what's up!?

James

toomany MBZ · #2 11-23-2009, 07:11 PM

I don't think it has any effect on the temp gauge, maybe a failing thermostat.

MercFan · #3 11-23-2009, 11:53 PM

Quote:

Originally Posted by toomany MBZ

I don't think it has any effect on the temp gauge, maybe a failing thermostat.

Failing thermostat or the temperature sensor?!

andrewjtx · #4 11-24-2009, 12:44 AM

If you turn the air on in econo mode, with the temp set to cold, do you still get heat?

Did you check the new monovalce with a 12v source to make sure it was working?

It's possible that it is stuck open and you have coolant flowing through the heater core when you don't want it to.

MercFan · #5 11-24-2009, 09:56 AM

Quote:

Originally Posted by andrewjtx

If you turn the air on in econo mode, with the temp set to cold, do you still get heat? Did you check the new monovalce with a 12v source to make sure it was working? It's possible that it is stuck open and you have coolant flowing through the heater core when you don't want it to.

Yes - I get heat no matter what.... that's a separate problem - right now I think I've narrowed it down to a faulty cabin temp sensor - I'm working on sourcing a new one. So, yes, the monovalve is always open - thus the constant heat, but WHY would the newly installed monovalve have any affect on the temp gauge?! Perhaps coincidence? I don't know... I do have a new thermostat that I'm going to replace anyway (along with bypass hose) so perhaps we'll see then.

andrewjtx · #6 11-24-2009, 11:05 AM

When you have the heater core open during engine warm-up, you're heating up a larger volume of coolant. So it would take longer for the cylinder head temp sensor to warm up.

Quick experiment, since you're not getting heat anyhow:
Unplug the monovalve and go for a run when the car is cool. See how fast it comes up. With power not supplied to the monovalve, it should stay closed.

MercFan · #7 11-24-2009, 11:22 AM

Quote:

Originally Posted by andrewjtx

Quick experiment, since you're not getting heat anyhow: Unplug the monovalve and go for a run when the car is cool. See how fast it comes up. With power not supplied to the monovalve, it should stay closed.

Intersting - I'll try it. Should be a quick test to compare it. Small correction - I AM getting heat - in fact, I'm getting to much heat - can't stop it unless I turn off the control alltogether, but it's worth a try to compare.

Graplr · #8 11-24-2009, 11:23 AM

Quote:

Originally Posted by andrewjtx

When you have the heater core open during engine warm-up, you're heating up a larger volume of coolant. So it would take longer for the cylinder head temp sensor to warm up.

Your theory is true but the time difference on engine warm up would be almost negligable. For example if the engine takes 5 minutes of highway driving to get up to temp with the monovalve closed, it will maybe take 6 minutes tops of highway driving with it fully open.

James, it does seem like the issues are unrelated. Engine taking a long time to get up to temp or not getting up to temp at all is a sign of a thermostat problem. I would start there. Plus it is simple and cheap. I always start with the simple stuff first (or at least in theory I do

).

Graplr · #9 11-24-2009, 11:24 AM

Quote:

Originally Posted by MercFan

Intersting - I'll try it. Should be a quick test to compare it. Small correction - I AM getting heat - in fact, I'm getting to much heat - can't stop it unless I turn off the control alltogether, but it's worth a try to compare.

Is your monovalve unplugged?

MercFan · #10 11-24-2009, 11:54 AM

Quote:

Originally Posted by Graplr

Is your monovalve unplugged?

No, it was plugged in last night for the drive - when I observed the sluggishness... let's remember though that the cabin temp sensor is busted (open circuit), and that probably confuses the ECC unit, so there's no telling what voltage the monovalve sees...

I'll unplug the mono and take it for a ride starting with the cool car and observe the difference. Thx

andrewjtx · #11 11-24-2009, 01:13 PM

Also, try to get an infrared reading on the cylinder head. If your heat is working and HOT, then you're not getting a good reading from the sensor.

MercFan · #12 11-24-2009, 01:21 PM

Quote:

Originally Posted by andrewjtx

Also, try to get an infrared reading on the cylinder head. If your heat is working and HOT, then you're not getting a good reading from the sensor.

I think I know that without the thermometer reading - how? My cabin is boiling hot and I've got blast furnace heat coming out of the vents as I drive, but my gauge has barely risen off the bottom... it does rise but a glacial speeds.

Where is that temp sensor and what does it look like - is it screwed into the head on the driver's side?! WOuld it be ok to pull it out to do some visual inspection without loosing too much coolant?!

Graplr · #13 11-24-2009, 02:19 PM

Quote:

Originally Posted by MercFan

Where is that temp sensor and what does it look like - is it screwed into the head on the driver's side?! WOuld it be ok to pull it out to do some visual inspection without loosing too much coolant?!

Yes, it is in the middle of the block, underneath the injector lines/injectors. I didn't loose much coolant at all when I changed out mine. A rag in tucked in place below it did the trick.

Although I don't think you will be able to tell anything from a visual inspection. It seems to me that when the sensor fails it jumps around rather than reads at a slower pace. I'm not sure if the 603s use the same sender or not. If they do perhaps you could swap them to see if there is any change?

The fact that you get HOT heat doesn't necessarily mean that your engine is up to full operating temp. You could be running at 60-70C (140-158F) and still be getting HOT heat even though the engine is below operating temperature. To me it still seems like a thermostat issue. Since you mention you have a new one and were going to change it anyways, I would do that first.

An infrared thermometer would verify if your dash gauge/sender is accurate or not.

MercFan · #14 11-24-2009, 02:35 PM

Quote:

Originally Posted by Graplr

An infrared thermometer would verify if your dash gauge/sender is accurate or not.

I see - that makes sense.. my bad...

toomany MBZ · #15 11-24-2009, 02:50 PM

Quote:

Originally Posted by MercFan

Intersting - I'll try it. Should be a quick test to compare it. Small correction - I AM getting heat - in fact, I'm getting to much heat - can't stop it unless I turn off the control alltogether, but it's worth a try to compare.

If the diaphragm is torn on the mono valve, you'll get heat all the time, is that okay?