w123 lukewarm heat
 

Seems like my heat isn't as hot as usual. It's just lukewarm.

Coolant is good, hoses at monovalve hot, 12v when set to MAX cold, Monovalve seems to be working.

I do have intermittent clicking from the CCU, i cleaned the contacts and it clicks less often.

Most threads on heat are about a failed monovalve and no heat, what's up with luke warm heat?

What is your engine temperature? Should be right around the 80C/175F marker after driving a few miles.

If it is significantly below that, your thermostat is probably stuck open and the engine can't heat up to optimal operating temperature - this is bad both for you and for the engine.

Feeling the hoses won't tell you anything, the difference between 130 F and 175 F is hard to tell since both are painfully hot to a human.

Back flush the heater core and thoroughly bleed the cooling system with the front of the car raised above the rear and the heat on full blast. Run the engine at 2000 rpm until it settles at operating temp and the lower rad hose gets hot. Once that is done let the engine cool down, and check the coolant level.

In my experience, when the coolant is low you won't feel much heat. It doesn't take much. My son was complaining, "heat doesn't work" in my 1985 300D when he came home last week. I had told him to check the coolant when he left a month ago since I had drawn it down to check the upper hose and knew it would drop a bit as air worked its way out, but he doesn't listen. I only needed to add ~1 cup to hit the full mark (using Evans Waterless) and the heat was much hotter then.

I have also seen the temperature read too high when there is an air pocket. I have used a HF IR gun to measure next to the temperature sensor in the head (goes to dash gage) and the reading agrees quite close w/ the gage. You can get one on sale for <$30 and they prove very useful.

Maybe nothing to do with the coolant. Could it be that some flap in the heater box is not operating correctly. Maybe the recirculate flap is not closing all the way? This time of year that would allow cold air into the mix would it not?

- Peter.

It could be a flap issue, but the worst case scenario failure is FULL heat and full defrost. :) It's a safety thing.

Car runs at 100C... always has, runs fine. At one point I replaced the thermostat, same running temp.

Coolant is topped off, added a "cup" just so you couldn't say I told you so. Still luke warm!

Is your auxiliary water pump working?

I was going to ask, "how do I tell?",

Here's a thread on it, I'll report back: Auxiliary Water Pump - Mercedes-Benz Forum.


HOWEVER, the pump is to provide heat at idle speed, and I'm not getting significant heat at ANY speed. It's warm, but not retina blasting hot like it should be at max heat.

You need to backflush the heater core like I said.

True, My 79 had a bad one and I had very poor heat at idle and acceptable heat when moving. However, my heat got noticeably hotter at all speeds after I replaced it.

Feel your hoses, it can tell you a lot. Quick touch, let go, then slightly longer touch, let go, before it gets uncomfortable. You will not be able to measure temperature accurately by touch, but you can tell if one hose is hotter than another by how long you can your keep hand on them.

Feel the in/ out hose on the monovalve as well as the hoses into the firewall to/from the heater core. They should be similar in temperature. If not, monovalve may be blocked or heater core may be blocked. At full heat, monovalve/ heater core hoses should be less hot than the upper radiator hose (because it is further away from it), but not by a lot. If it is very cool in comparison, you may have some coolant flow issues.

I find it very useful to start the engine cold and feel all coolant hoses as the engine warms up, which shows you the flow path.

After I reconnected the plug on my aux heater, I noticed a small increase in heat. Seems to come to temp a little faster. Not a huge amount, but measurable.

I'm due for a coolant flush; I think I'll back flush the core as well as do the MB flush as recommended, something about citric acid?


EDIT To add, I found this link after posting this... http://dieselgiant.com/mercedes_citrus_flush.htm

He mentions that his engine temp dropped about 10* after doing the flush, whereas mine normally runs a bit above 90*, not at 100* like others have mentioned. That makes it seem like mine doesn't have much corrosion in it... I'd be interested in thoughts on that, especially if the OP doesn't mine the sideline.

I had this problem this fall on a 1983 300TD and checked the usual suspects--turned out to be a loose connector on the box of electronics that lives behind the glove box, not sure if that is the CCU you refer to or not,--the connector fell out in my hand as soon as I touched it. Easy to check if you haven't.

Peter

Are you saying flush the whole system with a hose, or just blast the heat with the front of the car raised above the rear?