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?

Basically you need to disconnect the two rubber hoses to the heater core and using a hose on the heater core OUTLET flush water through it. Hence back flushing as the water is flowing backwards through the heater core.

You'll need to do it steps.

First, locate the in and out hoses of the heater core. The in comes off the head, and the out goes to the water pump housing. Remove the two lines. Now, take your garden hose and flush the heater core backwards, putting the garden hose onto the output hose and letting the water flush the crud out through the input hose. Don't use too much pressure since the heater core is probably fragile at this point. Flush it until the water runs nice and clear. Then, blow the water out of the heater core with an air compressor and low pressure, or with your mouth (carefully, to avoid ingesting coolant). Hook the lines back up to where they came from. Top off the coolant. Then get the car situated so that the front of the car is up higher than the rear. You can jack up the front, put the car on ramps, park on the end of your driveway if it's a steep incline, etc. Start the engine and run it at ~2000 rpm with the heat on full hot and low fan speed. Once the engine comes up to operating temp (the temp you see every day driving) make sure you have nice hot heat, check to see if the coolant level has dropped, and check the upper rad hose to see if it's solid or full of squishy air. Put the car back on level ground, let it cool, and top off the coolant.

Done.

Better yet, watch this
https://www.youtube.com/watch?v=baAAYM5Cl00
(and we can skip all the comments about the guy that made it, regardless, its a good video)

Dave has great vids, but I would skip using any chemicals until you try plain water first. These heater cores are old and burst just from normal use. I wouldn't put anything stronger than vinegar in one of them. That said, I'd need to flush with water a couple times before I even thought about using even vinegar. Heater core R&R is such a PITA that it should be avoided at all cost. I did it once on my 84 190D. The only way I'm taking the dash out again is for a crack free 87+ replacement.

Haven't backflushed the core yet because it's been freezing. Raining now so i might try today... my coolant looks clean and new though... if the core was full of crud wouldn't my coolant look nasty too?

In other news, opened up the monovalve and the diaphram is ripped. It pops out when I turn the CCU off, and pops back in with any of the air buttons. I'm assuming it pops out to seal off the hot coolant, and the ripped diaphram lets hot coolant into the system... so this isn't my problem.

I could feel the aux pump running with the key in the on position. I tried with the monovalve open but didn't notice any coolant flow...

No, not necessarily. The heater cores are normally made from Copper or aluminum, and as a consequence, tend to corrode differently than the steel of the block. You made not see much in the rest of the cooling system. Also, water flow is truncated away from it when it is off, therefore it is more sedimentary in the core vs the rest of the system.

I know this sounds strange but the next time you drive it at highway speeds do this when its safe of course- apply your brakes hard and see if temp increases coming out of the vents. I've had 2 MBs that would only produce heat at idle or during rapid deceleration.

...and what the hell is the explanation for that

good luck

The heater core collects all the junk that circulates in the cooling system. It isn't a high flow components and it's out of the loop a bit. Back flushing loosens all the crap and blows it back out the input.

how do you know the monovalve is working? its the weakest link in the system, i'd thoroughly rule it out before doing complicated flushing of this and that. they break easily. if yours is still original to the car it could very well be shot. they can fail in multiple ways such that you get full heat all the time, or no heat, or weak heat, etc. even if you're getting voltage to it the valve itself could be faulty

I would repair/replace the mono valve before proceeding.

It could mean the rapid deceleration may be:

1. Causing a blend door to shift to max heat position temporarily.

2. Causing the monovalve to temporarily shift into a position to let max coolant flow.

3. Causing sediment blockage in the heater core, to/ from the heater core to temporarily shift to allow more coolant flow through the heater core.

Any more guesses?

Agree with replacing the torn diaphragm monovalve with a known good unit before doing anything else.

One more thing: I would never do a citrus acid flush unless I am prepared to replace the heater core- a big job from what I've read. Radiator may also need replacing after the citrus flush.

Awaiting the monovalve diaphragm part... i'll pop that in and see if it helps.

@ivandrocco - THIS IS THE EXACT SAME PROBLEM I AM HAVING NOW

I am having clicking from the CCU, lukewarm heat, and very low power (3.5v) to the monovalve on defrost position.

The clicking is driving me mad...

The clicking completely stops ONLY when I disconnect fuse 8 at the fuse block (heater blower motor/AC compressor)

I put in a good working CCU and the same clicking sound occurred, so the CCU is not the problem.

Can't figure out what is wrong, but this is what I've found so far.

Also should note-

The clicking in the CCU only occurs when the car is warmed up to operating temperatures. It gets worse as the car warms up....

Does this happen to you @ivandrocco?

What year is your car?
 

Re: clicking.....try unplugging the aux. water pump. It may be having problems and tripping CCU circuit protection.

UPDATE: PROBLEM SOLVED!

Poking around the engine bay I found the thermo switch by the egr broken in the plug, sending an intermittent signal to the CCU, thus creating the clicking. For now its unplugged, no clicking and full heat. Will replace as soon as possible.

Hope this helps-

Can you send a photo? not sure what switch you're talking about. You mean the aux pump connector?

Popped new innards in the monovalve... and now my heat is cold all the time instead of just lukewarm. Thanks obama!

I'll try it. I checked this out the other day and the plug is very corroded, was planning on trying to clean it up.

http://www.peachparts.com/shopforum/...03-medium-.jpg

It is this Thermo switch (the red capped one in this picture, by the EGR that was causing my CCU to click repeatedly. Once the new one comes I assume everything will be back to normal, will update when installed

nice block off plate

nice clean motor too

unplugged the aforementioned plug, no difference. cold air on all settings.

But did you test the plug with an ohm meter? the wiring? the pump its connected too? the pump its connected to is triggered by this switch, no switch no full heat. I've been able to get lukewarm heat when switch is unplugged, but you have to troubleshoot these issues further than an unplug or not.

red switch unplugged - heat with key in on position, no heat with car running

aux pump unplugged - heat with engine running, no heat at highway speed

I believe that connector you unplugged on the thermostat housing is for controlling the blower fan so it does not blow cold air when the engine is cold.

Can you feel all the coolant hoses by the firewall ( from a cold start), starting from the hose coming out of the cylinder head by the oil filter and report back with your results? Make a video of it if you can. If the temp wheel is set to max, the hose coming out of the cylinder head should get warm first within a few minutes, the other hoses in and out of the firewall should follow, then hose to the mono valve to the aux pump back to the engine.

seems like this is what is happening. i now have heat at idle with a new monovalve diaphragm the aux pump unplugged, but heat goes cold at highway speeds.

Still dealing with this issue. I think a flap is the culprit, allowing cold air in that overwhelms the heat at high speeds. Winter is coming! help!

Flushed the core today... cleaner than my dog's mouth. Definitely not the culprit. Any ideas about investigating the flaps?