W126 300se Luke warm heaters

[design guru]

I have a 1991 w126 300se and the heating is very poor, it only gives out a luke warm breeze when the car is at full temperature and if I turn the speed up it is hardly noticeable. I have purchased some 2 stage coolant flush but need to locate the drain plugs on the engine.

I have done a search and some people say there are 2 and some say there is one plug. Can someone direct me to where I can find the plug on the engine block as I would like to do a full flush of the system.

any other pointers on the process to be aware of would be much appreciated and also which is the best replacement coolant brand.

 

[jefrs]

What is the engine temperature? - if the engine is not getting hot then the heater won't work. Could be thermostat.

 

[ianrandom]

Changing your duovalve is very easy and very likely to be the prob. Its what the prb was with mine.

 

[design guru]

I have done a fully flush of the system and fitted a new thermostat a and yet the heater only blows a warm air flow. I have even removed the plug on the duo valve and it makes no difference.

When flushing the system the water flowed out smoothly so there did not seem to be any blockages in the circulation.

I am not sure what the reason could be? The car warms up to 80 no problems but the heater only give a slight amount of heat and when the heater is put on level 2 or above the heat disappears.

 

[dinoy]

have u checked the fan is operating correctly if it does not operate properly it might that this yr problem, if non air con probably same as w124 and around £20 to replace, did u try replacing duo valve also, i am sorry it is a process elmination here

 

[design guru]

have u checked the fan is operating correctly if it does not operate properly it might that this yr problem, if non air con probably same as w124 and around £20 to replace, did u try replacing duo valve also, i am sorry it is a process elmination here

I will open the duo valve and see if the seals are intact. this seems to be the most common and likely source of the problem according to most people who have had this problem.

Thanks for the mention of the fan, I'll check that next if the duo valve is ok.

 

[jonnyboy]

Also worth checking the temp senders on the head aren't airlocked. Then look inside next to the sunroof switch and interior lights on the overhead panel. There is a little grille to the left if I remember correctly. Theres a little under dash v acuum thing that sucks cabin in through the grille over a temp sensor which controls cabin heat output. Tear off an inch square of toilet paper (single ply) run the car and see if when you hold the paper to the the grille the suction keeps it there. You need to use the thinnest lightest paper (for instance kitchen roll is too heavy). If it won't stay there take the assembly down, give it a good clean and vacuum out, maybe some rubbing alcohol/solvent cleaner or the likes on a cotton bud over the sensor too). Replace and try again. If the paper then sticks you may have found the fault. If the paper doesn't stick then possibly the tube/pump is u/s.

All that said, it's worked on 3 out of say ten cars for me. Mostly otherwise its the duovalve or the control gear behind the dash.

Treat the above as preventative maintenance as a poor signal from that vent will skew any "fixes".

Good luck - it's an easy job (easier than fitting a bloody duovalve!!!)

 

[126-valeter]

im not sure about mine ether but ive tryed to get the light switch and sunroof control of wiht no look but last night i drove past a big cloud of smoke from a fire so pushed the recirc air switch and after it finished its coursce wow it got nice and warm and all vents was blowing out hot air so myadvise is try the recirc air button worked a treat on mine not sure why tho

 

[design guru]

Thanks for the tip. I have had the car fully of a citric acid mix and have been running it for the lasts few weeks and it is starting to show some improvement. Mine does have 140k on the clock so I think it may well be just full of dirt.

Will flush it this weekend and refill with a clean batch of cleaner and leave it for another week and then refill with coolant and see how it does.

 

[johnsco]

Can you feel the temperature of the supply and return pipes to the heater matrix ?
This should tell you if hot coolant is actually circulating in the heater unit.
They should be accessible to feel in the engine compartment.

 

[ianrandom]

im not sure about mine ether but ive tryed to get the light switch and sunroof control of wiht no look but last night i drove past a big cloud of smoke from a fire so pushed the recirc air switch and after it finished its coursce wow it got nice and warm and all vents was blowing out hot air so myadvise is try the recirc air button worked a treat on mine not sure why tho

Just for reference, that sunroof switch assembly comes off by prizing it (Carefully) from the drivers side with something that wont damage the surrounding. It pops out on one side and then the other side follows after.

 

[126-valeter]

ianrandom thanks for that i will give it another go in the morning when i give it its full weekly valet

 

[design guru]

The heating is now working fine, I filled the cooling system with a citric acid mix just before Christmas and its still in the system, it seems to have cleaned the system really well as the heaters are blowing hot even when on full power.

I am going to flush it out and maybe refill one more time and leave it for a few more weeks. I used the normal cooking citric acid. Even though its been in the system for several weeks it has not caused any leaks, the only problem I have is the coolant plug in the block is stuck and I don't want to risk trying to open it with force incase it breaks.

 

[ianrandom]

Cool. I'd be way too worried to leave it in any length of time. I've heard of a bad experience with that. But it sounds like its worked for you so thats cool. Or warm in fact
Nothing more annoying than having a big luxury engineering work of art that has a rubbish heater..