W126 sudden heat loss - Page 2 - PeachParts Mercedes-Benz Forum

draglink · #16 02-22-2013, 07:00 PM

you are in the same boat as me....or close. Good luck to you, I havent soved my issue yet....been throwing parts at it since begining of Jan

LandYaghtLover · #17 02-22-2013, 07:03 PM

Before when I had a bad t-stat, yes I had heat just not intense heat and temp was just below 80, as it is now. But now the only way I get heat, when it drops out, its to recirculate air and have the fan on low. Then I get some heat.

Oddly now when its below 80, I get better heat than last year when heat does work. So its not the same "formula" as last time with a bad t-stat. Also before it would take a while to get heat. Now I still get heat quickly. It just eventually goes away, particularly on the highway.

As for milage, all summer I was getting 18 or more easily. My goal was 22mpg highway only. Got that up to 20 at least. But average was always near 19. I know winter will kill economy, but the first half of winter I was doing just above 18 and seldom in the 17s.

As for climate system, the mono valve is sure flooded with coolant, but when heat dies both the lines and windshield heater lines are just warm, not hot like the washer tank lines.

I think my biggest concern is overall engine heat. It has to be a clue. If engine heat was normal, just above 80, then I would suspect something clogged or air trapped. But the cooler running engine has to be a hint at something.

EDIT: note, I do not have a viscus fan install and have not for months now. Its sorta stuck on all the time, so I took it off this winter. I wonder if the new expansion tank cap is the issue. I dont think these issues started with a new one exactly, but when I remove the new one it will launch off the tank! The only one did not vent much air when the engine was hot. So the new one certainly is holding a lot more pressure. The new one is rated 20lbs and it some aftermarket brand. I did order an OEM unit that should be here next week.

Gilly · #18 02-22-2013, 09:18 PM

I wonder if the cap pressure being too high would make air stay in the cooling system especially the heater system?

LandYaghtLover · #19 02-23-2013, 12:01 PM

Well I put the old cap on. Its hard to say if things changed, the engine did get above 80 but did not quite hold there perfectly. When I left this morning for work, I lost heat after reving up the ramp to the highway and before engine op temp was reached. Had no heat until the exit ramp, then it came back. As it did, engine temp dropped close to 80 but not below. As if the sudden change in HVAC was cooling the engine a bit.

After work heat came on quick, as usual, and I cranked the heat to max and it held on the highway until it hit normal full operating temp, then heat went away quick. Did not come back until exit ramp. One a stretch of road from the ramp to my house I sped up and lost heat. Came to a stop, had heat. Seems getting over 3k rpm is a bad thing for this problem although engine temp always stayed above 80.

So I really have no clue. In my driveway I decided to try something. In park I got the engine up over 3k rpm and held it there. The heat I had was once again gone quick. After holding for a while engine temp climbed slowly. Nothing that I would worry about since I have no viscus fan. If temps did climb quick I would think water pump.

So... still no clue. Hot engine, cool heater core. Especially after getting RPM up. And it seems to be an issue after the t-stat does its initial opening - but not always, as I found this morning.

The cooling system is so simple. Coolant is new, clean and the proper stuff. I guess the only thing next is a water pump and t-stat. I may do the t-stat quick. If I do both and things work fine, I will never know for sure which was the issue.

This is no fun!

draglink · #20 02-23-2013, 01:09 PM

Keep us posted....Im watching your progress! T-stat and brand new radiator didnt help me at all. I have not replaced water pump

Gilly · #21 02-23-2013, 05:26 PM

It's sounding more like the heater system then the cooling system itself, unless you mean really drastic changes in the engine temp display. Almost like you are really scrutinizing the engine temp. I would disconnect the monovalve connector and see if it goes to ful heat once the engine warms up and report back, if the heater temp is still going up and down after that I would almost have to think it's the monovalve itself,unless others have ideas. But as a test, i would say next step is now to disconnect the monovalve electrically and report back.

Gilly · #22 02-23-2013, 05:28 PM

Second thought would be a plugged (or partially blocked/gunked up) heater core.

LandYaghtLover · #23 02-23-2013, 05:41 PM

Gilly. The engine temp has always been rock solid at above 80. Needle goes through the upper part of the 8. That has not changed since I replaced the original bad t-stat last spring or summer. I forget. But it never would drop below 80. And that is what often happens now.

If it was not for engine temp being wonky I would assume the same, a clogged core, etc. Maybe its multiple issues.

I will try unplugging the new mono valve again and see what happens though. I will also keep the old cap on until the new one comes in.

Just wish the symptoms were more repeatable, reliable.

LandYaghtLover · #24 02-24-2013, 08:02 AM

I was doing some more thinking. I wonder if I blew a head gasket. Exhaust getting into the coolant would explain a few things. Fluctuating temp as air is trapped by the sensor housing, air blockage for heat, etc. I hope not, but it might make sense. Higher rpm forces exhaust into the system and only reaching lower rpm lets the water pump properly expell the gasses. Also lower rpm means less exhaust pressure anyway.

LandYaghtLover · #25 03-01-2013, 05:54 PM

Replaced the fairly new t-stat, no change. Compression test will be next.

Hirnbeiss · #26 03-02-2013, 09:51 AM

There could be something going on with one of the vacuum actuators that lets cold air in at speed. Engine vacuum drops with throttle, so the vacuum could drop to a point where the flap shifts. At idle you have high enough vacuum to shut the flap. Just a theory, but I have read that those vac elements fail after a decade or two.

LandYaghtLover · #27 03-02-2013, 10:16 AM

Something along those lines was brought up on another site. I know they are a PAIN to service. Since this is clearly an RPM related issue, there is a very good chance. Im going to dig up the diagrams and see what I can disconnect to test the theory.

cbrown888 · #28 03-06-2013, 11:55 AM

I also have a 1985 300SD with same problem, replaced mono valve insert, thermostat, pushbuttons, temp control unit and aspirator motor. still same problem.
Last week when the aspirator motor seized and popped the fuse I had plenty of heat. Whenever I buy one of these old german cars I make it a point to clean all the fuse contacts and replace the fuses. having a number of these cars in the past I'm pretty well versed but this one has me for a loop. Last night I was driving stop and go, had plenty of heat but once I hit 65mph on the highway again no heat. Pulled off at a rest area and unplugged the mono valve and had so much heat I had to cycle the off and EC buttons to keep from getting cooked.
Has anybody had an experience with a mono valve magnet getting weak, I'm almost thinking that the plunger is getting sucked down at higher speeds and shutting off water flow.

Gilly · #29 03-06-2013, 06:55 PM

Since the valve is capable of working, it would sort of point to the controller in the dash.
Interior temp sensor; maybe. Those seem to last the life of the car though, not much can go wrong with them.

LandYaghtLover · #30 03-06-2013, 08:01 PM

I re soldered my HVAC controls. No change. I did not have time to pull the plug on the mono valve since installing a new one. I need to try that.

I re-soldered the HVAC controls beacause a few days ago I had a hard fault on the controls. The air recirculation switch was steadily flashing and I had no control over the system. Not even able to turn it off. But get this, I had plenty of hot heat coming out.

Odd, huh? So thats why I need to try pulling the valve next.

But so far a new t-stat, new valve and re-soldering the controls has not helped. All vacuum lines look good. Idle is normal. I just need to keep tinkering when I have time, which is little.