'91 300D 2.5 minor heating issue...where to start?

[dablk1]

Lets see. We've had a few chilly mornings recently, my car starts fine, I have the climate control set on auto at about 70ish. I let the car warm up for 5-10 mins and everything is fine. The issue occurs while driving or even when idling I suppose after the interior warms up. The air temperature would gradually become cooler and cooler. I find myself turning up the temperature just to stay warm. The blower motor would maintain the same speed, but just start blowing cooler air. I've changed the primary and auxiliary water pumps within the last month....no change. Now, if I put the car in park and rev the engine it will bring the interior temperature back up. Just to experiment...I drove my car about 30 miles after it was fully warm and had the sunroof open about 1/3 of the way just to see what would happen to the interior temp..the temperature stayed dead on. I'm not sure what this proved, but while the roof was open and the ambient temp was about 38 degrees the interior temp stayed at a nice and cozy 70ish. It seems as if the interior temp is accurate, but the blower isn't responding correctly or something. Where is it coming from and why does it start blowing cooler/cold air? As always any and all suggestions are welcomed. Thanks

By the way..I replaced the radiator, waterpump and auxiliary waterpump...as preventive maintenance when I started to have higher operating temperatures. The problem existed before then...I meant to address last winter.

[New2MB]

Does the engine coolant temperature drop as well?

Just making sure it's not a thermostat...

[dablk1]

No

[New2MB]

When it stops blowing warm and starts blowing cold, does it go on blowing cold forever or will the warm air come back after a bit?

[dablk1]

Well it will come back if I increase my speed by about 10mph. Also if I put it in park and rev the engine.

[Billybob]

Originally Posted by dablk1

Lets see. We've had a few chilly mornings recently, my car starts fine, I have the climate control set on auto at about 70ish. I let the car warm up for 5-10 mins and everything is fine. The issue occurs while driving or even when idling I suppose after the interior warms up. The air temperature would gradually become cooler and cooler. I find myself turning up the temperature just to stay warm. The blower motor would maintain the same speed, but just start blowing cooler air. I've changed the primary and auxiliary water pumps within the last month....no change. Now, if I put the car in park and rev the engine it will bring the interior temperature back up. Just to experiment...I drove my car about 30 miles after it was fully warm and had the sunroof open about 1/3 of the way just to see what would happen to the interior temp..the temperature stayed dead on. I'm not sure what this proved, but while the roof was open and the ambient temp was about 38 degrees the interior temp stayed at a nice and cozy 70ish. It seems as if the interior temp is accurate, but the blower isn't responding correctly or something. Where is it coming from and why does it start blowing cooler/cold air? As always any and all suggestions are welcomed. Thanks

By the way..I replaced the radiator, waterpump and auxiliary waterpump...as preventive maintenance when I started to have higher operating temperatures. The problem existed before then...I meant to address last winter.

It's a long shot but I'm placing my bet it's the CCU failing to operate correctly. Sounds like there is either some low cabin temp set point that is either not being triggered or not being responded to correctly.

You've already replaced the parts that provide a flow of heated coolant so it appears that it's not a lack of positive flow. So it stands to reason that there is some obstruction to flow, this appears to be overcome when engine speed (and hence coolant flow) is increased. That could point to a monovalve malfunction either a physical malfunction or a control malfunction.

It appears that the fan speed does not alter, it should slow once temperature is reached (assuming the CCU is in the AUTO mode) it appears yours does not. The blowing of cold air seems to be caused by the lack of coolant flowing through the heater core. When you open the sunroof it sounds like the system is responding by increasing the heat output in response to the cabin heat loss. The cabin temp sensor appears to function causing the system increase cabin heat. Now it could be that the temp sensor is dirty enough to cause it to fail sensing small changes or the aspirator motor/fan or the tube connecting to the temp sensor could be failing somehow to develop sufficient flow across that sensor.

With the car running and the CCU on AUTO you should be able to detect air being pulled into the sensor, a cigarette or ever an extinguished smoking match held near the sensor should show the smoke being drawn into it.

[dablk1]

Well you're definitely on to something. The sensor failed the test. It's not drawing anything into it. It even failed the paper test. I've some other posts and they refer to some type of foam tubing or something. I didn't notice this foam tubing the last time I was in there. All I noticed was a rubber hose from the sensor to the aspiration which went down from the a-frame. I guess I need to check the aspiration and then the hose. If I have to remove the hose I have no idea how I would get it back in there(R+R)

[dablk1]

Since one have issues have been identified, do you think it would be necessary to pull the CCU and look for some bad soldering points also?

[Billybob]

Originally Posted by dablk1

Well you're definitely on to something. The sensor failed the test. It's not drawing anything into it. It even failed the paper test. I've some other posts and they refer to some type of foam tubing or something. I didn't notice this foam tubing the last time I was in there. All I noticed was a rubber hose from the sensor to the aspiration which went down from the a-frame. I guess I need to check the aspiration and then the hose. If I have to remove the hose I have no idea how I would get it back in there(R+R)

The aspirator fan should be on whenever the car is on I'm pretty sure, I think you'll need to remove the glove box to replace it but you might be able to see it or get close to it if you remove the underdash panel on passengers side, that way you can feel or hear if its running. The sensor is located in the overhead light module you can blow that out with air or vacuum it without even disassembling it probably. The sensor can get covered by dust.

The hose between the sensor and the fan is rubber it doesn't degrade like the foam hose/tube used in 123/126 cars, so you shouldn't have any problem with it. If you get the fan working and for some reason there is no flow into the sensor I suppose you could blow it out or push a wire or plastic weed wacker string through it, I've just never seen one clogged.

I would hold off screwing with the CCU until I was sure the sensor was working, still might be the CCU it's 18 years old but I'd try to get the fan sensor straightened out first and eliminate an obvious problem before I started looking for "cold solder joints"!

[jt20]

ditto on the sensor first.

incense works great.... but there is no substitute for opening it and cleaning it.

I can hear my my little vacuum thingy running with my ear to the passenger vent.

[dablk1]

Ok, let me make sure that I understand you. You can hear it running with your ear to the vent? Is it constantly running with/without the CCU on? When I turn my car on I don't hear it. If I turn on the CCU all I can hear is air coming through the vent. I'll pull the glove box and take a closer look.

[jt20]

I believe Mr.Bob is right.... it goes on when the glowplug light comes on.

even before you start the car, even if the CCU is off

[dablk1]

I'm getting no sound.

[Billybob]

Originally Posted by dablk1

Ok, let me make sure that I understand you. You can hear it running with your ear to the vent? Is it constantly running with/without the CCU on? When I turn my car on I don't hear it. If I turn on the CCU all I can hear is air coming through the vent. I'll pull the glove box and take a closer look.

No, you might be able to hear it running with the key turned to the Run position if you hold your ear to the far passenger side dash vent. The aspirator motor/fan is located in that general area behind and below that vent, a good fan will be difficult to hear usually the fan's squirrel cage gets dirty with dust over time and makes more noise as it turns eventually possibly hitting the housing and squealing.

I have a car with a noisy aspirator fan and because of that noiseyness I'mall but certain it runs all the time the key is on!

You can once you get it out from the dash open up the fan's plastic frame and clean it up, sometimes that is enough to make things better sometimes the motor will either not work or will be very noisy, I suspect having the fan blades seized/not moving and power being applied to the motor armature through the brushes can cause a problem after a significant time of non-operation.

[dablk1]

Well gentlemen. It's the aspirator. I'll try to clean it up. It actually did work when I fumbled with it a bit. If it's not working properly after I clean it up. I'll be heading to PNP in the morning to grab another. Thanks for all your help.