Tan Qu wrote:
Wife has been the one who drove the car. I
periodically drove the car and took care of the things
I notice. But you know how women are - as long as the
car seems to run they think everything is just fine.
Anyhow I was aware the operating temp sat at 84c (with
a 80c Wahler thermostat) and in the summer it goes up
to 90 - 93c after I replaced the water pump and
thermostat about 2 years ago. I noticed the 120C just
this Tueday when she picked me up for lunch. She said
that just happened that day. I'll just leave it there.
Anyhow, what I think we need to focus on is that the
lower radiator hose stayed cold after the test run
during which the temp shot up to 120C again. So the
coolant wasn't flowing through the radiator. If it had
flowed at all I shouldn't have a cold lower hose,
right?
Thermostat - this was ruled out by the boil test.
Plugged up radiator - very unlikely although possible.
I bought this car while it had 103,000 miles. Replaced
water pump around 180k miles. Between 103,000 and
180,000 miles, the temp gauge was at 80C or 82c solid
even with A/C on in hot summer.
Trapped air bubble - wouldn't they start to show up as
soon as they were there?
Top hose should be HOT. Lower hose should be cool/cold when the system
is working as it should work. The engine fan should come on at about
95-100 deg. C and the aux fans should engage at 105 deg. C.
80 deg. C is too cold for the engine. It should run at a minimum of
85-87 deg. C. Anywhere between 85-95 deg. C is just fine and the fans
should NOT come on unless the AC is running under light loads.
Hank van Cleef's outline is the best troubleshooting guide I've ever seen:
-------------------------------------------------
Some thoughts on diagnosing cooling system questions:
Relying on absolute indications of electrical temp gauges is risky. I
have a nice Fisher Scientific dial thermometer with a long probe that
I use to test top tank temp in cars that have the filler cap on the
radiator (side tank jobs always have it on the upper hose tank). Not
too sure how will this would work with the M-B reservoir filler.
You should be able to see the thermostat opening on the temp gauge.
>From cold, drive the car at about 35-40 mph while keeping an eye on
the temp gauge. It should come up fairly quickly, then stop moving
and hold that temp with light throttle moderate speed running. On my
car, that is about 83C indicated. Do this with the air conditioning
turned off.
Now, kick the car up to 60-70 mph and go up a couple of hills. Some
small temp rise is normal, depending on the outside air temp. On my
car, it will hold the indication at 30C/80F outside temps. If the
temp walks up, let it coast down (downhill is best) to around 30 mph.
over a half mile or so, and resume the light throttle running. If the
temp comes back and stops, you've probably got poor coolant flow.
After this (car is nice and warm now), slow down to 20-25 mph, and do
some city driving (30-40 second waits at stop lights every quarter
mile). If outside air temp is above 80F, A/C off, my car will wander
up into the 85C range. Resuming 30-40 MPH will lower temp to the
thermostat point. If the temp goes up (you need a high outside temp
for this), you should look for air flow problems. These may be
fan-related, but don't ignore dirt and bugs in the radiator and A/C
evaporator fins.
Repeat the above tests with the A/C on and cooling. For this you need
enough outside air temp and humidity to assure that the evaporator
temp sensor is not shutting off the compressor most of the time.
This should produce a fair amount of temp rise in the "city driving"
test. With outside air temp at 30C/80F and dew point above 10C/50F,
my car will walk up to about 100C, engage the fan clutch, and turn on
the electric aux fan. Operation of both is pretty obvious. The
engine fan will make a lot of noise at 1500-2000 RPM, and you can open
the hood with the temp indicating high and tweak the throttle linkage
if in doubt. The fan will speed up with the engine at moderate RPM,
blow lots of air, and roar, if working properly. You can check aux
fan operation very quickly by disconnecting the sensor connectors and
grounding each with a clip lead. On my car, the A/C sensor makes the
fan run at reduced speed, and the coolant sensor makes it run at full
speed.
Keep in mind that the thermostat does not snap open and shut at the
rated temp, but has about a 10C range where it is throttling, between
fully closed and fully open. Thus, a variation of 10C on the gauge
around the point where you can see the thermostat open during warmup
is normal.
As outlined, in general, loss of cooling control at high speed is
generally related to coolant flow problems; at low speed, air flow
problems. Dirty fins will act like high speed troubles. If you can't
see an obvious "thermostat is opening" point, which is your reference
point for all other tests, then the thermostat may be failed open.
While I've outlined a bunch of tests, these probably can't be
conducted on the same day, or on the same stretch of road. However,
since you are driving the car regularly, you should be able to find
suitable times and places for observing behaviour under various
conditions.
A couple of things to remember, and use while testing:
The heater adds about 10% more cooling when operating at full tilt.
The air conditioner adds about 20% more heating when operating at full
tilt. Keep in mind that the worst heat load condition for the air
conditioner is high humidity.
Proper refilling of the cooling system, if it has been drained, can
have its tricks. In particular, you have to "burp" the system when
refilling it by waiting until the thermostat is open before topping it
off. My car recently had the engine out and the cooling system about
as completely empty as you can get it. I had plenty of heater
gurgling and water sipping for the first 100 miles or so after getting
the car back. Also, holding residual pressure when cold seems to be a
"feature" of Mercedes Benz cooling systems.
--
Marshall Booth (who doesn't respond to unsigned questions)
"der Dieseling Doktor" [EMAIL PROTECTED]
'87 300TD 182Kmi, '84 190D 2.2 229Kmi, '85 190D 2.0 161Kmi, '87 190D 2.5
turbo 237kmi
