I did a pressure test on my E420's cooling system and found a leak in
the upper radiator hose where it connects to the engine. It didn't
leak fluid before, but with about 20lbs of air pressure, I heard air
hissing out of the top near clamp. I re-tightened clamp and air leak
stopped. Went on road test and car still heated up, but when I stopped
car and it cooled for a while, I checked hose, it had collapsed flat.
When car was running and hot the upper hose was full and tight; I
noticed the lower elbow hose connected to the thermostat seemed
Does the above point to a water pump or thermostat issue? or both
"If at first you don't succeed, you succeeded with failure."
Follow-up--- After a couple cycles of on and off and adding additional
fluid, the last 5 mile test ride car stayed cool, well under the 100c
mark. I think IT'S FIXED..................
WOOOOOOOOOOOHEEEEEEEEEEE!!!!!!!! (no party yet till later highway
The collapsed hose was the engine vacuum that wanting more
fluid....still going to replace puffy elbow hose....
I made a homemade cooling system pressure tester with a bicycle foot
pump (with gauge) connected to the overflow tube going to the radiator
(used rubber gas hose that fitted inside overflow tube at one end and
slipped over foot pump air nipple). I just blocked the overflow hose
nipple at the overflow reservoir with a short piece of hose, clamped
to nipple, and with a bolt screwed in open end of hose, as a stop.
This test is great because it tests the cap too! MB's factory pressure
tester has a modified radiator cap that is used and you have to remove
your existing cap to test.
As much as I'm learning about my MB, I hate to even think about
selling it.... This car cost $54,000 new in 1994 (I paid a fraction)
and drives smoother than new. I tried the newer series E models 96-
later and they don't drive/feel like the original W124 series or older
MB classics.....also the design of newer MBs you have to see MB logo
to know it's a Mercedes... the classics stand out as MBs without a
I'll post a follow-up after highway test....
After highway test, engine stayed cool longer but eventually heated
up. Ballooned hose was defective (split inside and outer rubber skin
was barely holding fluid and eventually sprung a dripping leak). Also
did another pressure test and same upper hose was releasing air. I
replaced ballooning hose and upper hose. Rechecked pressure and
refilled with fluid. Now I'm awaiting a couple cycles of on and off to
One thing I did notice is that after heat up, radiator was cool in
center, but when I turned on cabin heat to circulate coolant, radiator
center became hot... not sure what that indicates??? I guess that
rules out clogged radiator???? Unless radiator is clogged at top where
fluid returns from engine???
Stay tuned for next road test after a couple of on and off cycles....
The saga continues.... Car still overheats after 20 minutes or full
warm up...Drives great until that point... ...upper hose still
collapses after cool down. I think Tiger might be right about radiator
clog, but why would loosening rad cap release vacuum even when I
pinched close upper overflow tube leading to collapsed upper
hose...shouldn't rad cap release vacuum without loosening it?
My overheating problem may be a clogged catalytic converter. When cold
the cat has not expanded fully and when hot closes up creating back
pressure... Anyone know of clogged cat causing overheating?
Also, I noticed the main overflow tank has it's own overflow tank with
a vacuum line attached. What's that about? Could that be the culprit?
All help truly appreciated...Derf
"Wisdom is gained by determination."
| The saga continues.... Car still overheats after 20 minutes or full
| warm up...Drives great until that point... ...upper hose still
| collapses after cool down. I think Tiger might be right about
| clog, but why would loosening rad cap release vacuum even when I
| pinched close upper overflow tube leading to collapsed upper
| hose...shouldn't rad cap release vacuum without loosening it?
| My overheating problem may be a clogged catalytic converter. When
| the cat has not expanded fully and when hot closes up creating back
| pressure... Anyone know of clogged cat causing overheating?
| Also, I noticed the main overflow tank has it's own overflow tank
| a vacuum line attached. What's that about? Could that be the
| All help truly appreciated...Derf
| "Wisdom is gained by determination."
I think you have a thermostat that is stuck in the almost closed
I've seen that symptom before and the thermostat was stuck.
Partially plugged catalytic convertors cause loss of power, not
Just my 2¢.
Thanks Steve... I did the thermostat in pot of heated water test and
it opened up (1/2") at around 80c, but now that you mentioned it, that
test only shows large round plate opening at base plate and not the
operation of the smaller round springed plate at opposite end of base
(which controls flow from engine)...... hhhhmmmmmm....
I did install a new thermostat (and still had overheating so I put
back old one) but that was before I had discovered a leak in top hose,
through a pressure test. When I swapped out the old thermostat, during
my diagnostics, I removed upper radiator hose and it didn't seal
properly, compounding my problem.... sounds logical.
I'll swap old thermostat with new one and keep everyone posted
(below link is a coolant flow diagram I've been using as a reference)
It would hang at 120c and red line at stops, then when moving temp
dropped just under 120 mark.
I borrowed a cooling system pressure tester that tests caps and found
that the new cap (and old cap) weren't consistently holding pressure
during different tests (newer cap would hold sometimes, old cap nada)
ugh! So I'm getting a replacement today.
Also, with pressure tester attached to overflow tank it was holding
pressure and car stayed at normal temp. Didn't get any blow back
either (a good sign).... I think I'm close to nailing this snafu....
Look for update
"If man can build a car, man can fix a car."
I doubt a faulty coolant cap would produce the overheating that you
are experiencing. Having the thermostat out, and thermostats being
cheap, I would have replaced it regardless of what the water pan test
showed. It's possible it's hanging up somehow when it's actually in
I don't think so Tiger, I already blocked CAT converter manually, it's supposed to feed-back to your engine 5-15% of exhaust air to reduce NOX gas on your exhaust output. Do you mean clogged exhaust pipe? That I can agree, it will raise the heat.
Derf, try to remember what did you do a month or two before having the problem. 120C is a lot. How high is your idle speed?
Are you aware of the fact that Mercedes 3 liter diesel thermostats
have to be aligned when put in? There is a small hole in the rim of
the thermostat which has to be aligned with some sort of mark in the
housing. I can find that in the manual and send you a scan of the
picture if you don't know about it and can't figure it out.
If your thermostat is not aligned properly it won't open completely
and the engine will run hot at speed or on a grade, etc.
Say what? It's news to me that any cat converter feedsback exhaust
gas to the engine. That's done by the EGR system no? The catalytic
converter is there to cause a chemical reaction that reduces
hydrocarbons, CO, and NO directly. The ones that do all 3 are the 3
way type. I know I've seen plenty of cat converters that have one
pipe in one pipe out.
There's a hole in my thermostat's base plate with a tiny bb within a
tube that acts like a flow check valve (no hole at rim). I read
somewhere it needs to be at the top, 12 o'clock? (someone please
Update: Put on newly bought cap that passed pressure test of 20lbs
(Mercedes dealer wanted $29 for cap and Autozone wanted $5) I'm now
going through cycling/burping process. Noticeable improvement on first
highway test (car ran at 100c at highway speed and under 110c in stop
and go traffic (never reached 120c). I read that it takes a couple of
cycles to fully bleed coolant and should eventually purge all air
traps. Future test drives hopefully will show cooler results....Stay
Car idles around 750 RPM in neutral with just under 1 bar oil pressure
after warm up...
Something is clearly wrong here. If cars needed multiple road trips
to bleed air and avoid overheating, a hell of a lot of cars would be
overheating following coolant changes and similar. I've also never
seen a car overheat during normal driving around town from a bad
The notion that every last bit of air has to be out of the system or
the car will overheat doesn't make sense. Years ago, cars didn't
have expansion tank reservoirs and the radiator cap and fill was on
the radiator itself. The rest of the cooling system was very similar
to what you have today. It was not unusual to see coolant level down
a couple of inches back then and these cars did not over heat.
I suspect you have either a plugged radiator, a faulty thermostat, or
less likely a water pump that has disintegrated. I haven't seen the
last one, but someone did report on this topic that they had seen one
where the impeller had disintegrated.