OK, I'm a good amateur wrench, but I have a
distinct feeling that the MB faithful
here can probably teach me some tricks.
Most of the advice I see from the techs
looks pretty solid. On to the tale of woe:

A woman who trusts my mechanical judgement
(hah!) asked me to take a look at the
steering in her 83 380SL, which has a big
on-center dead spot. I am not entirely up
on SL's, but diagnosed it as probably
(hopefully) a steering damper gone bad.

I'll tell her to have that replaced first
rather than go with an expensive new steering box right away. Damper has apparently never been done at 180K miles
so if it doesn't need it yet, it ought to
pretty soon.

But what I can't explain is what I noticed
when taking the car on the freeway for a
check ride. This was uphill in serious 95F
heat with the AC on. Temp gauge crept up
to just under 100C. Then (I tend to watch
coolant in older cars), I caught it jumping
madly up and down between 50C and redzone
pegged. Yipe! Immediate deceleration, left
freeway soonest, drove at low speed to keep
airflow going.

Temps fell back into normal range. Huh.
Got back on fwy, no problem. Later, on the
return leg, same thing. The engine showed no
ill effects nor any classic overheating
symptoms beyond the jumpy gauge. Runs great
when not under load. Low mileage on a full
rebuild with a retrofitted 2-row chain.

Frankly, I've never seen this in any other
car that I've worked on (my own vehicles
receive full coolant turnover with 50-50 antifreeze and distilled water once per year
rigorously and it's very rare for them to
have any overheating). I'm going to reach a
bit and diagnose cavitation in the coolant
flow due to boiling. I assume a couple of
big air bubbles would freak out the
thermocouple and cause the gauge to flick
around the way that it did. Sound reasonable?

The car also has (fairly fresh) green
Prestone-style coolant in the main
reservoir. I can't recall if standard
green Prestone is OK in an MB of this
vintage or not. I do recall that the Benz factory coolant is not green.

I will swap that out shortly as a test.
Will try an MB citric acid flush and/or
augment the mixture with Redline Water Wetter if it doesn't help.

Actually, secondary question there: is
Dex-Cool type extended use coolant OK in
an older MB? I use it in several other
vehicles, not so much for 150K drain
intervals (nuts, in my opinion) but to
be nice to the water pump impeller.

If the coolant replacement has no effect
I'll start in with electrical diagnostics
and work backwards from the thermo sensor.

Wondering also if this older rad is just
not up for hauling this heavy car up steep
hills with full AC on in desert heat. I tend
to try to rework cars to fit drivers rather
than the other way round. Perhaps an MB dismantler might come through with a 560SL rad? Can't imagine it would be a problem to
fit one of those and I imagine it would
expel a lot more BTUs.

Ron H

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