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#1
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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 -- |
#2
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Couple of things come to mind. If you suspect boiling, then the most likely culprit is the cap on the expansion tank not holding pressure, but.. you would be losing coolant. As far as the gauge, I would look for an intermittent faulty ground, or a bad sensor. if the engine doesn't have any signs of overheating, then you're getting bad information for tne monitoring system. Any good antifreeze in a 50 50 mix should not be a problem. Check the thermostat as well.. As far as steering goes, the damper would not produce a dead zone at full center. Try removing it and seeing is anything changes (unlikely). Sounds more like excessive wear someplace. Pitman arm, tierod ends or the box itself..
------------------ Jeff L 1987 300e 1989 300e 1987 BMW 325 [This message has been edited by jeffsr (edited 05-31-2000).] |
#3
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If the cap checks out OK take another look inside the radiator. Could the tubes possibly be partially blocked with deposits, which would slow the circulation at higher speeds and revs? If they tubes are, I'd check into getting the radiator cored and rodded.
------------------ Mike Tangas 73 280 SEL 4.5 |
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