Hi all.

My '87 300sdl is having cooling system issues that I can't seem to track down. Hopefully someone has a bright idea that I haven't thought of...

Ambient temps have been between 90 and 100F during the day, but symptoms persist even in 70-75 degree temps at night.

Symptoms:

-On flat ground on the highway it stays fairly steady at around 95
-Downhill on the hwy it cools off to ~85
-Any incline on the hwy it heats up well past 100, often as high as 110 and sometimes as high as 115
-It town it runs between 90 and 100 depending on traffic and terrain
-Temps stay fairly similar whether AC is on or not
-Turning heater on high has some cooling effect on engine temp, but not drastic


What I've done so far:

-Replaced thermostat twice (OEM from Mercedes)
-Replaced radiator cap (OEM from Mercedes)
-Replaced sending units for temperature gauge and AUX fan switch
-Citrus flush, twice
-Coolant change, at least 3 times, always 50/50 Zerex G05 or Mercedes antifreeze
-Replaced radiator with new Behr/Hella unit
-New primary/secondary fuel filters

System is fully burped as far as I can tell. No air bubbles from top radiator hose when squeezed, almost no air in reservoir (I know this isn't proper but I wanted to be sure there wasn't an air bubble). I spent probably two hours trying every method of burping I could find on the forums until I was satisfied that there was no air left.

Viscous clutch fan was replaced by PO with MBZ OEM, new style (plastic). Fan engages and disengages properly ("roars" when running, then stops as car cools down).

AUX fan functions properly with AC running as well as when engine temp is >105 (turns on high).

HVAC functions properly (cold AC, hot heater - no monovalve problems as far as I can tell).

Temp at top radiator hose and at head are within a few degrees as indicated by temperature gauge. Tested this with non-contact IR thermometer.

Temp difference between upper and lower radiator hoses is within a few degrees. Tested this with non-contact IR thermometer.

When thermostat is open and the top radiator hose is removed from radiator, coolant pours out freely (no apparent circulation issue).

No oil in drained coolant as far as I can tell, no milky oil on filler cap or dipstick, no milky oil at last oil change. No white smoke from exhaust.

No hesitation on acceleration, rough idle, etc.


That's all I can think of at the moment.


The only real anomaly with the car that I can think of is that, when given fuel, it smokes at the joint in the exhaust manifold circled here:



Far as I know it has the "can" thing replaced (name eludes me at the moment dang it) with the new setup under the recall:



Also, I run it on B100.

Any ideas? This is driving me nuts. I'd appreciate any suggestions. Thanks!

Try removing the thermostat completely. If you still have problems then from your list the only thing I can think of is that it is a bad water pump.

This is not something I would advise anyone to do but I've thought in the past of putting a glass section in the line somewhere to actually see the flow of fluid when tracking down a cooling problem. Never had the guts, though.

I would put a pressure tester on the cooling system and check for pressure rise when the engine is running. If the pressure rises fast and keeps climbing I would suspect a blown head gasket.

__________________
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1969 220D 4 speed (parts car)
1976 280C
1976 280S
1981 240D 4 speed (parts car)
1982 300D
1983 300TD (Ivory)
1983 300TD (gold)
1985 300TD (gray)
1987 190D 2.5
1970 280SEL (sold)
1977 240D 4 speed (sold)
1974 280 (rusted to death)
---
1927 Chevy 4 door sedan
1938 Chevy 2 door sedan
1950 Willys Jeepster
1955 Studebaker President 4 door
1977 Ford F250
1979 Glastron 17'
1948 John Deere A
1960 John Deere 2010
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2000 Bichon (Doby)

Could it be a faulty guage in the cluster? What's the readout on your IR thermometer for the top hose, when the car is running? Does it match the guage? Is the engine all gunky and dirty (probably not from your post).

95 seems rather hot anyway.

Hows your oil pressure at idle when temp shows 95C

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Thanks DieselBone - can I get a pressure tester at like Napa or Kragen or is it some sort of specialty thing?

Something I forgot to mention in my initial post was that the system seems to pressurize normally and hold pressure without puking out any coolant (as you might expect with a blown head gasket). It does not hold residual pressure once cool - sucks some air back in when opened.

Certainly not to say that a blown head gasket is out of the question, but I have investigated that and couldn't find any tell-tales. Maybe tomorrow I'll pick up one of those chemical kits that turn yellow or whatever when oil is present in the coolant.

The readout on the cluster temp gauge is within a degree or three of the IR thermometer readout at the top radiator hose. Hose temp was about 89C and the cluster gauge was right around 90 when I tested earlier this afternoon.

I'm not 100% sure on the oil pressure at idle - to be honest I haven't looked at it lately. Last time I clearly remember checking I know it dropped about 1/3 of the way down from being pegged, but I don't remember when that was. I'll go take it out for a spin and let you know.

Engine isn't exactly sparkling, but it's not exactly caked with grease either.

Thanks for the suggestion . I actually tried this before I replaced the radiator - didn't want to blow $350 to find out the problem was a $30 part, heh.

Regarding the water pump, like I said water pours out if you remove the top hose with the thermostat open, though I admit that I have no frame of reference for how fast it should be pouring out on a properly functioning system.

Unfortunately I cant seem to figure any better way of testing it myself.

Is the car using any coolant? Are you adding? I'm driving one now with a bad gasket and the coolant isn't getting into the oil... it's just getting out of the gasket in the rear. This happens frequently with these cars. A pressure tester would tell you it's leaving then you know. You may want to put some clean cardboard under the car after it heats up. Maybe you'll find it dripping.

Still, you don't have any air moving through the radiator. Perhaps the condenser fins are clogged (radiator fins should be clean, as it's new, right?).

Don't remove the thermostat. It's my understanding that, with the double-flange MB design, that jacks up the flow/bypass operation leading to all kinds of wierditude.

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1987 300SDL junker 170k
1982 300SD junker, 265k

With the exception of the water pump, your cooling seems to be in good shape. However there is a possibility of sediment and crud build up in the block and covering the passages in the head gasket, however, I feel that is a longshot. I might start looking into the possibility of overfueling. Do you know what condition your injector nozzles are in? How is your fuel mileage? BTW, the smoke from your exhaust manifold is most likely because the metal tube connecting the two halves of the manifold is missing.

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1987 300TD Turbo 5-spd
1991 350 SD Turbo (Frankenstein 603.970 crank, rods and head on a .50 bored 603.963 block )
1982 300D euro 617.951, 4-spd
2003 E500

2000 GMC C2500 6.5 TD (Double ott, ex DOT truck)
1980 Scout Traveller SD33T (Resting!)
1997 GMC K2500 Burban 6.5 TD (mid engine rebuild)

It's not losing any coolant as far as I can tell - have never had to add any and the level remains constant. I will monitor this more closely now that you mention it. Will also get some cardboard to put underneath.

I checked the condenser fins when I had the radiator pulled. There were a few bugs and road "stuff" but no major blockages. I hit it with my pressure washer for good measure though and I can easily see through it everywhere I looked at it.

Hmmmm...didn't consider overfueling at all, and it's a distinct possibility because I removed the ALDA. I'll put it back on this morning and see what happens.

Fuel economy is around 20, but most of my driving is in-town and I live in the mountains, so a lot of hills. Plus with B100 my understanding is that you'll get about 5% poorer fuel economy to begin with.

The reason I mentioned the smoke was because I was starting to wonder if the replacement for the trap oxidizer can crud up and cause an excessive backflow issue. I might pull it and the intake manifold today on general principle.

Nice idea - may be you could use 'steam engine grade' glass. Steam engines have lots of these types of things on them - usually to measure water levels.

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I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I haven't heard mention of the updated trap oxidizer exhaust system having any issues, but there is a first for everything. Do you mind my asking why you removed your alda? I thought it was common knowledge they could be adjusted but one should NOT do it without installing a pyrometer! The 603 cooling system is barely adequate to handle the heat this powerhouse puts out, turn up the fuel, add hot weather, then turn your AC on and your gonna have some major problems. However, I'm assuming your AC doesn't work because you made no mention of it making a difference. My 350 heats up a considerable amount when the AC is running, however the 126 350 has an even more inadequate cooling system, MB fixed this when they introduced the 140.

__________________
1987 300TD Turbo 5-spd
1991 350 SD Turbo (Frankenstein 603.970 crank, rods and head on a .50 bored 603.963 block )
1982 300D euro 617.951, 4-spd
2003 E500

2000 GMC C2500 6.5 TD (Double ott, ex DOT truck)
1980 Scout Traveller SD33T (Resting!)
1997 GMC K2500 Burban 6.5 TD (mid engine rebuild)

There are several threads regarding removal of the ALDA on the 603 - one long one in particular made me decide it was ok as long as I wasn't too heavy-footed. I've never heard mention of a pyrometer, but that could just mean I did something dumb without enough research...

The AC actually works fine. It was converted to 134a by the PO (no documentation on this so I don't know if it was done correctly or not) and I would say it is adequate.

It has some effect on the coolant temp but it doesn't seem to be drastic. It does, however, drag the engine RPMs down quite a bit when the compressor kicks on.

I'm sure you'll let us know if it makes a difference when you re-install the alda, I know it's a pita but I have a feeling this could be your issue. I don't know why there are all these threads around for removing the alda when all you really need to do is remove the cap on the adjustment screw (sealed at the factory) and turn the screw counter-clockwise about 1/2 to 1 turn, usually makes a world of difference.

__________________
1987 300TD Turbo 5-spd
1991 350 SD Turbo (Frankenstein 603.970 crank, rods and head on a .50 bored 603.963 block )
1982 300D euro 617.951, 4-spd
2003 E500

2000 GMC C2500 6.5 TD (Double ott, ex DOT truck)
1980 Scout Traveller SD33T (Resting!)
1997 GMC K2500 Burban 6.5 TD (mid engine rebuild)

Well...

Reinstalled the ALDA, no luck. Still running about 95C average. I can't make black smoke anymore even if I try (which makes my inner man-child kinda sad...), so as far as I can tell the fuel mixture is at least ok enough.

I also bought an exhaust gas test kit and ran that for about 30 minutes (kit recommends 2 minutes) and it came back OK. Fluid stayed blue. Autozone didn't have a pressure tester in stock but I will look around for one of those tomorrow.

I pulled the trap ox replacement thing and it was clean, though the EGR hole was almost completely clogged with gunk. The part of the intake that goes across the top of the engine was totally clear.

Double checked the fins on the AC condenser just to be certain and they were toally clear.

Cut the radiator box open and left it under the car last night and today and there was nothing on it.

Thinking I should order a water pump? At this point I dunno what's left lol.

Thanks everyone for the suggestions so far .