Anyone have a recommendation on a good Mercedes garage in Charlotte, NC? My '91 350SDL (117K miles) is running warmer than it should - block check shows no exhaust gasses in coolant; oil is clean and shows no contamination. Local garage put in a new, oversized 100% aluminum HD radiator, 85c Stant thermostat, serpentine belt, fan clutch, fan blade. Aux fans are running; AC is charged and triggering the different fan speeds.

Not seriously hot - but right around 100 - 105C. Everyone I've talked to who owns a 350SDL, and my previous experience with 3 other Mercedes diesels says it should be hovering around 82 on a decent temp day. Two local shops have just said "Maybe that's normal for the 6-cylinder diesel engine" and "oh, 15 - 20 degrees warmer than normal is fine" - With this engine, it is not OK - and we're heading into summer and I need to get this sorted, because it will start inching up to the 110-120 mark. Never been into the red and I don't want it even getting close.

My suspicion is that it's the water pump. This car sat for a long time and didn't get put back on the road until last September. It also may have an aftermarket water pump that's not moving enough coolant?

I have the correct MB/Zerex coolant and coolant ratio, and I did leave it up on ramps for a few hours when I did the last coolant flush & fill, to elevate the nose and get any air out of the system....heat and defrost always blow hot.. so I don't think it's an air bubble...

I need a good shop who can diagnose this. Tempted to swap out the Stant thermostat for an OEM from MB.

If it is a head gasket, then I need to get it fixed before I bend a connecting rod from hydrolock. But I also don't want to put $2K+ into a head R&R only to have it turn out to just be a $450 water pump replacement. Any thoughts/suggestions? Love this car and want it to be right - and able to drive it during the hotter months.

Sounds like you've got a good handle on most of the basics for diagnosing the problem.

If this were my car, I would:

First and foremost, don't bring it back to any shop that proceeds to tell you that something abnormal may be normal. (!)

Use a non-contact IR thermometer to check to see that it is actually running hot - the temp sender or gauge calibration could be off. (Not too common, but it does happen.)

Pull the thermostat and test it with a boiling pot of water and a thermometer - or better yet just spend $20 and put a Wahler (OEM) thermostat in it anyways. Make sure it is seated correctly too.

Water pump: There are (rare) instances where the impeller comes loose and spins on the shaft. I once had a replacement pump where the impeller was too short and not coming close enough to the housing to create adequate flow; it was poorly designed. Stick with a good (preferably OEM) quality pump - they are not that expensive. Believe it or not, sometimes the Stealership prices for a rebuilt pump are pretty good.

If this happens primarily at highway speed: Another rare oddball thing that could happen is the lower radiator hose could collapse inward under suction, thereby restricting flow. Again, not very common, but it has happened.

Two questions: 1.) Is it loosing *any* coolant? 2.) Is there high residual pressure in the system when it's dead cold in the morning? If neither of those, then it reduces the likelihood that the headgasket is leaking.

__________________
Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

You've probably already checked this, but pay close attention to the lower radiator hose. It kinks easily.

Those temps are definitely not normal. My auxiliary fans don't work at all and I'm only a needle width above the 80 mark, occasionally a little higher but not much.

__________________
1991 350SDL 350k+ miles

Thank you both for your quick replies. Definitely not listening to anyone who tells me this could be normal. I've driven Mercedes diesels well over 750,000 miles now and I know this is NOT normal. I'm losing sleep over this one....but not losing any coolant at all. It was leaking from the bottom composite tank before I replaced the radiator. Both upper and lower hoses are new.

I got home from the office right around 6pm; car's been sitting since. It's now 8:40pm - I just went out and checked. Not a 'lot' of residual pressure when I removed the cap - but I did hear it release some pressure and the cap was definitely nice and tight and hard to turn.

It was fairly cool here today - mid 70s and is down in the low 60s now - the intake manifold is just barely warm to the touch, so the engine is cool. I've asked both mechanics about head gasket and neither seem to feel that there is a problem there. i have previously tested it with an IR thermometer, but will do it again tomorrow - I aimed it on the top tank of the new radiator (which is shiny - and that can give weird readings), but it came back at 221F, so I need to check that again. I'm going to go ahead and order an OEM thermostat for it - maybe Stant isn't the greatest option, although that is all I have used in the past with no problem. 85C should be correct, yes?

One point I did neglect to mention - this car sat for almost 10 years and was hardly ever driven. Had the wrong coolant in it, as well, so I did a radiator flush & fill - didn't get any signs of contamination out of the system but I have read of cases where the impeller gets basically corroded, and have also read horror stories about aftermarket pumps - I don't know that this one is original or if it is a replacement. Hopefully, I am lucky and it'll be water pump just not moving enough coolant.

I have done everything I know to try and diagnose it - and these are really my last two potential issues. Head gasket would be fixable. Cracked head may or may not be - I don't know that I could easily find a good one with similar mileage on it - possibly a rebuilder out there somewhere?

This car is in excellent condition - maybe 1/4 quart of oil between oil changes at most. Body is excellent - no rust, paint is still beautiful. Interior is 99% new - couple of minor cracks in the clearcoat on the wood trim, but otherwise looks like its never even had anyone sit in it. It does have a very annoying vibration/rattle in the exhaust system right around 1800-2000 RPMs but from what I hear, that is normal in this car and they all do it.

I know (and knew when I bought it) that the 603 engines aren't the best - but the rod-bending issues don't seem to be that prevalent in the earlier versions, and the head gaskets were far worse in the previous iteration. I'd be tempted to put a good rebuild in it if this one goes just because the car is so incredibly well-crafted and the W126 long wheelbase is my all-time favorite body style.

Thanks again for your help - if anything in this reply rings a bell or triggers anything else I can check, I'd really appreciate it - otherwise, I got a lead on a good mechanic about 30 miles away. A hassle, but if he can fix it.....

Feel the radiator hose for hardness after the car has sat for at least eight hours. There may be a slight residual pressure in the system in my experience with cars.

Nowhere enough to have a radiator hose hard or pretty stiff the next morning though with these engines . Preliminary things you can still do are both cheaper and do it yourself. If that radiator hose is pretty firm the next morning get a kit to test for the presence of exhaust gasses in the coolant.

I am not positive one would see oil itself in the coolant with a slight crack or slight head gasket leak. Also to make sure the hose test has some validity.

On a cold engine I would pressure up the coolant system to say ten pounds to check that it is tight. Leaving it for hours and checking that the pressure remained. Even though there is no apparent loss of coolant. I also think many head gaskets simply fail from age related interactions between the materials.

I guess on the 603 engines one should use every method and test that can be thought of to clear or blame the head or head gasket before much of anything with over temperature issues. The tests are almost free, fairly easy to do, and involve little time. I have always disliked the shotgunning approaches to problems when avoidable.

No I have no or very little suspicion from what you have posted that there may be an issue with the head or head gasket for example. Just not enough that there is not either. Another thing that might be considered is to ask another member on site with the same engine. To remove his expansion container cap the next morning and tell you if there was the slightest indication of coolant system residual pressure remaining. This is not conclusive exactly but might provide a clue to look further.

I know where there is an 87 station wagon with relatively low miles. It has the #14 head and I really cannot make up my mind about buying it. Finding a new head when and if it became required from my location in eastern Canada might be pretty difficult for a reasonable price if ever required.

The wagon is rust free so I would pretty much round up a good spare head anyways if I decide to purchase. It would be one of those cases if you have it the old one will last forever. Without it on hand probably a much shorter time.

Logically to me at least the newer 3.5 litre 603 has a better chance of head gasket issues than the 3 litre 603. A picture of the tops of the lower blocks in comparison to each other is pretty revealing to me.Your engine was developed basically on demand of the north American mercedes dealers for more power I think. Mercedes solution for the problem was engineered too fast in desperation. The proper response should have been a totally new engine to replace the existing 603. Heads and head gaskets will always remain a concern to me with both versions of the 603 engines with rising operational temperatures. Or at least much more than on say the 617 engines.

If I do land up purchasing that 87 wagon. I will make a simple test device that is easy to use and produce conclusive results very fast for issues like these. Basically a stacking tower that fastens to the expansion tanks threads. It will have a pressure gauge and a feed port to a cheap household carbon monoxide detector. Plus a port to apply external air pressure.

It will only take one known good car to calibrate or find out what normal readings are. We are talking a device that to me is both really cheap and probably very effective.

You just cannot get the coolant overheated from an interaction between exhaust gasses and the coolant without producing some carbon monoxide reading. Those household detectors are sensitive enough to do the job well I believe. There should be no carbon monoxide present in the vapors coming off the hot engine coolant if the isolation between the coolant and the exhaust gasses are totally isolated. Because of operational pressure differentials it is much more probable to have exhaust gasses in the coolant rather than coolant loss with small cracks.

A couple of hours is not enough time. It takes a lot longer for the engine and coolant to reduce the temperature enough. Overnight is considered the normal test of at least eight hours to get back to ambient temperatures.

Shoot the IR thermometer on the head as close to the temp sensor as you can. Should give a more accurate reading.

__________________
Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

Have you tried contacting local (to you) forum member vstech? He's in Mt. Holly...

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Thanks, guys - will leave the car sitting until morning and check for pressure, but based on what I saw last night when I tried to remove the cap, it was very minimal pressure. It's a new radiator cap so it's stiff as the rubber is sealing very well. I did shoot the outlet on the top of the engine just past the thermostat this afternoon - the gauge was reading 100C or just slightly higher, and the highest reading I could get on the outlet fitting was 207. Mostly it was 197 - 202. Wondering if perhaps my temp sender is off. 200 degrees for coolant is not that bad - and 82C equates to 180degrees; I'd had the air running and was driving in city traffic. So maybe this is not a big of an issue as I suspected. That part is cheap - getting to it to change it may be a challenge, but that may be my next attempt. Car just runs too damned good to go ripping off the head unless we absolutely have to. I will reach out to vstech, too - good to know there's another dieselhead in the area. I sure do miss my old mechanic - he kept my '84 300D turbodiesel humming along for 400K. I sure do miss that car.... should've never sold it. But a 2007 E320 bluetec was beckoning. Great car - but it might as well be a Toyota. Hence the purchase of the '91. It 'feels' like a Mercedes. Have a great weekend, guys - and thanks so much for your help. I will keep you posted!

Well at least the ir reader indicates a little hope. I am one also that never wants to take anything apart until I have a solid indicator it is really needed.

Germans do the strangest things. The cooling system on the much cheaper diesel volkswagons of that period for example could function in desert conditions with little issue in my opinion.

I even drove them with the electric fan disabled with no issues in the hottest parts of our eastern Canadian temperatures. The early to mid eighty mercedes diesels seem to have adaquate cooling ability as well but not the tremendous reserve the volkswagons had.

My 77 300d had large segments of the fins between the water passages missing when first aquired and did not show any real problem. Other than a slight temperature rise on uphill grades. Just putting the heater motor on pulled the temperature back down easily.

The car was in pretty good condition otherwise generally and it was hard to figure why the rad was in the condition it was.

The car had been owned by a celebrity and they had taken it to many countries during their period of ownership. What is strange is that same celebrity driving her 560SL goes past our property every year on her way to her cottage. She would have no ideal she each year passes by the place where her old 77 300d is currently.

Ok. So I let the car sit overnight Friday night. Saturday morning, went out and just a tiny bit of resistance on the hoses. So it's not over-pressurizing. That's a good sign, I take it... Just need to get time to track down the correct gauge sensor, and swap that out. If that doesn't solve my issue, then it's water pump time. If that doesn't fix it, I'm selling the car...lol

Ok guys - I am back. Been doing some additional reading, and the sending unit for the gauge might just be my problem. Cheap enough to fix - but any suggestions on how to get it out of there? I can see it - but getting to it might be a bit of a challenge? Any input on how to do this with minimal busted knuckles and tearing up something else in the process would be appreciated.

Ken

I would guess that the intake manifold would have to come off to be able to get at it.

__________________
Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"