What will running hot do?
 

I am going to look at a 1991 300E 2.6 later today. The owner states that the car has just recently (since last Friday) started running hot (120 degrees) about 10 minutes after driving. He believes the heater core is on its way out. The AC still works. His wife drives the car everyday on a 20 min. commute. Will driving the car everyday at this temp detonate the engine? He states that the car does not overheat, only runs hot. Does this sound like a heater core or possibly a greater problem.
The car is going for $500 because of the difficulty in a heater core replacement. I am not scared of this DIY (even though I should be).:)
What are your thoughts?

As long as it has not actually overheated, I wouldn't be scared of it. Running hot for several weeks or months could probably have some detrimental effects, but for $500 I'd go for it.

I would expect that a plugged heater core would have essentially no or very minimal affect on creating a high engine temperature. On the other hand, a stuck thermostat or dying water pump will provide high engine temperatures.

Get a Pre-Purchase Inspection. In this case, I'd have the PPI do a compression check of the engine. Even then I'd be VERY suspect of an engine that has been run to the top of the temperature gauge numerous times. What if the gauge calibration is off a bit? Maybe it did overheat several times but the driver didn't see it??

Keep in mind that there is nothing more expensive than a cheap Mercedes. This one could be a simple, inexpensive repair, or have a damaged engine and you have a really nice parts car. Do the PPI, even on a $500 car.

120C isn't really super hot, though it's hot enough. What color is the coolant? Do all the fans work?

Sounds more like a head issue. Do you get hot air out of the climate control system? When you crank it to defrost, does the temp drop? Is there coolant under the carpet @ the footwells on either side of the dash?

Could be the headgasket....

Could, but there's a lot of troubleshooting to be done yet.

Yeah, I would check more simple things first.

When I first got my 91 300E 2.6 it ran fine (temp) then when the head gasket started to go it ran hot, many times just about touching the red. It even got into the red a few times sitting in traffic (with the aux fans running). Most any mechanic (including myself) would tell you that overheating, especially with aluminum heads, is very bad and will often result in irreparable damage to some engine components. However, I did a lot of reading and the consensus was, from most of the hardcore Benz techs and engineers that the 80s-90s inline six could more than handle temperatures well over the normal range and even occasional brief overheating should pose little if any detriment. I wish I could find the articles and threads so I could post those opinions/statements and site the sources.

Pacified, I drove it for many months like that (I only paid $500 for mine as well so I didn't care at the time) until the oil in coolant got so bad it would overheat in minutes and I parked it for about a year. I liked the ruggedness and dependability so much I decided to finally put a new head gasket in it instead of selling/scrapping it and it's been running beautifully ever since. The running hot/overheating had absolutely no adverse affects that I can notice so far and it's my favorite car right now.

Take off the coolant tank cap, It should be very clean in there. If there is grey-brown sludge on the cap, floating in the coolant or a film of such on the walls of the coolant tank it means the head gasket is starting to go. Probably half the private sale M103 Benzes I've looked since at had this symptom and I'm almost sure that the owners knew it but didn't mention it. One guy even became irate that I pointed it out and claimed it was just old coolant (his car was advertised as "needing nothing")!
If you're mechanically inclined the head gasket isn't that bad to do yourself and a great way to familiarize yourself with the car. Once you've done one the next one would take half the time. You can get some GREAT deals on older M103 Benzes that need head gaskets and the owners don't want to sink the $2500 into it (about $600 DIY cost incl. the machine shop). I even keep a spare head gasket set in my garage in case I spot a good deal.

A clogged/bad heater core would likely have little to do with it running hot. On the other hand, if it's running hot and not putting out good heat then hot coolant is not circulating through the core which could be MANY things including being restricted by oil gunk from a bad head gasket.

The above poster is right though, you can pretty much plan on your $500 Benz to cost at least $2000 (DIY) by the time it's comfortably road worthy.

He probably meant to say clogged radiator core, not heater core. A decent possibility especially if it is original.

I love $500 Mercedes. Just be aware that I expect to spend a minimum of $2-3K to make it what I consider to be road-worthy when I buy one. Done it twice now, and not disappointed. There's always stuff like rubber suspension stuff, brake hoses, bad calipers, warped rotors, etc. etc. etc. etc.

how can it be a 300 with a 2.6 in it? shouldn't it be a 260?

No. It's a 300E 2.6

The 1990-92 300E could have a 2.6 or a 3.0. In 1993 there was also a 2.8.