1998 Mercedes-Benz M-Class - Leaking Freon

Dear Car Talk,

I am the first owner of a 1998 Mercedes ML 320 with 252,000 miles. (I’ve driven it figuratively to the moon and am on the return trip. Some troubles along the way, but nothing complicated, until now: the AC is only retaining a Freon charge for a couple of months and I live in Arkansas which can get warmish at times. My mechanic tells me he thinks the problem is a Freon leak around the firewall: 20 hours of labor (@$85 per) to take apart the dash, steering column, etc. plus a $750 part. I think the I asked if there are any Freon-stop-leak additives, and he says his experience is that they don’t work. I am hesitant to invest so much into a likely ‘fix’ knowing that the engine and/or transmission are liable to be the next expensive problem. I hate to emit more ozone-depleting chemicals into the environment if I do nothing except replenish the Freon when needed. Is it time to take some selfies and bid farewell?

Warmest regards, from a long-time listener and follower,

Jeff Short

That statement tells me you need an actual air conditioner shop .

The “he thinks” is the operative phrase. As Volvo_V70 says, try an AC specialist. If the repair estimate is in the same ballpark as your mechanic, the financial decision is yours to make. At this mileage, in spite of previous faithful service, your vehicle may turn into a money pit.


Does the mechanic “think” the evaporator is leaking . . . or does he know it’s leaking because he used a leak detector or was he somehow able to see the leak?

That said . . . I wouldn’t be at all surprised that the evaporator is leaking on such an old vehicle

Has he carefully inspected all the ac components . . . ?!


Assuming the evaporator core is leaking, that would be a very expensive repair. If this was my car, and the leak was slow enough that the A/C could function for at least 3 weeks on a charge, I would simply buy a charging manifold (if I didn’t already own one) and the R134a adapters, and top off the A/C whenever it started to lose effectiveness.

At Wal-Mart, you can buy a 12 oz. can of SuperTech R134a for $4.88 plus tax. You can top off your A/C many, many times for the $2450 your mechanic wants to charge!

Yes, my local shade-tree mechanic, that specializes in foreign autos, used a leak detector on my MERZ. I first went to a MERZ dealership with it, with no definite diagnosis. The dealership is about 50 miles away and the costs are over $100 more than the local. This has been an on-going problem for at least two years and the accessible components and line don’t appear to be leaking.

I like the idea of topping-off the freon myself; or, just using the old 80 x 4 method: 80 mph and 4 windows down.

Thanks for the replies.

Jeff Short

rules say you have to try to fix the leak. a good effort. than you can add freon. if you add 1 can every 2 months than that is almost cheaper than putting in 1 qt of oil. cost wise.

Have you tried to find a repair shop that specializes in a/c?

I’ve installed refrigerant stop-leak in vehicles many times, and depending on the size of the leak, it works.

I have a friend from Texas that was visiting, and said they had a leak in the AC system. When checked for leaks, it was the evaporator leaking.

Added the stop leak, topped off the refrigerant, and after three years, the AC hasn’t leaked.


Is that what you really meant to say?

Thank you. I will use one of the better products.

Jeff Short

MERZ ?? I have never hears of a Mercedes Benz referred to that way . Merc , yes and MB but not Merz.

I’m glad you said that

That means he actually diagnosed the car, instead of just guessing

No offense, but I think the cost to repair exceeds the value of the car

you’ve got a lot of miles . . . this repair could very well be just the tip of the iceberg. Given the age and mileage, you could be at the point where the car needs expensive repairs every year from now on

what condition is the vehicle in?

be honest, please

Arkansas Revenue Dept uses the “MERZ” destination. I suppose “MERC” (to them) may refer to “Mercury"


The value is probably less than $3,000. I have put at least twice that much into it over the last couple of years, mostly tie rods, suspension, etc. Body and interior are nearly perfect. Drive train and engine have been stellar, but I agree about the looming failures and costs.

I “rationalize” the investment, thusly: 1) original owner, 2) no accidents, or significant dings (it could use a re-painting), 3) safe and reliable (so far); 4) spacious and rugged; 5) no worries about using the MERZ to perform unbecoming tasks like taking the garbage to the waste station or to haul recycling.


figuratively? lol … we here didn’t think you actually drove your Benz to the moon … lol … but if you could have, that would do a whole lot to prove the MB’s reliability under adverse conditions !

As far as the AC problem, you need to find somebody who can come up with a creative solution. For example rather that repairing a leak in an existing part it may make more sense $$-wise to simply replace or bypass that part, even if that entails replacing other parts too. It’s sort of like if you have a leak in a pipe under your house. You wouldn’t jack up the house to repair the pipe, you’d just run a new pipe around the perimeter. Here’s a good 4-part explanation of automobile AC, how it works, how it is repaired etc, fyi

thanks for the link.

Elon Musk’s Tesla is already heading for the Sun, so that is getting all the headlines. I have no doubt my ML could make that trip, but I would definitely need the AC working properly.


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I prefer using a sniffer but there is one area in which it can be problematic. That is in tight spots.
In other words, it may be picking up a leak from a nearby hose or hose fitting and the evaporator may not be leaking.

I use the sniffer to probe the outlet vents inside the car to tell if it’s an evaporator leak. I have no idea how your guy did it.

As Tester mentioned, the use of refrigerant with a Stop Leak additive can certainly work. It’c cheap and easy to try and would be my choice before 20 hours of labor replacing the evaporator.

Another thing to consider is teh weak link in the chain scenario. What if you spend 20 hours of labor replacing the evaporator and the compressor shaft seals start leaking? Just food for though.