Has anyone posted a fix for the monovalve spring issue? (w123) - PeachParts Mercedes-Benz Forum

Zacharias · #1 11-08-2016, 11:38 AM

I have acquired a spare complete w123 monovalve assembly from a parts car that I am going to pull apart and clean.

I am also considering pulling the trigger on a Mercedes OE insert replacement (assuming this is the old Bosch part), which I am able to get here in Canada considerably cheaper than the posted US prices (this happens from time to time). It would be approx. 3-4 times the cost of the MTC aftermarket item, but comes with a two-year replacement warranty (including third party labour charges, if applicable).

So two questions:

- Has anyone else opted for the Mercedes OE part and what was your experience?

- Has anyone come up with something definitive on the weak spring issue. I did read a thread on it some time ago but it doesn't seem to be popping up when I search.

EDIT: Maybe I should add that I am addressing a sudden problem whereby I have almost no heat after a hot start. I was already living with no heat at highway speeds (don't use the car in winter so not a huge issue).

Thanks

Demothen · #2 11-08-2016, 11:56 AM

I've started a thread with my experience and troubleshooting, including a disassembled core at W123 Monovalve disassembly and troubleshooting - but I still have more information to post. I'm going to look into a w124 unit this weekend (assuming I can find one at pick 'n pull). I'm really leaning towards thermal expansion and clearances as the problem, not the spring. I'll keep updating that post with more information as I can.

I am seeing the inverse of your problem, at high temperatures the valve sticks open, meaning the spring is not the problem, my solenoid is not producing enough force to close the valve. I am using a rebuilt bosch core, not the MTC unit - the bosch core does have a stronger spring than the MTC one though.

Zacharias · #3 11-08-2016, 12:09 PM

I actually meant to reference your thread in my post and forgot. I look forward to reading more....

I was leaning toward installing a w124 unit for a while but finally decided to try to make the stock setup work. I have read elsewhere, where one or two guys reported that just pulling the monovalve apart and cleaning it up solved their issues.

Dare I hope LOL? I don't have that kind of luck, frankly.

The more I think about the OE insert, the better the idea gets. I have only been getting two years out of the MTC parts anyway (and I think one only lasted a summer).

Demothen · #4 11-08-2016, 12:48 PM

I am currently soaking the core of mine in rust soak. I thi k i am going to try citric acid or CLR to see if its full of calcium. Worth a try to clean it

BillGrissom · #5 11-08-2016, 12:48 PM

Haven't heard of the weak spring issue. Can you elaborate? I installed one or two MTC inserts (~$30 ebay) years ago and the rubber diaphragms are still intact, which seems to be the main failure in these. Usually the problem is "full heat". Sounds like pressure can overwhelm an opening spring to keep the valve shut. I removed the aux pump in both my 300D's, so perhaps less dP in my cars. Still more than enough heat for our climate (orange trees).

A few basics, have you felt the heater hoses? Perhaps the heater is doing its job, but is overwhelmed by outside air coming in the climate door (worse at hwy speed). Try blocking off the cowl vents. Is the AC compressor engaging too often?

Zacharias · #6 11-08-2016, 02:11 PM

One theory behind no (or weak) heat at highway speeds is that the spring is weak in the MTC units, so at speed the insert isn't able to work against the force of the coolant. Someone said they were experimenting with a fix, which I think was just a washer added to the other fittings on the insert.

I read it when I was just passing time, I didn't have an issue, so I only half filed it away upstairs. I so far have only found a post by someone else, also referencing what was said. (Looks like it may have been on the BW list.) Not by the originator. Someone, possibly the person with the washer fix, also mentioned that he had carefully examined and compared the Bosch and MTC parts, and there is an issue with the measurement of the MTC diaphragm. It is every so slightly too small.

I am in Canada so AC/compressor isn't relevant, the system is on ECON most of the year. The auxiliary pump isn't relevant for coolant flow at highway speeds. When functional this system provides ample heat on the highway at -30 deg F, I know as I drove this car through several Canadian winters. Getting heat out of it at our current temps, around 50 deg F, shouldn't be any issue at all.

Steve_in_NV · #7 11-08-2016, 02:34 PM

Quote:

Originally Posted by Demothen

I am currently soaking the core of mine in rust soak. I thi k i am going to try citric acid or CLR to see if its full of calcium. Worth a try to clean it

I have good success with "evapo-rust" it's safe and re-usable, just pour back in container.

Evapo-Rust Rust Remover - Evapo-Rust Super Safe Rust Remover

Just don't

will_t · #8 11-13-2016, 08:28 PM

Quote:

Originally Posted by Zacharias

I am also considering pulling the trigger on a Mercedes OE insert replacement (assuming this is the old Bosch part), which I am able to get here in Canada considerably cheaper than the posted US prices (this happens from time to time).

Thanks

Hi, can you provide a link to the OE insert? Or send me a private message with the info? I would like to be able to purchase one next time I need one. Thanks!

Zacharias · #9 11-14-2016, 02:40 PM

Quote:

Originally Posted by will_t

Hi, can you provide a link to the OE insert? Or send me a private message with the info? I would like to be able to purchase one next time I need one. Thanks!

If you mean the cheaper one in Canada, it's a price I got from my local dealer, which gives me shop discount. There are no mail-order Mercedes parts sources worth mentioning in Canada.

PelicanParts can sell you the OE insert at a discount over the US dealer list. It's in their catalogue.