I have a history of car and bike problems that no one can figure out, so this actually is normal for me.
I have a 1987 Mercedes 300SDL.
Two years ago (before I bought it), it was converted to 134a. Compressor, expansion valve, dryer.
I got the car a few weeks ago and was unimpressed by the 70 degree air coming out of the vents.
I called the shop that did the work 2 years ago and they said that they advised a new condenser, but the owner had not taken their advice.
The shop that has the car, measured the pressures and they were 350/50. He said that it was too high and that it needed a new condenser. I had it changed. He recharged the system at 70% capacity with 134a. It was 250/38.
The system didn’t cool one bit better.
I brought the car back and we switched the refrigerant to Duracool. We used the 40% volume vs. the stock amount of refrigerant.
The mechanic felt that the car was cooling. I picked it up and it was cooling ok. Pressures were 150/38. As I drove the car on the freeway, it became real cold and I decided that it needed a better condenser fan. I put a 1900cfm fan on the condenser. (i checked to make sure that it was putting air through the condenser).
After the installation, the system stopped cooling as well. I don’t think it had anything to do with the fan. It was obviously moving a lot more air. I think it was a coincidence.
Took the car back and the mechanic tried to up the duracool. It had no effect.
I found a post on the mecedes boards, which said that excess oil in the system can cause it to have correct pressures, but not cool.
We pulled the compressor and had it checked. It was new, like the previous owner claimed. While the compressor was out, we flushed the system.
There was excess oil in the system. We reinstalled the compressor and recharged the system. It cooled well for the first 15 minutes, then started to warm again.
So the pattern, since Duracool, has been cooling OK (not good) for 15 minutes, then deteriorates.
We have now pulled the lines off the compressor and filled the system with flush solvent. It is sitting like that for a couple of days.
Someone on the mercedes boards mentioned that the expansion valve might be broken, once flooded with oil.
I don’t know.
I went on the Mercedes forums and someone suggested duracool
I never charge a system by volume; always by the pressure.
That figure of 150/38 is not good as there needs to be more of a pressure drop. An A/C system gets cold because the system is basically a controlled, perpetual leak that relies on a pressure drop and 150/38 is not good enough.
250/38 could be about right but that also depends on a number of other factors. 350/50 is way too high.
Was the system being properly evacuated? One drop of moisture (a molecule at a time) can cause a problem like this when it starts building up in the expansion valve.
Too much oil can also cause problems and in some cases can even destroy a compressor, although I don’t think this is the case here.
Let me add this. Has anyone considered the possibility of a blend door problem?
Not being too familiar with this model I assume it uses a blend door instead of a heater valve.
The blend door allows air to pass through the heater core and if there’s a glitch the air passing through the A/C evaporator will be cold but will then pass through the heater core which in turn warms it up.
With the engine idling for a while and the A/C on try touching the evaporator to compressor line near the firewall to see if it’s cold and condensing water. If it is, then it’s possible there’s a blend door problem. Hope that helps.
We should really take a look at how many “blend door” problems we end up telling the OP’s ,way out of line with what I saw in 35 years of shop experience.
Under certain conditions 350/50 would be in spec.
There is no situation where 150/38 whould be in spec.
I have no idea what the relation between “the car being real cold” and needing a better condensor fan.
How did you conclude there was excess oil in system,this is a hard one to qualify.
Did the conversion shop tell you why a new condensor was recommended?
It is OK to experiment on refridgerant charge amounts but deciding on less than half is something new to me.
You will need a new drier now that the system has been opened.
I always stick with R134A.
Get system pressures when cooling “deteriorates”
Well, regarding the car being really cold and needing a condenser fan. On the freeway, the car initially blew really cold. When I would get off the freeway, and the air wasn’t moving through the condenser, the temp went way. up.
This vehicle doesn’t perform under city driving conditions, so I put a fan on which moved a lot more air.
The conversion shop said that they recommended a new condenser because they felt that the unit on the car was undersized and not working properly. As soon as we replaced the condenser, the pressure came way down.
The oil in the system was pointed out as a possible problem, because of another Mercedes person, who had a similar problem.
Regarding refrigerant. My understanding is that when a system is designed for r12 and converted to 134a, you use 70% weight of the original spec for refrigerant.
Using Duracool, which is a hydrocarbon based refrigerant, only requires 35% to 50% of the original refrigerant weight. Duracool cools about 35% better than 134a. I went for this, because these cars are notorious for not working well after 134a conversions.
We vacuumed the system real well.
We sent the compressor out and had it checked, it is almost new.
This car does have doors, but they are working. There is a monovalve for the heater core, but I changed it.
The tube going into the firewall is not cold.
I have to respectfully disagree with a few points.
When converting to R134 one should not do this based on refrigerant volume. It should be done based on pressure and personally, I’ve never run into a situation where 134 failed to perform as well as R12.
Maybe the problem with the condenser was not that it was bad but simply oil clogged instead. The condenser is the most likely place for an oil clog to occur.
I also still have to respectfully disagree that 350 PSI is normal and acceptable.
Even with bug clogged condensers and 105-110 degree heat here in OK I’ve never seen nor heard of a unit hitting those figures unless that system had a problem. (inoperative fans, overcharged, etc.)
Whenever I see a system heading towards 300 PSI I shut it off and start checking a few things rather than risk blowing the compressor seals/hose or burning a belt off. 300 PSI makes me cringe; a lot.
The 250/38 PSI is about right and acceptable and if the outlet tube of the evap is not cold then I don’t think the problem is related to 134. Someone has to be overlooking something. What, I don’t know.
With pressure like that the system does not have a clog obviously so this would rule out moisture in the system more than likely.
(Note. Sometimes an inert system may need to be evacuated more thoroughly than what is commonly done. An evacuation removes air quickly but moisture is a different thing altogether. Sometimes it can take 10 hours and may even require partially charging the system and bleeding it several times to remove all traces of moisture. This is called “sweeping”.)
Well, we are at the point, where my mechanic wants to do a murder suicide with my car.
So, as previously stated. The dryer / compressor / and expansion valve were changed over for 134a equipment 2 years ago.
Currently, we have changed:
Condenser
Upgraded the condenser fan to a 1900cfm unit.
Dryer
and now… the expansion valve.
It turns out that the shop wasn’t lying, it did have a 134a expansion valve in it.
We have also flushed the system twice and sent the compressor out to be checked for defects.
We have also charged the system with Duracool, which claims to be about 35% more efficient than 134a.
Per specs provided by Duracool, they say that I should be looking for 25/140, based on the fact that Duracool runs at lower pressure.
We currently are getting 44/150. This seems high on the low side. Duracool says that we ad one can for every pound of stock system capacity. We only added two cans, which should be low. Given that we were already at our target pressure, we decided to stop.
We have also charged the system with 134a. No go.
NOTHING HAS WORKED. I am getting 60f to 70f in the city, when the outside temp is barely 80f.
I am starting to get a little crazy over this.
p
I’ll think about this a bit and post back because I’m still having an issue with this high side pressure.
A/C systems work on a “leaking can principle”. If one knocked a hole in a can of refrigerant which allowed the gas inside to escape quickly that rapid pressure drop will turn the can into a lump of ice.
An A/C works the same way by creating a constant pressure drop either through an expansion valve or an orifice tube and I’m having a hard time seeing a drop from 140 to 44 as being enough to create cold air.
Do those same pressures exist with the R134 also?
Something is wrong because a decent A/C system should create a 45-55 degree drop in temperature when compared to the ambient temperature outside.
Before signing off here for a while I took a quick look at the Dura Cool site.
Where does the 140 for the high side come from?
The DC site states a roughly 10% lower figure on the high side as compared to R12 so this means if one had a 250 PSI high side reading the system should have about 225 give or take a bit with the percentage a bit larger when compared to 134.
the california rep gave me those figures. i will call duracool tomorrow and see what the company says.
what is strange is that the low pressure is too high at 44.
I am thinking a blockage in the evaporator.
I skimmed the DC site again and they state on there a 5-10% drop in high side pressures with DC as compared to R12 with 20-30% as compared to R134 although I might disagree with the 134 percentage being that high.
At the tail end they do mention a high side press. of “approx. 150 PSI” so it appears to me they’re contradicting themselves a bit.
Agreed with you the low side is a bit high for my tastes anyway and I generally shoot for 35 or so. The low side pressure is roughly equivalent to the temperature inside the evaporator so in your case this would mean a 44 degree temperature. That won’t do much as far as removing heat from the air passing through the evaporator.
I’m still a bit undecided on this but I just cannot buy a 140 PSI as being an acceptable figure as this would only mean a roughly 100 PSI pressure drop.
Along with several comments on their site that I disagree with the one that jumped out to me was the one about DC being propane based. They state their product has been further refined for added safety along with having Mercaptan added as a safety feature. This safety feature is a stench chemical added to let you know if there’s a dangerous and/or flammable leak involved; a la natural gas in your home.
I’m not a fan of explosives under high pressure although they do state theirs is safer and more stable, which I take to mean that one would feel safer is a grenade went off in the next room rather than the one you’re standing in. 
these types of refrigerants are in use in every other country. They don’t blow up .
I am thinking the evaporator might be the problem.
Just my opinion but I don’t care what anyone else in any other country does. Some people run pure propane in their A/C systems too.
Wonder how many accident caused fires were immediately blamed on the electrics and/or fuel with no consideration given to the flammable refrigerant factor?
That’s a question that would never be answered because any evidence is up in smoke.
The DC site has several comments that I disagree with (one being the contradictory statements about high side pressure) and I don’t see how an evaporator would cause this problem at all.
Well I called them and this is what they say.
With Duracool, the high pressure should be ambient + 100f. The low should be 28f to 32f.
Propane blend refrigerants have been used for a long time and haven’t proven explosive under accident conditions.
What is really good about these hydrocarbon refrigerants, is that they don’t contribute to ozone depletion.
OK, in an effort to remove some of the variables, I have removed the Duracool.
As I know that all of the 134a equipment is replaced, I wanted to get a baseline to figure out if there was another problem. The Duracool wasn’t giving me readings that would allow a diagnosis.
So we re-charged the system with 1 pound of 134a. Immediately the pressure went up to 27/270.
At that pressure, the interior vent runs a a weak 55 at stopped idle. Under city conditions, it get about 50f.
The system should have about 2.1 lbs in it, so there is obviously something wrong here.
My mechanic had the compressor checked, but he is going to put a new one in to see if there is some invisible problem. He is trying to avoid getting into the evaporator, as it is a 2 day job, involving removing the dash.
ok, changed the compressor… fail.
system still running 10f too hot.
I’m not convinced the evaporator is the problem so I agree that tearing into that should be a last resort.
What does the outlet line from the evaporator feel like after the A/C has been running for a little bit? It should feel pretty cold and any exposed metal on the line should be sweating.
Is there a condensation drip from the evaporator drain tube?
If any of the above is true then this could fall back onto a blend door problem, etc.
If not, there’s several possibilities.
One could be a system with too much oil in it.
Another could be a matter of condenser size. This would not be related to the heat being removed by the condenser but the actual physical size of the condenser.
Does it appear to be at least roughly the same size as the one that it replaced?
(Just pointing that out because we had a Subaru (only a year old) come into the shop one time in which the A/C was not cooling well. The A/C unit was an aftermarket add on that would fill up with slightly less than one can of refrigerant. After several stabs at it, I got to looking underneath and behind the grill and found out the condenser was about half the size of a “normal” Subaru condenser. After dropping the passenger side kick panel and glove box I also discovered the evaporator was a half size unit. No wonder it wouldn’t cool; and the owner of the car was HOT. Can’t say as I blame him.)