Not getting full vacuum - PeachParts Mercedes-Benz Forum

Demothen · #1 09-25-2016, 05:44 PM

I am vacuuming my AC after reassembly and before I nitrogen purge it, using a HF 2 stage vacuum pump. My pump does not seem to want to reach a "full" vacuum (~30inHG), even with the manifold set disconnected from the car, relying on the quick connect seals. I've tried two manifold sets and they both agree. I tried changing the oil using fresh vacuum pump oil. I am getting to about 25inHg. I checked the local weather and we are at 30.12 inches pressure. Is this a normal reading or is it likely that my pump has failed? I have used this pump a few times before, and don't recall having any problems with getting the gauges bottomed out.
Sort of tempted to eat the cost and take it in to an indy to be filled on their machine. The local import shop is pretty pricey but there is a small shop that doesnt specialize and has pretty fair prices.

funola · #2 09-25-2016, 07:41 PM

Describe how the manifold gauges and the 3 hoses were hooked up to the car/ vac pump and direct to vac pump . Include both valve positions. You have a leak somewhere.

I say don't take it to a shop, because you can't claim DIY anymore and you learn less.

funola · #3 09-25-2016, 07:57 PM

Was the Nitrogen bottle ever hooked up to the manifold gauges? Nitrogen max pressure is around 2000 psi and if you're not careful with the regulator, you could have damaged the gauges by exceeding its pressure.

Demothen · #4 09-25-2016, 07:59 PM

Right now I have the manifold disconnected from the car. I pulled vacuum for about 5 minutes with the low side valve open, high side valve closed, and both quick connects removed from the car. After looking more carefully I would say the gauge indicates about 27-28 atm vacuum. Once i had the vacuum established, and the needle no londer moved i shut the blue valve and let it sit for a few hours. No change.
Connected was the same procedure with the low side quick connect on the car.

99% sure it is not a leak. However my pump does make different noises depending on whether it is at max vacuum or not. Its more rattley when its still pumping down, then smooths out once its at or near max vacuum. Suspecting I may have got a dud pump.

funola · #5 09-25-2016, 08:10 PM

When connected to the car while evacuating, both hoses must be hooked up to their respective fittings on the car, both valves on manifold must be open. It is normal for the vac pump to be rattling until full vacuum is reached, then it quiets down.

At the beginning of rattling, you should see a mist coming out of the exhaust. I think that is vacuum pump oil mist and is normal.

Demothen · #6 09-25-2016, 08:23 PM

I will try it again with both lines hooked up. I didnt do that this last time, but I did with my other manifold set and got the same results.
At this point I don't really care to learn, I just want it done. The car has been off the road for about 4 months, except for one weekend when I drove it until the compressor split open. It should just need to be vacuumed, purged, vacuumed, and filled now. Once I get the dash back together that is.

funola · #7 09-25-2016, 08:41 PM

On the compressor that blew up, did you evacuate/charge with only the low side hose hooked up? If so, that could explain why it blew up.

I still say finish the job yourself and not get a shop involved. You have all the tools needed. You are almost there.

Describe your charging procedure, I and others could go over it with you to make sure you got it right before doing it.

Demothen · #8 09-26-2016, 11:59 AM

I have to ask. What's the technical reason for having the high side hooked up when vacuuming? Doesn't the TXV always always stay slightly open? If so, that would let the pressure equalize on both sides of the system. (Not trying to argue, I just am trying to learn if I'm misunderstanding how the system works)

I honestly don't remember how we vacuumed it last time, that was ~2 months ago.

I'll probably miss a step trying to go through this..

Vacuum system 30 minutes or more (low & high side open), check that it holds for at least 30 minutes.
Fill system with Nitrogen - check for leaks with soapy water, make sure it holds same pressure for several hours.
Vacuum system for 1hr, check again that it holds.
EDIT
Close low side line, leave high side line open
Purge lines (my manifold has a shraeder valve on the yellow line)
Add first can with engine off, can upside down (using 12oz cans of r134a) into the high side of the system, working slowly
Close high side line.
Check for leaks with HF leak tester (can't hurt to look)
Run engine - 18" high power fan pointed at radiators, auxilliary fan jumped to run full time.
END EDIT
Turn off car.
Check for leaks with UV light, soapy water
Disconnect can
Hook up fresh can
Purge lines again
Add second can, vapor only, can partially submerged in warm water. Watch pressures
Repeat for third can.
4th can should be a partial fill to get proper pressures. (believe I figured ~37oz to get to 80% fill, may be slightly different since I have oversized condenser.

Any tips?

Probably going to try to take the vacuum pump back. Want to run one more test to make sure it can't achieve proper vacuum before I do. I think it's still under the 90 day warrantee.

Demothen · #9 09-26-2016, 12:17 PM

Local indy that did the initial r134a recovery wants about 125$ to do the pull down and fill for me including their 134a. Thinking strongly about letting them do it. They should be able to fill by weight and not have to worry about purging between cans. I really feel like i have learned enough, and they have better equipment to do the job right. I havent been able to find anyone with equipment to do a nitrogen purge locally, so i may vacuum and pressure test with nitrogen before I give it to them.

Demothen · #10 09-26-2016, 05:17 PM

Not really sure this answers my question about the TXV ever being completely closed, but here's an interesting read:
How Thermostatic Expansion Valves (TXV) Work - AC & Heating Connect

Diseasel300 · #11 09-26-2016, 06:53 PM

Have you tried another set of gauge hoses? Sometimes they can get a pinhole or one of the rubber seals can leak just enough to prevent perfect vacuum.

Demothen · #12 09-26-2016, 06:59 PM

Yep, two different manifolds @ hoses. Same result

Demothen · #13 09-26-2016, 08:31 PM

So manifold set A is not resetting to 0 when vented to atmosphere.
Manifold set B resets to zero, but has a poorly designed coupler to the vac pump on the yellow hose, wont let me tighten it properly.
Manifold set C is the gauges, red and blue lines from set A, with the yellow hose from set B.
I am getting roughly -.9 bar on manifold set C with the pump running connected to the car with both hoses hooked up. Going to let it run for 30 minutes, then close both valves, then turn off pump and see what it reads tomorrow. Also tried pulling down the manifold only, also getting about -.9bar.

I have checked all the fittings on the manifold and pump to ensure they are tight. The only other failure point that I see which might be leaking is the brass fitting on the compressor itself. It is tight though.

No receipt on vac pump, likely to just pay a shop at this point instead of buying more gear.

Demothen · #14 09-27-2016, 10:11 AM

Here's the gauge after ~30 minutes of running the pump. I wish I had a better gauge for reading vacuum via the manifold, the graduations are a bit hard to read accurately in this scale.

funola · #15 09-27-2016, 10:24 AM

With no leaks in the AC system or faults with the test equipment (manifold gauge set, vac pump), the gauge needle should be centered over the brass rivet= 30 in Hg on the gauge in less than 2 minutes of running the vac pump. If it was me, I'd be dying to find out what is going on and be trying different tests to determine the cause of the low vacuum reading.