Not getting full vacuum - Page 2 - PeachParts Mercedes-Benz Forum

Demothen · #16 09-27-2016, 10:28 AM

I have been trying literally every test I can think of. At this point the only common point of failure is the vacuum pump itself, which I cant return unless I can find the receipt.
At this point I think I am planning on purging the system from the strongest vacuum I can achieve with my small nitrogen bottle, then bringing it to the shop to let them pull a better final vacuum and charge it.

funola · #17 09-27-2016, 10:36 AM

What tests have you done? Have you verified your manifold gauge sets are bubble tight? I pressurized mine and put it under water up to the brass fittings of the gauge and found small/ slow bubbles in one of the hoses at the crimp ferrule.

The vacuum pump is unlikely to be bad since it is a new pump with fresh oil and it was good the last time you used it. Unless something was done to it that it does not like, such as using it to suck out remnants of the flush solvent, discharging high pressure into the inlet port.

Demothen · #18 09-27-2016, 10:50 AM

Haven't tried submerging them under pressure. That's a good idea, if I can rig up something to hook them up to my compressor instead of wasting my limited supply of nitrogen. Mostly I've ensured that they hold a vacuum overnight. In my experience troubleshooting (and I do stuff like this for a living, though not in the mechanical field), the simplest solution is usually the most likely. In this case I have common ground that the vacuum pump will not pull a full vacuum against either manifold set. It's unlikely that both manifolds exhibit the exact same problem, so the simplest solution is the most likely, which means that the pump itself is probably bad. We are talking about a cheap pump here.

One thing I might try tonight is unscrewing the brass fitting from the pump (it's a Y fitting with two different threads). It's possible that part has a leak. It's also possible that the second fitting's cap is not sealing well. I might be able to replace that fitting, or find something else to cap the second fitting with. The last alternative is to try with a different pump. Conveniently there's a DIY repair place that has a pump that I can use without spending much. I just need to get the car on the road to take it by there.

funola · #19 09-27-2016, 10:52 AM

Quote:

Originally Posted by Demothen

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 have perused many examples of AC system evacuation on the web and youtube videos that always had both hi/lo side hoses hooked up with both valves open during evacuation. Some sites says to remove both Schrader valve cores for faster/better evacuation since they are restrictions. I'd guess the TXV is a much bigger restriction than the Schrader valve cores and that is the reason you cannot achieve full vacuum with just the lo side hose connected and vac pump running for 1/2 hour. I experienced the same thing with my R4 system while vacuum/ pressure testing my system with just the lo side hose hooked up. I could not reach full vacuum (running the vac pup for at most an hour) and when I shut off the ball valve to cut the vac pump off, the digital vacuum gauge dropped slowly when it should be stable (if it did not have any leaks). Hooking up both hi/lo hoses while evacuating solved the "problems" I was experiencing with just the lo side hose hooked up.

funola · #20 09-27-2016, 11:17 AM

Autozone rents out vacuum pumps for free. That's what I used for all the evacuations I have done. I have rented 3 different pumps from different stores just to see if one pump was different than the other 2. Two were used pumps and one was brand new one in the box. These are OEM brand single stage pumps and the quality is probably the same as your HF pump. One of the used pumps actually pulled a better vacuum than the new one per my digital pressure/vacuum gauge. Maybe the new pump needs to be broken in but I ended up using the old pump that pulled a better vacuum.

My digital pressure/vacuum gauge has a resolution of 0.001 MPa (0.29 in Hg) so I was able to see minute differences in vacuum/pressure levels that a Bourdan tube gauge could not.

Demothen · #21 09-27-2016, 11:21 AM

Oh, I didnt know they rented them out! I will stop by tonight!

funola · #22 09-27-2016, 11:34 AM

Quote:

Originally Posted by Demothen

Oh, I didnt know they rented them out! I will stop by tonight!

I've mentioned multiple times in threads you've participated in that I rent my vac pump from Autozone. I guess you missed that? SKU 90059-RAZ Call before you go, some Autozone's may not have them or have them but rented out already. Make sure there is oil in the site glass and ask them for oil if it doesn't. They probably want you to buy the oil but ask them if they really want you to run it without oil and they may give in and give you a bottle.

Demothen · #23 09-27-2016, 11:59 AM

I don't memorize all of your threads, sorry. Also AutoZone's site won't navigate to the vacuum pump from their rental tool page, you need to search for it.

vstech · #24 09-27-2016, 12:32 PM

nearly ALL manifolds I've used SUCK! especially for displaying vacuum. the "compound" gauge has too much swing. a micron gauge is needed.

I have 2000.00 manifolds, with digital readouts, and full micron displays... junk!

I've come to the conclusion, that quality hoses, connected to a vacuum tree is the best way to get a quality vacuum...

but this is my JOB...

for the DIY... as long as the PURGE is done correctly, and the system is LEAK FREE UNDER PRESSURE... normal manifolds are fine for vacuum... but don't expect repeat use from them. expect worn readings, and such.

certainly leak test all the fittings, and connections, but on automotive, pulling a sub 1000 micron vacuum is fine to verify you are moisture free. weigh in the liquid on the HIGH SIDE ONLY!!! with the compressor OFF!!! and final charge on the low side with the compressor running.

as for why to pull from BOTH sides when vacuuming the system, it's because you want equal flow out both sides, with no impediments. the compressor valves will SERIOUSLY reduce the vacuum flow. pull from both sides.

Demothen · #25 09-27-2016, 01:11 PM

Thanks vstech.
Got a pump from AutoZone. Will try it out after work. Even if I can get a good vacuum, going to let a shop do the fill on their machine, though i will need to be there to help with my modified electronics to make sure everything is running right.

funola · #26 09-27-2016, 01:24 PM

Quote:

Originally Posted by vstech

nearly ALL manifolds I've used SUCK! especially for displaying vacuum. the "compound" gauge has too much swing. a micron gauge is needed.

I have 2000.00 manifolds, with digital readouts, and full micron displays... junk!

I've come to the conclusion, that quality hoses, connected to a vacuum tree is the best way to get a quality vacuum...

but this is my JOB...

for the DIY... as long as the PURGE is done correctly, and the system is LEAK FREE UNDER PRESSURE... normal manifolds are fine for vacuum... but don't expect repeat use from them. expect worn readings, and such.

certainly leak test all the fittings, and connections, but on automotive, pulling a sub 1000 micron vacuum is fine to verify you are moisture free. weigh in the liquid on the HIGH SIDE ONLY!!! with the compressor OFF!!! and final charge on the low side with the compressor running.

as for why to pull from BOTH sides when vacuuming the system, it's because you want equal flow out both sides, with no impediments. the compressor valves will SERIOUSLY reduce the vacuum flow. pull from both sides.

I thought about the compressor's effect depending on the position of the stroke that it's in.

Youtube member Grayfurnaceman has some excellent videos on HVAC, much of it applicable to MVAC. I recall one episode where he says the best vacuum is pulled when the vacuum pump is connected to the system being evacuated with copper pipes, no hoses.

vstech · #27 09-27-2016, 01:32 PM

Yup. Copper is best. All rubber passes some.

As for compressor stroke... valves open when pressure pushes them open. If comp isnt running, valves are closed.

funola · #28 09-27-2016, 01:40 PM

Quote:

Originally Posted by Demothen

Thanks vstech.
Got a pump from AutoZone. Will try it out after work. Even if I can get a good vacuum, going to let a shop do the fill on their machine, though i will need to be there to help with my modified electronics to make sure everything is running right.

You don't trust yourself doing the fill? Is the system filled with oil already? Does the shop use virgin R134a or reclaimed? I would suggest not use them if reclaimed. What if the shop does the fill and the vent temps do not meet expectations? Who would you blame?

funola · #29 09-27-2016, 01:48 PM

Quote:

Originally Posted by vstech

Yup. Copper is best. All rubber passes some.

As for compressor stroke... valves open when pressure pushes them open. If comp isnt running, valves are closed.

In this case, we are talking vacuum, not pressure. What does the reed valves do under vacuum? I really thought (though I am not sure) that the effect I was seeing has more to do with the compressor and not the TXV. The effect was that when I shut off vacuum to the system (with a bubble tight ball valve) that was evacuated from just the lo side port , the vacuum level in the system immediately drops slowly in a tight system with no leaks.

Demothen · #30 09-27-2016, 02:01 PM

I have a manifold set, which has already been noted to not be very accurate. I have 12oz cans instead of a tank, which makes filling by weight accurately difficult. I also have no experience except for supposedy causing my compressor to split in half.

Not sure about whether they use fresh r134a or recovered. Will be sure to ask them
May call a mb indy to see what they would charge, they did a recovery for me at one point and creditted me with the r134a they recovered. Not sure if they would be willing to use fresh, even if I supplied them with the cans.

And if they cant get the vent temp right, what makes you think I can? Every single thing I have done has been nitpicked, so why should I be confident in my skills at this point?