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More VW Woes

fasto on Sat May 23, 2009 11:16 PM User is offline

Year: 1993
Make: VW
Model: Corrado
Engine Size: 2.8
Refrigerant Type: R134a
Ambient Temp: 70
Pressure Low: 26
Pressure High: 130
Country of Origin: United States

I last posted in This Thread about some puzzling problems I am having with my VW.

I've had some time to work on it today, and I am not enlightened... I apologize in advance for a really long post.

First, the AC circuit of this car.

HS & LS are the service valves. SG is a sightglass. PS is a three-way pressure switch. There are only 3 hoses in the system, which I indicated by putting the hose length above the line that indicates a hose. All other connections are "close coupled".
The pressure switch disables the compressor clutch if pressure is less than 30 PSI, or greater than ~425 PSI. It also switches the fans to "medium" speed if the pressure is greater than 250 PSI.
The car has two radiator fans driven from a single motor. One fan's driven directly the other by a "micro-V" belt. The fan runs on low whenever the AC switch in the cabin is requesting AC (and also based on coolant temp). The fan switches to medium should the pressure switch see greater than 250 PSI (or some other reasons not related to AC). Should the coolant sensor detect "very hot", the AC is disabled and the fan runs on high.

OK, this is the story. Last year I replaced the compressor (defective RCV), condensor (wrong type), R/D, and TXV. I drained all the oil I could from the three hoses and blew air through them. I blew air through the evap which I realize was not useful. I reassembled the system with all new o-rings + nylog. I can't remember if I deducted the amount of oil supposedly left in the evap when I added new oil. The system takes about 4oz of oil & the evap is quoted as holding 0.5 oz. I evacuated & charged the correct amount of refrigerant. All was well, the system cooled as good as you'd expect a European car to cool.

This spring, about 3 weeks ago, I noticed a lack of cooling (I only used the AC 2 or 3 times over the winter for defogging). I found the TVX had a leaky seal, and when I recovered the refrigerant I found that about 1/3 of the charge has been lost. I replaced the TXV & recharged (no oil added). I saw a little bit of greyish film in the oil (aluminum), but no big chunks of debris or anything like that.
After charging the temperature outside was 65F, pressures were 26/130, air temp measured at the center vent was 59F. I checked for reheating and ruled that out. I measured the temperature of each collector tube on the evap. and found 3 of 4 were at 65F, one was at 35F. I shut the engine off (to post on the forum - see above). When I went back & restarted the engine, the low side pulled down to 26 & the high side went up to 450 within 10 seconds or so. The HPCO tripped, the high pressure dropped, then the cycle repeated. At this point I had to stop troubleshooting because it started raining.

Today I had a chance to go back & work on this again. I cleaned the radiator & condensor by blowing shop air backwards through them, I think I blew out about 5 pounds of sand. I checked the radiator fans were working correctly. I recovered the refrigerant & weighed in a new charge. Outside temperature was 70F. When I started up & switched the AC on, I got 26/130 for pressures & vent temp of 60F. I let this run for 5-10 minutes with no change. I switched off the AC for one minute to check something, when I switched it back on I got 26/400+ and vent temp of 40F. The high side climbed to 400+ within 5 seconds of the compressor clutch engaging. It was again cycling on the HPCO. I misted the condenser which did not change the high side pressure at all. I drenched the condenser which changed nothing. The condenser was cold to the touch. When the HPCO tripped I saw the high side pressure go down, then back up when the HPCO reset so the gauges were hooked up correctly. The pressure was actually climbing because the radiator fans would shift to medium at the correct pressure and the HPCO was triping. I had been testing with AC on max, blower on high, but I changed to blower on low thinking that the AC system couldn't reject the heat the evap was picking up - there was no change.

I measured the temperature of the pipe adjacent to the pressure switch (inlet side of condenser) and found only 120F! The pressure must have been high enough to trip the HPCO but the temp was only 120F.
I checked the sight glass which was showing nothing, no bubbles, no foam, nothing. The inlet & outlet side of the evap were cold, and the vent temp stayed around 40F. I only ran the AC for a minute or so at a time to try to keep from damaging something.

After a few minutes of this, suddenly I saw the high side pressure drop. The compressor clutch stayed in, not an HPCO trip cycle. The high side stabilized at 130 which took 5 seconds from 400+, low side stayed 26, vent temp climbed back to 60F. I didn't look at the sight glass. I ran the car like this for a few minutes with no additional changes, then had to quit for the day.

I'm at a loss; I don't know where to look next. I have at least 4 different old TXVs, I may try to repair one of the leakers and try that. I have several used receiver/driers, I could put one of those in (it's a 5 hour job). I'm trying to find a flexable line set out of a wrecked car to try that. I have considered replacing the evap (an 8 hour job).

I would make any additional measurements & tests that could add anything, I just don't know what those might be.
Any insight is appreciated,
Aaron

bohica2xo on Sun May 24, 2009 11:20 AM User is offline

There must be too much of "something" in your system.

At that low ambient temperature, with a receiver in the system you could weld the txv shut & not see those pressures. When you tag 400 psi against a blockage, the refrigerant should stack up in the receiver, leaving the compressor precious little to pump.

The intermittient nature of the problem suggests either debris, or excessive oil in the system.

Time to flush things back to bare metal, and start over again. With every component completely clean, and a new receiver dryer you can add the factory oil charge to the system.

That system is tiny, and too much oil will make it a dog. Too much refrigerant is bad too, and 2 extra ounces of oil means you are overcharged with refrigerant if you put the proper charge in.

Do a proper cleaning of the air side of the heat exchanger stack as well. If you could blow out "sand" with shop air, it is filthy with other things too.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

fasto on Sun May 24, 2009 12:31 PM User is offline

Quote
Originally posted by: bohica2xo

At that low ambient temperature, with a receiver in the system you could weld the txv shut & not see those pressures. When you tag 400 psi against a blockage, the refrigerant should stack up in the receiver, leaving the compressor precious little to pump.

Which means the low side should pull into a vacuum, which doesn't happen, so you are right there can't be a hard blockage.

Quote

Time to flush things back to bare metal, and start over again. With every component completely clean, and a new receiver dryer you can add the factory oil charge to the system.

I will do this. I have a new R/D in stock.
Quote

Do a proper cleaning of the air side of the heat exchanger stack as well. If you could blow out "sand" with shop air, it is filthy with other things too.

I did as good of an air side cleaning as can be done, short of replacing the radiator. The sand is from 17 of our New England winters, picked up from the road. The radiator & condenser are pretty clean, now. It's easy to see light through the stack. I will check this again, though.

Many thanks for your suggestions.

Regards,
Aaron


fasto on Mon May 25, 2009 11:08 AM User is offline

This car uses a Sanden SD7V16 compressor. The compressor has suction & discharge ports, as well as a belly drain. I will drain the oil from both suction & discharge ports and the belly drain. Should I add oil to the belly drain before closing it up? I have read that the crankcase of the compressor is connected to the suction & discharge ports, so perhaps it is not required to add oil to the belly drain?

I ask because I think this is where any excess oil must have come from. When I installed the compressor I drained it as much as possible of any shipping oil, but very little drained (0.5 oz perhaps). If the Sanden units come with more oil than that - and I have read that some come with the complete system oil charge - I have WAY too much oil onboard and I now know where it came from.

Thanks,
Aaron

TRB on Mon May 25, 2009 12:00 PM User is offlineView users profile

If a compressor has a drain plug that is where I always drain and refill the oil.

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bohica2xo on Mon May 25, 2009 2:32 PM User is offline

A lot of oil can stay in an evaporator like yours. Once air passes through one channel, the others can still be full. A flush to bare metal takes a decent solvent, and a LOT of air volume. Take one of your old TXV's, and drill it out to take a pipe thread. Use a standard shop air quick coupler on the high side, and a 1/2" (or larger) hose to your catch can.

Be sure you use the correct quantity and viscosity for your vehicle.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

fasto on Mon June 15, 2009 1:27 PM User is offline

Update: Replaced the TXV with one from a wreck. System now works fabulously.

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