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[patsmx5]

The A/C in my 99' miata is going whack-o. Ok, so a while back I fucked up an A/C line and all the freon leaked out. I unhooked the line and taped up the fittings well to keep dirt/moisture, etc out of the system. 2 weeks later I got a new (read, welded the old one up) line installed, pulled the system under full vacuum for 2 hours, and then charged it properly, adding 2 ounces of oil. Worked perfect for probably a month.

Then it started leaking again at a connection on the high side where I changed the line originally. After a month, the air was noticeably lacking, so I bought some thing that had 22oz of freon and probably 3oz of oil. I put like 1/3 of that in and it was back to perfect. Next month, put another 1/3 in and it was back to perfect. After a month, it was low yet again.

Then I cleaned up that leaky/oily fitting and JB Welded over that bitch. Twice. No more leakage. System is low, but no longer loosing freon.

I drive it for probably a month like this and it's not leaking anymore. Then one day, about a week ago, something weird happens. After running the A/C down the highway for about a minute, the compressor begins to cycle on-off-on-off every second. Weird as shit, so I turned the A/C off.

Now I don't have my pump and manifold right now, all I have is a low pressure gauge. Yesterday I check it with the car idling at ~1500 and it was showing ~35 PSI on the low side at 80*F ambient. Air was cool, but not really cold. I put maybe 3 oz of freon in it and now it's showing 40-45 PSI on the low side at 1500 RPM. Blows mediocre air, but not cold like it should be for 80*F ambient. Still doing this crazy cycle shit ever second though. When I first bought the car it showed 55 PSI on the low side at ~1500 RPM idle on a 90*F ambient day.

So WTF is wrong?

[Conceptz-X]

possibly the shit you infected your system with.  Most OTC referigerant contains stop leak, and it does not agree with expansion valves.  or you now have too much oil in the system.  Change the drier(add only 1/2oz. oil) vacuum and recharge, if its still acting fucked, the system is likely contaminated with stop leak.

[patsmx5]

Yeah, I'm thinking it could be moisture freezing up at the expansion valve or too much oil. This summer I'm gonna completely flush the system, remove and clean the evaporator, and then vac. and charge the system to spec. But for now, if I could figure out what's fluxored, I'd fix that so I can have some air.    The stuff I put in didn't say it had stop leak, but who knows.

[Conceptz-X]

  The stuff I put in didn't say it had stop leak, but who knows.

It is usually in fine print on the back

[bryantaylor]

find a friend that works at any decent shop and use their a/c machine.  we have this huge ass mac machine at work, i just recovered, vac'ed, and charged my lincoln.  its cold as fuck now.

[snarf]

My 96 integra does the same thing (cycles on and off) It may be due to your compressor clutch starting to go out.  The high pressure could be due to non-condensable in the system but you really should evacuate the hole system, then weigh in a charge.  asfar as the oil goes you shouldn't have to keep adding oil just take the compressor off fill it up with oil then reinstall it.

[SloS13]

my '96 miata air sucks ASSS.  I've replaced the receiver/dryer and expansion valve, pulled a vacuum and re-did all the shit.  Barely cold at all.  I think im gonna bring it to somebody with a machine to try it.

[patsmx5]

Update:

Problem got worse. And I've found a couple interesting patterns. At first, the air is 1/2 ass. Cool air, like it's low on freon. But it runs like normal, with no abnormal cycling. But the coldness of the Air gets shittier and shittier, and then it begins cycling like crazy. And when it begins to cycle, the A/C belt will sometimes squeak, and you can tell the engine is under more load than normal. And it stalled once.

So today I got out the laptop, hooked it up to my megasquirt and datalogged all this shit while checking out the sensors on the A/C system with a meter. And I also had my pressure gauge on the low side.

65*F ambient
Engine up to temp
Hook up low side gauge
begin datalog

I turn on the A/C. Blows cool air, but not great. Return line from evaporator is barely cool with little condensation. Low side ~40 PSI at 1500 RPM idle. Bringing RPM up has a corresponding drop in low side pressure. Above 3k it pulls the low side down to 18 PSI, at which point the low side pressure switch kicks out VERIFIED with my meter. At this point, A/C on my RPM was 1020 with map at 40kPa.

After running for a few minutes, low side pressure is now sitting around 25 PSI at 1500 RPM idle, but changes insignificantly with a change in RPM. How weird... RPM now sitting at 1140 map at 36kPa.

I elevate engine RPM to ~2700 for 30 seconds, then return to idle. RPM now at 870 RPM map at 47kPa.

After 100 seconds of idling, we are now at 960 RPM and map at 41kPa.

A/C is off for 120 seconds.

Turn A/C back on.  RPM at 1060 map at 40kPa. After idling for 50 seconds RPM is now 1100 and map at 37kPa.

Bring RPM to 3k for 30 seconds, then back to idle. 840 RPM idle, 48kPa.

A/C off for 120 seconds.

A/C back on for 60 seconds at idle.

Then I put it to 3k and hold it there. After about 20 seconds, it begins cycling with a period of ~5 seconds. Low side is sitting around 25 PSI (the cutoff point of 18). Probe the high side switch and sure enough, it's the one kicking in and out, turning the comp. on and off.

Fans are kickass and work well. High side line going through firewall is roughly ambient, showing me the condenser is rejecting heat well, fans are working well, etc. Low side line is barely cool at all, with minimal condensation. Low side line was cooler when I first started the A/C. Air inside the car went from 1/2 ass to like 1/10 ass. By the time it began cycling, the air exiting the vents was maybe 10*F below ambient, where when the procedure began, it was probably 25-30*F below ambient.

Conclusion:
Compressor works. Condenser works. Evaporator works. We're low on freon. We have a restriction in the system, probably a clogged up expansion valve. Could possibly be moisture in the system, and it's freezing up at the expansion valve after it runs for a few minutes.