Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Air conditioner keeps blowing the high pressure hose. The shop is stumped

My car is a 2006 Mazda Miata, stock except for being lowered and having a different muffler. Nothing really noteworthy about it. I did run over a big chunk of something a few years ago that cracked the plastic undertray under the nose of the car and dented the bottom row of the condenser, but the problem I’m having is more recent.

About a year ago the AC stopped working. I took it to one shop that found there was no refrigerant in the system. They refilled it, and a few hours later it stopped working again. I took it to another shop that detected a leak in one of the schrader valves (the low pressure I think). He replaced that, filled it, and it worked again for another few months.

Recently it stopped working again. I took it to the same guy that replaced the schrader valve and he found that the high pressure hose had failed. He replaced it and refilled the system. It worked for a few hours. Then when I was sitting at a drive thru, a massive cloud of refrigerant erupted from under the hood and the AC stopped working. Took it back to the guy and he found the high pressure hose had blown off. He put it back on, refilled it, and the same thing happened again. Sitting in traffic, the high pressure hose blew and all the refrigerant poured out.

I took it back to him again and he put the hose back on and refilled it again. We let the car idle for about 30 minutes with the AC on and it worked fine. During this he had the high pressure gauge hooked up and it read between 150 and 175 with the car idling (about 70 degrees F ambient temp). He didn’t have the low side gauge hooked up. We did see that the fan is working and kicked up to the higher speed. He did say he checked the dryer and that it looked like the bag that held the desiccant was intact, but he didn’t mention replacing the dryer in any of these visits.

The guy is stumped so I won’t be going back there. I need to find an AC Ninja but I want to go in with an idea of what to ask and some possible solutions…

Any ideas?

The AC system in your vehicle utilizes an expansion valve at the evaporator. The expansion valve is what converts the high pressure liquid refrigerant back into a low pressure gas into the evaporator. The expansion valve also meters the amount of low pressure gas that enters the evaporator. If the expansion valve malfunctions where it fails to open while the AC compressor is operating, this will dead-head the compressor where the pressure builds in the high pressure hose and blows the hose off.

I’d be looking for a defective expansion valve.


I’m in agreement about the possibility of a stuck, or sticking, expansion valve but would add that maybe there’s a problem with debris piling up in the system or oil logging.

There are some things that point to the ability of the mechanic maybe not being up to par on A/C work.

  1. That 150-175 PSI is way too low for the high side on an idling engine.
  2. There is no reason not to connect the low side gauge at the same time as the high side.
  3. How does he check a dessicant bag visually?
  4. Anytime an A/C system is opened, especially catastrophically, the drier should be replaced.

Theorizing for a minute, it could be that the damaged condenser has a small leak. This can lead to refrigerant oil loss and in turn can damage the compressor. Compressor debris to some extent could be behind this problem.

In the hvac world an expansion valve would never fail and cause a high head condition. The condenser can usually hold the complete refrigerant charge. If it keeps blowing the high side line it almost has to be a condenser fan or switch failure. High head is almost aways caused by overcharging, noncondensibles, or control failure. Plus, I thought all lines on auto ac’s were premaid with crimped fittings. If a line blows off, doesen’t it need to be replaced? You can’t just “put it back on” can you?

There’s a lot of issues with the complaint on this problem though, not the least of which is the high side pressure which is too low.

I respectfully disagree that an automotive A/C line will not blow up if there is a restriction (stuck expansion valve, clogged orifice tube, etc) because I’ve seen a few that suffered this problem; especially after some age sets in and this car’s A/C lines are going on 7 years old.

The puzzler to me at this point is why that shop would consider 150-175 PSI acceptable and normal at idle on a 70 degree day. Once the RPMs are elevated that high side pressure will drop even more; sans a restriction in the lines or condenser.

The only reason I can think of for a hose to repeatedly be blown off would be if the threads in the affected area are stripped to some degree. One would hope (?) that the person doing this repair repeatedly would have taken that into consideration and checked the threads but based on what has been related that is not a given.

Who in their right mind would reinstall a high pressure hose that “blew off” even if inspection of the threads seemed “ok”? I cannot believe someone faced with this would even consider inspecting anything other than to understand if the mating threads were OK for a new hose. Obviously if it blew off they cannot be ok and a professional shop to boot!!

Okay, wow. So the consensus is I need to find a new shop, haha. It’s interesting how someone can LOOK and SOUND like they know what they’re doing and be so wrong… He had the big machine and hoses and it beeped and everything!

I found a temperature/pressure chart for the car, so I plan on renting/borrowing a gauge from AutoZone this weekend to see what the pressures are at the recommended engine speed (1500rpm) so I can take all this info to whatever shop I take it to next.

So my next question is - How the heck do I find a GOOD auto AC shop? Nobody I’ve talked to seems to have a good auto AC guy so all I really have to go on is Google reviews and those are mostly unreliable…

I don’t know what the factory specs are on this particular car but in mild weather this time of year and at elevated RPMs you should see about 225 PSI on the high side and 30-40 on the low side, all depending.
Finding a good A/C shop can involve some leg work so I can’t help much there.

I would strongly advise that if you check the pressures yourself that you wear safetly glasses and gloves. Last year while servicing an A/C (and I’ve done a bunch of it) I got careless and accidentally discharged some refrigerant onto the finger of a bare hand. This created frostbite instantly, hurt like hxxx for several days afterwards, and to this day the end of that finger is still numb to a certain extent.

Just pointing out what a little inattentiveness can lead to and thankfully it wasn’t the eyes.

Many times the A/C experts in an area will be auto “HVAC” experts. I have usually looked for places that call themselves “radiator shops.”

Just to clarify what OK4450 said about the pressures - those are not only with the car running but also with the AC compressor running. Put the switch on defrost and make sure the A/C button or light in the car is on and crank it up to high. If the compressor isn’t running the hi/low sides should be about the same, probably in the neighborhood of 120psi

Although, I’m not sure what you’ll accomplish with a loaner set of gauges. If you want to do it for a hoot that’s fine but be careful as noted. But there’s almost no AC work that can be done outside of a fully equipped shop by well-trained tech.

I second the idea of using gloves and goggles, especially since your car has a history of misbehaving.

Sounds like the hose was repaired to begin with. Not replaced with a factory hose. The fittings don’t have the barbs they used to have when we could replace just the rubber hose and use “A/C” style clamps. R134 runs higher pressure than R12 175-300 high side depending on engine speed,charge and ambient temp. Put on a factory Mazda hose the correct amt of 134 and make sure the condensor fan is running. Watch the gauges…350-450 is trouble on the high side and will blow off at the pressure releaf valve on the compressor…or find a shop with the right know how

Since the A/C works normally (when the hose is on), I also think you have a bad part. Get a NEW part and put that on. Where exactly is it coming apart when it blows off?

oblivion… I’m not sure what exactly is happening when the hose blows. All I know is that the shop was looking at the driver’s side of the condenser… That’s where he was working.

The car is at the dealer as we speak. I’m sure I’ll get bent over but I don’t know where else to take it. We’ll see what they say later today.

“I did run over a big chunk of something a few years ago that cracked the plastic undertray under the nose of the car and dented the bottom row of the condenser, but the problem I’m having is more recent.”

Maybe yes and maybe no…Also, don’t automotive A/C systems have a cut-off switch on the high side to turn off the compressor in the event of over-pressure?

I would follow Kriley’s insights to the source of the problem…

The attached link shows how your AC system is set up.

You can see how a malfunctioning or plugged expansion valve could cause a hose to blow.

You can also clearly see how a restricted lower portion of the condensor could cause a high pressure to blow the line between the compressor and the condensor is this where it’s blowing?

It’s very possible that the damage you did to the bottom row of the condesor has now manifested itself as a foreign object in the expansion valve or even in the bottom line of the condensor that’s creating an intermittant plug. If it were in the bottom of the condensor, that would also answer the “low pressure at the tap” question that otheres here have asked.

Clearly you need a new shop. Automotive HVAC shops, and many radiator shops, do AC work. Don’t forget to tell them about the damage to the condenser.

I don’t think your vehicle was originally in the hands of an A/C capable person. Acts such as not connecting the low side of the gauge set and assuming that 150-175 PSI is an acceptable high side pressure on an idling engine makes things very suspect.

If something blows apart (whether it’s excessive pressure or a weakened connection) it’s incumbent upon the guy doing the repair to solve the problem of why it’s failing, not just put it back together and hope for the best.
Even 275 PSI should not blow anything apart, much less 175.

I’ve seen “DIY” re-charges result in 400psi on the high side and nothing blows…Maybe the first step here is just using a FACTORY hose and not one made up locally…That and having the system repaired by a pro…

Okay, I’m even more baffled now. I took it to the dealer and they said that other than the refrigerant level being low and having a non-OEM hose on it, it was in perfect working order. They wanted to charge me $450 to put an OEM hose on, to which I said “HAAAAAAAAAHAHAHAHAHAHAHAHA, No.”

But the weird part is that I’ve used the AC three times since then and it’s worked fine. Same situation, around town driving and sitting in traffic, and it works fine. The tech did say he drained the system and refilled it. I assume he has access to the Mazda spec book… So maybe the other guy wasn’t putting the right amount of refrigerant in it? Who knows.

So I give up for now… I did talk to a different AC guy (combination radiator shop/AC shop) that seemed to have some theories about why it kept blowing that hose. So if it does blow again I’ll take it there I guess.

Thanks for all your input!

Have you taken the car to a real AC/Radiator shop, or are you using a general auto repair or the guy down the street that does repairs from time to time.

While the generalist auto repair person may do fine, for A/C I recommend finding a real A/C or Radiator shop for this work. In general in the south the shop will be called an A/C shop and in the north it will be called an radiator shop, but they are the same thing.