Underwhelming cooling performance after complete A/C restoration

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pete_89t2
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Underwhelming cooling performance after complete A/C restoration

Post by pete_89t2 »

Greetings,

I recently completed the A/C restoration on my '93 Mazda RX7 project car, and finally got around to charging the new system with R134A. Bottom line is the cooling performance is rather weak - coldest it would get at the blower vents was 15*C (59*F) on a humid day with 86*F temps. Looking for some tips & suggestions on what to try next to improve the results. According to other 3rd gen RX7 owners who still have the factory system with R134A, I should expect vent outlet temps in the low 40's F* range.

To reiterate the A/C restoration project, reference this post here: https://www.autoacforum.com/viewtopic.p ... 3&start=30

In summary, this restoration is a mix of OEM Mazda parts, some aftermarket parts and custom A/C plumbing to work around the big turbo (OEM hard pipes would not fit). Compressor is a brand new unit purchased form this site's sponsor. The evaporator unit was flushed (Mazda OEM), and while it was out I replaced the expansion valve with a new Mazda OEM part for the R134A system. All of the HVAC blowers, wiring and temp control doors/actuators are OEM and were tested per the Mazda service manual; all of that stuff tested out good. The condenser selected was a generic aftermarket parallel flow unit, 24"x12" core, which is the biggest I can fit in the OEM location & orientation - the larger fitting (gas in from the compressor) up top, and smaller fitting (liquid out to dryer) on the bottom. Plumbing was all reduced barrier hose & bead lock fittings, and I had to weld some MIO fittings onto the end of modified OEM compressor pipe/pad fittings to get everything connected up. The dryer selected was an aftermarket part, which was selected by size/shape to be able to mount in the OEM bracket and same OEM location.

Upon getting the system fully assembled, and after FINALLY getting it leak free and able to hold a vacuum overnight, I ran the vacuum pump on it again for over an hour to ensure the system was completely evacuated of any air/moisture that might be in there before charging it with R134A. Since I lacked the equipment to charge by weight, and the OEM weight specs would be a bit off anyway given the aftermarket condenser, dryer and plumbing, I tried to charge by keeping an eye on low & high side pressures while monitoring my A/C vent outlet temps, and charged to the point where the temp bottomed out. That turned out to be 14~15*C (about 58*F), at stable low & high side pressures of 35psi & 198psi, respectively. I ran the engine at about a 1500RPM idle the whole time per Mazda's FSM procedure for charging.

Those pressure numbers for the 86*F ambient temp seems to match with what the Mazda FSM says to expect, so I think my charge is in the ballpark? After weighing the 2 used R134A cans, it looks like I got approximately 15oz of R134A into the system.

Any suggestions on a DIY charging procedure re-do that might get better results, before I give up and bring it to a shop to try?

Thanks!
DetroitAC
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Re: Underwhelming cooling performance after complete A/C restoration

Post by DetroitAC »

At these conditions what is the temperature of the suction line (evap out going to compressor)?
pete_89t2
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Re: Underwhelming cooling performance after complete A/C restoration

Post by pete_89t2 »

DetroitAC wrote: Wed Jul 21, 2021 7:42 am At these conditions what is the temperature of the suction line (evap out going to compressor)?
I didn't actually measure the temperature there, but the aluminum bead lock fitting where the suction line connects to the evaporator was cold to the touch - not quite freezing cold, to the point where it would collect condensation or frost, but definitely colder than ambient.

That said, the location of the piping is close to the turbo & down pipe, so its fighting the radiant heat from the exhaust/turbo in the area. No different than the OEM installation, except I used hoses in lieu of aluminum hard piping there, and I was able to route the hoses a little further away from the turbo than the OEM routing.
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bohica2xo
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Re: Underwhelming cooling performance after complete A/C restoration

Post by bohica2xo »

Your pressures and charge level look pretty close. I would expect a high side between 200 & 220 in those conditions @ 1500 engine rpm.

The 35psi low side indicates a refrigerant Temperature around 40f So I would suspect reheating or air bypassing the evaporator with vents nearly 20 degrees warmer.

Start with the obvious and bypass or block the coolant to the heater core.
pete_89t2
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Re: Underwhelming cooling performance after complete A/C restoration

Post by pete_89t2 »

bohica2xo wrote: Wed Jul 21, 2021 1:58 pm
The 35psi low side indicates a refrigerant Temperature around 40f So I would suspect reheating or air bypassing the evaporator with vents nearly 20 degrees warmer.
^You may be on to something here. This afternoon I took the car out for some tuning pulls on the freeway; ambient temps in the mid-80's, windows open. On my way back to the house, I flipped on the A/C for a short time on a whim - no thermometer to check the vent outlet temps, but the air blowing out definitely felt a lot colder than it did yesterday when I was charging it in the shop. So perhaps the HVAC air mix box/door mechanism is buggered up. The heater core on this car always has coolant flowing thru it regardless of your HVAC cold/hot settings, so a re-heating situation is a possibility.
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bohica2xo
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Re: Underwhelming cooling performance after complete A/C restoration

Post by bohica2xo »

Cooling systems where the bypass runs through the heater core should not be blocked with a single valve. There are 4 port / 2 way coolant valves in many newer cars that can be used to keep the hot water out of the passenger compartment. Both vacuum and electric operation units have been used by Ford and GM as well as others.

Insulating anything the turbo is baking is always a good idea too. Hoses, wire harness etc. can take a real beating from the radiant heat.
pete_89t2
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Re: Underwhelming cooling performance after complete A/C restoration

Post by pete_89t2 »

bohica2xo wrote: Wed Jul 21, 2021 5:40 pm Cooling systems where the bypass runs through the heater core should not be blocked with a single valve. There are 4 port / 2 way coolant valves in many newer cars that can be used to keep the hot water out of the passenger compartment. Both vacuum and electric operation units have been used by Ford and GM as well as others.
On the FD RX7, the coolant always flows thru the heater core. There are no valves, and with the space limitations, I doubt I'd be able to figure out a way to add in a 4 port/2 way coolant valve to do the job.
bohica2xo wrote: Wed Jul 21, 2021 5:40 pmInsulating anything the turbo is baking is always a good idea too. Hoses, wire harness etc. can take a real beating from the radiant heat.
I've already insulated the turbo, exhaust down pipe and tubular exhaust manifold to reduce radiant heating with a turbo blanket and header wrap, respectively. These are the so called "lava" rated insulators that can handle the higher heat of a rotary exhaust; does a good job of reducing radiant heat as much as possible, but it still gets pretty toasty under there. Haven't put any additional insulation on the A/C hoses yet, but that may be worth doing.

As a quick test to see if the HVAC air mix door has gone wonky, I'm going to just drive around with the A/C on, highest blower speed, recirculate on, then gradually cycle the temp setting dial from cold to hot, while monitoring my dash vent outlet temps. Outlet temps should be proportional to the dial, and repeatable once stabilized at the hot & cold setting extremes.
tbirdtbird
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Re: Underwhelming cooling performance after complete A/C restoration

Post by tbirdtbird »

This for next time.
We did a job where the hoses were going to exposed to extreme heat. We insulated the hoses with woven ceramic cloth sleeving, and slipped it on before we installed the hoses. Pictured is good to 1100° but I believe they have an even higher temp rating product. McMaster Carr (Industrial supplier) was our source
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cyfi66
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Re: Underwhelming cooling performance after complete A/C restoration

Post by cyfi66 »

bohica2xo wrote: Wed Jul 21, 2021 1:58 pm Your pressures and charge level look pretty close. I would expect a high side between 200 & 220 in those conditions @ 1500 engine rpm.

The 35psi low side indicates a refrigerant Temperature around 40f So I would suspect reheating or air bypassing the evaporator with vents nearly 20 degrees warmer.

Start with the obvious and bypass or block the coolant to the heater core.
Not to derail, but doesnt this indicate a high superheat? IE only the first portion of the evap is 40f? He said the suction line was cool not cold which would sound like there was a lot of superheat.
FabricGATOR
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Re: Underwhelming cooling performance after complete A/C restoration

Post by FabricGATOR »

How about the oil quantity? These systems are small so adding an extra ounce or two of oil will take the place of the much needed refrigerant (R134a)

With these "oil chargers" being sold in auto part stores I used to be guilty of aimlessly just giving it a can of oil.... oblivious :roll:
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