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I can still hear the blower when accelerating, although there might be a very slight change in pitch that I can’t tell without listening very closely

My car does something very similar when under prolonged load. I have just chalked it up to a bad check valve or slight vacuum leak somewhere. While under moderate load for several seconds my AC air flow will slowly drop off then immediately come back when there is higher vacuum. I also verified that all connections are visually good and replaced most of the lines.

Curious to see what you find.

No, it's not normal for the AC to cut out completely during acceleration after an R134a conversion. While some AC cycling is normal, a complete cutout with even slight throttle suggests a problem, potentially related to refrigerant levels, pressure, or a faulty component like the compressor clutch.

If the issue is air flow and it’s rpm dependent, look to the vacuum lines on the servos (gold colored bulbs) attached to the hvac box under the hood. They control the flaps for diverting airflow from dash to defrost or feet.

Inspect any and all other vacuum condition while you’re there.

Petethepug wrote: Mon Aug 04, 2025 9:04 pm If the issue is air flow and it’s rpm dependent, look to the vacuum lines on the servos (gold colored bulbs) attached to the hvac box under the hood. They control the flaps for diverting airflow from dash to defrost or feet.

Inspect any and all other vacuum condition while you’re there.

^^ This ^^

Relating to my stuck-up 928 cousin, where there's a check-valve between the manifold and a vacuum reservoir for the HVAC and cruise control systems, used to maintain vacuum in that reservoir even while the manifold pressure is high. If/when that check-valve leaks, or more commonly when there's an actuator servo leak, vacuum in the reservoir bleeds down quickly and vents go closed. There's nothing particularly magical about the check valve beyond making sure it's installed in the correct direction -- air flows from the reservoir to the intake. If you find that you can blow both ways through it, you can replace with a common parts store valve that fits in the hose. As part of a regular service check, it's a Good Idea to use your MitiVac or equivalent to verify that all your actuators and valves do in fact hold vacuum.

@Poorsche44
Any updates?

I’m in bear lake for the weekend, hopefully I can get to it sometime on Sunday. Where exactly in the bay are those check valves located?

PSU_Crash wrote: Mon Aug 04, 2025 11:21 am My car does something very similar when under prolonged load. I have just chalked it up to a bad check valve or slight vacuum leak somewhere. While under moderate load for several seconds my AC air flow will slowly drop off then immediately come back when there is higher vacuum. I also verified that all connections are visually good and replaced most of the lines.

Curious to see what you find.

Do your headlights or interior lights dim at the same time? In theory, the alternator should put out more with rpms, but at one point my voltage would drop under boost... Replacing everything fixed it.

Tom wrote: Fri Aug 08, 2025 5:59 pm

PSU_Crash wrote: Mon Aug 04, 2025 11:21 am My car does something very similar when under prolonged load. I have just chalked it up to a bad check valve or slight vacuum leak somewhere. While under moderate load for several seconds my AC air flow will slowly drop off then immediately come back when there is higher vacuum. I also verified that all connections are visually good and replaced most of the lines.

Curious to see what you find.

Do your headlights or interior lights dim at the same time? In theory, the alternator should put out more with rpms, but at one point my voltage would drop under boost... Replacing everything fixed it.

I don't think voltage was dipping. To be honest I was driving a bit "spirited" when I noticed it, so I wasn't watching the meter and it was daylight. If my lady wasn't riding shotgun and pointed it out, I may have never noticed. Everything is fine when just cruising. Prolonged WOT, or close to it, is the only situation where is drops.

PSU_Crash wrote: Sat Aug 09, 2025 9:45 am

Tom wrote: Fri Aug 08, 2025 5:59 pm

PSU_Crash wrote: Mon Aug 04, 2025 11:21 am My car does something very similar when under prolonged load. I have just chalked it up to a bad check valve or slight vacuum leak somewhere. While under moderate load for several seconds my AC air flow will slowly drop off then immediately come back when there is higher vacuum. I also verified that all connections are visually good and replaced most of the lines.

Curious to see what you find.

Do your headlights or interior lights dim at the same time? In theory, the alternator should put out more with rpms, but at one point my voltage would drop under boost... Replacing everything fixed it.

I don't think voltage was dipping. To be honest I was driving a bit "spirited" when I noticed it, so I wasn't watching the meter and it was daylight. If my lady wasn't riding shotgun and pointed it out, I may have never noticed. Everything is fine when just cruising. Prolonged WOT, or close to it, is the only situation where is drops.

Yeah, probably vacuum related -- check valve, hoses, etc. Those vacuum reservoir tanks like to crack too...