1970 LS5 stalls with AC on

[Mathew S.]

If my idle is set at around 800 rpm warm and no load, when the AC is turned on it stalls. Did they do anything back in the day to mitigate this?

I am not a fan of idling 1200+ rpm all the time to keep from stalling. Is this expected behavior?

[Leif A.]

Re: 1970 LS5 stalls with AC on

Mathew,
Is your idle solenoid installed and functioning?

https://www.ecklerscorvette.com/solenoid-carb-idle-71-72.html

[Mathew S.]

Re: 1970 LS5 stalls with AC on

It’s a 1970 and to my knowledge doesn’t use solenoid originally. I can add one, but trying to keep original, and not sure if the behavior is normal.

[Mark E.]

Re: 1970 LS5 stalls with AC on

Leif,

I don't believe an idle solenoid is used in 1970; at least not with 454. Instead, a crude dashpot is used in place of an idle speed screw, but all it does is slow down the release of the throttle its last bit of travel (for emissions reasons).

Mathew,

You didn't say if this was an auto or manual car, but either way these cars didn't have anything to "mitigate" idle speed drop other than a lot of engine torque. If I remember correctly (AC not working now), my LS5 M20 dropped 150-250 RPM when I switched on the AC. The CSM specifies idle speed as 750 rpm, but doesn't say if this is with AC on or off. I adjust it to 700-750 rpm with AC on, which gave 850 - 1,000 rpm with it off.

You suggest experiencing a drop of 400 rpm, which may indicate excessive load from the AC. You may start by checking its charge and condition of the compressor.

[Mathew S.]

Re: 1970 LS5 stalls with AC on

Mark, it’s a manual. The compressor is new and system is mostly new/rebuilt and converted to 134a. The RPM drop is a guess And have only had it working since Tuesday. I haven’t found one of those pots for the carb, and not sure really worth it. To be honest, didn’t even think about setting idle with AC on, sometimes the most simple thing eludes us all.

[Leif A.]

Re: 1970 LS5 stalls with AC on

Mark, it’s a manual. The compressor is new and system is mostly new/rebuilt and converted to 134a. The RPM drop is a guess And have only had it working since Tuesday. I haven’t found one of those pots for the carb, and not sure really worth it. To be honest, didn’t even think about setting idle with AC on, sometimes the most simple thing eludes us all.

Mark,
Typical recommendation is to set your RPM about 150 above the setting used without the A/C. What I do for mine is to drive the car until it warms to normal operating temperature, choke having relieved. I then park the car (still running), engage the A/C and adjust the idle speed up to 700-750 RPM, or whatever is required to accomplish a smooth idle with the A/C engaged. Obviously, when not operating the A/C the idle will be 150-200 RPM over normal. The other thing that some do is to simply leave the idle speed and, then, feather the gas when you come to a stop to keep the revs up to stop the car from stalling. Whatever is most comfortable for you. Happy driving.

[Paul Y.]

Re: 1970 LS5 stalls with AC on

If I recall correctly, the corvette compressor was not the same as the passenger compressor and took less power when new. Who knows now what one gets from the suppliers if it has a different one specified for the corvette than passenger autos. Paul

[Tom L.]

Re: 1970 LS5 stalls with AC on

I had wanted to reply to this thread when it was current but my car wasn't running yet, just completed "winter" projects. Anyway, it's a '72 LS-5 w/ auto. I know yours is a '70 manual but they should be similar. My car idles in neutral at 700 rpm, 600 when in drive and drops to 550 with the a/c on and in drive. No stalling issues of any kind. Oh, also using 134a and no problems with cooling capacity. My thoughts were perhaps an overcharge or maybe just something to do with engine tuning. If you haven't resolved your problem I hope this helps. have fun!!

[Duke W.]

Re: 1970 LS5 stalls with AC on

If somewhere along the way the OE cam was replaced with a high overlap aftermarket cam, that may be the issue. Normal idle behavior for the LS-5 should be about 15"@600 in neutral with the compressor off. The quick fix is to set the idle speed as low as possible with the compressor on and live with the higher idle speed when it's off.

I accompanied the owner of a '65 365 HP Coupe with OE AC to the national convention earlier this month. Idle was set at about 900, but with the AC on would drop and cause the engine to stall unless the owner added some throttle. It was only an issue on our return when we got back to the LA basin and ran into some traffic.

I'm surprised GM didn't install an idle-up solenoid. This was likely a problem when the cars were new, so I guess it was left up to the dealer or owner to set an idle speed that wouldn't stall the engine when the compressor was engaged. That's the way it was sometimes back in the day.

Duke

[Mathew S.]

Re: 1970 LS5 stalls with AC on

I know the cam is different, but no clue what the cam is. At idle of about 800 RPM, I get about 13-15” of vacuum, with 13 deg of initial timing. The idle quality sounds similar to the LT-1. It is a hydraulic cam though, and for time being will turn off the AC at idle.

[Rick N.]

Re: 1970 LS5 stalls with AC on

Aftermarket camshaft with an automatic transmission and A/C will require a higher stall (looser) torque converter.

[Michael G.]

Re: 1970 LS5 stalls with AC on

If somewhere along the way the OE cam was replaced with a high overlap aftermarket cam, that may be the issue. Normal idle behavior for the LS-5 should be about 15"@600 in neutral with the compressor off. The quick fix is to set the idle speed as low as possible with the compressor on and live with the higher idle speed when it's off.

I accompanied the owner of a '65 365 HP Coupe with OE AC to the national convention earlier this month. Idle was set at about 900, but with the AC on would drop and cause the engine to stall unless the owner added some throttle. It was only an issue on our return when we got back to the LA basin and ran into some traffic.

I'm surprised GM didn't install an idle-up solenoid. This was likely a problem when the cars were new, so I guess it was left up to the dealer or owner to set an idle speed that wouldn't stall the engine when the compressor was engaged. That's the way it was sometimes back in the day.

Duke

Duke, since our trip I've retorqued a couple of loose hose clamps which had allowed some refrigerant to leak out, topped off the system with 3.25 lbs of R12, and reset idle to 850 with A/C on. It's made a world of difference in the drive ability but as others have mentioned, the trade-off is an idle of 1100 with the A/C off. This is with the L76 running what feels like a proper 30-30 cam; no doubt the idle drop with a hydraulic cam or a big block would be less. It doesn't bother me much because I'll be running the air all summer and when the weather cools off, I'll readjust the idle RPM, easy peasy.

[Duke W.]

Re: 1970 LS5 stalls with AC on

That's a lot of R-12. My MR2 and Merc are only a little over two pounds, but I think your OE system is about four. Given how much the system took, I'm surprised the AC worked, but it did seem a little weak.

BTW, what did you pay per pound for the R-12, and what shop. Since both the MR2 and Merc need to be recharged, I'm looking for shops that still service R-12, and they are getting few and far between.

Duke