# Rough Idle and Stalling when AC engaged



## Mike O’Neill (Feb 5, 2019)

The car is a 1967 GTO with factory AC and TH400. Carburetor is the correct 7027262 Quadrajet, although not the one that came with the car. It lacks the throttle return check valve.
The AC works very well - maybe even a little too well. Even on the hottest days I will quickly turn the temperature up some when AC in use. And it bogs the engine, slowing idle to about 500 and rough idling, even occasionally stalling. Previous owner says he did nothing to it, and I don't know what refrigerant is in it. I have not had it freeze up. I don't like to simply turn idle speed up as it then idles too fast when AC not in use, and slams into gear when shifting out of park.
1. would installing the throttle return check valve help with this?
2. would the high idle solenoid from a 1968 or later GTO fit and help with it?
3. anything I should check about the AC - overcharge, etc.?

Thanks


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## LATECH (Jul 18, 2018)

Sounds like the HVAC system is fine. 
Your issue is simply with Idle speed compensation

Not sure what the "check" valve you mean.

I think a throttle Idle solenoid is the way to go.


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## Mike O’Neill (Feb 5, 2019)

Thanks for the reply. The throttle return check was used on 1967 and some earlier years. It looks like a vacuum dashpot and mounts where the solenoid would otherwise go. I only learned about its existence from the service manual. 

According to the service manual:
_"The throttle return check is mounted on the carburetor and is designed to open the throttle valves to increase engine speed slightly and prevent stalling when engine vacuum drops. It also acts to slow throttle closing when the driver suddenly takes his foot off the accelerator pedal. The throttle return check is standard on most V-8 engines with automatic transmissions."_


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## PontiacJim (Dec 29, 2012)

That should be what you want. Does it work/actuate? It will run off vacuum.


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## BearGFR (Aug 25, 2008)

What do you know about the engine, specifically "how much" cam does it have? Is it stock or more "rowdy"?

Reason I ask: If it's got a cam with some personality (idles with some attitude) then what could be happening is that you're on the "knee of the curve" as far as idle vacuum goes - meaning that vacuum drops off significantly with even a slight reduction in RPM. Perhaps what happens is when the A/C compressor kicks in, it drops idle vacuum enough so that the idle mixture goes significantly lean, which can cause the idle symptoms you reported. Something you might try just for grins: Open up/back out both idle screws exactly 1/2 or even 3/4 turn and see if that improves things any when the compressor kicks in. If you're idle is good "normally" that'll be too rich to leave it that way so you'll want to put them back where they were, but it might tell you something.

Bear


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## Mike O’Neill (Feb 5, 2019)

Don't know what's in it for a cam, but sounds a little livelier than stock. I just tried the idle screw test, but did not improve the idle with AC. I did earlier find that vacuum drops a lot with AC. It will start about 15 without AC, then when AC first turned on it drops to 10, with idle slowed somewhat, then after about 5-10 seconds the engine slows more and starts idling rough, and vacuum drops to about 6. I did that test to see whether a "throttle return check" as mentioned above would help. The big drop in vacuum suggests that it would help. 
I don't currently have a throttle return check to try. Most of them listed on ebay look like they would not fit. There are several styles. Would like to go that way to keep car original.
If I luck out there, I'll get the solenoid and bracket for a 1968 GTO. It looks like it would bolt right on.
Thanks for the replies.


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## Colorado67GTO (Sep 11, 2014)

Do you have the original throttle check return or at least the bracket? If so, there is a thread that explains how to replace it and what parts to use.

67 Dashpot Question 

If you don’t have the original, I can send pictures and measurements and you could fab one.


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## Mike O’Neill (Feb 5, 2019)

I don't have original throttle check or bracket. Pictures would be great.


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## BearGFR (Aug 25, 2008)

Mike O’Neill said:


> Don't know what's in it for a cam, but sounds a little livelier than stock. I just tried the idle screw test, but did not improve the idle with AC. I did earlier find that vacuum drops a lot with AC


Ok - well now you know and it didn't cost anything to find that out. I assume you've already used your vacuum gauge to adjust the idle mixure?


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## Colorado67GTO (Sep 11, 2014)

Here are some pictures. Let me know if you need more.


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## Mine'sa66 (Oct 30, 2019)

It may be overcharged with r12 or maybe charged with propane or something else. Be careful if you don't know for sure. Assuming it's been that way for a while, so if it ain't broke don't fix it would be my advice regarding the ac. Looks like you have some good advice about the carb components.


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## Mike O’Neill (Feb 5, 2019)

Colorado67GTO said:


> Here are some pictures. Let me know if you need more.


Thanks for the pictures. I ordered a throttle return check dashpot from an ebay seller, and it looks like the one you picture, so it must be the right one. Is that steel vacuum tubing original? Mine just has rubber hose everywhere but I would like to be as original as practical.


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## Mike O’Neill (Feb 5, 2019)

Mike O’Neill said:


> ..... Is that steel vacuum tubing original? Mine just has rubber hose everywhere but I would like to be as original as practical.


Answered my own question - found the tubes in the Ames and OPGI catalogs.


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## Montreux (Mar 8, 2009)

It can be really hard to differentiate between ignition and carburetion problems—especially on modified engines!
You’ve verified that vacuum is dropping off as idle speed drops. Ignition timing will also be changing. As the vacuum drops, the vacuum advance retards the timing and the rpm drops further so the vacuum drops more...
A bigger cam will require less mechanical advance and more initial advance. It may also benefit from a vacuum advance can with less advance. Compare factory specs for “standard” engines vs. HO/RA engines. 
On the carburetor side you might try fine tuning the mixture with the car in gear and A/C on. As a preliminary indication of too lean/too rich, when the engine starts to labor, cup your hand across the top of the carb to restrict the airflow. If the engine speeds up, it’s too lean. Back the idle screws out 1/8 turn. And vice versa; if the engine slows down with your hands over the carb, it’s too rich. Turn the screws in 1/8 turn. 
Before you even start fiddling with the carb, turn the idle screws in until they are LIGHTLY bottomed, counting the turns. Set both screws to the average of the two. (If one was 2-1/2 turns and one was 3, set them to 2-3/4 turns out) You will probably find them roughly 3 turns out. 
If fine tuning doesn’t solve your problem, you’ll just have to two-foot it, or knock it into Neutral when you hear it start loading up.


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## Montreux (Mar 8, 2009)

Another option (for a few dollars more😁) is a slightly looser torque converter. The one I chose was a Banks (I think—it was red) BPO25 from Summit. One step above stock, and just enough to let the car idle. It also softens the bump when you put the car in gear. It does raise the RPM a little when leaving a stop sign, but definitely doesn’t feel “too loose”.


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## Montreux (Mar 8, 2009)

Sorry...Hughes GM25-BPO.








Hughes Performance GM25-BPO Hughes Performance Street Master Torque Converters | Summit Racing


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www.summitracing.com


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## Mike O’Neill (Feb 5, 2019)

Thanks for all the suggestions. I will be trying the carburetor tuning ideas this evening. I got (from eBay) a throttle check return like the one Colorado67GTO pictures above and plumbed it to the hollow stud at the back of the carb. It helps, although the engine will speed up to about 800 and slow to about 550 occasionally when idling with AC on. I can see the plunger on the throttle check return going in and out when this happens, so vacuum is changing. I haven't yet adjusted it per the service manual - was getting late.


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## Mike O’Neill (Feb 5, 2019)

Just to follow up, I've tinkered with idle mixture and the adjustment of the throttle check return, and it no longer stalls and idles reasonably well with or without AC engaged. I have a really strong AC system - I think it could cool a bus.


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## BearGFR (Aug 25, 2008)

Mike O’Neill said:


> ... I have a really strong AC system - I think it could cool a bus.


Congrats on getting it sorted out. Which A/C system are you using?

Bear


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## Mike O’Neill (Feb 5, 2019)

BearGFR said:


> Congrats on getting it sorted out. Which A/C system are you using?
> 
> Bear


'67 GTO with factory a/c.


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## BearGFR (Aug 25, 2008)

Of course...  There are times that I wish I'd kept my factory system instead of going aftermarket. Are you still running with R12?


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## Mike O’Neill (Feb 5, 2019)

BearGFR said:


> Of course...  There are times that I wish I'd kept my factory system instead of going aftermarket. Are you still running with R12?


Don't know what refrigerant is in it, nor does previous owner. PHS documentation show that it came with A/C. Now that the stalling problem is fixed, I'll just turn up the temperature a little.


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## Lemans guy (Oct 14, 2014)

Nice work Mike....! Stayed with it!..I used to fix my kids cars AC. They were off to College driving college clunkers and I would make sure that they were reliable and that the AC was Ice cold....when they would complain about the car, I would say yeah it is old,..

but it also the coldest place in Tennessee!.....


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## Mike O’Neill (Feb 5, 2019)

Lemans guy said:


> Nice work Mike....! Stayed with it!..I used to fix my kids cars AC. They were off to College driving college clunkers and I would make sure that they were reliable and that the AC was Ice cold....when they would complain about the car, I would say yeah it is old,..
> 
> but it also the coldest place in Tennessee!.....


Your post reminds me of a very hot, humid summer afternoon at Penn State when I was in engineering grad school. I had a -67 LeMans which I loved and memories of it inspired me to buy a GTO more recently. Dorm rooms lacked A/C, so 4 of us sat in my LeMans with the A/C running for a couple hours, and had a beer or 2.


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## Lemans guy (Oct 14, 2014)

Like Humphrey Bogart said;....”The whole world is three drinks behind”


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