Iraq 69

Hey All,
73 cutlass 350/350 factory air car with 70,000 miles. First warm day out and turned on the A/C. It blows very cold but creates an engine miss/hesitation. Happens at idle and all speeds up to 60MPH. Once at idle, the engine actually stalled. I cycled the A/C on and off several times during the trip and had the same results.
I have done a search and cannot find a similar issue in the forums. I am guessing electrical since it only happens with the air on. The fan speed makes no difference and the miss/hesitation is random and happens every 10-20 seconds whether at idle or driving and the amount of throttle does not affect to problem.
Plugs, wires, points, rotor, cap all new in last 1000 miles. Carb rebuilt and cooling system too. New alternator and battery within the last 1000 miles.
The car runs and drives fine other than this issue. I am hoping some of you have some ideas for me.
Thanks in advance for your help.
Marc

Ozzie

The A/C compressor is a significant load on the engine and will aggravate any existing shortfall or near shortfall. While it's misfiring try to isolate which cylinder is the culprit, if it is just on one cylinder. The beginning to resolving this will require lots of analysis. If a comprehensive compression test has been performed recently, please state the results. If it's electrical in nature, it could be something affecting the voltage available to the ignition system. More checking will likely be in order to get an accurate resolution.

nsnarsk65cutlass

There should be an idle kick up solenoid on the lh front of carb that increases idle speed when ac is on.Also does the compressor turn freely maybe it's putting excessive load on the engine.

MDchanic

Are you using a POA valve or a pressure cycling valve?

- Eric

Iraq 69

Hey all,
Thanks for the replies.
I think I have a POA valve (looks like the pics for the upgrade kits) The A/C was upgraded to R-134 before I got the car.


I will loosen the belt tomorrow and see if the compressor spins. I do have the kick up solenoid on the carb. I turned the very end a few times to see if it would screw in and out. did not notice anything so screwed it back the other way to about where it was.


I pulled the plugs today and checked the gap. Only 2 were at .40 with 4 less than .40 and 2 over .40. They all are a light tan to white color. I checked that all plug wires were in good shape and routed clear of anything. I pulled the cap and cleaned the plastic and contacts. The points looked OK too. I also cleaned the rotor end. The vacuum advance works at about 15 in of vac and the centrifugal advance is clean and moves freely. I looked at the wires going to and from the coil. Tomorrow I will check the dwell and timing, then set the idle speed. I will also hook up a vacuum gauge and set the screws(at the base of the carb) to get max vacuum.
I did find the fuel line going into the carb twisted almost closed but that would not cause my issue. I will get a replacement and change the filter at the same time.

I have not done a compression test but will look at borrowing the tools to do one.

I will continue to work through the trouble shooting and hope to get it fixed.


Thanks again for your support!!


Marc

MDchanic

Just to be clear -

POA valve:




Pressure cycling valve:



- Eric

Ozzie

Iraq 69

Hey all,


Ozzie, I do mean the compressor and will give it a spin, thanks.
I will make sure I read the info I can find here on a post. I had to work late today and have done nothing to the car. Will probably have to work late tomorrow too but will get with it by Friday afternoon for sure and keep you all up to speed.


Eric, I have the pressure cycling valve. Is that a bad thing?
Thanks again,


Marc

MDchanic

Well... We could discuss whether it's bad, but that's been done, and opinions vary.
They usually do their job well enough.

BUT, the normal action of he pressure cycling valve is to abruptly turn the compressor on and off, which is a mode of operation that the A6 compressor was not designed for, and which can cause quite a repetitive jolt to the engine every time the compressor cycles on -- My thought was that you could be experiencing the sudden jerk of the engine when the compressor cycles and interpreting it as a miss, especially if the compressor is a bit tighter than most.

If you observe the operation of the compressor, you should be able to determine whether this is happening.

- Eric

Iraq 69

Hey Eric,
Thanks for the thought and I will check. I have to work late again today so it may be tomorrow or Sat.


Then tune up specs I have for the 350 4brl carb are as follows; plug gap .40,
dwell angle 30, ignition timing 12 degrees BTDC or Mark at 1100 rpm and idle speed 650 in drive hot.
I know to pull the vac hose from the vac advance and plug it for the timing and to set dwell first.
Anything I am missing? Does the timing seem slow to you at 12 degrees BTDC?


Thanks again for your help.
Marc

Ozzie

Your numbers seem to be correct. The 2BBL. 350 engines in the "A" bodies called for 14 degrees, but all others were 8-12. It's just an initial setting.

Iraq 69

Hey Eric,
Thanks.
Is closer to 8 a good place to start and see if it runs better or should I see where it is and knowing it runs OK, then move it closer to the 8 a couple degrees at a time?


Marc

MDchanic

It should run best around 12-14 (or possibly even a bit higher), but it should crank best closer to 8.

- Eric

Fun71

That's way, way too big of a gap. Try .040 instead.

Iraq 69

Hey Fun71
That is what I meant. Just a rookie typo


Thanks for noticing, could have been bad on another issue


Marc

Iraq 69

Hey all,
I took time today and worked on the cutlass. Changed the fuel line that was bent and installed a new filter. Checked the dwell, it was 25. It is now 30. Checked timing and it was 6 degrees BTDC, now it is 12 degrees BTDC. I have 18 in of vac and have set the idle speed to 650 in drive hot.
The A/C clutch is tight when spun by hand, but it does turn. With the engine running and the A/C on, the clutch cycles on and off every few seconds and at 2500 RPM cycles much less frequent. It no longer misses at idle or any speed up to 2500 or so with the A/C on. Sooooooo, it seems that a combination of plug gap, dwell and timing issues were the culprit. I won't be able to go for a drive for a few days but it seems fine and does not miss when it did before.
By the way, I was listening to classic rock groups on the 8-track while working on the car.


Thanks for all your help and will post after the test rive later on.


Marc

MDchanic

Great! Glad you got it fixed!

- Eric

oldcutlass

Proof that sometimes its the simple things. Good work.

Iraq 69

Hey all,
About 100 miles for a cruise, lunch and ice cream. The car runs great and the miss is gone. The A/C compressor cycles all the time and you can sure feel when it comes on as it loads the engine. Should it cycle all the time? I figured that once it came on it would stay on..............................
I guess I will have to do some research on that kind of valve to see it mine is operating as it should.


Thanks again for your help with the issue I had.


Marc

MDchanic

Well.... That's the thing with the pressure cycling valve - it goes on and off all the time.

All modern cars do this, but their compressors are designed for it.

The A6 compressor is designed to turn on and stay on constantly, without cycling.
The POA valve that your car was born with regulated the cooling by varying the amount of refrigerant supplied to the compressor, which is entirely different.

If you don't want your compressor to cycle, you'll have to install a POA valve that is calibrated for R134

Glad it's working well! It's amazing what a tuneup will do.

- Eric

Ozzie

The A6 was a compressor of considerable capacity. The displacement of the (large) A6 was 12.6 cu. in., and was rated as developing 3 tons of capacity at 3000 RPM. While it may cycle a bit less during the hottest part of the year, something has to regulate the pressure (& hence temperature) of the low side of the system. Without some control, the pressure of the low side would drop sufficiently (especially at highway speeds) to cause the temperature of the evaporator be be consistently below 32F and any humidity in the air would condense on the coil and freeze. After the coil becomes covered with ice, air flow and cooling would essentially cease. That is what the cycling valve and the POA are there to prevent, though they do it in different ways. The POA will allow the compressor to run continuously without icing the evaporator. Even the old Frigidaire 5 cylinder compressor used a valve to control the low side pressure, though the pumping capacity was significantly lower than the A6.