While AC on, Engine Bogs Down

[Trstno1]

Hey guys -

Looking to bounce some ideas off you. When my AC compressor kicks on, my engine seems to bog down. So if I set the engine to idle at 750 RPM as it should, when the compressor kicks in the car will idle down somewhere between 550 - 600 RPM making things pretty shaky because it wants to die. Turn AC off, engine returns to 750 RPM and all is right with the world. So....I have adapted to raising my idle to about 900 RPM when I want to use the AC so that when the compressor kicks in the idle bogs down to about 700...still a little shaky but ok.

What could be causing such a bog down? Is this a sign of a compressor going out? a bad idle speed unit? I know the engine will normally bog down with the compressor running, but it should be compensating for the additional load and allowing more air into the system to maintain the idle. Why would it not be?

I just want to fix this once and for all. I spent a long time getting my car to idle properly at 750 RPM and now I'm purposefully mucking it up just to have AC.

As always,

Any and all help is much appreciated

[bytes311]

My engine would actually die with the AC on. The problem was my regulator had gone bad so the alternator couldn't keep up. I replaced the alternator and the problem went away. I also installed modern radiator fans so the draw is considerably less now than with the stock fans.

[Bitsyncmaster]

A stock idle system you can not adjust idle speed (775 RPM) because the idle system will try to keep that RPM. If your adjusting the throttle open wider than the idle system can adjust than you can increase RPM.

It sounds like your idle system is not working. When you turn the key to on, does the idle motor buzz or vibrate?

[Ron]

Hey guys -

Looking to bounce some ideas off you. When my AC compressor kicks on, my engine seems to bog down. So if I set the engine to idle at 750 RPM as it should, when the compressor kicks in the car will idle down somewhere between 550 - 600 RPM making things pretty shaky because it wants to die. Turn AC off, engine returns to 750 RPM and all is right with the world. So....I have adapted to raising my idle to about 900 RPM when I want to use the AC so that when the compressor kicks in the idle bogs down to about 700...still a little shaky but ok.

What could be causing such a bog down? Is this a sign of a compressor going out? a bad idle speed unit? I know the engine will normally bog down with the compressor running, but it should be compensating for the additional load and allowing more air into the system to maintain the idle. Why would it not be?

I just want to fix this once and for all. I spent a long time getting my car to idle properly at 750 RPM and now I'm purposefully mucking it up just to have AC.

As always,

Any and all help is much appreciated

It sounds like you are using the brass screw(s) to set your idle as in WS D:04:02-19. Some owners do this but D:01:13 says they are not to be used on a DeLorean because it has an electronic idle speed control system (idle speed motor, etc.). If that's the case, try fully seating the screws and see how it acts. (If no luck, you may want to inspect/wash the ISM out too.)

[Ron]

My engine would actually die with the AC on. The problem was my regulator had gone bad so the alternator couldn't keep up. I replaced the alternator and the problem went away. I also installed modern radiator fans so the draw is considerably less now than with the stock fans.

I don't want to sidetrack the thread but mention that, if the alternator/regulator was allowing a good battery to get low enough to kill the engine, then it would be because the ignition was suffering and you would also have trouble starting the engine immediately afterwards. (IE, if there was enough power in the battery to start the engine when it died, then it had way more power left than needed to keep it running.) The battery light should not have came on when the key was in the Run position OR turned on as soon as soon as the alternator was unable to keep up.

[Trstno1]

It sounds like you are using the brass screw(s) to set your idle as in WS D:04:02-19. Some owners do this but D:01:13 says they are not to be used on a DeLorean because it has an electronic idle speed control system (idle speed motor, etc.). If that's the case, try fully seating the screws and see how it acts. (If no luck, you may want to inspect/wash the ISM out too.)

You are exactly right. I was adjusting the brass screws in order to give me a little more air into the system, thus raising the engine idle RPM. If I were to seat all of the brass screws like it should be, the engine will idle at 750 rpm just fine, but as soon as the AC compressor kicks on it bogs the engine down so much the car shakes. I'm trying to get that to stop happening... Understandably what I am doing is a band aid to get the results I need. I just need to trouble shoot why the engine would be bogging down in the first place when the compressor kicks on.

[Trstno1]

I don't want to sidetrack the thread but mention that, if the alternator/regulator was allowing a good battery to get low enough to kill the engine, then it would be because the ignition was suffering and you would also have trouble starting the engine immediately afterwards. (IE, if there was enough power in the battery to start the engine when it died, then it had way more power left than needed to keep it running.) The battery light should not have came on when the key was in the Run position OR turned on as soon as soon as the alternator was unable to keep up.

I have no issues with either the battery or alternator. When the car is running I am charging at nearly 14 volts. If I were to guess, I would say my Idle speed motor is not reacting quick enough. Which sucks, because I had just replaced it about a year or two back....

[Ron]

I have no issues with either the battery or alternator. When the car is running I am charging at nearly 14 volts. If I were to guess, I would say my Idle speed motor is not reacting quick enough. Which sucks, because I had just replaced it about a year or two back....

Good. (I just mentioned that because his alternator shouldn't have caused his to die w/o the other symptoms he didn't mention....maybe clear something up/not your D's problem, imho.)

How are you "setting the idle"??

[Trstno1]

Good. (I just mentioned that because his alternator shouldn't have caused his to die w/o the other symptoms he didn't mention....maybe clear something up/not your D's problem, imho.)

How are you "setting the idle"??

I have just realized that if you open the brass screw closest to the middle of the engine and adjust the other two a bit, I can get the idle RPM of my car to increase from the 750. I really don't want that to be the focus here. I will be re-torqueing the brass screws down to a closed position and start the troubleshoot from there. When the brass screws are where they should be, the engine idles fine at 750 - 775.

how quick should the idle speed motor react to the ac compressor kicking on? And is there a chance I could have some sort of vacuum leak only when the ac is on?

ideas?

[Ron]

I have just realized that if you open the brass screw closest to the middle of the engine and adjust the other two a bit, I can get the idle RPM of my car to increase from the 750. I really don't want that to be the focus here. I will be re-torqueing the brass screws down to a closed position and start the troubleshoot from there. When the brass screws are where they should be, the engine idles fine at 750 - 775.

how quick should the idle speed motor react to the ac compressor kicking on? And is there a chance I could have some sort of vacuum leak only when the ac is on?

ideas?

Sorry, I missed post #6.
Good!

The ISM should react ~instantaneously and settle down quickly, eg, as when you tap the throttle lightly.

Yes, when the AC is on, the Mode Switch and anything it de/activates are good candidates for vacuum leaks.
Good thinking!
Try removing its vacuum feed line at the intake manifold and cap the port off.

Load the ISM with carb spray and shake it so that the rotor gets a good workout, when clean, repeat with WD or similar lube...