Also, since I’m assuming the wheather was warm, your power production is probably down anyway, due to knock thresholds and density.
The air conditioner provides an additional load which slows down the engine. Many engines have an idle solenoid that will engage when the A/C is on. This solenoid will attempt increase the speed of the engine. Depending on how it is adjusted, it may or may not cause the engine to speed up.
Ken’s explanation is correct. Both the Honda’s and the Dodge van I own have throttle positioners that are controlled by the computer. All of the various sensors on the engine are looking at idle speed, all of the environmentals (i.e. outside temp, engine temp, etc.) transmission selector, etc. When at idle, in gear or out of gear, lean or rich, cold or hot, the computer adjusts the idle setting based on the RPM sensor and whatever the software guys programmed in. The service manuals for the above cars specify that engine RPM when warm should not change more that 50 RPM as the lights are turned on, the A/C cycles, the brakes are applied (brake lights on) or the fan is set to high. There is some delay from one manufacturer to the next but they should all recover within a few seconds. If not, there is a good chance that the throttle positioner is loose or worn, the throttle plate is dirty, the mass air sensor is dirty or not up to temp, the injectors are dirty, the fuel pressure is not within spec, or you have a manifold leak. (Assuming you have kept to the maintenance schedule and have good plugs and clean filters) Most of these problems are minor and they result in a small loss in economy. There are a number of shops that are able to clean intake parts and I know of one shop that has equipment for cleaning the intake, injectors and fuel rail for a very reasonable price.If your RPM drops 200 or more (quite common with post '96 Dodge big V6 engines with dirty throttle parts) you might have a problem with engine stall in the future.
I noticed that mine likes to bounce. When there is a load, the tach drops. When the control system tries to recover, it overshoots. Then it realizes it went too high and comes back down. The A/C does this when it turns on. If I turn the bass on the radio up, it’ll follow the music.
On a side note, I did notice that there’s a certain temperature that my engine doesn’t like. The problem is that it’s a little above ambient for this time of year. When the engine’s at this temperature (at start up), it revs really low. Every so often, it’s so low that it feels like it’s getting ready to stall. It’s fine when I give it some gas, but it doesn’t like to idle properly. Once the engine warms up, it’s fine though.
Phil,
The first one is easy, the music is too loud. It might be an engine response or it might just be accoustic coupling.
The second one is a prime suspect for a temp sensor gone bad. It probably works above a certain temp and is way off below that temp.
I was thinking that the music thing might be from extra power consumption. The electrical system is connected to the alternator which is connected to the engine.
As for the temp sensor, it just doesn’t work at that temperature. When the engine is really cold or already warm, it’s fine.
It’s possible that you are drawing too much current through the dash power wiring. As the current goes up the voltage drop on the wire goes up. (just like the wiring to speed controllers and motors.)
I had the same problem with a normally aspirated engine years ago. The carb just didn’t like wet weather between 32 and 45 when it was cold. Seemed that ice would form across the venturis as the air was accelerated.
I dont agree entirely
I think every car on the US market has been fuel injected for some time now. The engine controller (computer) holds the engine at the idle RPM when you have your foot off the gas. It compensates for air temp, engine temp, air pressure, the load on the alternator, and keeps the engine at its proper idle speed.
the on/off button on the air conditioner is just another input to the engine controller.
also, on most cars, when you accelerate with the pedal all the way to the floor, the engine controller turns the A/C off, because you obviously want full power to the drive train (or you wouldnt be mashing the gas pedal).
My Saturn Vue has a electic gas pedal. There is no cable to the intake butterfly, its controlled by the engine computer. (and my Vue is by no means an expensive vehicle: $14,500 new).
I adressed that in the fancier post that i originally posted but i had accidentally closed the browser and didnt feel like typing it again. Yes some newer cars infact the number is increasing do more “thinking” for throttle input but many cars(my experience with new cars runs till 2000 on Jeeps so try not to hold that against me) but what i posted earlier is mainly exident in less advanced fuel injection and carbueration not so much on todays more electronically controlled machines
slightly off topic but kinda on. my truck when i rev it in neutral and then as i let go of the throttle, the speedometer whips up about 5mph and then settles down again. this is when i have my foot on the brakes and the truck hasnt moved a cm. how can it? the transmission is not in gear… why is this?
instrumentation error
or
your engine is warping the time/space continuum!
yea. figured as much.
Flux capacitor is obviously uncalibrated…
The reason it follows the music is because those bass notes draw a lot of current, causing the alternator to draw more power from the engine. If you can see an effect from 63 watts of brake lights, 500 watts of bass will do it, too. Note that many A/C compressors draw up to 5 HP off the engine. Modern electronic engine controls should compensate for the load, but not all cars are the same. As for the bounce, what you are seeing is the control system feedback loop frequency response, which is either in need of repair (see below) or wasn’t designed right in the first place (which I highly doubt).
For the poor idle regulation at a certain temperature, there is obviously something wrong with the warm-up system. Given enough money, the car will run fine at any temperature, but now it becomes a question of how much you want to spend to make it right. A decent mechanic can tell you which part needs replacement, but I would hazard a guess it is the mass air flow sensor that is fouled a little.
Don
This is more along the line of your question and not so much along the original thread, but I think it fits in here.
Talk about odd behavior! When I have my truck in neutral and rev the engine to about 3500-4000 RPM, if I then slide the transmission into gear, the tires let out an awful howling sound and sometimes smoke! The part I really can’t explain is that almost every time, my head bounces off the head rest. It’s all very curious!
Ive seen this before. There is a nut with a screw loose behind the steering wheel! :^)
OK, I got it. That explains the rattling sound I hear right after my head bounces off the head rest. 
lol for those of you who have never heard a transmission ripping apart and a rear end cracking, try this, its the kind of sound that is about equal to the smell of a burnt victor right by your nose. the things you see and hear in a highschool parking lot… shudders
My best friend did this with his moms Gremlin in '77. It was a 6 cylinder auto, way underpowered, but he was determined to spin the tires.
They chirped for a second, then the engine broke free from all its mounts, and the fan went through the radiator.
He had some ‘splaining’ to do when he got home!
Knowing all the things me and my friends did when we were young, Im now thankfull everytime I see my son pull his car in the driveway in one piece.
Happy Thanksgiving everyone! ]
…or so you think… :rolleyes:
lol yup. friend’s 02 4.3 zr2. never a good idea
Yeah my trans am not even running to well at the moment(way too rich) can spin tires in reverse without me trying hard(granted thats on gravely dirtlike stuff) Also if you do as above and you get lucky you will only explode U-Joints or a driveshaft(on rear wheel drive vehicals only unless your really trying)