cllatgmail

93 Pickup with 22RE and 5-speed, 112k miles. Just obtained 2 weeks ago. Very good condition, loving it so far. Here's a problem I have observed: when the AC compressor is on, as I decelerate coming up to a stop sign or traffic light, having been running at 45-55 MPH, the tachometer will indicate the engine's idle speed is bouncing from around 800 RPM up to 1500 RPM or so. If I turn the AC compressor off, idle speed drops down to around 600 RPM and remains steady. If I tap the accelerator to get the engine speed up a bit, maybe to 1500 RPM or so, and then take my foot off the accelerator, then idle speed will become steady. The AC is working fine, by the way.

How can I smooth out this idle speed while the AC is running?

2ToyGuy

Ok, I think, rarely, that you have 2 things going on at the same time.
1) I might be wrong, I often am, but the ac, once it gets the cabin down to the set temp, will cycle the compressor on and off. When it comes on, it kicks the idle RPM up a little to compensate for the load the compressor puts on the engine.

2) When decelerating, if the idle RPM is above a certain point, usually 1100 RPM, and the brakes are applied, the ECM cuts the pulse to the injectors to slow the idle to below the set point. Once the IDLE drops, usually down to 800 or so, the ECM restores the injector pulse. Toyota decided that, if you're slowing, you don't need to waste the gas to keep the engine RPM high, so they cut it down for you.

Could it be that the AC is kicking on, raising the idle RPM, and the ECU is slowing it down? If the AC is raising the RPM above the deceleration trigger point, while the brakes are applied, you essentially have 2 systems "fighting" over engine RPM.

Alternatively, could the butterfly valve in the TB be sticking open slightly, and when you tap te pedal it releases and drops everything down to normal? These TB's get a lot of crud built up in them rapidly due to te EGR system. The throttle cable can get sticky as it ages as well, soooo.......

Bear in mind, too, that the on-board tach is notorious for it's inaccuracy.

Just my couple of thoughts. I get so few....
Pat☺

cllatgmail

In fact you turned out to be exactly right. The AC idle speed control was speeding the engine up too much so the ECU was compensating by dropping the idle speed down...as soon as idle was below the threshold, then it sped back up again. I solved the problem by turning the AC idle control valve down to where the engine idles right at 1k when the AC compressor is on and the engine is hot. Problem is gone. I do need to put an analyzer on it to see how far off the tach is from being accurate.

2ToyGuy

I'm glad the problem is solved!

Even a cheapie Tach/dwell meter from Harbor Freight is a decent check of the actual vs displayed RPM. I have one I bought in te late 70's when I bought a Chevy Luv 4WD pickup. Had to set the dwell on the points once a week or so. I still use it on my Toys to this day.
I think I spent a blistering $19.99 for it, although then it was relatively more than it is now.
Even some relatively inexpensive timing lights have tachs built in...

I'm not sure Toyota went to OBDII test ports yet for your year. Check before you buy an expensive tester you can't use. I don't know for sure though.

Have fun!
Pat☺

cllatgmail

Nope, the '93 is equipped with OBD I...that's a new thing for me to get used to. I have gotten pretty comfortable with the other autos in my fleet (2013 Toyota Sienna and 2006 Mazda MPV) and the OBD-II reader I have. I'll be on the lookout for a tach next time I go to HF...sounds like a good tool to add to the box.