Idle question on 1987 740 B230F

I've come to believe that a "24 hour" rule exists on my Volvo. It's
like I make an adjustment and I don't notice anything. 24 hours later,
I start the car and the changes take effect.

One example is my idle. I think I am fighting pinhole vacuum leaks and
I am trying to isolate them - but here's the deal. I've cleaned my
throttle body & IAC and she idles well under the circumstances. But,
when I turn the black knob under the throttle body - it doesn't affect
the idle - until later in the day when I restart the car, or 24 hours
later. By 24 hours I generally mean I let the car sit until the next
day.

Anyway - let's say my car is idling at about 600 rpm. I open the knob
1/2 turn, then another and another and another and 2-3 turns later the
engine is idling exactly the same.

I shut the car off, come back a few hours later, or the next day and
the idle is up according to the turns.

Why the delay?

Thanks!

 

I think the difference is warm/cold. The idle adjustment knob controls the
minimum amount of idle air, but the electronically controlled Idle Air
Control valve should be doing the main adjustment. If the throttle body and
idle air control valve need to be cleaned out (as they need every couple
years) the cold idle becomes unsteady.

Mike

 

Thanks Mike,
What throws me is that I thought the purpose of that knob was to speed
up or slow down the idle. For example, when I my car is running and I
see the idle on the dash around 600 rpm and I want it to be 900 rpm. So
I open the hood and turn the knob open until the car idle increases to
900 rpm.

Is this not correct?

I do recall reading something where I was supposed to disconnect the
electronic idle control to set the idle, but didn't quite understand
that.

I figured that knob was to manually set the idle speed. Is this not so?

 

I've never done the electronic disable myself; I understand it is done by
grounding one of the wires in a 2-pin connector that is floating around
behind the air cleaner.

But the electronic part will take over if it can. That screw is a needle
valve that controls a passage around the throttle plate, and the electronic
control is a valve that does the very same thing. When the screw is adjusted
right and the throttle body and IAC valve are clean, the idle will hold
steady around 600 rpm as it should.

Mike

 

Hmmm. Ok, thanks. It's hovering around 600 rpm, but when I first engage
into gear I am getting the dreaded one-time stall.

Then there is an intermittant, mild miss I am getting at idle. I
recently pulled the throttle body off to clean it, checked the openings
and did make sure the idle knob holes were clean.

I still think I have some tubes to check for vacuum, and the miss only
started after I pulled the plugs and wires off several times to replace
the valve shims. I spent a fortune on those Bosch wires so I hate to go
replace them know - so we'll see.

Thanks again.

 

Stumbling at idle is very often the result of deposits in the throttle body
and the Idle Air Control valve. Both are easy enough to remove, and between
them it takes about a can of carb cleaner to get the goo out. The critical
area in the throttle body seems (to me) to be that passage the black screw
controls. The throttle body can be cleaned somewhat in place, washing out
the throat with a rag soaked in carb cleaner and spraying it through that
idle passage.

Mike

 

I did all that Mike. I've already cleaned all of the above very well.
The only thing different was that I ran Sea Foam through the system
later. I pulled the throttle body again and cleaned it, but not the IAC
again.

 

I cleaned the IAC again and checked the vacuum tubes. It seems to be
better. I'll test further.

thanks,