Hi all,

I've cleaned my IAC valve, throttle body, and intake manifold. Now i want to do an idle adjustment with the black **** on the manifold which regulates the air mixture. There is a sticker in my engine bay that says the idle must be 750 RPM +- 20 rpm. I have some questions about this.

Must the car be at full operating temperature when adjusting the idle speed?

Must it be 750 RPM when in park, or drive? Currently its about 950 - 1000 RPM when in park, but when its in drive its about 750 - 800. So which one do i go with? Should i turn it down to be 750 RPM when in park, or just keep it as it is now since its already around 750 when in drive?

Thanks.

 

what year 740 is this? LH2.2 (up to 1988, or 1989 turbo) has an idle adjustment screw, LH2.4 (1989+ non-turbo, 1990+ turbo) is fixed by the computer.

the idle should NOT be that high in neutral with a warmed up engine, that indicates the IAC is not functioning correctly.

if you have the screw, the procedure is to put the engine in neutral or park when its fully warmed up, turn off all accessories (headlights, air conditioning, etc), and then unplug the IAC, and adjust the throttle bypass screw for a 600-650 rpm idle. plug the IAC back in, and it should idle 700-750 RPM. put it in drive, it should idle 700-750 RPM. turn on the A/C, and it should idle 700-750 RPM

when the engine is cold, it should idle 1000-1200 rpm.

 

Its an '87 740. It has the Bosch LH 2.2. I've also noticed that very rarely the idle may get up to 1400 RPM, but that may happen once 3 weeks or so. After driving for a couple of minuets and putting it back to park, it will go back to 950 - 1000 again. Is this a symptom of a faulty IAC?

 

could well be a faulty IAC.... or a leaky IAC hose... or a leaky throttle body gasket... or a maladjusted idle air screw... or a hung throttle thats not fully returning to idle. or something else along those lines.

has the wiring harness been replaced? 87 was the last year for faulty wiring with biodegradable insulation. early 87 cars were problematic, late 87 were not.

 

It was built in August 1987, so I'm not sure if it has the problematic wiring. Not really an early 87 car, nor a late 87 car. I smoke tested it and there were 2 leaks in the vacuum hoses - one in the brake booster hose, and i don't quite remember where the 2nd one was - but both of the hoses were replaced so there are zero vacuum leaks.

I think it might be the IAC, but it could also be the wiring harness as you said. I am going to try to do the idle adjustment as per your instructions, and see if that goes well.

Seeing as how my vehicle is August, would have the wiring harnesses been corrected by then?

 

Also just to note - the actual insulation around the wiring harness is perfectly intact and flexible, so i don't know if its been replaced before, or its the late 1987 harness that's been kept in very good condition.

 

Update: I just experimented with the IAC. When the IAC is plugged in, idle is roughly ~950 rpm. When the IAC is plugged out, idle is exactly the same - does not change by a single RPM. So at this point i think the IAC has gone completely bust. Having a look, the IAC is about $200 so before i definitely know its the problem i don't want to spend $200. Could it potentially be the harness connection? Is there actually a way to test the IAC and to know if its faulty or not?

 

the IAC is a simple air valve, 12V opens it, no 12V closes it. in operation its opened and closed really fast over and over to fine tune the amount of idle air flow.

but first, with it disconnected, the engine running, and all accessories off, turn down that idle air screw on the side of the throttle body until the idle drops to like 600 RPM.... then plug it back in and the idle should go to about 750 RPM.

its also important that the idle switch on the side of the throttle body is clicking closed when the throttle is at idle, and clicking open as soon as the throttle is cracked.

shadetree mechanics often mess with the idle screw when they are trying to fix another problem

 

Yep. Did exactly that. Warmed the car up to operating temp, unplugged the IAC, set idle to ~600 rpm, plug the IAC back in and it does not go up to 750 rpm. Stays at the exact same rpm. So this is why i don't think the IAC is working. Either that, or the harness connection to the IAC is faulty.

 

or the idle switch on the throttle isn't functioning. the idle will only be controlled when that switch is closed.

 

Is there a way to test the idle switch?

 

continuity or ohm meter, connect across switch, move throttle manually off idle and back (with everything shut off). switch should be closed (0 ohm) when the throttle is at the idle position, and open (infinite ohms, but it might read some high number if its still hooked up) when the throttle is off idle.