My 85 240s ignition and idle timing appear to be fluctuating in parallel. The ignition timing hovers around 12 degree mark, but will drop to 7-9degrees and up to 15-17 degrees varyingly. The base idle also fluctuates, down or up one from 25Hz, which is 750rpm. (my car lacks a tach) Does this indicate unmetered air entering the engine or some other problem? I can take a video if that would be helpful to show.

thanks yall!

 

It could be caused by unmetered air, but not the only possibility.
You changed the Hall sensor, right? Was the problem present before the Hall change?

 

yes, and this problem has been present for about half a year or more maybe.

i cleaned the throttle body today and i 'discovered' the air intake boot from the MAF to throttle has been damaged as the collar where it meets the throttle body. it had been taped up and clamped - i hadn't bothered to investigate it previously. I glued, taped and re-clamped it with no change in the idle fluctuating character. so it could be that. the rest of the intake hosing looks fine from the box on down. i'll troubleshoot the area tomorrow with wd40.

 

its amazing how little the air leaks can be to mess up idle, yet have an engine that runs OK once its rolling.

but another thing that can make an erratic idle is a gummed up idle air controller.

 

this may be an artifact of the erratic idle, but the entire engine seems to move a bit during the fluctuations in the idle. i presume that's normal since it's getting hit with fluctuating fuel, etc., but i did replace the motor mounts in Feb with genuine volvo. they helped but i thought it may eliminate that movement completely.

looking in the manual, it doesn't describe any procedure to clean the IAC valve. is it possible to do or is it a replace if faulty part?

 

It can be cleaned, use carb cleaner spray, make sure the springy thing moves freely.

 

so i misspoke, the car is actually idling around 1050rpm with the ignition timing at 12 degrees. I have the idle air screw on the throttle body screwed all the way in, so I need to adjust the throttle switch position to get it correct at 750rpm.

i cleaned the IAC and the fluctuation in idle rpm doesn't appear much different. it's dipping less drastically but still bouncing within a Hz.

i sprayed WD around the idle air hose to the intake and around the throttle body, where the intake boot was damaged, with no change in the idle character.

the car has been sitting for almost two weeks and i drove it today for about 30min, getting it up to 75mph for a mile on the highway and mostly around town 25-40mph. it had three instances where it had a same problem of the acceleration going out. twice out of the hole, it would 'buck' as the acceleration cut out and back in; the third time, crusing around 45pmh, the acceleration just went out and the car was coasting; I gave it full throttle and it came back. the car did not die.

I have no idea what this intermittent problem could be. I really thought it was the hall sensor, as that appeared to be what caused this same issue before when the acceleration would cut out and the car would die. But now it's back....

 

when you adjust the idle screw, you should first unplug the IAC, the engine should be warmed up, and all accessories (headlights, a/c, etc) off. you adjust it to about 600 RPM, then when the IAC is plugged back in, it brings it up to the 700 or so control point.

 

right, but if you cannot obtain the correct idle speed with the bypass screw, you must alter the throttle switch. per bentley, anyway.

the bypass screw is basically all the way in and i'm still idling high ~1000rpm.

i don't suspect this has any connection to the intermittent loss of acceleration though.

 

if the idle screw can't achieve the correct idle, something is gummed up, or the throttle plate isn't closing fully.

 

the throttle plate is closing fully. no click when i engage the pulley, though, also indicating a problem with the throttle switch per bentley?

 

the real test for the throttle idle switch is to measure continuity... should be closed at idle, and open as soon as you get off idle. when the switch is closed, the ECU uses the idle control valve, when the switch is open, it doesn't.

 

so the throttle switch has nothing to do with the idle speed? if I understand correctly, the manual says that adjusting the throttle switch position adjusts the idle speed if the bypass screw doesn't obtain the correct speed.

any thoughts on the acceleration loss/problem?

 

the switch turns on the idle control loop that opens the IAC when the RPMs are below the desired idle speed. thats all. never heard of having to adjust the throttle plate position to achieve idle, although I've certainly seem plenty of cars where this has been mangled due to someone not knowing what they were doing

 

I adjusted the throttle position sensor, getting a click when the throttle opens, so that's good - but car is still idling high, around 900rpm. the bypass screw on throttle body is in as far as I can get it with my hands - should I try channel locks? put silicone lube on it, didn't seem to help.

cleaned IAC but didn't check the valve itself - that required prying back some metal tabs that I didn't want to mess with.

also - found the culprit for the loss of acceleration. loss engine harness grounding bolt on fuel rail. lucky i am the idiot and not the car.

 

alright, the high idle persists. here's what I know:

-throttle butterfly valve is closing all the way at rest
-idle air screw on throttle body is in about as far as I can get it
-high idle only present in P and N

BUT - I think I found the culprit - a faulty IAC. the IAC piston itself functions normally but my resistance readings are off.

resistance measurement between: (3 wire terminal, 3 4 5 left to right)
across 3 - 5 : 20ohms
4 - 3 : 42ohms
4 - 5 : 21.8ohms

whereas the spec is 40ohms across the outer terminals, 3 to 5, and 20ohms across center to each outer, either 4 to 3 or 4 to 5.

what could've caused this failure suddenly? I had no high idle problem nor apparent IAC fault a few weeks ago. I installed a new engine harness and hall effect sensor, and cleaned the throttle body. then the high idle was present. trying to troubleshoot that, i cleaned the IAC and reinstalled. Still present. Now I got the check the piston and resistance measurements and it appears faulty - but does that mean it failed at some point before cleaning the throttle body, if in fact this faulty IAC is what is causing the high idle?

OR could it be that I failed to ground the IAC test lead when adjusting the idle bypass screw, thus shorting out the IAC and creating a high idle situation???

if I look at what changed, I am less inclined to think that the IAC failure is causing it. on the other hand, barring some obscure vac line leak or air leak, i don't know where else to look.

hmm....