Date: Sun, 15 Mar 2015 11:55:28 -0400
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: Idle Control System Explanation, please?
In-Reply-To: <CAHQe0n5BSnFUJs5-3u6S9oqm=4vRh8VrHRV=bW=ATbg8Hf6j-Q@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
I doubt you really need a throttle body. Unless it is worn to the point the throttle plate is binding or sticking it is probably OK. The system can handle a small amount of leakage at that point. Vacuum leaks become more of an issue near the injectors as a leak can effect a single cylinder making for a real mess.
The throttle switch and cam was updated many years ago to provide better action. They get replaced as a set when needed.
The throttle has almost no effect on actual idle operation. When the throttle switch closes and the airflow-rpm indicates that the engine is being over-run such as coasting down a hill, the injectors are disabled until the engine gets below some preset speed which seems to be around 1,600 rpm for the Digifant, (2.1L) and only 1,300 rpm for Digijet (1.9L). Once the injectors get turned back on the O2 sensor is ignored for some number of cycles, crankshaft revolutions, 25. If the switch is failed open you won't get the fuel shutoff or O2 sensor recovery. Under a high enough vacuum you may get an overly rich mixture which the O2 sensor will try to compensate for which could give a lousy idle recovery. The throttle switch is also used for full throttle enrichment so it should work at both closed and open throttle.
If the switch is failed closed then acceleration will be effected as the injectors get briefly cut out while trying to accelerate.
The idle control system provides a number of functions. It assists with engine starting, warm up idle boost, and idle compensation for various things such as the O2 sensor changing mixture, accessory loads including electrical consumers, power steering and air conditioning compressor compensation. For a vehicle of this era I am amazed it doesn't not have in input for the automatic transmission not in neutral.
There are a few different versions of idle control units out there. Some of them have a different "pulse" frequency. The valves barely hum. For testing you want to measure the duty or dwell cycle. For the system to be in control, the base settings are such that some boost is always needed. The system can only increase air flow-idle speed. So with the valve closed the idle at best should only be ~800 rpm. Many of use set it closer to 850 -900 since the engines feels better and that also seems to help with idle emissions testing. However now if the O2 sensor gets lazy or the engine is warm and we do not have any loads such as battery charging etc. the idle may creep up a bit. For a manual transmissions who cares. For the automatics you won’t detect it once in drive. Now if you start getting idle speeds like 1,100 or 1,200 you should do something.
A common failure of the idle control unit is for the boost circuits to override the feedback. For example turn on the AC and engine starts to idle at 2,000 rpm. Same can happen for warm up and power steering boost.
Most idle issues I find are the result of something besides the idle control system. You need to learn to isolate and test things beginning with the basics. It's all about the process and the idle system is just one small piece.
Dennis
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Kevin Duvernay
Sent: Friday, March 13, 2015 8:52 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Idle Control System Explanation, please?
Posted this on the Samba, but not getting much action there.
Okay, get this:
87 2.1, all stock. Trying to diagnose some minor idle issues. Noticed my idle control valve wasent vibrating with ignition on, instead it was making a very faint whine (could really only hear it with my ear right next to it). So, I put a known working idle control valve in, same thing. Then I swapped in a known working idle control module (behind the right tail
light) and boom, the idle control valve starts vibrating.
However, it didn't really effect the idle of my van at all. If I had a bad idle control module in there that was also causing the idle control valve to malfunction, how could getting both working again possibly have little to no effect on my idle?? just looking for an explanation. Repose off idle does seem to have slightly improved however.
For those curious, my idle issues are: idles slightly high, at or just above 1000. Also has a "pulsing" sound but rpms don't seem to pulse with it. Can only hear it from the inside. Ive got a rebuilt throttle body with a brand new TPS from Gowesty in the mail now, as I highly suspect I have a slightly worn throttle body and I know my TPS isn't perfectly adjusted and is incapable of being properly adjusted.
Anyways, Ill save those questions till after i get the new throttle body and switch in, mostly curious about the module and valve effects on idle (or lack there of??) Is it possible that my finicky throttle switch is somehow not alowing the idle contol module to work as/when it should?
