Further idle troubleshooting

[jAndrew]

I've done a few things to get the poor idle right, but still have issues with high and wandering idle on my 89 Syncro Doka with a DJ and Digijet.

I've cleaned the air filter housing and inspected the Idle Control unit, it all looks good.

I removed the throttle body and cleaned with parts cleaner. It looks pretty good and no obvious signs of wear. No loose shaft, no real light showing thru. Replaced the gasket and put it back on. Also did a new throttle cable and return spring. I adjusted it as shown in the tech video with a meter as well.

The ICV doesn't make any noise or vibrate but has some voltage at the connection but if I understand correctly this should be variable? So I took the ICV off and connected to 12 volts. It snapped open. I also cleaned and blew it out.

All this had little effect on idle. The cleaned throttle body is smoother though. Turning the big screw on the TB seems to have little effect, although turning it clockwise a little more than one turn might have decreased it a hundred or so. Engine still idles mostly at 1800 and will dip and come back every 10 seconds or so. A little lower RPM when first started.

So what should be next in diagnosing it? I've read many pages of posts but still not clear on the order of things to do next. Have no desire to throw new parts if not needed but will do what I have to do.
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'89 Syncro Doka

[dobryan]

[Steve M.]

Check out this video from Tencentlife about checking the AFM.


Link

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This free advice is worth exactly what you paid for it.

There are seven days in a week. Someday is not one of them.

[Steve M.]

Try sticking a 1-1/4" vacuum cleaner hose into the exhaust and blowing air back up into the exhaust. This might help you find any leaks giving the O2 sensor false readings.
Hide the vacuum cleaner behind the rear passenger tire so you can hear a leak and not the vacuum cleaner!
_________________
This free advice is worth exactly what you paid for it.

There are seven days in a week. Someday is not one of them.

[jAndrew]

[jAndrew]

[jAndrew]

I also disconnected the hoses from the ICV and blocked the openings in the air box and the other below the ICV but that did nothing.
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'89 Syncro Doka

[Vanagon Nut]

Have you tried disconnecting and bypassing the idle control system?

Neil.
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1981 Westy DIY 15º ABA

1988 West DIY 50º ABA

VE7TBN

[jAndrew]

[zoti]

Does the DJ have an idea control unit behind the rear right tail light?

The signal that the IACV gets is a pulse with modulation (PWM) that the unit sends to it. That means it turns on and off and changes the time is is on in relation to the time it is off.

In idle, it has a PWM of roughly 32%. That means 32% of the time it is on and the rest it is off. This happens at 150hz (150 times a second). So it is fast. Hence, the valve feels like it is vibrating or humming.

When the Idle Air Control Unit behind the rear tail light goes bad, it will do weird things.

If you want to test it properly (the manual has a test you can run on it but it's simple) then you need a scope.

The green shows you the PWM signal the unit is sending out.

[CessnaJon]

I have a known good spare ICV from a MV you can borrow.

I had a similar issue with the fluctuating/high idle a few years back. In the end, I found that the timing was too advanced. This freaked out the ECU I guess. My problem only surfaced with when the AC was turned on.
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1973 Westy (shared birthday)
1990 Westy Multivan
2015 Passat TDI 6sp
2015 Sportwagon TDI 6sp
LR-JET, Gulfstream 159

[Vanagon Nut]

[jAndrew]

[jAndrew]

[jAndrew]

[Vanagon Nut]

Just so we're clear on terms used, by "air box" do you mean the S shaped hose between air filter box and throttle body?

Does the engine idle better (~ 1050 RPM) with just one of the ISV hoses closed off?

Try clamping off just that "T" hose or hose between S shaped hose and ISV.

Don't clamp off the second hose between crank case breather "tower" and S hose.

Neil.

[jAndrew]

Yes, sorry, the ones that go to the S shaped hose.

I'll try clamp just the one and see.

[jAndrew]

Now, with the breather hose back in place and the other hose closed, as well and just the bottom where the T hose, it idles a bit lower at around 950.
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'89 Syncro Doka

[Vanagon Nut]

I'm not sure if a failed oil breather tower (crank case breather) would contribute to a poor idle. Just wanted to leave that part out of the testing for now.

If closing off just the ISV hose brings idle down, no surging, that doesn't mean the fault lies at the ISV. Those idle symptoms could be due to idle control module or, a combination of things.

Tencentlife, along with others, have posted a bunch of info on these parts. I'm not certain of test procedures to determine if the ISV or ICU is at fault. Sometimes, a visual inspection of ICU innards can confirm if that part has failed. There are test procedures shown in Bentley to measure current draw (duty cycle) of the ISV.

Part of the problem with idle can be the order in which someone does the tests and adjustments. They have to be done in the right order.

Neil.

[Vanagon Nut]

Ok so with all hoses installed, if either the T hose or other longer hose connected to ISV are closed, the idle is 950?