[T3] rough idle at cold start up

[Jim Adney]

On 9 Feb 2014 at 22:06, Dennis Stiefel wrote:

> The only thing about the running part has been going on since we got it
> going. When you first crank it up cold it pufs out black to dark gray
> smoke Idles rough then after about a minute it smooth's out and all is
> well. It was worse when we first put it on the road but got a little
> better after the engine reached over 100 miles. Then last week I done a
> 500 mile oil change and valve adjustment after that the cold rough got
> back worse like it was when with the first 100 miles. 

The smoke is due to oil getting into the combustion chambers and 
burning when the engine is first started. A certain amount of this is 
normal, and a little more common on freshly rebuilt engines, where 
the rings have not fully seated. It can be worse if the car tends to 
be parted on a slope to one side, but mine sometimes do this for no 
particular cause.

I would check a couple of things:

1) Make sure you have not been overfilling the air cleaner. It should 
be filled up to the "step" and no farther. If you put in more, you're 
likely to suck some into the intake. Also, NEVER turn the upper half 
of the air cleaner upside down while you're servicing the air 
cleaner. Turning it upside down, allows oil to drain into the intake 
passage where it will get sucked into the  engine when you next start 
it.

2) Make sure you're not overfilling the crankcase. The dipstick is 
spring loaded so that it always measures off the bottom of the 
dipstick tube. I've seen dipsticks which had gotten bent, so that 
they were stuck "retracted upwards." That would put an extra half 
quart or more oil in the crankcase.

3) If you left out, or mis-installed, the louvered plate under the 
breather stand, this will cause excess oil to be thrown up into the 
breather box. This oil could then be sucked into the intake, 
especially on '72-3 engines with the PCV system.

> I've tested the aar by when it is cold and just been cranked I take
> pliers and squeeze one of the hoses going from it ti the intake and the
> engine just bogs down. Then I do it with it warmed up at operating
> temperature and no change. I guess the aar is working but I wonder if
> it is partly stopped up not letting enough air (but some) in. Has
> anybody seen this? After the first minute or so it's all good. Maybe
> I'll take a video and post it sometime this week to show what its
> doing.   

Forget the pliers trick: That will just ruin the hose. Instead, pull 
off the hose that goes to the air cleaner (at the air cleaner end) 
and cover it with your thumb. That is more effective and 
non-destructive.

When the engine is cold shutting off that air should make the idle 
speed drop. When the engine is hot, this should have no effect. 
That's all you need to do to check the AAR. Efforts to be more subtle 
than this are just a waste of time, as the AAR has absolutely NO 
effect on the engine's performance except at idle.

The AAR has NOTHING to do with richness. It is simply a device to 
give you a little more power when the engine is cold in an attempt to 
hold the idle speed somewhat constant. It's not sophisticated, so the 
cold idle speed always tends to be slightly elevated, which is not a 
problem unless the warm idle speed has already been adjusted too 
high. The air that comes into the IAD via the AAR is 
indistinguishable from that which comes in via the throttle butterfly 
or the idle air passage. The pressure sensor adjusts the fuel 
delivery for whatever air gets in, and doesn't care how that air gets 
there.  

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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