OK, there's a lot of information you give here, so here goes...vwduud wrote:The last "bug" I am chasing, is an inconsistent idle. It can be as high as 1200 RPM's, and drop to where I have it set, at approximately 750 - 800 RPM's. I initially suspected the Aux Air Valve. After pulling it off, inspecting, and testing it, it is doing it's job just fine. Heats up quickly, and completely blocks air flow within a few minutes. I then suspected the crankcase vent valve, and it's 90 degree elbows and hose, as it makes it's way to the airbox. Sure enough, if I pull the hose from the elbow, at the vent valve, on the breather, and put my finger over the hose, the idle drops to normal (from 1100 - 1200 RPM). This indicated either a leaky 90 degree elbow, (loose fit, and old), or the valve within the breather. Can someone give me some insight toward how this valve is suppose to behave? When rebuilding the engine, I completely cleaned the breather, and this spring loaded valve. Should this valve be completely closed at idle? Does the type 4 need this much suction to overcome crankcase pressure? Again, this may simply be a hose/fitting issue, but I would like to understand more about this, as it could present a sizeable vacuum leak to the air box. Oh, and the oil filler cap does have it's original rubber seal. It is complete, and seems to completely seal the oil filler.
First, let's talk about how one sets the idle speed in the first place. The first thing you have to make certain of is that you do not have any hidden problems that will cause idle issues. Your motor must have good, consistent compression and the valves must be properly adjusted. The ignition system must be in perfect condition and set to proper specs. Your fuel system needs to be in good order, with sufficient fuel delivery (no clogged filter) and at exactly the spec pressure of 29.4 psig. Lastly, the FI system must be all of the correct p/n for your car (see my table at my web page for the p/n's by year) and must be properly installed and adjusted. All hoses must be in good condition, plumbed correctly (see Dave Darling's diagram at the Pelican Parts site) and your injector seals must be in good condition and not leak. Injectors must be clean and in good condition with good spray patterns and matched delivery.
Once you've assured all is in good order, the next step is to set the idle mixture to the proper CO level. For 73-74' 914 2.0L's, this means to set it to 3.0%. Other years have different CO specifications, but 3.0% is a good starting point for all cars. To set the CO correctly, you must either own or have access to a shop-quality HC/CO meter - nothing else will do. Set the CO using the idle mixture adjustment knob on the ECU. This must be done after the car is fully warmed up after 30 minutes or more of driving on the street.
Once the CO is set, the idle is then set to 950 rpm using the air bleed screw on the throttle body. That's it. When it's all working right, you get a stable idle.
In your case, from what I can read, you get a high idle at some point when driving, apparently after a "long" period of driving. Pulling the rubber elbow from the PCV and blocking the port on the elbow drops your idle - that's normal. The PCV valve permits some bypass at idle, therefore when you block it, you reduce the air bleed and you get a lower idle. The PCV valve is designed for very specific flow characteristics so that the flow is low (but not zero) at idle and high at high engine speeds when blowby is greatest.
Just for kicks, I tested mine earlier today. If I set the idle to 1300 rpm, pulling the rubber elbow off of the PCV valve and capping it with my thumb drops the idle to 1000 rpm - normal operation. My idle is normally 1000 rpm, I had to open the air bleed screw first to get it up to 1300 rpm (which I'm guessing is about where yours is when it's "high").
Here are a couple of good web articles on what a PCV valve does and how it's supposed to work:
http://www.hastingsfilter.com/engineeri ... 94-2r.html
http://www.napaechlin.com/cpc7c.htm
http://pergatory.mit.edu/2.75/projects/ ... ummary.pdf
(the last one has some great diagrams of PCV valve design and operation)
You will note that they say that an improperly operating PCV valve will cause idle problems - absolutely true. But when operating properly, you will have good idle stability and characteristics. Checking the PCV is very simple - remove it, clean it, and verify that it opens under positive crankcase pressure (blow through it), and that it closes (blow on the other side). That's it. If it doesn't seal correctly, make certain that there are no deposits or debris on the valve seat that prevent it from closing.
One thing that's not very obvious when playing with the PCV valve is that it actually works in BOTH directions - that is, when there is a high vacuum on it, it's sucked up against the limit stop. You can't generate this much vacuum by sucking, only a constant vacuum source (like your motor) can generate enough vacuum for this condition. When near the limit stop, only a small amount of bypass is permitted. That's why it doesn't look like a big vacuum leak to the motor when at idle conditions. Maximum bypass occurs when there is low manifold vacuum and high crankcase pressure - at high engine speeds under significant load. Note also that during starting, it doesn't act like an air leak, either. That's because during cranking, the engine does not generate sufficient vacuum to open the valve. See the pictures in the third reference above for examples of the behaviors I describe here.
The D-Jet PCV valve is crude in comparison to modern valves with regard to the precision of metering. Perhaps it would make sense to replace it with a modern PCV valve, but it really does work as it should when it's clean and in good condition.
I think the issue here is that your idle is too high to start with - or that you have some other underlying problem - follow the procedure above for setting the idle speed properly to begin with.
Brad Anders
http://members.rennlist.com/pbanders/djetparts.htm
