D-Jetronic Idle Stability

[pbanders]

OK, after my recent rebuild, I've finally gotten a setup that idles reasonably well. There's just one more bug I'm chasing and I'm wondering how many other people have the same issue.

After your motor is fully warmed-up (at least 20 minutes of driving), do you have a problem where your idle floats at about 1500 rpm, instead of dropping to 1000 rpm? And can you bring it down to 1000 rpm (and it stays there) while sitting at idle, by putting the car in gear and releasing the clutch just enough to drag the motor down to 1000 rpm?

If your car does this, check one more thing - do you have a PCV valve, and is the hose connected to it going to the air box or to the intake manifold?

My car does this, and as according to the Porsche Workshop Manual and other published vacuum hose diagrams, my PCV is connected to the intake manifold. My PCV is new and in good shape. What I think is happening is that as the engine speed drops from higher RPM's, the PCV throttles too much air to the intake manifold, acting like a secondary aux air regulator. If you can get it past the threshold where it holds the idle at 1500 rpm, then the flow through it stays low and the idle stays at 1000 rpm.

You can read my earlier analysis of the PCV system at this link:

http://members.rennlist.com/pbanders/PCV.htm

I'm going to try replumbing it this weekend, connecting it to the air box instead, and seeing if that eliminates the behavior I descirbe above. It also may be possible to put some form of flow restriction into the PCV line (short section of decreased diameter) to limit the flow such that the idle hanging is eliminated.

From a pollution control perspective, it's advantageous to scavenge the crankcase under idle and part load conditions, when manifold vacuum is high or relatively high. If you connect the PCV to the air box, the system becomes passive under those running conditions.

From what I've heard, 75-76 D-Jet cars lacked a PCV and the breather was connected directly to the air box. Perhaps Porsche encountered the same problem with idle stability and solved it by eliminating the active system and going to passive crankcase ventilation.

[pbanders]

I've been thinking about my posting above and it's led me to change some of my ideas about how the PCV valve is actually metering flow. I'm going to update my document with this info later today.

Clarifying my comments above, I think what's happening is that for virtually all running conditions, the PCV valve is in a state where the disk inside is pressed up against the seat on the intake manifold side, where there are four metering slots that restrict flow through the valve. What's happening when the idle hangs is that as engine speed decreases when you let off the throttle, there's still sufficient crankcase pressure such that you get a significant air flow through the PCV valve and into the intake manifold. From a combination of the way the ECU reacts to this overrun situation, the air flow through the valve, and the engine speed, conditions are such that the idle hangs at about 1500 rpm. Dragging the motor down to 1000 rpm gets you through this point, and the idle stays at the setpoint of 1000 rpm.

[pbanders]

OK, I've updated my PCV document with new information, it's located at:

http://members.rennlist.com/pbanders/PCV.htm

[porsche914blue]

Hey Brad,

Anything new regarding the Decel valve? Do you need any other tests done?

[pbanders]

Still working on the decel valve thing, stay tuned, thanks for the offer.

[magaoidh]

Brad,very impressive stuff,I have been having a problem with idle stability and I thought it was fluctuation between rich and lean which is what the the PCV valve appears to be doing(as you have explained).Have you tried connecting to the air filter rather than the manifold?Probably the fastest way to check is the 75/76 straight connection to the manifold.
Regards Geoff

[RustyWa]

Brad,

Have you conversed with Ray Greenwood over on ShopTalkForums.com? He frequents the Fuel Injection section quite a bit.

[pbanders]

Yes, I've conversed with Ray in the past.

Well, there's nothing like good, solid, experimental results to blow a good theory to bits. I tried swapping the PCV hose between the intake manifold and the air box - no effect on the hanging idle, still there. Next thought was that it might be due to my vacuum retard hanging - checked that by pulling the dizzy cap and using my hand vac pump to check the action of the retard cell - smooth as silk and at the vacuum levels I expected. Nothing wrong with the advance cell, either. Ahh, the search continues....

[Dave at Pelican Parts]

Sticking centrifugal advance? Have you tried checking the timing when the idle is "hanging" versus when it isn't? The problem with that test is that the "hanging" idle might be high enough to start getting into the centrifugal adance anyway, but you might be able to detect that there is "too much" advance for the RPM, which would mean that the advance is sticking...

Does it do the same thing with the advance and retard hoses plugged? How about with the Decel Valve hoses plugged? Or the AAR ones? Or...

--DD

[sean_V8_914]

I gave up on that issue and call it good. it only does it from time to time. does yours do this consistently?

[Demick]

Brad

This is typical behavior for a car with a bad decel valve. I know you have been messing with the decel valve lately (I haven't kept up enough to see what you have been doing). But I would first take the decel valve out of the equation by plugging all of the lines that go to the valve. If your problem goes away - then you need a new decel valve.

Demick

[pbanders]

Demick, thanks, I've completely removed the Decel valve from the system and blocked the lines - still hangs. It was one of the first things I checked.