PCV Solution Options

[Not_A_Six]

I have a '73 2.0 with stock d-jet, and have struggled for years with idle issues that I suspected were related to the NLA OEM PCV valve. I've finally come up with a solution that fixed the various problems and wanted to share here in the hopes that it may help somebody else down the line.

My engine is now 2056cc with a Webcam 73 cam. But, my idle problems plagued me even when the engine was previously stock. The original PCV had worn so that it flowed more air than it should when "open", and was sticky as well, causing inconsistent airflow. I tried to purchase a replacement, but had no luck both here and via the usual online vendors. I also tried cleaning the one I had several times, but to no avail.

Before anybody shouts "vacuum leaks", I thought the same for a long time. But, I've now completely rebuilt the engine and have replaced every hose, tested every component by pulling vacuums, and finally did a smoke test to confirm that there are no vac leaks. It was the old PCV valve along.

The problems that the old valve caused were:

1) It flowed too much air to get the idle speed below about 1500 rpm without very retarded idle timing helped by vacuum retard from the distributor. (I have a 123Ignition distributor, and one must choose between vacuum retard at idle (e.g. with profile "A"), or vacuum advance at light load (e.g. with profile "1")). For anyone interested, details on the 123 issues can be found here:

http://www.914world.com/bbs2/index.php?act...=2&t=347291

2) The inconsistent flow thru the PCV caused the idle to be correspondingly inconsistent regardless of what other adjustments were made to timing, idle screw, air-fuel mix, etc. Sometimes it would idle high, sometimes it would idle low.

From @pbanders excellent resource:

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

"Porsche used a PCV valve on some 1.7L and 2.0L D-Jetronic engines, mostly in the 1973 and 1974 models. Early 1.7L and late (1975 and 1976) 2.0L engines did not have a PCV valve, and the crankcase breather on the oil filler was connected to the air box."

Indeed, I experimented with connecting the PCV valve to the airbox as above, and it did "fix" my idle problems. But, I didn't want to sacrifice the crankcase scavenging that a properly operating and plumbed PCV would provide.

The solution I finally came up with (again taking a cue from pbanders), was to adapt a modern, readily available PCV valve to the car. This provides apparently similar airflow as stock at high engine loads (low vacuum), and lower airflow than stock at idle (high vacuum). If you try this, you'll likely need to adjust the idle screw on the throttle body to compensate for the reduced flow.

The components I used are as follows:

PCV Valve: Microgard PCV 279 (cross for FRAM FV279) which is a generic valve for a 1990 Toyota Corolla

Grommet: dormanproducts.com p/n 42057 PCV Grommet (cross for Toyota 90480-18180)

The grommet fits the hole perfectly in the 914 breather box. The nipple on the valve is around 11mm diameter, which is a bit too small for the 13mm PCV hose (or elbow) on the 914. I currently just have the nipple on the valve wrapped with electrical tape to size it up to 13mm, but I plan to fit a proper adapter to size down the 13mm hose and connect directly to the valve. I suppose I could also fit the original elbow, but plan to just leave the hose exiting the valve vertically.

Here are the relevant pics:







I hope that somebody may find this useful. Comments are welcome and appreciated.

Cheers. (IMG:style_emoticons/default/beerchug.gif)

[DRPHIL914]

do you still have the hose on the pvc going to air box?

btw i have a 75 2.0 and have never had the pvc per stock, but would like to put this in place, just making sure hose is hooked up correctly for correct result

[Not_A_Six]

do you still have the hose on the pvc going to air box?

btw i have a 75 2.0 and have never had the pvc per stock, but would like to put this in place, just making sure hose is hooked up correctly for correct result

No, I just had it connected to the airbox temporarily as an experiment. It's correctly (for '73) plumbed to the plenum per Jeff Bowlsby's diagram ( @JeffBowlsby ). That was sort of the whole point of this exercise. Plumbing the PCV valve to the airbox along with the hoses from the heads via the anti-flashback valve sort of defeats the whole operation of the PCV system. Though, I think that may be what Porsche did on later model years (maybe your '75...), likely due to limitations of the primitive PCV valves of the day (I.e., too much flow at idle to get reasonable flow at higher load.) The link to pbanders' page in the OP describes the details.

If you try this, please post back and let us know how it worked for you.

Plumbing to the plenum sucks much better. (IMG:style_emoticons/default/biggrin.gif)

[DRPHIL914]

i ordered them and were delivered today. i have not had the anti flashback valve hooked up but i ah w one. the 75’s didn’t have it , but my car had the 75 smog stuff removed when i got it, so i e been thinking about this for a while. i am going to install these as shown on that diagram, will let you know. what should i see,
experience observe?

do you still have the hose on the pvc going to air box?

btw i have a 75 2.0 and have never had the pvc per stock, but would like to put this in place, just making sure hose is hooked up correctly for correct result

No, I just had it connected to the airbox temporarily as an experiment. It's correctly (for '73) plumbed to the plenum per Jeff Bowlsby's diagram ( @JeffBowlsby ). That was sort of the whole point of this exercise. Plumbing the PCV valve to the airbox along with the hoses from the heads via the anti-flashback valve sort of defeats the whole operation of the PCV system. Though, I think that may be what Porsche did on later model years (maybe your '75...), likely due to limitations of the primitive PCV valves of the day (I.e., too much flow at idle to get reasonable flow at higher load.) The link to pbanders' page in the OP describes the details.

If you try this, please post back and let us know how it worked for you.

Plumbing to the plenum sucks much better. (IMG:style_emoticons/default/biggrin.gif)

[Not_A_Six]

i ordered them and were delivered today. i have not had the anti flashback valve hooked up but i ah w one. the 75’s didn’t have it , but my car had the 75 smog stuff removed when i got it, so i e been thinking about this for a while. i am going to install these as shown on that diagram, will let you know. what should i see,
experience observe?

Good luck. Enter At Your Own Risk. (IMG:style_emoticons/default/smile.gif)

I'm not sure what the plumbing on a '75 is supposed to be like. I'd guess that best case is that you'll just have to adjust the idle screw on the throttle body to account for the change in airflow through the valve at idle.

There are lots of spectacular worst case scenarios out there... (IMG:style_emoticons/default/blowup.gif)

[DRPHIL914]

i ordered them and were delivered today. i have not had the anti flashback valve hooked up but i ah w one. the 75’s didn’t have it , but my car had the 75 smog stuff removed when i got it, so i e been thinking about this for a while. i am going to install these as shown on that diagram, will let you know. what should i see,
experience observe?

Good luck. Enter At Your Own Risk. (IMG:style_emoticons/default/smile.gif)

I'm not sure what the plumbing on a '75 is supposed to be like. I'd guess that best case is that you'll just have to adjust the idle screw on the throttle body to account for the change in airflow through the valve at idle.

There are lots of spectacular worst case scenarios out there... (IMG:style_emoticons/default/blowup.gif)

(IMG:style_emoticons/default/blink.gif) (IMG:style_emoticons/default/idea.gif) can anyone else explain why placement of a PCV valve could have a serious or disasterous effect?

[Not_A_Six]

(IMG:style_emoticons/default/blink.gif) (IMG:style_emoticons/default/idea.gif) can anyone else explain why placement of a PCV valve could have a serious or disasterous effect?

These are probably ridiculous scenarios, but if the PCV system -- hoses, valve, anti-flashback, etc. -- is done wrong somehow, you might:

1) Build up combustion byproducts in the crankcase -- corroding stuff, building sludge, messing with the oil properties...

2) Create a fire/explosion risk from a backfire into the throttle body.

3) Create too much negative pressure in the crankcase, sucking stuff past the rings, valve guides, etc.

4) Create a positive pressure in the crankcase, leading to oil leaks at various locations.

5) Throw off your AFR and cause poor running problems (not likely w/ D-Jet).

6) Somehow create idle issues that can't be fixed with the idle screw.

(IMG:style_emoticons/default/blink.gif)