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)

[porschetub]

Interesting (IMG:style_emoticons/default/idea.gif) ,maybe that valve is designed to operate @ a different pressure and not matched the engine perhaps (IMG:style_emoticons/default/confused24.gif) would be hard to know unless you had a stock one to test against,IMO they need to be working @ the same range.
Maybe further experimentation would yield a good result ?

[Not_A_Six]

Interesting (IMG:style_emoticons/default/idea.gif) ,maybe that valve is designed to operate @ a different pressure and not matched the engine perhaps (IMG:style_emoticons/default/confused24.gif) would be hard to know unless you had a stock one to test against,IMO they need to be working @ the same range.
Maybe further experimentation would yield a good result ?

I don't have quantitative flow data for either the original or new valve. The new one certainly does have different flow characteristics. However, I can say that in my '73 2.0, with plumbing matching Jeff Bowlsby's diagram, that it works great for me and fixed my idle issues mentioned previously. YMMV.

I don't know how DRPHIL914's car is configured, but I suspect that this is the root cause of his leak:

"i had not put the anti flashback valve in like the 73-74 have it shown"

When plumbed like Bowlsby's diagram, the PCV valve sucks air from the crankcase, and makeup air gets back in through the heads via the anti-flashback valve. If you just re-route the oil filler box hose from the airbox to the plenum, and add a pcv valve without the anti-flashback plumbing, you'll create negative pressure in the crankcase due to lack of makeup air at idle/low load. Then at wide-open throttle, you might create positive crankcase pressure if the pcv valve can't keep up with the volume of piston ring blow-by gas.

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