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:

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

Cheers.

Excellent!

Added to my important bookmarks. Thanks!

You can use "adhesive lined shrink tubing" instead of electrical tape, it won't come off when it gets oil on it. You need to get all of the oil off the fitting before you start. You may need multiple layers to get a tight fit with the hose, so get a size of the shrink tubing that is big enough to slide over itself and the fitting before shrinking it. I have used it on several fittings with good results. If anyone has a better solution please comment.

Bob

That's a great idea. Thanks!

I think a couple layers of non-adhesive shrink wrap might work too given the bulge on the end of the nipple.

I'm a bit bothered by the kludginess of the electrical tape. But, other than the fact that it shouldn't belong on a proper Porsche, I wonder if there are any real long term problems that it might cause. It shouldn't ever unravel with the hose attached. Maybe it will eventually degrade from the heat or oil exposure... Anybody know?

I was planning to just insert an inline 11-13mm fitting between the existing 13mm hose and add a short 11mm hose that would connect to the valve. But, I really like your shrink wrap idea.

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.

Click to view attachment

Very helpful and a cheap fix. PCV valve ($1.99) grommet ($3.49) at O'Reilly Auto Parts.

Thanks

Thanks. Good luck. If you try it, please post back how it works for you.

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.

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...

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.

first test result: option#4- immediate pressure issue, idle and big oil leak. removed pvc and put it back to no pvc all other things same, normal perfect idle, have to burn off some oil now.

Phil

BTW that was a big puddle before i soaked it up, and FAST! way too much positive crank case pressure! so i geuss thats a big no for me- back to stock '75 set up.

Hmmm...

Sorry it didn't work for you.

Like I said earlier, I'm not sure what the plumbing on a '75 is supposed to be like. And, I don't know how your particular car was plumbed, or what you've changed to add the PCV.

Out of curiosity, where was it leaking? Valve covers? Under what conditions? (I'm guessing WOT if it was an excess pressure problem...) Can you post a pic of the plumbing?

If the anti-flashback valve is plumbed right (to both heads and the airbox), crankcase pressure shouldn't exceed atmospheric pressure. That's the first placed that I'd look if you still want to experiment.

EDIT: If it was leaking at idle, there might be another possibility: The PCV valve should slightly *reduce* idle crankcase pressure over your prior setup. So, under normal operation, it may slightly change the way that the valve covers mate with the heads. If your leak was at the valve covers, it might just be a gasket problem that was masked before. Or, your heads or valve covers might be warped a little.

Usual disclaimers apply: YMMV. Enter At Your Own Risk. Good luck.

of course all disclaimers understood,
2 things, but the biggest issue is i had not put the anti flashback valve in like the 73-74 have it shown, the 75’s just put the hose to the air box, - so i might try again but putting all hoses and flash back valve in place exactly as your diagram. leaks were not from the valve covers those are tight , so i have to look closer , might be push rod tube seal but i can tell you that before this try my motor was leak free, no leaks - i re torques the heads etc and put all new seals in 2 years ago and had been zero leaks . so i am also i tersted to see where the leak was. but after putting back to stock, no leak again.

Was it leaking at idle, wide-open-throttle, or something else?

Pushrod tubes is interesting. I imagine that it might leak there under both over- and under-pressure scenarios...

On the 75 there is no PCV valve. The oil fill box has the port and the hose just goes to the air filter housing. (vent to atmosphere) Adding a pcv valve there, would potentially increase crank case pressure because there is no vacuum to the pcv valve. I think the pcv valve, if installed at the oil fill box should be piped to an intake manifold vacuum port like the earlier years.
Sorry to hear that this caused a leak. It's always at the weakest sealing point. Hopefully, it's not a difficult fix.
There must be a reason that the manufacturer didn't engineer an pcv valve into this system. May be better left alone.

Interesting ,maybe that valve is designed to operate @ a different pressure and not matched the engine perhaps 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.

Sorry most likely didn't read it all to know you had success,well done,one thing we have in our favour is that this is a very simply system ,remember my E39 BMW was a real bastard to sort when all the plastic bits fell apart and various hoses failed .

*** UPDATE ***

So, the electrical tape didn't last long. Either the heat or the oil caused it to unstick. I took Bob's advice and wrapped the nipple with three layers of 2:1 3/4" to 3/8" heat shrink tubing. I just used regular heat shrink tubing, not the adhesive-lined version he recommended. But, the flare on the nipple appears to hold it very secure. I'll have to check again after some miles to see how it's holding up. The tubing is rated to temps up to 125C, so I think/hope that will be sufficient.

The factory elbow fits nicely over the heat shrink if you wanted a more original appearance, but I think I'm just gonna leave the 13mm hose hooked up directly for the time being.

As to flow rates, a bit of seat-of-the-pants experimentation by blowing through the original and replacement valves suggests that the original one opens fully somewhere around 2-3 In-Hg differential pressure. Although, the original one that I have is pretty old and worn.

The replacement appears to flow less across the entire vacuum range. I wish I could compare it to a known-good original one, but I only have my old worn one that seems to flow too much. However, flow through the replacement appears sufficient as evidenced by the fact that I see no oil residue in the make-up air lines connected to the anti-flashback valve. But, the engine has just been rebuilt, so as the rings wear, it's possible that the replacement PCV valve won't be able to keep up with the increased blow-by. I intend to monitor the setup and see how it goes.

Even at the apparently reduced flow vs the original (worn) valve, I think it should provide more flow than the "passive" setup used on later model years where the oil filler box was just plumbed to the air filter box. YMMV.

Cheers.

I have a good used one if you want to test it.

Thanks for the offer. That's tempting. But, to do it right, I'd need to rig a test fixture, maybe out of welding stuff, to measure the flow at different pressures. Sadly, I have to spend more time on *real* work now (I.e., work that actually results in an eventual paycheck).

More excellent work, this time, on the PCV. For quite a while, replacement PVC valves were available directly from Porsche! I assume this is no longer the case, glad I bought two when I had the chance so that I have a spare. This solution is even better, because it uses a modern PCV that should be available for a while.

@pbanders Thanks, Brad! I consider that high praise from you. Your work was invaluable for this. I hope this all helps somebody down the road. (So to speak...)

@pbanders Thanks, Brad! I consider that high praise from you. Your work was invaluable for this. I hope this all helps somebody down the road. (So to speak...)

As I say, "you can never have too many spare parts", I remembered this topic, and I just ordered both parts from Amazon.

PCV Valve
https://www.amazon.com/gp/product/B000C2ZJK2/

$7.15

PCV Grommet
https://www.amazon.com/gp/product/B000CO78FE

$3.19

You feel good about these? Tested and approved?

After seeing this post, I ordered the PCV valve and the grommet and installed them in my '74 2.0 today. Works like a champ!

Just got them, not tested. First impression is that the flow rate is much lower than the stock PCV valve.

Wondering if anyone has used this pcv without the head vents/flashback valve? Does there need to be a fresh air intake?

If you are running without the head vents I would run one of the later style oil towers and vent directly to the air cleaner with no PCV the way the factory did in later years.