Crankcase Vent Valve - D-Jet Vacuum Leak

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vwduud
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Crankcase Vent Valve - D-Jet Vacuum Leak

Post by vwduud » Sun Mar 30, 2003 12:44 am

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.

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raygreenwood
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Crankcase Vent Valve - D-Jet Vacuum Leak

Post by raygreenwood » Sun Mar 30, 2003 10:10 am

ALL RIIIIIGHT! After months of ranting....someone finally did what I asked and discovered a major D-jet flaw!...Oh sorry!
That valve is supposed to be closed at idle. It is your PCV valve obviously. It is defective. Here is what happens: It has crankcase pressure on one side, and vacume on the other. It has too much vacume. Between the two, it is opening and closing at RANDOM. There is no input to the ECU about when the the PCV opens. It dumps right into the plenum next to the MPS vacume line. It causes radical fuel mixture changes. The combination of vacume and pressure causes a flutter in the PCV valve. It wears out and destroys the plastic phenolic disc that sits on the spring inside the PCV valve.
The solution: Remove the disc altogether from the PCV. Use a 1/2" plastic marble or something like it, with about a 3mm hole drilled in it, to plug that 12mm line between the PCVand the plenum. This way you get a constantly metered flow of PCV...which you need. It stops the up and down idle, and a lot of the "bucking" syndrome. You will need to readjust the idle downward once you have a constant flow vent. Ray

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vwduud
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Crankcase Vent Valve - D-Jet Vacuum Leak

Post by vwduud » Sun Mar 30, 2003 10:46 am

Would it be possible to open up the PCV valve completely (make it just a fitting), then buy an inline PCV valve from a FLAPS? I have never researched PCV valves (hadn't had a need to). Are they something you can get specs to, if trying to locate a suitable replacement? Also, can the original be repaired? Stronger spring? Revive the seal?

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vwduud
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Crankcase Vent Valve - D-Jet Vacuum Leak

Post by vwduud » Sun Mar 30, 2003 11:18 am

Or, is there any chance that a new PCV valve can be purchased? Or is this NLA?

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Re: Crankcase Vent Valve - D-Jet Vacuum Leak

Post by Guest » Fri Apr 04, 2003 5:29 pm

raygreenwood wrote:ALL RIIIIIGHT! After months of ranting....someone finally did what I asked and discovered a major D-jet flaw!...Oh sorry!
That valve is supposed to be closed at idle. It is your PCV valve obviously. It is defective. Here is what happens: It has crankcase pressure on one side, and vacume on the other. It has too much vacume. Between the two, it is opening and closing at RANDOM. There is no input to the ECU about when the the PCV opens. It dumps right into the plenum next to the MPS vacume line. It causes radical fuel mixture changes. The combination of vacume and pressure causes a flutter in the PCV valve. It wears out and destroys the plastic phenolic disc that sits on the spring inside the PCV valve.
The solution: Remove the disc altogether from the PCV. Use a 1/2" plastic marble or something like it, with about a 3mm hole drilled in it, to plug that 12mm line between the PCVand the plenum. This way you get a constantly metered flow of PCV...which you need. It stops the up and down idle, and a lot of the "bucking" syndrome. You will need to readjust the idle downward once you have a constant flow vent. Ray

well well well......I have been experiencing a problem like this with my 72 square (bucking, and inconsistant idle) though my idle varies by much less. This may be the solution for me as well.

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raygreenwood
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Post by raygreenwood » Sat Apr 05, 2003 1:07 am

Nope...an after market PCV valve will be even worse. Lets see if I can make this clearer: You must understand, the PCV was designed with a carburater in mind. It has little effect on the metering system of the carb.
ANY outside addition of air...other than your foot moving the throttle...or the Aux. air reg. being open in cold weather.....is translated as a throttle movement by the MPS. You get enrichment via the MPS...but not enough extra air...because you have not actually moved the throttle. What this really causes is a flexing of the copper plate in the back of the MPS. It is not really enough change ususally, to upset the barometric chambers. So what it is screwing up is the sensitivity adjustment of the entire fuel mixture stack. Thats why at any given instance off the line, it may take off smooth...and another it may buck or have a flat spot. The PCV valve is random. The ECU has no knowlage or control over it. But you have to have PCV....so make it a constant...but lowervolume flow...and adjust around it. Ray

volksguy
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Post by volksguy » Sat Apr 05, 2003 1:33 am

what years we talking about here would my 71 1.7 fall into this i have the same problems i thought i was dealing with a mps problem maybe this i my problem as well.

so where would i locate this part on mine if its there ??? and how do all of us that have this problem go about fixing it ???

would bucking be considered when you lift off the throttle the car like to i guess buck ?? or is this a cruising or acceleration problem ??

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Bleyseng
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Post by Bleyseng » Sat Apr 05, 2003 11:23 am

The PVC valve is located on the oil breather with a hose attached to it.
Bucking cruising along at 2500-3000 rpms is caused by the TPS being worn or out of adjustment.
Geoff

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raygreenwood
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Post by raygreenwood » Sat Apr 05, 2003 12:08 pm

Bleyseng...yes...thethrottle valve switch is PART of the problem. Correctly adjusting the trottle valve switch, or making a smallmodification to make the whisker contacts more sensitive (less gap) can any times almost eliminate bucking. But...the PCV is also a major source of this "bucking" syndrome. They are actually causing the same thing ,improperly timed enrichment or the lack therof,but by very differnt methods.
The problem with the throttle valve switch is the built in gap bewteen the two pins that the floating whisker switch contacts. If the T-valve is improperly adjusted, the momentary float at part throttle may either leave the wiper arms half on or half off a contact, or leave whisker contacts with not enough tension to be either securely on either the on pin or off pin...so vibration causes intermitant contact. So it either gives an enrichment signal or a fuel cut-off signal at light or part throttle at inopportune times.

The PCV causes the same problem. The main problem is that it enters the plenum less than 1/4" from the vacume port for the MPS. It also is out of time to any functions of the ECU. It is uncontrolled.
If your engine has D-jet and has the oil breather box WITHOUT the mushroom shaped breather vent....and has a 12mm nipple and hose coming from it and going to the plenum.....you have a PCV. The nipple itself is the PCV valve. It is accessed by pulling the D-shaped plastic plug out of the breather from underneath. Ray

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Bleyseng
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Post by Bleyseng » Sat Apr 05, 2003 2:10 pm

Ray, I have yet to try your marble trick, sounds good. I am refering to the standard bucking syndrom that effects the djet car 90% of the time. You have seen it "At 3000rpms my car bucks when I am just cruising, why? All the fixes I have done involved the TPS being worn out or mis-adjusted. I would always try this avenue first then go the PVC fix route second.
Geoff

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raygreenwood
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Post by raygreenwood » Sat Apr 05, 2003 6:06 pm

Bleyseng, the TPS and the PCV both cause various degrees of this problem at any rpm from idle to redline. Some of the later MPS that have the long spring horn, have less sensitivity problems because they have more spring load to the main armatutre stack. The TPS being out of adjustment is immediately noticable. I modify my TPS's to be more sensitive because as you tune D-jet to higher levels, the slop in the TPS's becomes more noticable and no amount of proper adjusting will totally stop the problem that the slop in the whisker switches causes.

The PCV thing is not a wait and see problem. It was defective from the factory. They are all defective. Some of the early 411's did not have a pcv. The late model 412's and some buses came with an oil breather tower that has a steel nipple coming of the side for the 12mm hose...and a constant flow breather vent (that mushroom shaped plastic piece on the breather towers of 1.8L engines). That is the part/ effect you want to duplicate. VW learned that this method of PCV (constant flow) worked better than the spring load method. This same part is also used on several watercooled cars as a valve cover vent for the same purpose
Even if you are not encountering bucking from the PCV valve, by correcting the intermittant operation...you will get much better control over idle mixture adjustment and off the line throttle response, and adjustment of the MPS becomes easier. It also allows opening up of the outer diaphram stop of the copper plate in the MPS by about 1/4-1/2 extra turns. Ths allows better part throttle tuning. Otherwise, you will have to have this adjustment screwed in tighter than necessary in order to control the pulses from the PCV valve. The PCV valve was an afterthought. Its requirement came after the original production of these cars. Its main problem is its adjacent location to the MPS intake line. Ray

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Post by Guest » Sun Apr 06, 2003 8:51 pm

raygreenwood wrote: You get enrichment via the MPS...but not enough extra air...because you have not actually moved the throttle. What this really causes is a flexing of the copper plate in the back of the MPS. It is not really enough change ususally, to upset the barometric chambers.
What you describe is not correct. Under idle conditions, the MPS is at 12 in. Hg of vacuum at a minimum and usually somewhat more. For the full-load diaphragm to move and to unseat the adjuster from the part-load stop plate requires the vacuum to drop to below 5 in. Hg - something that clearly does not happen under idle condtions, regardless of the condition of the PCV valve. 5 in. Hg corresponds to a large throttle angle condition, close to WOT. If you experienced this low a vacuum in your intake manifold while idling, you would immediately stall from the extremely rich mixture that would result. For example, normal idle injector pulse width is about 3 ms. At idle engine speeds and 5 in. Hg of vacuum, the injector pulse width would be about 6 ms, or 100% richer!

Minor fluctuations in the manifold vacuum cause the aneroid cells to respond, affecting the mixture. If your PCV valve is in poor condition, and is either stuck open or closed, or improperly plumbed (e.g. connected to the air box instead of the plenum), then you can expect idle problems. But when in good condition, it works fine and does not adversely affect the idle stability.

Brad Anders

pbanders
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Post by pbanders » Sun Apr 06, 2003 9:22 pm

raygreenwood wrote:Bleyseng...yes...thethrottle valve switch is PART of the problem. Correctly adjusting the trottle valve switch, or making a smallmodification to make the whisker contacts more sensitive (less gap) can any times almost eliminate bucking. But...the PCV is also a major source of this "bucking" syndrome. They are actually causing the same thing ,improperly timed enrichment or the lack therof,but by very differnt methods.
The problem with the throttle valve switch is the built in gap bewteen the two pins that the floating whisker switch contacts. If the T-valve is improperly adjusted, the momentary float at part throttle may either leave the wiper arms half on or half off a contact, or leave whisker contacts with not enough tension to be either securely on either the on pin or off pin...so vibration causes intermitant contact. So it either gives an enrichment signal or a fuel cut-off signal at light or part throttle at inopportune times.
This is not an accurate description of why a worn TPS causes part-load, constant throttle "bucking". Bucking is caused by inadvertant injection pulses stimulated by the TPS due to track wear. As the tracks where the contact wipers wear, they transport bits of contact material into the insulating space betwen the contact fingers. Eventually, enough material is moved so that intermittant contact occurs when holding constant throttle. This contact causes the ECU to add unneeded 1.0 ms acceleration pulses to the normal injection cycle, causing mixture enrichment which affects motor output, causing bucking.

The link below a 60X photomicrograph of a partly worn TPS trace, showing the material transport already underway:

http://members.rennlist.com/pbanders/TP ... s%2003.jpg
The PCV causes the same problem. The main problem is that it enters the plenum less than 1/4" from the vacume port for the MPS.
This may be true in some D-Jet applications, but it is not true in the 2.0L 914 plennum, and I do not believe it is true in the 1.7L plennum. In the 2.0L plennum, the MPS attaches to the nipple on the top of the plennum, and the PCV port attaches on the side of the plennum, at least 3" apart. Regardless, it's not like there is some kind of differential vacuum across the plennum.
It also is out of time to any functions of the ECU. It is uncontrolled.
If your engine has D-jet and has the oil breather box WITHOUT the mushroom shaped breather vent....and has a 12mm nipple and hose coming from it and going to the plenum.....you have a PCV. The nipple itself is the PCV valve. It is accessed by pulling the D-shaped plastic plug out of the breather from underneath. Ray
In a D-Jet 914 with all FI components correct and working well, as well as a good ignition system and decent compression and valve adjustment, the idle is stable and you don't get bucking. There is nothing inherently wrong with the system, it works as it should when all is in order, including the PCV valve.

My car is a totally stock 2.0L with about 40K on the motor. I have done no modifications to the MPS, TPS, PCV valve, or any other part of the FI system. I have replaced the stock ignition with a Crane XR-3000, but the stock ignition works fine (I just got tired of adjusting it for wear). My car starts on first turn, idles cold at 1500 rpm until the AAR closes, then idles stably at 1000 rpm under all operating conditions. There is no magic here, just a matter of making certain that the mechanical, electrical, and fuel systems on the car in good working order and are properly adjusted.

Brad Anders

pbanders
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Re: Crankcase Vent Valve - D-Jet Vacuum Leak

Post by pbanders » Sun Apr 06, 2003 9:26 pm

vwduud wrote:Or, is there any chance that a new PCV valve can be purchased? Or is this NLA?
Automobile Atlanta lists the item as "in stock". For a 2.0L 914, the p/n is 022 115 542.

http://www.autoatlanta.com/model/914cat ... ine11.html

It's part number "4" in the diagram

I have no idea of if they actually have it, or if this part is new or used.

Brad Anders

pbanders
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Post by pbanders » Sun Apr 06, 2003 9:28 pm

Bleyseng wrote:Ray, I have yet to try your marble trick, sounds good. I am refering to the standard bucking syndrom that effects the djet car 90% of the time. You have seen it "At 3000rpms my car bucks when I am just cruising, why? All the fixes I have done involved the TPS being worn out or mis-adjusted. I would always try this avenue first then go the PVC fix route second.
Geoff
Geoff, several things have been identified as causing bucking. First, the classic worn TPS problem we all know about. Next, ignition - more often the cause than you might expect. Lastly, the trigger contact points. In this case, the bucking gets worse at higher engine speeds, apparently due to the contacts "floating" due to wear.

Brad Anders

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