Date:         Fri, 9 Oct 2020 14:26:52 -0700
Reply-To:     Neil N <musomuso@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Neil N <musomuso@GMAIL.COM>
Subject:      VW engine conversion. Cause of intermittent stall (solved?): air
              intake path length-design?
Content-Type: text/plain; charset="UTF-8"

Hi all.

For an intermittent stall (hopefully solved) on a 1988 Vanagon with
self rebuilt 18,000 mile OBD1 "Jetta" 2.0 inline 4 gas engine. Valve
guides showed wear, valve guide seals were replaced, no other head
work done by machine shop. A PWM type idle control valve controls all
idle air. Idle spec: 840 RPM. I installed a ~ 2.5' long pipe between
MAF outlet and intake bellows to throttle body.

Intermittent issue: as clutch disengaged when coming to a stop,
especially if done at ~ 1200 RPM, idle would dip low, "slowly" bounce
back up, or idle remained low and engine would stall.

My test drive route was nearly the same every day and I could recreate
the stall at the same time/location by decelerating ~ 20 seconds at ~
2500 - 2000 RPM then disengaging clutch at ~ 1200 RPM.

Repairing other issues that masked this stall issue, re-testing
parts/wiring, swapping parts, didn't help. I'm a newb at interpreting
VCDS data but logs taken while driving didn't show anything unusual.
(IAC duty cycle, MAF, etc etc.)

I hesitate to say this, as watch what happens (issue returns! LOL) but
removing the 2.5' pipe thus establishing intake length to OE (and
swapping the MAF) seems to have solved the idle issue; I can't
recreate it and when idle dips, it returns to spec more quickly. Of
note, the air cleaner is currently an open to air Ford air filter
element.

The 2.5' pipe seems to have been the cause; engine running, voltage
readings at replaced MAF seemed ok and VCDS MAF load reports appear to
be similar or same for both MAF's.

To help confirm that the increased length in air intake path was the issue:

while decelerating in gear, and at or near low RPM's, could increased
length (~ 2.5' ) in intake path between MAF and throttle body cause:

- back pressure, poor laminar flow or ? thus cause a rich mix which
causes engine to stall?

- poor post MAF air flow causing MAF to provide inaccurate readings?

- or ?

Possible factors affecting air flow in now removed pipe:

- heat soak into pipe (from muffler) cause air in pipe to warm up
which affects air flow?
- 4" section of pipe is oval shaped to clear PS pump. This shape
reduces laminar air flow?
- during long period of deceleration, back pressure is created in pipe
at lower RPM ?
- during deceleration, un-metered air passes by worn valve guides in head?

I can provide pics of the long steel pipe etc.

Thanks,

Neil.





-- 
Neil n

VE7TBN

1988 Westy 50º ABA swap: https://tinyurl.com/yap5hpwt

1981 Westy 15º ABA swap: https://tinyurl.com/y9n4xob8

VAG Gas Engine Swap Group <http://tinyurl.com/khalbay>