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> |
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