Saturday was gorgeous. Short sleeves, top off, I went for a ride in the mountains. The sky was blue, the golden aspen leaves were quivering in a gentle breeze, set against a deep blue sky. Then an old friend paid a visit. As I started rounding a twisty, the engine began bucking. Like it died for half a second, then recovered, died, recovered, etc. If I pushed in the clutch, the engine idled and revved fine. Let out the clutch, bucking started up again. I tried all sorts of RPM/gear combos, still bucking. I pulled over and tried holding RPM constant at various levels; at about 2000 RPM, the speed would oscillate up and down by about 100 RPM, with the pedal rock steady. On previous occasions when I asked about this, people suggested the throttle position sensor might be flaking out. But with or without the TPS connected, the sysmptoms were exactly the same. Finally, on about the third or fourth pull-over, I tried wiggling the fuel injector hoses. THAT WAS IT ! Killed the engine instantly. So I restarted and wiggled much more gently and the engine choked and bucked all over. The problem was something Brant recounted having on his car : the tiny short FI lines that lead from the fuel "rail" to each individual injector have a small amount of bending force on them, and over time develop a fold which can restrict flow. Brant said his car only had problems when hot, and then only intermittently; same for me.

(Skip this paragraph)
The physics geek in me wanted to figure out how this caused oscillations, and what I came up with was this. A slight restriction in the FI hose upstream from the injector and FPR has an effect that depends on flow rate (like the voltage drop across a resistor depends on current), so at low fuel demand, the pressure at the injector is correct. As a load is placed on the engine, the flow rate increases, so the pressure drops by a corresponding amount, and the ECU has no way of knowing this. So the engine starts running lean. Pretty soon, the engine is dying, and either via tach signal or MPS, the ECU compensates by easing up on fuel delivery. Before long, the requested rate is low enough so that the effect of the restriction is small, and the engine is running smoothly again. But the throttle is in the same position, so the RPMs begin to increase, and the whole cycle starts over. This may have nothing at all to do what was really happening, but it gave me something to think about on the way home.

(Start reading again)
When I got home, I picked up a bunch of 5/16" FI hose. I also had a fresh set of injector seals on hand. I replaced nearly all of the FI hose, even though it was probably only the short stubs that lead to the injectors that was causing the problem. On Sunday I went back up into the mountains. Top on, wearing a sweatshirt, I drove through grey mist-shrouded mountains as the temperature kept going down. By the time I got out onto an isolated scenic back road, the windshield was fogging up, so I cracked the window down an inch despite the increasing rain and started to get pretty chilled (no heat or defroster). But the bucking problem seems to be solved (fingers crossed). I turned around, thought for an hour about what would be the best way to warm up (soup ? cocoa ? mexican food ?) and finally ended up stumbling into a great Polish restaurant. Talk about a great warm-you-up cuisine !

Anyway, it'll take several more drives, and maybe some warmer weather to make sure that this was the fix, but I'm pretty sure I fixed at least part of the problem. And that makes for a good day in the life of a 914 owner.

physics major?

QUOTE (type47 @ Oct 10 2005, 02:32 PM)

physics major?

yeah, plus way too many years in grad school, then teaching. And they say nicotine is a hard habit to shake...

Sounds like a story from "Zen and the Art of Motorcycle Maintenance".

Good job.

one of my best friends in HS went to Boulder for graduate school in physics, i suppose that is your school too. well, you could have taught HS physics for 29.25 years..... ummm..like me

I've been wondering how long the crowd around here would take to realize Jeff's background. Or more accurately for Jeff's occupation to become obvious. I can't wait for him to get up to 914 speed. LOL He will probably teach some of us old dogs a few new tricks.

Nothing like a physicist in the neighborhood!

Hey Jeff - When do you want to do ball joints?