F.I. Flat Spot? Resolved

I posted a while back about a flatspot between 3000 and 4000 rpm. Because of its behavior I assumed it had to be related to the throttle position sensor. I modified a TPS to make it adjustable by simply turning a screwdriver. I thought I had solved the problem because the flatspot was diminished to almost nothing. I thought it was solved, that is, until I was showing off going up a long steep climb at 80+ mph in

3rd, grabbed 4th and flooded the engine!

Back to the drawing board. I had discounted the idea of the airflow meter causing the flatspot even though on stock VW aircooled FI systems that is the #1 culprit. On stock L-Jetronic bus and bug systems, the airflow meter works with electrical contact points brushing across a series of fields on a printed circuit board. It is common for them to actually wear through in a position that relates to normal cruising speed. The symptoms are very clear: You are driving along at "cruising speed" on a flat road with no wind, holding a steady speed and the engine cuts out. If you do not react in any way, just keep your foot steady on the throttle, the engine will stay "not running." If you either depress the gas pedal slightly or back off slightly, away the engine goes once again, as if nothing ever happened. It may be a month of driving before you match that condition perfectly again, or it might be as soon as you settle back in to cruising speed. The solution is a remanufactured airflow meter from Bosche and take it to a mechanic who knows how to adjust L-Jetronic FI systems on aircooled cars and has the proper exhaust gas analyzer equipment to do the job. The alternative is likely what I did when I learned this lesson: a new set of heads!

Back to my flatspot: I assumed because the airflow meter on my engine is the hot-wire type and not mechanical like the stock VW systems, that it would not fail in this manner. I was wrong. It was rather amusing though, as I replaced the airflow meter at the wrecking yard, thirty miles from home. The engine barely ran the first block. On leaving the next intersection, it ran a little better. About a mile down the road it was running pretty well, after a few stops and starts and going up through the gears! I had never actually experienced the "learning" mode of a modern engine management system before (1992 is modern to me!) It never quite got back to normal, so when I got home, I disconnected the battery for a few minutes then went on a test drive. Its old smoothness was back. I now find myself doing second gear starts more often than first gear starts! The big thing, though is that the flatspot was eliminated. It had been the airflow meter all along.

I guess the other moral of the story is that whenever yer showin' off, sumthin' BAD is much more likely to happen than at any other time! I'm relatively certain that there will be only a very temporary behavioral adjustment regarding that, as the next time the opportunity arises for me to pass a bunch of cars up a steepass hill, I'm probably gonna be pushing that pedal to the metal once again! My best (strongest) bus motor was good for 55mph up that same hill, wound up in third gear. It's awfully hard to resist the temptation!

-BH

Reply to
Busahaulic
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That's alot of speed in 3rd gear! What's the engine revving under those conditions? Fred67bug

Reply to
Vw67lives

It'll go to 7200 before the cutout, but I don't go that high normally - just by mistake! (It's not a VW motor, BTW! the post was for the info of people with VW's with L-Jetronics that might have flatspots and for those who have followed my follies and might be interested in the resolution.) 5200 in

4th is 88mph, peak hp and when it passes through the upper 80's, you can feel a definite kick in acceleration!

revving under those

Reply to
Busahaulic

I think red line in 3rd is about 62 mph. I smell a bit of exaggeration. ;-)

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

Depends on the engine. With my experimental 1600 I would downshift at

80MPH for better acceleration when passing someone on the highway. It would pull to 90. This with a bone stock transmission, no freeway flyer, not even 3.88 diff. Yes it was a screamer :) I'm building another one from used parts soon, just because I have parts lying around that would make such a package.

Jan

Reply to
Jan Andersson

in

revving under those

Reply to
Busahaulic

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