Date: Sat, 10 Mar 2012 10:07:49 -0800
Reply-To: Don Hanson <dhanson928@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Don Hanson <dhanson928@GMAIL.COM>
Subject: Slight stumble on "resume throttle" (Digifant EFI van)
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I just finished my winter Snowbird trip with a beautiful drive from the
California low desert through the high deserts and along the east slope of
the Sierras up US 395. About 1300 miles, with nice spring weather. The
van ran almost perfect all winter, other than I've just began to notice a
small stumble or hesitation when the throttle is re-engaged after coasting
at high speeds. This is a Digifant system on an inline VW gas motor (2.0
block).
I've heard mention of a " throttle=stop switch" or a throttle position
switch, not sure the correct term. I suspect this, given the symptoms I am
seeing.
Other than this slight stumble or hesitation as I roll back into the
throttle after a longish coasting deceleration, there are no indications of
other problems. The spark plugs look just right, there is no smoke from
the tailpipe after a long downhill or any other visual indication that the
motor is not right. I can run it right up to about 6000rpm without any
problem and the power is undiminished at any load... The fuel consumption
has remained as normal....with maybe just a slight decrease in gas mileage
(which could simply from the van being full of weight...4600lbs on my
return trip on a ODOT highway scale, full fuel tank) Oil consumption has
not changed. Oil temp and Oil pressure are the same as usual. Starts
normally and idles well.
Here's what the van is doing....When I am running at speed, at normal
operating temperature, and have cause to lift my foot completely, such as
going down a long hill or approaching a signal....(Nevada has these cool
blinking yellow "pre-signals" on their parkways that warn you when a signal
is about to go to yellow and then red)....if I re-apply the throttle again
before I slow all the way down, the van seems to 'catch' just for a second
before it resumes strong throttle response. It's not a radical thing
(yet?) just a slight hesitation as I re-apply my foot to the pedal. If I
*really* take care, just feed in a tiny amount of gas before I ask for real
power, I can avoid that stumble. If the period of 'drop throttle' (when I
am coasting) is short....like say under a few seconds...there is no
hesitation. Last night I saw my first 'backfire'....flame coming out the
exhaust after a long sharp downhill at about 75mph, with a 'jolt' as the
engine 'cleared' I think this indicates there is gas being run into the
motor during deceleration somehow... I have also noticed some very slight
'burbling' from the exhaust as I decelerate....more so when the motor has
just been started....This has always been present on startup somewhat, but
if I am not imagining it, I think it has become a bit more frequent and
sometimes it seems to be happening even when the motor is fully warmed
up. If I coast down to a traffic signal or a stop sign and then go again
after just a second of idling, waiting for the light to change, a 'stop and
go', the throttle responds perfectly..the motor idles normally during the
stop.
The TPS...(throttle position switch?) I see...it is flat, has a small
"leaf spring" device that operates on a post as the throttle body valve is
opened...When I rotate the linkage to the throttle body, that switch
'clicks' softly as I just begin opening the throttle body. Mechanically,
this seems correct.... I know nothing about this device. I'm pretty lame
when it comes to electrical components and the Bentley books show a few
different types of switch. I don't make much sense of it. If I had to
replace it, to "throw parts at it" I wouldn't even be able to identify
which switch I was supposed to order, unless there is a part number
concealed somewhere on the switch...
Am I on the right track here? Anyone (Digifant Guys) care to provide any
input on this problem?
I'll be posting a trip report in a short while.
Don Hanson
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