[V8] engine surging at idle, jerk when decelerating - TPS troubleshooting

[Jeremy Ward]

Sounds like the Throttle Position Sensor to me, specifically the Idle
Switch.  The computer didn't realize you took your foot off the gas and
didn't compensate; this made for a rough transition from lots of fuel to
no fuel.  Did your transmission go into limp mode (stuck in 3rd gear)
with out lighting up all of your PRND321 lights?  This is what happened
to me...  

The TPS is located on the front of the engine and attaches to the post
that comes out the front to the throttle body.  There are 2 Phillips
screws that hold the TPS through the slotted mounting brackets to the
throttle body housing.  These are adjustable (to a certain extent) so
you might get lucky with loosening these screws, rotating the TPS to a
better position, and re-tightening the screws.  You will get a 'click'
sound then the idle switch makes contact at the 'foot off the
accelerator' position.  There is a little lever on the front of the
throttle body that you can use to open and close the butterfly valves,
thus simulating a foot on the gas pedal.  In my case, no matter how far
over I had the TPS adjusted, I didn't get the 'click' unless I pressed
the leaver down to the 'even more closed' position.

2 things were attributing to my problem.  1) The transmission shop
buggered up (bent) the throttle linkage (you have to take the air cover
and air box off to see it) that goes to the cruise control vacuum
diaphragm. 2) The PO had installed a K&N air filter and either
under/over oiled the thing...  My TB butterfly valves were so filthy
with oil and dirt, that the valves wouldn't close all the way on their
own.  I removed the TB (6 Allen bolts) and cleaned it real well with TB
cleaner.

You can verify your problem by pulling the engine codes.  You can use a
laptop, Palm Pilot, or a current-limited LED.  Instructions can be found
for the latter here (very easy to do):
http://www.sjmautotechnik.com/ecufv8.html 

After re-reading your post, I have my money on bent linkage, which would
not only cause the idle switch problem, but also a high idle.  Oh yeah,
and if you need it, a TPS can be found at AutoZone for $67 (or twice as
much from the stealership); they will tell you it is a 'Wells' part, but
mine was a Bosch OEM in a Wells bag...

Welcome to the brotherhood of penniless frustrated V8 nuts.  ;-)

Regards,

- Jeremy


-----Original Message-----
From: kyle leatherwood [mailto:kyle_leatherwood at hotmail.com] 
Sent: Tuesday, February 10, 2004 3:35 AM
To: v8 at audifans.com
Subject: [V8] engine surging at idle, jerk when decellerating

Hello,

Kyle Leathewood here.  I just purchased a 1990 V8 Quattro and it does a 
surging at idle, as well as idles high.  The surging is as if you are 
pressing the gas pedal a considerable bit (1000 RPM to 1500 RPM) and it 
sometimes does it when you are slowing down after driving.  Along with
this 
surging, it does a sort of jerk when you let up off of the gas pedal,
almost 
as if you are lightly tapping the brake each time.  It gets quite
annoying.

I am told the surging and high idle could be due to a recall on the
throttle 
linkage bushing not allowing the throttle to go back to full stop,
however 
the jerking has got me stumped.  Any suggestions would be greatly 
appreciated.

Thanks,

Kyle

90 V8 Quattro
90 Audi 80
85 Jaguar XJS
71 MG Midget
69 MGB

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