1981 FI Rabbit - Timing Vacuum Advance - Hesitation/Stumble

Basic Information
1981 vw rabbit fuel injected-1.7L-EN engine-with A/C --built in usa--
A westmoreland rabbit --- I am trying to fix the classic stutter/
stumble/hesitation problem only when the engine is warm, or has warmed
up and sat for a short while (i.e. 1/2 to 3/4 hr) and then is started
and driven -- this problem never happens when the engine is cold.
FYI the timing is 3 degrees ATDC, system pressure is 72psi, warm
control pressure is 49psi, cold control pressure is 14psi at outside
temp of 49 degrees, dwell on frequency valve is 45 degrees plus-minus
4 degrees or so. Residual pressure is 37psi after ten minutes --
Grounds all have continuity -- o2 sensor is 6000 miles old. Normal
things replaced, i.e. plugs, plug wires, rotor, distributor cap, fuel
filter -- Timing belt replaced within past 8 months, O-ring on idle
bypass valve replaced with a 6x10x2mm stock o-ring from autozone -- a
little sloppy but it sealed it --Have not done a gas analyzer test nor
a compression test.
The vehicle does not have any vacuum leaks. All the vacuum lines are
perfect. The engine idles well at 850 rpm. Static timing checked and
is OK.
NOW THE PROBLEM: The distributor is the standard bosch unit with a
vacuum cannister with two ports, one for the advance (on the front
which is activated by the ported vacuum on the throttle body) and one
for the retard (on the back of the vacuum cannister activated by the
manifold vacuum). By design when the engine is at idle there is no
vacuum to the distributor's vacuum advance, however as the throttle is
barely opened (i.e. on acceleration), the ported vacuum is at its
greatest and the distributor's vacuum advance is also at its greatest,
causing the timing to advance something in the neighborhood of 10 - 12
degrees. It is at this precise point that the engine stumbles. If
the throttle is opened wider (i.e. 1/3rd or more, the engine takes off
without hesitation, presumably because the ported vacuum is diminished
or substantially lost, causing the vacuum advance to return to its
normal position. This same hesitation can also happen while cruising
as once again the barely opened throttle (the increased ported vacuum
causes the vacuum advance in the distributor to advance the timing
beyond what the mechanical advance caused by the higher RPM's). If
one opens the throttle to a greater degree (not WOT --although this
always fixes the stumble but is not an ideal way to drive in town) the
stumble goes away. To verify that this is the problem or at least the
onset on the hesitation/stumble, I have blocked the vacuum line
leading to the vacuum advance and the car runs perfect.
I have tried three separate vacuum advance units on the distributor
(none of which leak, all of which have a properly operating return
spring, hold pressure and operate the base plate correctly), and i
have the same problem. Beyong diagnosing te cause of the hesitation/
stumble i am out of ideas about how to remedy the problem. I did run
a three foot longer ne to the unit ad it did lessen the stumble but it
did not fix it.
I would appreciate any suggestions re the vacuum advance -- stumble/
hesitation/bucking problem when warm.

Should I reset the timing to 5 degrees ATDC to overcompensate for the
vacuum advance?
Should i be looking for something else that is causing the problem?
Should I leave the distributor's vacuum advance permanently blocked?

Owner of '81, 82, &'83 rabbits and a lot of parts

It might be a fuel delivery problem.
Make sure your fuel delivery plate is correctly adjusted and clean. The
plate should sit at a certain height.
Check your frequency valve with your dwell meter to find out it's reading.

I have found that certain vehicles with 02 sensors will hesitate slightly if
the mixture is too rich or way too lean. I suspect your engine may need a
CO adjustment to lean it up a tad. CW turning of the CO screw will lean it
up AND please go slowly like 1/8 or 1/4 turn and wait for things to adjust.

OR
I will hope that your engine does not have deposits inside that absorb the
fuel as it is trying to enter the combustion chamber. Some good Top Engine
Cleaner could help reduce that. Or some SeaFoam in the gas tank will give
you a slow cleaning process of both the engine and the fuel system.
What do the spark plug electrodes look like? And which plugs are you
running?

"jfruniontown" <rothring (AT) peoplepc (DOT) com> wrote

Basic Information
1981 vw rabbit fuel injected-1.7L-EN engine-with A/C --built in usa--
A westmoreland rabbit --- I am trying to fix the classic stutter/
stumble/hesitation problem only when the engine is warm, or has warmed
up and sat for a short while (i.e. 1/2 to 3/4 hr) and then is started
and driven -- this problem never happens when the engine is cold.
FYI the timing is 3 degrees ATDC, system pressure is 72psi, warm
control pressure is 49psi, cold control pressure is 14psi at outside
temp of 49 degrees, dwell on frequency valve is 45 degrees plus-minus
4 degrees or so. Residual pressure is 37psi after ten minutes --
Grounds all have continuity -- o2 sensor is 6000 miles old. Normal
things replaced, i.e. plugs, plug wires, rotor, distributor cap, fuel
filter -- Timing belt replaced within past 8 months, O-ring on idle
bypass valve replaced with a 6x10x2mm stock o-ring from autozone -- a
little sloppy but it sealed it --Have not done a gas analyzer test nor
a compression test.
The vehicle does not have any vacuum leaks. All the vacuum lines are
perfect. The engine idles well at 850 rpm. Static timing checked and
is OK.
NOW THE PROBLEM: The distributor is the standard bosch unit with a
vacuum cannister with two ports, one for the advance (on the front
which is activated by the ported vacuum on the throttle body) and one
for the retard (on the back of the vacuum cannister activated by the
manifold vacuum). By design when the engine is at idle there is no
vacuum to the distributor's vacuum advance, however as the throttle is
barely opened (i.e. on acceleration), the ported vacuum is at its
greatest and the distributor's vacuum advance is also at its greatest,
causing the timing to advance something in the neighborhood of 10 - 12
degrees. It is at this precise point that the engine stumbles. If
the throttle is opened wider (i.e. 1/3rd or more, the engine takes off
without hesitation, presumably because the ported vacuum is diminished
or substantially lost, causing the vacuum advance to return to its
normal position. This same hesitation can also happen while cruising
as once again the barely opened throttle (the increased ported vacuum
causes the vacuum advance in the distributor to advance the timing
beyond what the mechanical advance caused by the higher RPM's). If
one opens the throttle to a greater degree (not WOT --although this
always fixes the stumble but is not an ideal way to drive in town) the
stumble goes away. To verify that this is the problem or at least the
onset on the hesitation/stumble, I have blocked the vacuum line
leading to the vacuum advance and the car runs perfect.
I have tried three separate vacuum advance units on the distributor
(none of which leak, all of which have a properly operating return
spring, hold pressure and operate the base plate correctly), and i
have the same problem. Beyong diagnosing te cause of the hesitation/
stumble i am out of ideas about how to remedy the problem. I did run
a three foot longer ne to the unit ad it did lessen the stumble but it
did not fix it.
I would appreciate any suggestions re the vacuum advance -- stumble/
hesitation/bucking problem when warm.

Should I reset the timing to 5 degrees ATDC to overcompensate for the
vacuum advance?
Should i be looking for something else that is causing the problem?
Should I leave the distributor's vacuum advance permanently blocked?

Owner of '81, 82, &'83 rabbits and a lot of parts

Thanks for the response!

The throttle plates (butterflys) are as clean as a pin, as is the
throttle chamber (assembly wherein the butterfly's are located). The
actual intake air distributor (THE BIG assembly to which the cold
start valve is attached) is dirty, with what i would assume to be
normal soot.

The dwell on the frequency valve is 45 degrees +/- 4 degrees of
fluctuation.

The plugs are autolight 63, with about 2000 miles on them and they
look good, with uniform gray deposits -- not glazed, not too hot,
not covered with ash or carbon deposits.

I have run two bottles of chevron techron thru it over the last 2000
miles.

I have not used any other engine cleaner. I usually run 87 or 89
octane in it, usually 87. The vehicle gets around 26 mpg. I have
always wondered if this was an indication of anything. good-bad-lean-
rich.

When I got this car about 9000 miles ago, it had been sitting in a
barn for 14 years, and it had a lot of problems with fuel delivery,
i.e. return line obstrucyed, gunk in the gas tank, rust in the tank.
I was changing the fuel filter every 200 miles and the gas in the
filter was brown with a lot of tiny rust particles. That has all been
fixed and now i get an appropriate flow of clean fuel.

I added three more feet of vacum line to the ported vacuum line and
it diminished the stumble, ( went from a bad stumble to a slight
stumble).

Do you have a specific recommendation to what you refer to as an
engine top cleaner.

Is it important to have the vacuum advance.

You probably have crud everywhere the fuel flows. I bought an 84 with
no hood or gas cap. The cylinders had corrosion. I replaced the
injectors twice on that car. Replaced the CPR. Replaced the fuel
distributor. Replaced the engine but that was another story.

I think he asked if you cleaned the crud off the sensor plate which
sits on top of the fuel distributor.

Have you done the flow test for the injectors as per the Bentley?

Have you removed the injector inserts replacing any broken inserts and
using new orings when reseating the injectors? Are you using the
proper Locktite when replacing the injector inserts?

Yes you need the vacuum advance. Your off idle performance suffers
greatly with no vacuum advance.

On 1 Apr 2007 11:17:50 -0700, "jfruniontown" <rothring (AT) peoplepc (DOT) com>
wrote:

Thanks for the response!

The throttle plates (butterflys) are as clean as a pin, as is the
throttle chamber (assembly wherein the butterfly's are located). The
actual intake air distributor (THE BIG assembly to which the cold
start valve is attached) is dirty, with what i would assume to be
normal soot.

The dwell on the frequency valve is 45 degrees +/- 4 degrees of
fluctuation.

The plugs are autolight 63, with about 2000 miles on them and they
look good, with uniform gray deposits -- not glazed, not too hot,
not covered with ash or carbon deposits.

I have run two bottles of chevron techron thru it over the last 2000
miles.

I have not used any other engine cleaner. I usually run 87 or 89
octane in it, usually 87. The vehicle gets around 26 mpg. I have
always wondered if this was an indication of anything. good-bad-lean-
rich.

When I got this car about 9000 miles ago, it had been sitting in a
barn for 14 years, and it had a lot of problems with fuel delivery,
i.e. return line obstrucyed, gunk in the gas tank, rust in the tank.
I was changing the fuel filter every 200 miles and the gas in the
filter was brown with a lot of tiny rust particles. That has all been
fixed and now i get an appropriate flow of clean fuel.

I added three more feet of vacum line to the ported vacuum line and
it diminished the stumble, ( went from a bad stumble to a slight
stumble).

Do you have a specific recommendation to what you refer to as an
engine top cleaner.

Is it important to have the vacuum advance.

Thanks for the interest in the subject matter!

Before I ever started the engine I flushed the tank of the debris
(tarballs. etc,) cleaned the in tank screen and lines in the tank, but
as you know it is impossible to get every speck of rust out. I have
an in-line prefilter installed that filters out the rust specks before
they get into the fuel pump, fouling it ang the parts upstream, i.e.
the check valve, accumulator, etc.

The output of the pump is 24oz in 30 seconds at the fuel return line,
while 30 would be perfect, 24 is acceptable, i think. i have run the 4
corona bottles test and the result revealed one less than adequate
injector which was replaced, a second test revealed that the output
was OK.

I installed new plastic injector inserts, & injector o-rings (the
green ones) when i was eradicating all the vacuum leaks. I did not
use loctite, ( a bottle of hi-temp air sealing loctite costs more than
i paid for the car) and there was no leak present, using the propane,
carb cleaner and Stereoscope ( vinly tubing nserted into my ear) test
at each injector.

The fuel distributor (& air flow metering plate) is as clean as a pin
-- i removed it and cleaned it before starting the engine, and it is
still clean 9000 miles later.

WUR/CPR in this car is right on spec --- original one is in my "83 and
it runs fine. i swaped them out to see if there was a difference.

Put some seafoam in the tank today, given the number of miles i
drive it will take a while to go thru a tank of gas.

Well you took care of the basics then. The issue of the Loctite is
twofold. First to seal up tiny leaks and the second to reduce the
chance of backing out.

The stumble can still be from the air flow plate being gunked up or
the plunger in the fuel distributor binding up a bit.Since the air
flow meter plate is clean that is one less thing to look at. If it
cannot move quickly responding to the throttle changes then you will
have some stumble. The Bentley shows the process to remove the plunger
to clean and inspect.

When you were doing the flow test of the injectors did you do just off
idle, part throttle and wide open? I need to send off the injectors in
my truck. I have a lack of power but my catalytic convertor looks ok
from the input side. They are electronic and I have no way to test. I
did toss them in the ultrasonic cleaner which seemed to help a bit but
that could just be my wallet speaking justiying the purchase of an
ultrasonic cleaner. My truck has a clean head and intake as the head
was rebuilt recently enough. That Techron may take some time to clean
up gunked up valves. About the only time I have seen gunk that worried
me on my VWs is the gunk from the egr clogging my intake but we are
not talking about TDIs today.

On 1 Apr 2007 19:44:26 -0700, "jfruniontown" <rothring (AT) peoplepc (DOT) com>
wrote:

Thanks for the interest in the subject matter!

Before I ever started the engine I flushed the tank of the debris
(tarballs. etc,) cleaned the in tank screen and lines in the tank, but
as you know it is impossible to get every speck of rust out. I have
an in-line prefilter installed that filters out the rust specks before
they get into the fuel pump, fouling it ang the parts upstream, i.e.
the check valve, accumulator, etc.

The output of the pump is 24oz in 30 seconds at the fuel return line,
while 30 would be perfect, 24 is acceptable, i think. i have run the 4
corona bottles test and the result revealed one less than adequate
injector which was replaced, a second test revealed that the output
was OK.

I installed new plastic injector inserts, & injector o-rings (the
green ones) when i was eradicating all the vacuum leaks. I did not
use loctite, ( a bottle of hi-temp air sealing loctite costs more than
i paid for the car) and there was no leak present, using the propane,
carb cleaner and Stereoscope ( vinly tubing nserted into my ear) test
at each injector.

The fuel distributor (& air flow metering plate) is as clean as a pin
-- i removed it and cleaned it before starting the engine, and it is
still clean 9000 miles later.

WUR/CPR in this car is right on spec --- original one is in my "83 and
it runs fine. i swaped them out to see if there was a difference.

Put some seafoam in the tank today, given the number of miles i
drive it will take a while to go thru a tank of gas.

Well you have done a fantastic job so far.

Two things..
1. Did you try setting the dwell of the frequency valve down to 30 degrees?
2. You have to DRIVE IT!!! And driving it hard should help since I still
think the engine might have deposits in it.

I have seen one cylinder head so clogged with buildup that I was shocked the
vehicle made it to me. After a mechanical cleaning of the head at the
machine shop that car, an '81 or '82 Audi 4000, was getting over 32 mpg.

JMHO
If your cylinder head has deposits...............the deposits will absorb
more fuel coming into the intake area especially when the engine is warm and
the fuel mixture is leaner. Setting the valve at 30 degrees and DRIVING IT
should help get things cleaned up quicker. Or I think that link I had
included in a prior post claims that some people slowly pour SeaFoam into
the intake to clean things up almost instantly. I doubt if your fuel system
is clogged since you have tested it well, so maybe the deposits are causing
the last bit of trouble. ;-)

BTW I saved a non-running Scirocco from the junkyard and it cost $1200 to
get it running perfectly. A quick turn of the key and the engine would
start and purr perfectly at all times.

On 1 Apr 2007 19:44:26 -0700, "jfruniontown" <rothring (AT) peoplepc (DOT) com
wrote:

Thanks for the interest in the subject matter!

Before I ever started the engine I flushed the tank of the debris
(tarballs. etc,) cleaned the in tank screen and lines in the tank, but
as you know it is impossible to get every speck of rust out. I have
an in-line prefilter installed that filters out the rust specks before
they get into the fuel pump, fouling it ang the parts upstream, i.e.
the check valve, accumulator, etc.

The output of the pump is 24oz in 30 seconds at the fuel return line,
while 30 would be perfect, 24 is acceptable, i think. i have run the 4
corona bottles test and the result revealed one less than adequate
injector which was replaced, a second test revealed that the output
was OK.

I installed new plastic injector inserts, & injector o-rings (the
green ones) when i was eradicating all the vacuum leaks. I did not
use loctite, ( a bottle of hi-temp air sealing loctite costs more than
i paid for the car) and there was no leak present, using the propane,
carb cleaner and Stereoscope ( vinly tubing nserted into my ear) test
at each injector.

The fuel distributor (& air flow metering plate) is as clean as a pin
-- i removed it and cleaned it before starting the engine, and it is
still clean 9000 miles later.

WUR/CPR in this car is right on spec --- original one is in my "83 and
it runs fine. i swaped them out to see if there was a difference.

Put some seafoam in the tank today, given the number of miles i
drive it will take a while to go thru a tank of gas.

Thanks for the relpies!!!

In the very beginning I pulled the fuel distributor and shot a lot of
carb cleaner into the outlets and inlets, I also let the plunger slip
out (of it's own weight) and cleaned it. I cleaned every part of the
air flow mechanism and relubricated the moving parts, with the result
being that it was and is as clean and pristine as it can be. I also
tried a completely different air box/fuel distributor from a working
'82, thnking that the stumble was the fuel distributor, It was not.
The replacement unit exhibited the same problems. After tyring that I
put the original unit back on, and moved on to the vacuum leaks, which
have been since been eradicated.

30 degrees on the frequency valve, as i understand it, is making the
valve run lean correcting for a rich setting, RIGHT?? which means
that I am making the mixture richer??? clockwise richens up the
mixture. So I need to run rich to help burn up the deposits? I'll
try it, and also the seafoam directly into the throttle body/main
manifold.

Hate to keep asking the question, is the vacuum advance that
important, as it runs great without it.

Are you positive you have the vacuum hoses correct and it is timed
correctly? My 80 Rabbit had that vacuum retard and I never had a
stumble I could recall. If it runs worse with the vacuum hoses
attached it suggests you have the hoses incorrect.

On 2 Apr 2007 20:20:44 -0700, "jfruniontown" <rothring (AT) peoplepc (DOT) com>
wrote:

Thanks for the relpies!!!

In the very beginning I pulled the fuel distributor and shot a lot of
carb cleaner into the outlets and inlets, I also let the plunger slip
out (of it's own weight) and cleaned it. I cleaned every part of the
air flow mechanism and relubricated the moving parts, with the result
being that it was and is as clean and pristine as it can be. I also
tried a completely different air box/fuel distributor from a working
'82, thnking that the stumble was the fuel distributor, It was not.
The replacement unit exhibited the same problems. After tyring that I
put the original unit back on, and moved on to the vacuum leaks, which
have been since been eradicated.

30 degrees on the frequency valve, as i understand it, is making the
valve run lean correcting for a rich setting, RIGHT?? which means
that I am making the mixture richer??? clockwise richens up the
mixture. So I need to run rich to help burn up the deposits? I'll
try it, and also the seafoam directly into the throttle body/main
manifold.

Hate to keep asking the question, is the vacuum advance that
important, as it runs great without it.

On Tue, 03 Apr 2007 10:54:47 GMT, Jim Behning
<jimbehning (AT) doesthisblockpork (DOT) mindspring.com> wrote:

Are you positive you have the vacuum hoses correct and it is timed
correctly? My 80 Rabbit had that vacuum retard and I never had a
stumble I could recall. If it runs worse with the vacuum hoses
attached it suggests you have the hoses incorrect.
I agree with checking timing and vacuum routing. 26 mpg

is very low for this vehicle, i'd expect more like 35.

On 2 Apr 2007 20:20:44 -0700, "jfruniontown" <rothring (AT) peoplepc (DOT) com
wrote:

Thanks for the relpies!!!

In the very beginning I pulled the fuel distributor and shot a lot of
carb cleaner into the outlets and inlets, I also let the plunger slip
out (of it's own weight) and cleaned it. I cleaned every part of the
air flow mechanism and relubricated the moving parts, with the result
being that it was and is as clean and pristine as it can be. I also
tried a completely different air box/fuel distributor from a working
'82, thnking that the stumble was the fuel distributor, It was not.
The replacement unit exhibited the same problems. After tyring that I
put the original unit back on, and moved on to the vacuum leaks, which
have been since been eradicated.

30 degrees on the frequency valve, as i understand it, is making the
valve run lean correcting for a rich setting, RIGHT?? which means
that I am making the mixture richer??? clockwise richens up the
mixture. So I need to run rich to help burn up the deposits? I'll
try it, and also the seafoam directly into the throttle body/main
manifold.

Hate to keep asking the question, is the vacuum advance that
important, as it runs great without it.