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Re[2]: '87CSTQW Engine under load cuts out momentarily.

Based on the replies from Scott, Wallace and Doyt(captured in this email to 
avoid WOB), it sounds like this is not a normal sypmtom to be experiencing - 
I'll check the hoses and diaphram as best as I can visually.  So if I understand
correctly, if the wastgate and related circuit is operating correctly, I should 
not experience an abrupt bucking, but more or less a topping off of power as 1.6
bar is reached and wastgate opens bleeding off a little bit of boost.  Since 
there may be a leak, the fuel pump shuts out as an overboost protection. 

Scott I'll review your site pages again.  What are the most likely hoses and 
components I need to check/replace - my initial going over this engine resulted 
in replacement of some hoses, perhaps I missed one.

Also what kind of vacuem/pressure  should I be seeing at idle on the barometer. 
Are there other places I should check with standard vacuem guage.

*** I've summarized replies below for list reference to this problem 
which is suspect is commonplace in our aging but still lively AUDI 
turbos ***

Thanks for the prompt replys.  Ben

reply from wallace...
Ben -

Glad to hear that your after-run system does work, after all. Just to be 
sure--do you know that the pump is working? It's hard to hear over the 
roar of the fan, and they say that impeller _sometimes_ breaks so that 
the motor runs but the pump doesn't pump. Just a thought.
< Yes I disconnected FAN to make sure I was hearing the pump run>

As for your cutout, that's happening because your car isn't supposed to 
hit 1.5 bar. 1.4 bar is the target boost when you're at wide-open 
throttle (WOT). My car has had the same problem, on and off. The fuel 
pump cuts out because 1.5 bar is considered an overboost condition.

It happens because the wastegate is not opening as it should under high 
manifold pressure. This could be because of a leaky wastegate diaphragm 
or hose attached to it.

- Wallace
  '87 5kcstq 154k
reply from - scottmo@teleport.com...


It sounds like you are hitting the overboost fuel pump cutout. The ECU will cut 
the fuel pump if the boost goes to high ~ 1.6 bar, the instrument panel boost 
gauge, usually responds too slowly, so it may only read 1.5 bar when the engine 
cuts out.

My web site has some info on the 1986-88 5000TQ, and 1989-91 200TQ. Information 
on the ECU system, and boost controls can be found at


Other boost related issues are described at


Scott Mockry

reply from Doyt...

............it is the notorious fuel pump cut out, which comes in at 1.6 bar. A 
designed safety limit feature.

Does the wastegate circuit cut the
>ignition or fuel pump too?  I've been following related posts, but I'd 
>appreciate finding out if this is normal for my stock vehicle in good >tune 
with vacuem hoses in good condition. 

Also, what are the steps to
>rectify this in order of dificulty and expense - eg, cut particular >wire to 
fuel pump relay causing cutout, re-chip, install stronger >wastgates spring, 

............................solder a small resistor between posts 1 and 10 on 
the board of the ECU.

You could try one rated at about 560 or higher ohms. The soldering takes some 

............. As with all owner-based mods, do it at your own risk. 
Not doing the mod incurs very little risk.

  This mod raises the fuel cut off and gives more advanced timing at low RPMs, 
resulting in better performance below the turbo cut-in range. Contact Paul 
Timmerman at  <ptimmerm@mashtun.JPL.NASA.GOV>       for full details. 
He invented the mod and markets an inexpensive kit. He supplies all 
parts and as much hand holding as you need.

BSWANN@arinc.com wrote:
> Greetings,
> I had posted a question awhile back re. the afterrun circuit on my > 
'87CSTQWagon, and now have another.  I hadn't had time to get back as > I have 
been going through the wonderful experience of my wife having a > baby girl.
> To recap. on the afterrun circuit, as it turns out I never had a > problem.  I
had finally taken a good cruise around town on a warm 80+ > degree day to the 
local AUDI dealer to buy the switch I was reluctant > to buy.  When I got back, 
lo and behold - the circuit kicked in and
> fan and pump were running/cycling.  I guess this upholds my theory > that even
when the engine appears to reach normal operating
> temperature and the fan is kicking on, there isn't always enough
> latent heat in the block to get the head temp. to rise enough to make > the 
afterrun kick on.  I replaced the switch anyway, since you can't > return 
electrical parts - it made no difference.
> Onto the next question.  When I'm accelerating and hit 1.5 on the
> barometer, the car bucks as if the ignition or fuel pump is shutting > out 
momentarily.  This is rather aggravating and seems almost
> dangerous especially when passing.  The engine kicks back in when
> boost drops to 1.4 and is insistant to not accelerate so hard - to me 
> it seems that the car is just getting into its power band. This
> happens more as a load condition, not RPM related, although problem is 
> usually noted 3300 RPM on up. The engine has around 137Kmi. Is this
> the waste gate opening up?  Does the wastegate circuit cut the
> ignition or fuel pump too?  I've been following related posts, but I'd 
> appreciate finding out if this is normal for my stock vehicle in good 
> tune with vacuem hoses in good condition.  Also, what are the steps to 
> rectify this in order of dificulty and expense - eg, cut particular
> wire to fuel pump relay causing cutout, re-chip, install stronger 
> wastgates spring, etc?
> I have obtained a set of Bentley manuals for reference, but appreciate 
> other owners experences and BTDT.
> TIA for your replys(direct replys appreciated, as I get the digest). 
> Ben Swann
> '85 4ksq
> '86 5kcstq (rebuilding)
> '87 5kcstq Wagon