Boost problems

[Ameer Antar]

There are a lot of possibilities here. First check your engine mounts. They 
don't usually go bad very quickly, but are vulnerable to oil and heat on 
the turbo side. When the engine is above 1 bar, it may vibrate a little due 
to the pressures and more if there is any internal imbalance. This 
imbalance could be mechanical or caused by one injector leaking or other 
engine control problem. Check the ECU pressure line for leaks. Also a small 
vac. leak in the intake can cause a lean mixture at high throttle and 
normal mix at low throttle. The vibration may be caused by lean running. 
Are you getting full boost? If so your leak will not be in the intake 
system between the turbo and throttle body. Any leaks in this area will 
reduce full boost. Leaks from the air-flow sensor to the turbo will cause 
lean mixtures. The hard starting hot or cold points to injectors. If you 
haven't changed these in 80k miles or so, you should. I found the best and 
easiest diagnostic tool is an Air-fuel ratio gauge. You can get a cheap one 
for $25 or so from JC Whitney or a nice autogauge for $50. All you do is 
crimp a 3-way splice onto the O2 sensor lead and connect 12V to the gauge. 
It will tell you everything about mixture conditions, under boost, accel, 
decel, whatever. It also shows when the ECU is controlling the mixture when 
you see the mixture cycle on and off. Once you figure out if it is a fuel 
issue, you can decide what to replace instead of having to guess. good luck 
and let us know what you find.



At 12:31 PM 9/25/00, you wrote:
>While driving the '91 200q (all-stock) around town this weekend, I noticed 
>the following problem:
>
>When I "get on it" and the engine develops boost above 1.2 bar, the engine 
>(and entire car) bucks like a bronco(!).  It can be quite disturbing if 
>you don't expect it.  And it's tough driving the car and intentionally 
>accelerating *slowly* :)
>
>I'd suspect vacuum (or actually pressure) leaks somewhere.  I'd think when 
>the engine is running vacuum (<1.0 bar) it doesn't care if it sucks air in 
>from the leak or from the airbox.  Once the turbo pressurizes the system, 
>the air gets pushed out of the leak because it's under positive pressure.
>
>Could this also be related to another problem I've been having 
>recently?  The car is hard to start.  It will take a considerable amount 
>of cranking for it to initially "catch".  Once it does, it idles 
>fine.  Does not seem to matter if the engine is cold or hot.  FWIW, once 
>started, the exhaust smells as if it's running rich.
>
>Any possible causes, or more importantly - solutions?
>
>State inspection is due at the end of this month.  I'm sure my mechanic 
>would love to charge me for some work...  I'd like to "cut him off at the 
>pass".
>
>(Another reason to own a quattro - they can't put the car on the NYS dyno 
>inspection machine because of the AWD.  They can only use the sniffer in 
>the tailpipe.)
>
>-RJM-