88 80Q, Runs rough upon starting//possibly resolved

[SJ]

> From: Huw Powell <audi at humanspeakers.com>
> Subject: Re: 88 80Q, Runs rough upon starting//possibly resolved
>
>
> > Turns out the brake booster is bad... Leak changes with pedal
application.
> > Thinking the booster was bleeding completely down and upon startup the
car
> > would run alright for a moment, but the vac demand and leak were too
much
> > for the system.
> >
> > Plausible? Seems to start and run fine with line pulled from booster and
> > plugged.
>
> Yes.
>
> The "not a problem" version of this you sometimes see is the engien
> stumbling (due to leanness) when the brake pedal is pressed just after
> starting.  Rare, but can happen.
>
> The brake booster is a big "vacuum leak" if it is bad.
> Huw Powell

**** Good Point. It's easy to forget that some Audi models have vacuum
booster brakes and not the 'Green Gold' hydraulic system.
Even the hydraulic system car models have engine vacuum operated parts. The
climate control system which works off vacuum. There is a small hard plastic
line that runs to the climate control system. The vacuum operates the
various HVAC flap doors actuators. Its a good idea to remove the hose and
block it off when testing for vacuum leaks . .also test that hose with a
vacuum hand pump to make sure its holding vacuum.

Oh yeah, speaking of vacuum leaks and propane testing, I forgot to mention
in a previous post that applying propane at the air inlet at the grill may
not work all the time. There is a air temp valve at the intake port of the
air filter box. If the engine is cold, the valve is closed and combustion
air is drawn from the heat shroud at the exhaust manifold. You can apply all
the propane you want at the intake hose at the grill and it wont make any
changes to the engine. When I was doing propane testing, the engine was
warmed up already and that air intake valve was open, so that applying
propane at the grill intake hose worked.

This vacuum leak problem reminds me of my trials and tribulations with my 85
Dodge pickup engine. The engine pinged from day one after I bought the truck
used with 40K miles on it.
I had to use the highest octane gas to keep it from pinging. Even then, I
had to use 'Octane boosters' to keep the pinging in check. Retarding the
distributor would fix the pinging, but then the engine had no power
whatsover.
I got desperate and had the heads rebuilt - thinking maybe carbon buildup in
the heads was causing the pinging. The valve stem seals need replacing
anyway.
$300 later, no difference, pinged merrily away. I gave up, thinking it was
just an engine quirk, and lived with the problem for 5 years.
Finally, I read a post regarding pinging on the Dodge truck site that I
belong to -  www.ramchargercentral.com. The lister said to check the vacuum
hose from intake manifold to brake booster.
I removed the hose, cleaned it and lubed it with silicone spray. The
connection at the booster port was just a rubber bung. Pressure from the
rubber against the booster metal was the sealing action. I removed the bung,
cleaned and lubricated the bung and the booster port (metal hole).
Installed the cleaned parts, started engine and took it for a test. No
Pinging!
I gassed up with 87 octane gas, advanced the distributor timing to what it
was supposed to be, and took it out on the highway . .no pinging . .problem
solved.

Enough of nostalgia. I gottta fix my alternator problem on the 88 5kq. Post
on that issue coming up next.

SJ
85 Dodge PU, D-250, 318, auto
85 Audi 4k - - sold but still on the road
88 Audi 5kq
90 Audi 100q



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