DIAGNOSIS:
Vacuum leak under plenum diagnosed by remote vacuum pump
running constantly.
PROCEDURES AND CURE:
Upon first startup of the day, when engine is cold and has
been sitting at least 6 hours, turn the ignition key to ON
but do not start the engine. You should hear the sound of
the remote vacuum pump (located under the headlight on
passenger side). This pump should run for a period and then
shut off, and stay shut off. (The service manual reports
that occasional short periods of running and shutting off is
normal).
*NOTE - if you do not hear the pump run at all, your pump
may be inactive and in need of replacement.
To track down the vacuum leak, disconnect the vacuum line
at the plenum connection and put your finger over the two
open ends. If the pump still runs, you need to check
further.
First, remove the vacuum hoses and plastic tube from the
MAP sensor and carefully inspect the hoses. Sometimes,
minute cracks, even smaller than you can see, will cause
problems, so replacement is recommended. Perform the vacuum
check again. If the pump still runs constantly, you will
have to remove the plenum to find the source.
The service manual recommends disconnecting the negative
battery cable and draining of the coolant for this
procedure. If you are very careful, you don't have to drain
the antifreeze, as long as you leave the cap on the upper
anti-freeze tank (over the A/C stuff). You'll get a little
anti-freeze when you remove the plenum, so be sure to have
rags already around the injector housings (over the heads)
when you pull the plenum. Use the GM Service manual ST-364,
page 6E3-C2-10 instructions to remove the plenum. * NOTE -
Remember that antifreeze stains the silver paint on the LT5
engine, so be certain to rinse with a wet cloth and dry any
spot that gets on the plenum or engine parts immediately. Of
course, you should never wash the top of your LT5 engine
with water or get water into the plenum chamber or starter
motor.
You will need a new set of Plenum gaskets (part
#10168684), MAP sensor hoses (part#10110989) the assembly of
Vacuum spider hoses with check valve (part #10168624). We
recommend replacing the entire assembly since the tubes may
be quite brittle with age. You may also need to replace the
secondary actuator solenoid (part #1999222) which has a
small foam air filter. At least, clean and dry the foam
filter if replacement of the solenoid is not necessary.
Test each component by blocking off the lines at various
spots. The vacuum pump should now run a bit when the
ignition key is first turned on, then shut off for good. If
the pump still runs constantly, check the secondary
actuators and other parts connected to vacuum. If you have a
vacuum gauge and vacuum pump, you can check as you go and
not have to turn the ignition key on each time. While you
are under the plenum, inspect, tighten to torque specs, and
replace any of the items that may show signs of wear - spark
plug wires, oil breather gasket, secondary throttle
actuators.
(*NOTE - correct position of these actuators is
described in another procedure by Gordon Killibrew).
