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).


-- Randy Schulkers

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