It's usually a rubber plug in one of the rocker covers.
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clockwise
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try your ignition control sensor located in your distributor
According to the Haynes repair manual for Chevy/GMC trucks 1988 thru 2000 the cap should be a 15 psi radiator cap.
It is under the distributor cap. mounted on the base plate and will have electrical connectors hooked to it.
It is bolted to the rear main cap on the bottom of the engine. You will have to remove the oil pan to get to the oil pump.
the cap doesn't have a #1 mark on it.
at 11 Oclock
Same place you check the level.
Look for a plug or cap in one of the rocker covers.
it is under the roter cap.