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**MikeB** · 08-23-2017, 10:30 AM

Could be that 91 octane is the only cure. However, it certainly won't hurt to check all your vacuum lines for cracks or loose fitting ends, as well as any vacuum-driven devices such as brake booster and EGR valve. And there are other emissions control devices to which vacuum lines connect, such as as TVS, EFE, TVS and more. Even a small vacuum leak can cause detonation and run-on after the engine is shut off. There's a good section on the operation of the emissions devices in my 82 factory service manual. Bottom line: Also, make sure your heat riser valve is opening when the engine warms up.

Hey, a good start might be checking manifold vacuum at idle when the engine is cold, and when it's warmed up. I would estimate that on a stock engine you should see around 20". That doesn't mean there's not a leak at highway RPM and high load, but it's a start.

The problem is worse on cold days because cold air is denser, causing a already lean condition to be leaner.

Finally, what do your spark plugs look like? If almost white, that tells you the mixture is too lean. What you're looking for is tan to brown.

08-23-2017, 10:30 AM	#2
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,665	Re: Detonation ping Could be that 91 octane is the only cure. However, it certainly won't hurt to check all your vacuum lines for cracks or loose fitting ends, as well as any vacuum-driven devices such as brake booster and EGR valve. And there are other emissions control devices to which vacuum lines connect, such as as TVS, EFE, TVS and more. Even a small vacuum leak can cause detonation and run-on after the engine is shut off. There's a good section on the operation of the emissions devices in my 82 factory service manual. Bottom line: Also, make sure your heat riser valve is opening when the engine warms up. Hey, a good start might be checking manifold vacuum at idle when the engine is cold, and when it's warmed up. I would estimate that on a stock engine you should see around 20". That doesn't mean there's not a leak at highway RPM and high load, but it's a start. The problem is worse on cold days because cold air is denser, causing a already lean condition to be leaner. Finally, what do your spark plugs look like? If almost white, that tells you the mixture is too lean. What you're looking for is tan to brown. __________________ Mike 1969 C10 LWB -- owned for 36 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- converted from 250-six to roller cam 350, Vortec heads -- sold 1981 C10 Silverado LWB, 305, TH350C -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!