Detonation ping

[MikeB]

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.

[Rednek13x]

Thanks for the reply! That's another thing I forgot to add! When I was running the 87 it would almost always "diesel" when I would turn the ignition off. With the 91 it doesn't do that. I will look over all the vacuum lines and devises that are hooked up. Last I checked the heater valve on the manifold was indeed working as should but I'll check again. I'll check plugs. Thanks again

[68longhorn]

I bought my 86 with 305/700r4 in 2008 at 105k, it currently has 197k and has always pinged for me. I ran 91 when i lived in MD and that helped but not perfect. Timing is 8degrees because less than that I have no power. Since moving to Florida i started running rec89 which is ethanol free and the pinging has gotten much better.

What is the ethanol content where you are? In MD we had 15 percent. Ethanol is a real problem for older vehicles