Engine knocking

[JoeR]

Latley my 85 720 has been pinging more than normal, mostly when climbing hills or if its warm out. When I first bought the truck it had 65K miles on it and ran fine with our 87 octane cat piss. I've had it now for almost 4 years and it has 126K on the odo. For the last year and half, I've started to run 89 octane in it which helps the pinging but the engine still pings when I'm climbing or if its hot out. I bought the truck in Norcal and had it up there for three years. I have moved back down to Socal and I'm thinking the warmer average temp has something to do with it. On the flat, the engine runs great but when I start climbing and the engine gets warmer, it starts to ping and I loosed power. I am using the reccomended NGK plugs and there is plently of coolant in the system. I haven't adjusted the valves in a while so that may be the culprit as well. Would using hotter plugs help? Any thought/reccomendations would be helpful!

[JoeCool]

Adjustment of the valves helps a lot. And I imagine CA is running ethanol blended gas, so hotter plugs such as NGK V-power are a good idea as well.

[MikeRL411]

That climb up Hawthorne or Crenshaw Blvd, or Vista del Mar will do that to you!  Check the timing.  The old rule of thumb was to adjust the timing so that the engine just pinged on  an incline, but your description sounds a little beyond that.  If you don't have the tools, Walton Motors at the Penninsula Center does good honest work.

[JoeR]

Yes! I do the Crenshaw climb 4 or 5 times a week, as well as the climb up Hawthorne from PV drive west every day. They can get the engine pretty warm! I've heard good things about Walton's but I have had good experiences with hilltop automotive as well. Thanks for the advice!

[datzenmike]

Stock timing is 3 degrees so check and stick to that.

 

There is a vacuum switch that under heavy load turns off the exhaust side plugs. The distributor automatically advances the timing to compensate for the longer burn time. This was introduced by Nissan to reduce engine noise under heavy load. The noise might be the two flame fronts from both plugs meeting and sounding like detonation.

 

Maybe the switch is stuck or the vacuum line to it is cracked. Might be worth checking out.

[JoeR]

I've checked the timing at its right on. Where should I be looking for this vacuum line?

Stock timing is 3 degrees so check and stick to that.

 

There is a vacuum switch that under heavy load turns off the exhaust side plugs. The distributor automatically advances the timing to compensate for the longer burn time. This was introduced by Nissan to reduce engine noise under heavy load. The noise might be the two flame fronts from both plugs meeting and sounding like detonation.

 

Maybe the switch is stuck or the vacuum line to it is cracked. Might be worth checking out.