Fuel knock

[DMC3165]

Recently I started noticing a slight fuel knock. It only happens when i'm running regular gas (87 octane) and the engine is under a heavy load. So I started running 89 octane and it went away. The weird thing is I've always run regular and never noticed it before. The car has about 56K miles on it right now and I've put every mile on this car since it had 12K and in all those years never noticed anything like that.

Last year I had a major tune up done and it was around six months after that I began to notice the knock. I recently changed the coil and ballast resistor and that only seemed to make it worse. At the time of the tune up I decided to go with OEM style plugs cap and wires and not the high performance Nology set. Not real sure if this has anything to do with it or not.

But like I said if I run the higher octane everything's good. But why would this happen now and what could it be a sighn of?

[Chris Burns]

My 1982 BMW did the same thing:banghead: European cars love high octane fuel!

[Soundkillr]

I have been told ethenol levels are higher in summer, and can contribute to poor fuel economy, as well as spark knock. Something I have noticed on all my daily drivers in summer for the last few years....

[WelmoedJ]

Over here (in Europe) we have two tastes of gasoline euro95 and euro 98.
Their RON (Octane numbers) are 95 reps. 98.

Much higher than the 87/89 Octane you are running.
As Chris mentioned European cars love higher Octane, but 95 is sufficient to make the car run smoothly.
The 98 version only empties your wallet a bit quicker.

With lower the Octane numbers you will have to adjust the spark advance.
So if you do want to run 88, adjust the timing advance to compensate for the loss of "spunk" of the gas.

[WelmoedJ]

If the Octane and Ignition timing is interesting you, you may want to read the following:
http://oldeloohuis.com/octane.html
http://oldeloohuis.com/octane2.html
http://oldeloohuis.com/octane3.html

[Ron]

But like I said if I run the higher octane everything's good. But why would this happen now and what could it be a sighn of?Odds are it is the ignition timing...distributors move (or "slip") and advance mechanisms get "sticky", etc.

Check your base ignition timing AND advance operation:
775 RPM: 13° ± 2°
2000 RPM: ~40°
(WS Manual D:04:01....more at M:01:04)

[WelmoedJ]

But like I said if I run the higher octane everything's good. But why would this happen now and what could it be a sign of?

+1
The higher octane probably ignites closer to the TDC+advance than the lesser 87 octane.
That points towards improper spark advance.

Did you by any chance already read the info I pointed to?
That explains IMO the process sufficiently.

[David T]

I would check the vacuum and mechanical advance.
David Teitelbaum



+1
The higher octane probably ignites closer to the TDC+advance than the lesser 87 octane.
That points towards improper spark advance.

Did you by any chance already read the info I pointed to?
That explains IMO the process sufficiently.

[Spittybug]

Knock = predetonation = too much advance for a given octane rating as others have pointed out.

"Under heavy load" I assume means when you are acceleration quickly. Under these circumstances the mechanical advance is kicking in (roughly linearly between 1000 and 4000 rpm) and the vacuum advance is dropping off like a rock since the amount of engine vacuum to the unit is dramatically reduced. At WOT the total advance will actually be ~15-20* less than it would at high RPM cruising. The vacuum advance is used when the charge inside the cylinders is less dense, thus needing more time for the flame front to move through it. When under load and more gas is flying in with all that air, the charge is denser and the flame front actually travels through it quicker, requiring less advance to complete the burn by the same point in the power stroke.

+1 for what David T. said. Either you've lost/weakened the spring on the mechanical advance and it is flying out too quickly or your vacuum advance arm somehow binds up in the advanced position. While a good idea to check the static advance, chances are you would have noticed this at idle or the car would be hard to start.

[David T]

It can be as simple as the hose is knocked off of the distributer or the hoses are mixed up on the solenoid valve. It can also be the mechanical advance is stuck and needs to be cleaned and lubricated. You will know in minutes after hooking up a timing light. It could also be your local gas station is "watering' the gas! They may have a leak in their tank and you are getting water. Other possibilities include bad ignition wires, worn spark plugs, or a bad coil. An overheating motor will also start knocking before it POPS.
David Teitelbaum