Spark Knock

[SkiBoy]

99 outback LS 5.7 350

Knock started out of the blue. Skied twice, let the boat sit for 45 minutes and then the noise started under load.

Lean knock was the consensus from several people. Had a mechanic come out and he said the same thing. Thought it was bad fuel, and running lean.

Rebuilt the carb, new plugs, cap, rotor, changed oil and all filters. Ran all the old gas out of it with the timing retarded. (This kept it from knocking) Lost power, but was able to run the boat out of fuel. Put 25 gallons of 93 octane in and advanced the timing a bit, no knock, more power. Advanced again and the knock came back.

Any ideas or hints? This is driving me crazy. Boat was idling poorly all year, still does. It didn't after new plugs, etc for a while but is again. I pulled the plugs, old and new, and they seem in good shape.

[mmandley]

Heres some info i found for you. Sounds liek you need to check out a few things on the boat to help narrow it down.

Mild detonation can occur in almost any engine and will not cause damage. Prolonged heavy detonation can crack pistons and rings, blow out head gaskets, damage spark plugs and valves, and flatten rod bearings.

Any of the following can cause detonation:

^ Too Much Compression: An accumulation of carbon deposits in the combustion chambers, on piston tops and valves can increase compression to the point where it exceeds fuel octane rating. If a top cleaner fuel additive fails to remove deposits, a new alternative is to blast the deposits loose by blowing crushed walnut shells through the spark plug hole. Otherwise, the head will have to be removed so the deposits can be scraped off.

^ Overadvanced Ignition Timing: Too much spark advance causes cylinder pressure to rise too rapidly. If resetting the timing to stock specifications does not help, retarding timing a couple of degrees may be necessary to eliminate knock.

^ Engine Overheating: A hot engine is more likely to suffer spark knock than one which runs at normal temperature. Overheating can be caused by low coolant, a defective fan clutch, too hot a thermostat, a bad water pump, etc. A buildup of lime and rust deposits in the head and block can also reduce heat transfer.

^ Overheated Air: The thermostatically controlled air cleaner provides the carburetor with hot air to aid fuel vaporization during engine warm-up. If the air control door sticks shut so that the carburetor continues to receive heated air after the engine is warm, detonation may occur, especially during hot weather. Check the operation of the air flow control door in the air cleaner to see that it opens as the engine warms up. No movement may mean a loose vacuum hose or a defective vacuum motor or thermostat.

^ Lean Fuel Mixture: Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor gaskets or fuel injection intake plumbing downstream of the throttle can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors, carburetor jets clogged with fuel deposits or dirt, a restricted fuel filter, or a weak fuel pump.

The air/fuel ratio can also be affected by changes in altitude. A carburetor calibrated for high altitude driving will run too lean if driven at a lower elevation. Altitude changes are generally compensated for on computer cars by the barometric pressure sensor.

A lean fuel condition can be diagnosed by watching for lean misfire on an ignition scope, or by using a four-gas infrared analyzer and watching exhaust oxygen levels. A reading over about 3% to 4% oxygen would indicate a lean fuel condition.

^ Spark Plug Too Hot: The wrong heat range plug can cause detonation as well as pre-ignition. Copper core plugs are less likely to cause detonation than standard spark plugs.

^ Loss of Exhaust Gas Recirculation (EGR): EGR keeps combustion temperatures down, reducing the tendency to detonate. If the EGR valve is inoperative or someone has disconnected or plugged its vacuum hose, higher combustion temperatures can cause pinging.

^ Low Octane Fuel: Burning cheap gas may be one way to save pennies, but switching to a higher grade of fuel may be necessary to eliminate a persistent knock problem.

^ Defective Knock Sensor: The knock sensor responds to frequency vibrations produced by detonation (typically 6 - 8 kHz), and signals the computer to momentarily retard ignition timing until detonation stops. A knock sensor can usually be tested by rapping a wrench on the manifold near the sensor (never hit the sensor itself). If there is no timing retard, the sensor may be defective

[SkiBoy]

Thanks for the reply.

From top to bottom:

Too much compression - I used Sea Foam to clean carbon deposits in the fuel and poured it directly on cylinders, as per instructions.

Advanced spark - timing was retarded to get rid of knock.

Engine runs at operating range of 160 -170

One vacuum hose, no leaks Engine has additional vents for clean air.

Carburetor rebuilt

Altitude not changed

Plugs are to spec


No egr

Running 93 octane only

No knock sensor

The only thing left on this list is the ignition scope for O2 levels.

[kaneboats]

Is it possible the 93 is too high? I think your engine is supposed to run on 87 or 89.

[Mikes]

Did you check the springs on the advance weights in the distributor? If one came off or broke it can cause this to happen.

[SkiBoy]

I changed the cap and rotor but did not check the distributor itself. Not sure how to check the advance weights. Any hints?

The motor was idling poorly all year, and now it fluctuates from 850 to 1100. After a run it sometimes jumps to 1500 before settling back to 900-1000. The Tach also moves around during operation but does not affect actual speed of the boat. No trans slip, no engine rev noise when the tach runs up or drops. I ski a lot, 3-400 times a year usually at 36 mph. I would feel the speed drop or increase while I'm cutting. I don't feel it, but the drivers see the needle move.

Thanks for your help.

[Mikes]

The weights are under the trigger wheel and pickup near the bottom of the distributor. Between the tach moving allover and the spark knock its most likely in the distributor,but not allways. If you really ski that much how many hours are on this boat?

[SkiBoy]

800 hours, and yes I do ski a lot. I live on a small lake in central NY and usually I'm on the water from early May to Late October, skiing 3-4 times a day, on the average 5 days a week.

This boat has been a champ for 12 years. Almost no service problems. Water pump, transmission seals, and a few hoses, and regular maintenance, etc. Nothing else.

I pulled off the rotor and did not see the spring weights. Are they under the wheel that holds the rotor, or below that assembly. Thanks, I'm hoping this is the problem, as I have done just about everything else I can think of.

[Mikes]

If you cant see them it may not have them the springs are very small. This boat may be too new. Grab the distributor shaft and see if it has any play in it. Can you take a pic of the distributor with the cap off? Also are you sure about vacuum leaks,have you checked it with a vacuum guage? Did you change the plug wires with tune-up?

[SkiBoy]

It's an electronic distributor. I just went down to see the mechanic that has been helping me and he asked about any play in the distributor. I'll check it. maybe the modules are bad or the whole thing just is out of whack.

We have not checked any vacuum leak with a gauge, just sprayed around the carb and manifold. Did not change plug wires yet. I'm pulling the plugs now to look at them again.

The odd thing to me is that the knock started so suddenly. All my drivers know the sound the engine is supposed to make, and will shut it down if there is a different noise than normal. I also pull skiers now and then and do check rides alone to see how it is performing. There was not an indication that anything was wrong until the first knock was heard, and it was loud and strong enough to make the driver stop before the boat was up to speed.

Thanks