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Old 07-08-2019, 07:10 AM
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Spark Knock (One Cylinder?)

I have a 2.7 liter 4 banger with TBI injection.
The last two or three times I've driven I've encountered spark knock when in 4th gear under load. This is a new problem that I have not encountered previously.

I topped off the tank with Sunoco 93 Octane, and also dialed back the timing via EBL Flash about 10% in the RPM range where I'm hearing the knock. However neither of these things seemed to resolve the problem.

I pulled the plugs last night and found 3 of the 4 looked pretty normal and one appeared to show signs of detonation.

I have a 180 degree thermostat and operating temperature is normal. There is no EGR.

Not sure if this is significant or not, but I've taken a few short drives (around 10 minutes) and don't hear the knock. It seems like it manifests only on longer drives 15 minutes or longer.

Why would only 1 plug show signs of knock?? (Its TBI so there is only 1 fuel injector)

I plan on going down one heat range on my plugs, but other than that, is there something else I should look for?? As mentioned this is a new problem, so not sure why the plug change would be needed.

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Last edited by edfiero; 07-08-2019 at 07:16 AM.
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Old 07-08-2019, 07:59 AM
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Oil getting past rings or valve guides could do it. Oil in the combustion chamber lowers the octane rating of the fuel.

Carbon or coked up oil on surfaces in the chamber can glow and cause preignition..

Was the plug from that cylinder also oily, shiny black glazed?? Or black dry carbon covered??

You could try a clean-out flush....running engine at 2000 rpm or so and slowly trickling about a pint of plain water into the throttle body. You might be surprised how much black crap will come out of the tail pipe. Steam cleans the valves and combustion chambers and piston tops.

Some guys like to do the same with Seafoam.

GM used to have a flush kit for sequential injected engines that hooked to the Schrader valve port on the fuel rail and used that to inject a can of cleaner through the intake tract over a 10-15 minute time span.
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Old 07-12-2019, 06:57 AM
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Thanks EricNova. I'll get some Seafoam (or some Berrymans) and see if that helps.

Some more info....though not really conclusive.

I tried to get a temp gun on all the headers. Each seemed to measure in the 500-600 degree range so I couldn't see that the cylinder with the questionable plug was any hotter or colder than the rest.

I checked my valve lash and found the INTAKE valve on that questionable cylinder to be just a little too tight. I backed it off less than a 1/4 turn and brought it back to where it should be. That didn't seem like enough to me to cause a problem.

Finally I did a cranking compression check. I had 2 cylinders measure right at 170. One was the that which had the bad plug, but the other one measuring 170 had a normal looking spark plug. The other two cylinders were 150 and 160.

Is there anything else I should be checking???
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Old 09-05-2019, 01:09 PM
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Hi Guys,
I'm still looking for input on this spark knock issue.

Since my original post, I've tried using some Royal Purple octane booster which is suppose to raise the octane by 3 full points (so 93+3 = 96). This had no effect. I've also tried reducing my timing around 2000 rpm area where I get the most knock by another 10%. But the problem is still there.

I used some Berrymans cleaner thru one of the vacuum ports. No effect.

Finally I am currently running some Berrymans fuels system cleaner mixed with the gas. Just started this, so no results yet, but tank is still full.

Short of pulling the cylinder head off, is there anything I can do to get a handle on this knock?
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Old 09-06-2019, 10:40 AM
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This is a good candidate for pre-ignition rather than detonation, they sound alike but have different causes. Pre-ignition is caused by something being incandescently hot that it starts a different ignition point and possibly timing of the ignition point from that of the spark plug. Where detonation is the end burn mixture spontaneously igniting under temperature and pressure increases before the flame front gets there.

Usually runnng cleaner through the engine, as you have, eliminates sources of pre-ignition where this has little to no effect on detonation, while increasing fuel octane, reducing timing lead, dropping coolant temp, etc also has little to no effect on pre-ignition. Where detonation is sensitive to increases in fuel octane, reductions to timng and temperature, etc.

Since engine cleaners had no effect, I expect they were unable to eliminate the source of the problem which while could be a persistent chunk of carbon also brings the possibility of a hard part like a damaged fire ring hanging into the bore. It also raises the possibility that this cylinder isn't cooling correctly to where the spark plug or exhaust valve is running too hot.

This can be caused by a failing EGR valve that either doesn't open at cruise which leans the mixture by the default of no exhaust dilution. Or leaks manifold vacuum on the control side which leans the mixture by adding unmeasured air. The fact that it's one not all cylinders reacting can be a quirk of manifold distribution on the TBI system which in that regard has all the vagrancies of a carb's to the cylinders distribution system.

Bogie
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