Spark Knock (One Cylinder only)

**edfiero1** · 07-17-2019

I have a 2.7 liter 4 banger with TBI injection.
The last few 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 and found 3 of the 4 looked pretty normal and while 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.

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.

I put in spark plugs that were 1 heat range cooler. That did not help the problem. Neither did the valve lash adjustment. Also, I ran a can of Berryman's cleaner thru the vacuum port for good measure, but no change. The engine only has about a 1000 miles on it. So I wouldn't expect a tremendous about of carbon build up.

Why would only 1 plug show signs of knock?? (Its TBI so there is only 1 fuel injector)
Anything else obvious I should be checking?

**PlayingWithTBI** · 07-17-2019

I don't know if this is it but I was have issues with surging and Kcs, it turned out the ICM module (only a couple thousand miles on it) was wigging out and the ECM thought it was seeing higher RPM so it threw more fuel at it. I bought an AC Delco module which fixed all the issues. Could your ICM be throwing spark at that cylinder when it shouldn't?

**edfiero1** · 07-20-2019

I've logged the data from the ECM and the RPM, TPS, AFR, etc all seem believable.

Yesterday I threw a bottle of Royal Purple Octane booster in the tank. This stuff claims to raise the octane 30 points or 3 full numbers. However it didn't appear to make any difference to my spark knock.

The problem is now very consistent. I don't hear any knock during the first 10 minutes of driving. Its only after I come up on 15 minutes and longer that the problem shows up. The coolant is at full temp (180) before the problem starts.
Here is my spark table. Knock occurs primarily in the highlighted area. I've already reduced the advance in this area 10% over what was working fine previously.

Capture.JPG

Any ideas??