Knock at WOT

Kania2k1 · April 19, 2002

I've noticed that if I'm running 87 and even sometimes 89 gas (usually from Mobil), that when I mash the gas, I can hear a knock in first gear. Asked the dealer about it and he said:

"Did you lease it or buy it?" To which I replied "Bought it", he said "Well, if you leased it, go a head and beat the h@ll out of it, but since you bought it, run out the tank and try running 93". My question is, why the heck do they recommend 87 if at WOT it knocks? And my second question is why the majority of car dealer emplyees freakin morons! What the heck sort of dumb@ss question was that, did I lease or buy, why should it matter, I don't want a truck that knocks, and I don't think he would either! Not that I make a habit of ripping off a full throttle acceleration that much, but I was just curious as to why this happens. Has anyone else had this happen? Thanks.

Matt in Albany, OUT

GreenRado · April 22, 2002

Don't have an answer to your question, but a comment about dealer service depts. I've gotten the "lease or buy it?" question also, and given the tenor of the discussion, I could only infer he was wondering why I was so picky if I was leasing it.

Some of these people are morons - and I agree a pi** poor attitude.

W/ respect to your knocking, is it maybe valve train noise, ie pinging? I have had this once or twice and run a couple of tanks of 93 octane, and or fuel injector cleaner, and it hasn't come back. Can't say whether it'll fix yours, but that's where it would start - now if you have a hard, loud knock, that sounds much more serious. my 2 cents.

CMNTMXR81 · April 22, 2002

Couple of questions.

1) Does it knock on the initial throttle tip in (as you are mashing it to the carpet)?

or

2) Do you already have it firmly planted in the carpet and it's knocking?

Without have a scanning tool on it, the knock you are experiencing may be perfectly legit, but I'll get to that in a second.

If it's #1, I have a feeling your EGR system may not be turning on or not doing it's job of injecting a slightly richer air/fuel mixture that was recirculated from the exhaust. The EGR system is injecting this into the incoming air stream pretty much all the time EXCEPT under WOT conditions. If it's not doing it's job, you can experience knock upon throttle tip in, light throttle and mid throttle applications.

If it's #2, it could be something as simple as a vacuum leak, primarily in the PCV system causing it to lean out too much. Other places to look is the MAF. HAve you de-screened it? Have you added a translator? Did you override the IAT sensor? All these could cause incorrect frequencies across the resistors in the MAF. If you overrode the IAT sensor, it could be setting to aggressive of a fuel/ignition curve than what works in reality. I'd also check O2's if you can.

Some knock is normal but that is dependant on INITIAL timing advance. These newer LS1 derived engines like to make the most power between 25-28 degrees of overall timing advance at WOT. If you are getting 25-28 degrees of initial timing there shouldn't be an knock and as such no knock retard. However, if you scan it, and see that you are getting 30+ degrees of initial advance then 6 degrees of knock retard, it is doing it's job by knocking off 6 degrees to bring it back into that 25-28 degrees of overall timing advance at WOT. But I would investigate as to why it's getting soo much initial timing advance.

As to what gas you use, all the Octane rating is, is it's resistance to igniting. Hence, an 87 octance fuel will flare up faster than a 93 will in the compbustion chamber if there are hotspots or heat due to compression. This could be part of your problem. You may have deposits in the combustion chamber that are causing these flareups with the 87. But without pulling the heads, this is just a long shot possibility and I'd only recommend as a last resort after all other avenues have been exhausted.

Hope this gives you a starting point.

MarkD · April 22, 2002

Excellent reply CMNTMXR81!

One thing you did not mention was temperature. I have noticed that Octane levels and timing that are perfectly safe in winter may cause detonation during the summer.

It has been unusually hot in SC this past week. How has it been in your area?

Regarding the dealership employee, they really do their best to conceal the fact that most enthusiasts know more about the vehicle than they do.

CMNTMXR81 · April 24, 2002

Thanks!

Yea, I did forget to mention temp as well as the possibility of the dreaded RFG gas like we get here in the midwest. :angry:

What is the temperature outside as well as the temp on your gauge when you notice this knocking?

Kania2k1 · April 25, 2002

CMNTMXR81, DUDE! You know your sh!t! thanks alot. The knock happens when I mash the peddle to the floor and you feel the trans downshift, then put you into the seat. It only does it for a couple of seconds, if that. It's noting major, because as I mentioned, I've done it maybe 5 times in the 18 months I've had the truck. In regards to the outside temp, I think it was about 45-50 outside and the truck had been running for about 30 minutes. I'll try a couple of tanks of 93 when it gets below $1.70 around here! Thanks for the great advise!

On the dealer dudes, Back in December, I was crusing through the truck lot and a sales slime rolled up to me. I was looking at ont of the 3500 crew cab sunset orange metallic dually's they had($45,000), and I asked him if he knew when the 4 wheel steering was coming out. Needless to say he had no I dea they even had that sort of technology. Thus I left him to ponder the question.

CMNTMXR81 · April 25, 2002

Thanks. Actually I had to go through this when tuning my PCM on the Camaro.

If it only happens slightly when you mash it but goes away, I think you are fine. Some "ping" under that condition is fine especially since it's doing some gear hunting in the transmission.

If it persists or is so loud that you sound like a diesel, that's a different story, but otherwise I think you're ok.