pre combustion

[jokken]

my 5spd ls sometimes has a bit of pre combustion when trying to accelerate in 5th at 60ish.

It also happens when I bog down the engine a little bit when I would be better off in a lower gear but I don't downshift.

This happens a lot worse when the arse in the gas station gives me 89 octane gas rather than 93. Or when they put 93 octane on the middle pump and 89 octane on the right pump in the self serve. A bottle of octane booster helpes those situations. Also there was one time that I did get 93 but heard the pre combustion like it was 89. I figured it was just a bad tank of gas, I ran it for a while and then topped off the tank with 94 octane at Sunoco.

I’ve since been through many tanks of real 93 octane gas and I just finished doing a tune up. plugs, all 6 coils, fuel, air and pcv. The problem is better but I still can hear it a little.

how do I start troubleshooting this?

 

[01lssport]

OBDII scan tool, a bad knock sensor could be a possibility for the detonation. It might not be backing any timing out of it.

my 5spd ls sometimes has a bit of pre combustion when trying to accelerate in 5th at 60ish.

It also happens when I bog down the engine a little bit when I would be better off in a lower gear but I don't downshift.

This happens a lot worse when the arse in the gas station gives me 89 octane gas rather than 93. Or when they put 93 octane on the middle pump and 89 octane on the right pump in the self serve. A bottle of octane booster helpes those situations. Also there was one time that I did get 93 but heard the pre combustion like it was 89. I figured it was just a bad tank of gas, I ran it for a while and then topped off the tank with 94 octane at Sunoco.

I’ve since been through many tanks of real 93 octane gas and I just finished doing a tune up. plugs, all 6 coils, fuel, air and pcv. The problem is better but I still can hear it a little.

how do I start troubleshooting this?

 

[jokken]

thanks for the info!
so if the ODBII scanner shows a code wouldn't the check engine light have come on also? or does the check engine light not come on for every code?

also for my own education could you please explain what a knock sensor is and what backing timing out is and when it would happen?

 

[01lssport]

Ive had times were the sensor is going out (not working properly) and not making the check engine light come on, but have a code in the bank.

The knock sensor does just that, senses when detonation is occuring and retards the timing to help stop the pre-combustion (detonation).

 

[jokken]

i see. I will get the car scanned and see what codes come up.

not that I am doubting what your telling me but I just want to ask a few things so I understand it.

Doesn't detonation occur when the the heat of the engine is enough to ignite the gas/air mix without the spark?

or is it when the spark fires too early and the cylinder isn't at top dead center?

or both?

 

[01lssport]

i see. I will get the car scanned and see what codes come up.

not that I am doubting what your telling me but I just want to ask a few things so I understand it.

Doesn't detonation occur when the the heat of the engine is enough to ignite the gas/air mix without the spark?

or is it when the spark fires too early and the cylinder isn't at top dead center?

or both?

both

 

[2001LS8Sport]

Your engine ALWAYS fires the spark prior to TDC. The car just wouldn't run otherwise. But it has to be very precise when it does it. You want the combustion pressures maximized right after TDC...not before. That takes time...and that's why you fire the mixture early. But carbon deposits, etc can also cause detonation just like you stated. They get hot spots and can fire the mixture before you want it to. I would recommend you put some good fuel in the car and take it out and run the crap out of it a couple times and see if that doesn't help blow some of the carbon out of it. These engines are high compression anyway and it doesn't take much to get them into detonation with carbon build up. An engine that is running too hot will also lead to detonation. Like was mentioned, you could have a knock sensor that is not pulling timing from the vehicle. That is VERY easy to detect with the proper scan tools. Just reading codes won't necessarily tell you a thing. You really need to see the data. You will actually be able to see the ECM pull timing. A knock sensor is nothing more than a "tuning" device. (pizzioelectric (sp)????) It will detect detonation by the sound waves they put out. They can be so sensitive that they will pull timing long before you can hear it with your own ears. I know of guys who have installed them in their own systems they have built for their street rods and can actually tune their sensitivity by torque amount and whether or not they use teflon tape to install them. It can be quite a complicated set up. That's why it is extremely important to install them in the exact same spot with the exact torque the factory calls for when you use the factory ECM for control.

 

[jokken]

thanks for more info

sounds pretty technical. and I guess precision is important

i don't run the car all the time. sometimes I won't drive it for a week. But I always do run it hard for some amount time when I do drive it. I want to keep the mileage low so I have a beater for a daily driver (93 dodge spirit, 150K, but that's a different story altogether, lol) In any case I realize I sacrifice certain things by keeping the mileage low and letting the car sit. Could this be one of the sacrifices? is letting the car sit causing or escalating this problem?

 

[1963jim]

my 5spd ls sometimes has a bit of pre combustion when trying to accelerate in 5th at 60ish.

It also happens when I bog down the engine a little bit when I would be better off in a lower gear but I don't downshift.

This happens a lot worse when the arse in the gas station gives me 89 octane gas rather than 93. Or when they put 93 octane on the middle pump and 89 octane on the right pump in the self serve. A bottle of octane booster helpes those situations. Also there was one time that I did get 93 but heard the pre combustion like it was 89. I figured it was just a bad tank of gas, I ran it for a while and then topped off the tank with 94 octane at Sunoco.

I’ve since been through many tanks of real 93 octane gas and I just finished doing a tune up. plugs, all 6 coils, fuel, air and pcv. The problem is better but I still can hear it a little.

how do I start troubleshooting this?

Check The EGR Valve or U have too much carbon build up on the valve/or the valve is not operating properly due to the lack of vacum..

 

[2001LS8Sport]

Check The EGR Valve or U have too much carbon build up on the valve/or the valve is not operating properly due to the lack of vacum..

You know what? That is an OUTSTANDING suggestion! One thing an EGR valve does that most don't understand is reduce the combustion temps. They do this buy introducing basically an inert gas into the combustion chamber (spent exhaust) Since this gas has little or no oxygen left in it, the mixture tends to richen a tad and the combustion temps are cooled and thus reduce emissions that are created from high exhaust temps. It also reduces detonation. Sometimes remarkably so. You have to remember that the ECM is programmed for EGR operation. It compensates for a properly functioning
EGR valve. If the valve is not allowing gasses to flow, your calibration is off. By the way, it is very possible for the EGR valve to move and yet not allow gasses to flow because the port is plugged. It doesn't always toss a code. And if the valve weren't closing, the car wouldn't idle right.

Great suggestion 1963Jim!!