Guys,

I've got a mostly-stock 2000 Camaro SS (LS1) with a pre-igntion problem. It tends to ping under moderate-to-heavy acceleration. Here's the strange part: It runs great after putting gas in. It will perform great for the first few miles (sometimes 5, sometimes 40).

I know the igntion system uses the knock sensors listen for a pre-igntion condition and retard the timing. The timing is reset when the fuel level increases. So, I'm thinking this must be a problem with the knock sensors. But everything I read on knock sensors says that a bad one will light the SES.

Before I go pulling the intake and replacing the knock sensors and wiring harness, is there anything else this could be? Is there any other part of the system that's reset when the fuel level increases?

In an attempt to anticipate any questions, the car has 105,000 miles on it, and I did a full tune-up at 90,000, which included plugs, wires, and the fuel filter. This problem has been happening since before then. I always run high-quality premium gasoline. I don't have any internal engine mods. The only "engine" mod I can think of is my SLP airbox lid.

Thanks in advance for any insight you can provide!

 

Could be a number of things. I'd suspect fuel first and try another station. No codes so knock sensors, 02's, etc are probably OK. Here's some ideas:

www.gearchatter.com/viewtopic8952.php

 

Clean your MAF sensor.

 

I'd do that and Seafoam the engine internals.

 

Thanks for the suggestions, guys!

I don't think it's the fuel, because it happens whenever or whereever I fill up. I only run Shell Premium, but I use a few different stations. A brand new station opened up down the street, and I've been using the lately, but this problem has persisted for at least a year. At least, that's when I made the connection with the fuel level change.

I had to replace the water pump last night, so I used some MAF cleaner on the MAF and some SeaFoam Deep Creep on the throttle body while I had it off. What's the consensus on SeaFoam these days? Last time, I sucked it in through the brake booster vacuum. I thought I read someone say to pour it in through the PCV in order to get to all cylinders. Any opinion which way works best?

 

I start the engine and let it reach operating temperature. I manually open the throttle and stick a penny inbetween the throttle body cam and the throttle stop (this keeps engine above idle). Disconnect the booster hose and dip it into the can letting the vacuum just barely suck up a little Seafoam. Once the can is 1/3 full, I put the hose all the way in and the engine consumes the rest, running poorly and nearly stalling. Then QUICKLY shut the car off.

 

Yes, one of my thoughts was carbon build up. Another thing to check is the condition of your cooling system, make sure you are getting full flow through the heads. Possible causes could be hot spots, or too much ignition advance, or fuel leaning out- have you replaced your fuel filter recently? If not, worth throwing a few $ at it, for a routine maintenance item any way.

Do you have a friend or mechanic buddy that could put it on a scanner, to see what data is being given to the PCM? See if it is accurately recording the pinging, and pulling timing like it should. Maybe see what temp the PCM sees, and the duty cycle on the injectors, too.

 

Wild ***** - Thanks for the tip on the cooling system.

I replaced the water pump this weekend. While I had the MAF and Throttle Body off, I cleaned them up. This morning, I found a stretch of road where I could open it up, and she seemed to run fine without any knock. I've still got my fingers crossed that it wasn't just a fluke, but one of those seems to have fixed the problem.

 

It was most likely a contaminated MAF sensor. Even with the cleaning it still may not be reading properly, mostly due to age and miles. I would strongly suggest replacing it.