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Discussion Starter #1
I have a 2007 Grand Prix GT, and I have had issues with a rattling spark knock type sound when accelerating while the engine is cold for quite some time. When I purchased the car in 2015, it had a bad spark knock problem to the point I ran premium grade fuel. Not long after purchase, I replaced the spark plugs and wires. This cleared the issue up to the point I could run on regular octane fuel.

As I mentioned, it now only does this when the engine is cold during acceleration- such as merging onto the interstate. I let off the gas when I hear this, and it returns to normal.
I have actually replaced my spark plugs since the time I mentioned, so the spark plug gap should be correct. I should also mention that I don't have this problem if I fill up with mid grade fuel.

I don't know if it is related, but my engine will also sometimes die immediately after a cold start unless I lightly press the gas pedal. Once it's running, it is perfectly fine. I assume this may be a vacuum issue, but I haven't looked into it much.

Any ideas are appreciated, thanks!
 

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For a vac leak, you can use some tubing as a stethoscope, and move it around all the lines/gasket mating areas to hear any whistling. This could pull un-metered air in and make your car run lean, causing problems when you get on the gas, and not good to do repeatedly.

I'd check your plugs to see if any got smashed while running cheap (reg) fuel. It's a supercharged car, so it's going to need premium to run right, or the knock is just the computer playing catch-up after it's already having an issue. Mid-grade is just pulling half&half from each barrel (reg/prem), so don't waste your money, there.

If your plugs are coming back closed shut, you've most likely got chipped pistons from whoever beat on the car with junk gas or a vacuum leak early on. You can also get a USB borescope camera to look into the plug holes and check the edges of the pistons for issues; you'd see a 1-2" section missing as you look at part of the piston ring. Worst case scenario, but only $15-20 to check. Same goes for a compression gauge to test the consistency/seal of your cylinders.
 

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Discussion Starter #3
For a vac leak, you can use some tubing as a stethoscope, and move it around all the lines/gasket mating areas to hear any whistling. This could pull un-metered air in and make your car run lean, causing problems when you get on the gas, and not good to do repeatedly.

I'd check your plugs to see if any got smashed while running cheap (reg) fuel. It's a supercharged car, so it's going to need premium to run right, or the knock is just the computer playing catch-up after it's already having an issue. Mid-grade is just pulling half&half from each barrel (reg/prem), so don't waste your money, there.

If your plugs are coming back closed shut, you've most likely got chipped pistons from whoever beat on the car with junk gas or a vacuum leak early on. You can also get a USB borescope camera to look into the plug holes and check the edges of the pistons for issues; you'd see a 1-2" section missing as you look at part of the piston ring. Worst case scenario, but only $15-20 to check. Same goes for a compression gauge to test the consistency/seal of your cylinders.
I might need to check for a vac leak. If I have one, its definitely not enough for me to notice without a stethoscope.

It's not supercharged, and the recommended fuel is regular grade fuel, so that's what's puzzling.

I check my spark plugs generally once or twice a year, and the gap is generally pretty close to where it should be. I replaced the plugs in May of 2019 with ACDelco Iridium plugs when three plugs (I believe on the back side of the engine) were a bit more deteriorated than I'd like. I didn't initially check them for this issue, but I saw very little performance change once I replaced them. However, I have never seen the plugs be smashed, but rather a slightly larger gap if anything. It's probably about a good time to check the gaps on those now...

I think it's strange that I have no issue with it if the engine is warm, or had even been ran that day. Makes me wonder if it is a sensor issue (whether a faulty sensor or it is sensing something else), causing the fuel air ratios to be off.

Thanks for your reply!
 

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This is what I use . Plenty cheap enough .

Ant check engine lite codes ?

Where is the knock sensor located ? Check the wiring . If it is defective , a new sensor is not crazy expensive .


Wyr
God bless
 
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