I bought a 2012 hs250h a couple of months ago. It had 129,000 miles on it. The car has been great 👍, but when I accelerate with the windows open, in a narrow street with barriers on each side, I can hear a lot of detonation / pre-ignition / knocking coming from the engine. I have 134,000 miles on it and would like to know what I should check and replace to prevent this detonation sound. Anyone with experience on such an issue?

 

It is normal, I encounter similar noises leaving the underground parking garage at work in the HS250h. Especially if the windows are open. I think it is normal noise that is reflected off of the close walls and echoing around to make it sound much louder and worse than it is in open spaces. The alley is an echo chamber. The real test is if the hood is open and the engine running do you hear any weird noises.

I would put a large bottle of Techron Fuel System Cleaner in your gas tank and drive until that tank is mostly gone. It is best poured in at the gas station right before you put the gas in. It will clean the fuel injectors, carbon on the valves etc and should reduce engine noise, improve performance and gas mileage. Repeat once or twice a year. Also check if your engine air filter is clean. They are inexpensive and easy to replace.

Also check your oil level as these engines get much louder when low on oil. Once it is warm, park on a level surface, shut off the car, wait 5 minutes, then check the oil. Pull the dipstick, wipe it clean, then insert it back in and pull it out to get the oil level reading. Some of them do burn more oil with increased mileage. It takes 0W20 synthetic oil.
The perfect time to check it is after you are done filling it with gas.

 

I totally agree with the suggestions 1) change the oil with proper synthetic 0w20, 2) check or replace the engine air filter.

I would just like to briefly disagree with the idea that detonation/knocking sounds are not "normal". They should be looked into.
Any OBD2 scan tool would be able to report the code recorded by the knock sensor. The ECU is supposed to adjust the timing and mixture to eliminate the knock when it is detected. Maybe something is preventing that correction, or the carbon build-up is more than the adjustment can handle?

After checking codes, trying the oil replacement, air filter replacement, (and maybe Techron) I would take it to a mechanic and have them analyze any stored codes, pull the spark plugs for inspection, and maybe inspect the timing chain and VVTI for proper operation.

 

My point was that the noises are not detonation but instead are normal noises that sound louder and strange because you are driving through a small enclosed echo chamber environment with the windows down. I too though there was a problem with my vehicle hearing those noises leaving the parking garage, but never heard them anywhere else. Also when I drove other vehicles to work, if I had the windows down, they too made strange noises that would cause one to think something was wrong with the vehicle.
If really concerned have a shop check for stored codes, note if the mileage is worse or performance worse than normal and check all the stuff mentioned above.

 

Thank you for all of your suggestions; I shall try them. Even if the check engine light is not on, you are saying that the odbc2 scan tool will pull stored codes as it pertains to all of the detonation, right?

 

Yes. Many of the big chain Auto parts stores will check for stored codes for free. It only takes a few minutes to plug into the OBD connector and scan for any stored codes that could be related to any engine etc issue.
If a code is found that does not mean run out and buy the part to replace it. The code is a guide to a system that has an issue, it is not always a guide to what part to replace or repair.
Knowing a code is the first step in a proper diagnosis. To proceed further you need a skilled, educated technician to properly diagnose the problem.