Hi everyone,

I have a 2003 Mercury Grand Marquis GS with the 4.6L engine, 122500 miles, and I genuinely love this car. I am hoping to get some guidance on an issue that has been bothering me for a while.

I experience a rattling or knocking noise under load at low RPM, most noticeably during hill climbs and during the 3rd to 4th gear upshift. When this happens, I can see spark advance drop to around 0 degrees. The noise does not occur at idle or when revving the engine in park.

Interestingly, the issue almost completely disappears when I run 91 octane fuel, during colder weather, or on rainy days. However, in warm/hot weather, the rattle is very prominant and will almost always show up under load. Once when it was 100 deg outside, she was rattling all the way on the highway, untill I put 91. It sounds exactly like the following video - https://www.reddit.com/r/MechanicAdv...attling_sound/

So far, I have done the following:

• Cleaned the EGR valve
• Cleaned the IAC
• Cleaned the MAF sensor
• Installed relatively recent spark plugs
• Ran Sea Foam through the system
• Consistently used clean Shell 87 octane fuel
• EGR is working
• Short-term fuel trim ~ +5% on both banks under high load (suspect?)
• Spark Advance ~ 20 deg even when pinging
  

I have also tried checking for vacuum leaks using both a propane torch and brake cleaner, but did not find anything conclusive.

At this point, a vacuum leak or injector spray pattern still seems like the main suspect, but I am running out of ideas. This is my first car, and I take good care of it. I would really like to keep it running well for a long time.

Any help or insight would be greatly appreciated. Thank you in advance.

Update (01/02/2026): The intake insulation between the intake and the engine valley is missing in my car. Is it possible that the intake air temp is getting just high enough to pre-ignite the mix? Just contemplating if this rising temperature during hot days is causing pre-ignition.