Knocking under hood on cold start...? - Toyota 4Runner Forum

merlinrunner · 04-30-2019, 11:11 PM

I've got a 2006 4Runner with a 4.7L engine , 132K on the clock. I'm beginning to hear a knocking/tapping under the hood on cold start. Once the engine runs for 4-5 mins the noise stops and I don't hear it the rest of the day. Any ideas what's going on?

AuSeeker · 05-01-2019, 12:51 AM

Quote:

Originally Posted by merlinrunner

I've got a 2006 4Runner with a 4.7 engine , 132K on the clock. I'm beginning to hear a knocking/tapping under the hood on cold start. Once the engine runs for 4-5 mins the noise stops and I don't hear it the rest of the day. Any ideas what's going on?

The V8 is known to sometimes have a "piston slap" when cold, if this is what you're hearing it's not known to be a issue with performance and longevity and shouldn't alarm you.

merlinrunner · 05-01-2019, 11:29 AM

Thanks! Is there anything I can do to further analyze the issue?

BTW, I grew up in Spartanburg...always enjoyed traveling around the Bluffton area, very nice down there.

KrisB410 · 05-01-2019, 11:48 AM

More likely a cracked exhaust manifold. Makes the sound until it heats up, expands, and closes the crack.

CJFONNER · 05-01-2019, 12:34 PM

Search for threads containing “fuel pressure regulator” as well. This is a common and harmless source of cold startup knock.

Sent from my iPhone using Tapatalk

blackoutt · 05-01-2019, 01:00 PM

I've heard of fuel pressure regulator causing this sound as well but there were mixed results with replacement of it, I never replaced mine. 169k miles now and still get occasional random startups with the knocking sound.

CJFONNER · 05-01-2019, 05:17 PM

Quote:

Originally Posted by blackoutt

I've heard of fuel pressure regulator causing this sound as well but there were mixed results with replacement of it, I never replaced mine. 169k miles now and still get occasional random startups with the knocking sound.

Same here, but with 140k now. First noticed at around 70k and no ill effects yet.

Sent from my iPhone using Tapatalk

alia176 · 05-01-2019, 05:40 PM

Quote:

Originally Posted by merlinrunner

I've got a 2006 4Runner with a 4.7L engine , 132K on the clock. I'm beginning to hear a knocking/tapping under the hood on cold start. Once the engine runs for 4-5 mins the noise stops and I don't hear it the rest of the day. Any ideas what's going on?

See if you can isolate the sound location. The exhaust header tick tick sound usually comes out of the passenger front wheel well while the piston slapping sounds come from the engine bay.

AuSeeker · 05-01-2019, 06:07 PM

Quote:

Originally Posted by merlinrunner

Thanks! Is there anything I can do to further analyze the issue?

BTW, I grew up in Spartanburg...always enjoyed traveling around the Bluffton area, very nice down there.

The other suggestions are also a possibility except the cracked exhaust, it would be a ticking sound and usually not that loud unless you're next to a wall, but none are serious issues and shouldn't ever cause you any trouble.

Yes the Bluffton area is a great place to be....if I was retired and not having to work 50 to 60+ hours a week, but all in all I like it here and have a good job, been here since 1974 so I almost qualify as a native.

One thing you might want to get to help pinpoint vehicle noises...a mechanic's stereoscope, you can get one at most auto part stores for around $10 to $15, it helps a lot to know exactly where noises are coming from in trying to diagnose any issues.