Hard knock or thump when cold starting

[W4R123]

Hi everyone,

I have a 99 4Runner SR5. Just a couple days ago my vehicle started to knock and stumble on cold starts. The knocking sound is a very loud thump that sounds like metal on metal slap. Just to clarify, this knock or thump only happens during cold cranking. After giving it a few tries the truck would start and run smoothly. And if I shut the truck off and start it right away after running for a little bit, it would start right up just fine without no knocking or thumping at all.

Can someone explain what could possibly be causing this? Any help would be greatly appreciated, thanks!

 

[reelamin]

Hi everyone,

Can someone explain what could possibly be causing this? Any help would be greatly appreciated, thanks!

More detailed info needed. What is the cold temp, one single noise, multiple times noise, every time it is cranked until it starts, is there a skip in the turning over from starter, never happens the rest of the day, does it make the noise every single time it's started, when did it start, any mechanical changes, etc. etc.

 

[JayRolla]

You can sometimes get a knock at cold startup when the fluid has all settled. Different weight oils can help stop this. Also if oil is old it can be super thin and cause a startup knock.

 

[19963.4lsr5]

Try shooting a video


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[W4R123]

Thanks guys. This morning after letting the truck sit for about a day, I tried shooting a video while starting the truck. To my surprise it came to life with no hiccups! I went around to the back and noticed a ton of white smoke pouring out of the exhaust. I think I could smell gasoline in it as well...

My guess is leaky injectors? Bad head gasket?

The knock and thump could it be from misfiring? It definitely felt like it.

 

[JayRolla]

Thanks guys. This morning after letting the truck sit for about a day, I tried shooting a video while starting the truck. To my surprise it came to life with no hiccups! I went around to the back and noticed a ton of white smoke pouring out of the exhaust. I think I could smell gasoline in it as well...

My guess is leaky injectors? Bad head gasket?

The knock and thump could it be from misfiring? It definitely felt like it.

Well its kind of normal for white smoke and gas smell on startup. The white smoke can be steam if cold. Its an explosion so it will put heat out the pipe causing white smoke. Also condensation in the pipes will burn off. Gas smell is normal because the car starts up in open loop giving a richer burn until its fully warmed up and goes into closed loop. This is why people get bad gas mileage that have short trips and dont warm up there vehicle.

Misfiring is a stumble, shake loss of power. Not a hard metal on metal knock. Also a bad head gasket wont cause a knock unless its been run long enough to wash the cylinder and now has caused a constant rod knock.

We need a video of the sound to really help. Or maybe bring it in for diagnostic.

 

[Moonbandit]

I recently purchased a runner that had a slight tapping, valve or piston clatter sound ONLY on cold start ups.
At the time of purchase the owner I'm sure had it all warmed up for me not to notice this.
Anyway, I changed all fluids, cleaned MAF, plugs, wires etc including oil twice with the cold start up sound still being there.

This being my 5th 4runner I knew something was different.

I actually saw a video by Scotty Kilmer (I know I know he's a little funny, but a few of his vids have proved useful) on youtube talking up Gumout multi tune up additive to alleviate some carbon build-up inside the engine.

After adding the can and filling up for gas - no exaggeration at all - after half tank was used up and the noise has totally stopped. Next fill up I added another can just for shts and giggles and so far so good. Seems to have done the trick.

For $6.5 at wally world, totally worth a try if the PO did not exactly take care of the truck.

 

[19963.4lsr5]

I recently purchased a runner that had a slight tapping, valve or piston clatter sound ONLY on cold start ups.

At the time of purchase the owner I'm sure had it all warmed up for me not to notice this.

Anyway, I changed all fluids, cleaned MAF, plugs, wires etc including oil twice with the cold start up sound still being there.



This being my 5th 4runner I knew something was different.



I actually saw a video by Scotty Kilmer (I know I know he's a little funny, but a few of his vids have proved useful) on youtube talking up Gumout multi tune up additive to alleviate some carbon build-up inside the engine.



After adding the can and filling up for gas - no exaggeration at all - after half tank was used up and the noise has totally stopped. Next fill up I added another can just for shts and giggles and so far so good. Seems to have done the trick.



For $6.5 at wally world, totally worth a try if the PO did not exactly take care of the truck.

Cold knock from carbon build up at startup is totally legit and goes away as it warms up. I used a GM upper engine cleaner when my Saturn was doing it. (Sounded like a rod knock......) I may look into that Gumout, I even added water injection to try and get rid of the carbon buildup.


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[baja1]

Cold knock from carbon build up at startup is totally legit and goes away as it warms up. I used a GM upper engine cleaner when my Saturn was doing it. (Sounded like a rod knock......) I may look into that Gumout, I even added water injection to try and get rid of the carbon buildup.


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what exactly does the carbon build up do to create this knocking sound and why does it go away after warm up?

 

[19963.4lsr5]

The carbon buildup hits the piston cold as things warm up and expand the interference becomes less and noise goes away. I googled "engine knock at start up" (or something similar) and found a few engines (Cadillac North Star V8 and a ford engine) had issues with this due to a tight combustion chamber. I'm pretty sure our 3.4l has a tight combustion chamber because of the EK plugs it comes with. This was around 5 years ago. The info should still be out there.


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[Moonbandit]

Yeah - I was certainly super worried about the cold start slapping sound thinking my engine was pretty much doomed.

Googled and found the carbon build up threads and went from there.

Gumout took care of my noise and now I can sleep at night :brushteeth: