Rattle Sound when starting

My wife’s 2014 Prius V has ~181,000 miles.

Last week she said there was a large rattle / clunky sound followed by an immediate CEL when she was sitting at a light heading to work. She said it went away a few seconds later. She has a long commute so it was around 50 miles before she got it home.

When she got home I pulled the codes and it came back with Cylinder 1 misfire. Her plugs have around 80k on them with the original coil packs so I changed all 4 spark plugs and coils and cleared the code.

Cylinder 1’s spark plug was pretty wet with oil but the other 3 were dry. I reset the check engine light but clunky sound didn’t go away during startup, it was hit or miss though. I figured it was leftover oil in the cylinder.

We waited a week (around 300 miles) and the startup rattling still hasn’t gone away, but the CEL is still off.

I re-ran the code reader anyway and it’s now reading 3 “pending” codes.

- Cylinder 1 Misfire
- Cylinder 2 Misfire
- Random Cylinder Misfire

Today I noticed her left hand coolant tank only had about 1-2” of coolant in it (WAY below the low line). The engine was still hot so I waited for it to cool down and I topped it off to the max line and I plan to keep an eye on it.

I pulled the engine dip stick and fill cap - no sign of milky residue - the oil looks very normal (translucent brown).

I’m not sure what to check next - I thought about pulling the plugs again to see if they are still wet and if so, maybe look into replacing the valve cover gasket.

My other thought was a blown head gasket but the oil looked fine. I don’t want to replace this unnecessary but I’m not really sure how to pressure check it from home either. The low coolant is concerning though.


Here is a video of the sound:

 

Forgot to mention the EGR valve and intake manifold was replaced at 160k miles.

 

The Prius lost ~1“ of coolant in just 2 days driving.

On Saturday I rented a coolant pressure pump from my local auto parts store and put a camera down each cylinder.

Cylinder 1 surprisingly had carbon but the plug was soaked and there were visible oil drops all along the cylinder walls. When I added pressure to the coolant tank you could see a stream start to come down the wall and pool at the bottom.

At the top of the cylinder near the gasket it all looked soaked.

Cylinder 2 and 3 looked normal and dry.

Cylinder 4 was dry from what I could tell, but the piston head looked really clean as if it was coolant washed but I couldn’t find any evidence of coolant.

I think we’re going to retire the Prius - we have an appointment to test drive a few 2025 Corolla Hybrids tomorrow.