Cylinder Misfire or problems with head gasket

I'd swap the coils/plugs around, see if the code follows. If not, borescope the combustion chambers, morning after a previous day driving. Preferably with cooling system pressurized.

Make-shift coolant pressurizing gizmo:




Also check condition of plugs.

 

did you make sure nothing was warped?

 

"I do not remember if it was a Denso or a fake"

You should find out. It could be the issue.
You could always take the "O" rings and seals from the new one and put it on
the original one.

What about the spark plugs? Where did you get them?
If they are the cheap chinese copies, they are likely bad.

If you aren't loosing coolant, the head gasket is likely okay.

Did you make sure the head was not warped?

 

Yes it can happen because in a simple gasket replacement not much has changed.

With these engines a cylinder misfire is obvious at low or no speed Engine On but not as obvious at road speed Engine Ons. If it is a plug, coil or injector the scanner will show a rising number of misfires per cylinder as it drives.



Oil burning rate in 5k miles (8k kms)?

Was the head professionally rebuilt?

Has it overheated ever and if so when?

Has an egr flow blocking test been run to rule out a stuck open egr?

The only good head gasket leak test is a quality borescope inspection when cold with coolant under pressure. However observation of a steam cleaned piston matching the misfire cylinder is not good anytime.

 

In my opinion people on this forum announce that your head is blown with insufficient evidence. These engines seem to rattle and misfire due to EGR clogging, bad PCV, and general air intake grunge. In my case, I had rattles and misfires, I cleaned the EGR and slapped in cheapo new plugs and coils, engine runs fine now.

Proper test for blown head is a compression test , which can be "faked" with a Techstream scan tool in 5min, or proper compression test by removing wipers, coils, and plugs. I got 120psi on all four cylinders. Then I ran a bore scope down into the cylinders for extra verification. After I posted those results, the yapping died down about "your head is blown, maybe it'll get better if I just wait haha". No substitute for proper diagnosis. Internet forums are filled with speculation. Don't let the armchair experts get into your head.

Also keep in mind that there's a torque dampening clutch between your engine and trans that tends to rattle. Not a problem unless its totally shot.

 

Things are different on a Prius. Putting a camera down the spark plug holes, and putting
pressure on the coolant system to see if coolant leaks in is the BEST way and most reliable
way to check for a bad head gasket.

You can have a bad head gasket and still have good compression. That has been reported several
times on Prius Chat. 90% of the time it leaks between the # 1 and #2 cylinders. And it's when the engine
cools off that allows the coolant to leak into the cylinder.

So just because you have good compression, doesn't mean you DON'T have a head gasket that is failing.

Yes, several things cause a misfire, but the most common is the failing head gasket.

Sometimes, it's just not worth answering some posts because people won't listen.
And people have spend hundreds of dollars replacing good parts because of an improper diagnoses...

 

Of course there are times that somebody won't accept that they need a repair that will cost them $3k. What's there to do in such cases? Move along and don't worry. Far worse to convince even one person that they need a $3k repair when basically a tuneup can fix their issues.

I'd like to see and engine with spec compression on all 4 cylinders and a blown head. Not saying it's impossible, but I'd like to understand how it could happen. Maybe if the cylinder only leaks into the water jacket and the thermostat is closed?

 

The 3rd generation Prius. You can see them all over the place.
Swoping coil and/or spark plug positions can help diagnose miss fires.

The Prius engine is different than a standard gasoline only engine.
It's very common for the head gasket to start to break down between cylinders 1 and 2.
When the engine is cold, coolant will leak into the either or both cylinders, the coolant keeps the
plug from firing, thus miss fires.
When it heats up the metal expands and compresses the gasket, and creates pressure that keeps the
coolant out.

So when you check the compression, on a hot engine, you'll get good compression.

When a person takes there Prius to a shop that doesn't understand that is how a Prius engine works,
they charge them for spark plugs and coils, they are miss diagnosing the PROBLEM, they are diagnosing
the symptoms. They don't seem to ask WHY is it miss firing???
It's also why the coolant level goes down.

Is it possible a coil or plug good go wrong? Certainly. Especially when the plugs are from amazon or ebay.
Cheap chinese copies fail withing a few thousand miles.

That is why you MUST use a bore scope in each cylinder and put pressure on the coolant system to rule out
the head gasket.

 

It happens because the gasket may only leak a few drops when it is cooling down and the coolant is pressurized.

This causes a few seconds of misfires from that cylinder until it burns off the coolant. The cylinder seals as it quickly heats up.

On a conventional car you normally don't notice because there is no hybrid ecvt transmission to rattle the flywheel to transaxle spring loaded damper.

This can go on for months with only occasional rattles on cold start. Misguided owners change plugs, coils and sometimes injectors because their mechanic says it's aok. The old school mechanic ran compression, leak down and combustion gasses in coolant tests successfully. He just does not get it when it clears for a week and comes right back.

Meanwhile hybrid shops who replace gen3 engines every week with exactly these symptoms try to pass the word. But the unnecessary parts cannon has been expended and the engine eventually gets worse and warps. It's a new week which means more replacements.

 

What’s “a tuneup”?

 

The Prius gasoline engine is basically a stock-standard Toyota internal combustion engine. It does use the Atkinson cycle rather than the Otto cycle, meaning the intake valves stay open a little later. This is done to improve fuel combustion at the expense of a small power loss. Toyota has used this design in literally billions of engines since 1997, and head gasket failures are occurring at a high rate only on 3rd gen Prii.

There's plenty of speculation about improper gasket selection, excessive heat cycles, improper bolt lubrication and torque during assembly, and poor air intake regulation as the root cause of these problems. The big question is: why does only the 3rd gen Prius suffer from so many blown heads? As far as I know the matter isn't settled yet. Many of these engines with blown heads aren't torn down, and many engines with head gasket repairs fail again after a short time.

The gasoline engine does start and stop a lot more in a hybrid vehicle, causing lots of rapid heating and cooling. Also the Prius's starter motors are the electric motors, which are much stronger than the usual ICE starter. The stronger starters can create higher pressure inside the cylinders during cold starts, especially if the air-fuel ratio is off. Rather than chugging or stalling like a normal ICE, the 2ZR is getting forcefully cranked and started. Also there is the matter of the torque dampening clutch, clearly it absorbs excess force during the starting procedure. But it could be creating harmful oscillations inside the engine, and the rattling it produces sounds terrible.

Once the gasket starts to fail, the mechanism of damage is fairly well understood. Hydrolocking is surely a factor when coolant starts to enter the cylinder, and very clean cylinder walls and pistons is a likely sign the gasket is breached.

That said, engines can misfire for many other reasons. Concluding that an engine is misfiring due to a blown head is jumping past 10 other known causes of misfires. Air, fuel, and spark problems routinely cause misfires, which can often be remedied with simple fixes. Replacing coils, plugs, injectors, and doing fuel treatments may not fix the misfires, but really what's the harm? And if it does fix the problem, then they were likely the cause of the misfires. Cleaning the EGR system and replacing the PCV valve is a strongly recommended maintenance task for these engines. All easy stuff that MIGHT save many of these engines. Again, not proven 100%, but what's the harm?

 

I concur with frequent EGR cleaning, every 50k at least. That “maintenance task” borders on onerous though, and is needed because Toyota rushed the newly-minted EGR system to market without running a few cars on a test track. They left that task to gen 3 buyers.

ditto for the low-friction piston rings.

 

This statement makes no sense to me. If you have two engines with the same bore size, stroke, and the compression ratios are the same, how does a stronger electric starter motor create higher pressure inside the cylinder versus the engine with a standard ICE starter motor?

Second question. How does cold engine versus a hot engine affect the pressure inside the cylinder during starting?

My point is that the engine has a certain compression ratio. The starter motor used has no effect on the pressure inside the cylinder during starting. A hot engine may have a very small increase in pressure in the cylinder during starting due to the rings on the piston sealing a little bit better in the cylinder bore versus a cold engine, but the increase will be very small.

 

Blah Blah Blah..... Already know this.....
You're not understanding what I'm typing.......

It's your engine, your money.... do whatever....

 

The questions are related. The seal between piston and cylinder is imperfect, Some of the charge is lost as blowby past the piston. Faster motion of the piston reduces that effect. And the seal is probably designed to be best when the aluminum piston and the iron sleeve are at their operating-temperature dimensions. (Also, once operating, there's oil there, which can contribute to the seal.)

 

The concept isn't too wild, think of starting a standard ICE engine with an improper air-fuel ratio, the engine will stall or not start because there's too much air or fuel in the combustion chamber. Now say there's too much fuel and you force the engine to start, this creates high pressure in the cylinders, albeit for a short time. But this could increase blow-by and run down your rings over time.

The starter creates pressure inside the cylinder by turning the flywheel/crank, that's how it's designed. Remember the old videos of guys hand cranking Model T fords? Sometimes the engine would backfire and break their elbow. That's why people used an open hand to crank.

Cold starts would usually have lower pressure because the engine isn't warm so the seals and rings are looser. But as I said if you're forcefully starting the engine and the fuel isn't combusting, that's creating higher pressure.