My first Gen 3 Prius came to me from Facebook Marketplace. The ad said, new water pump, new spark plugs, and coils, the engine is making a strange noise. It was then I knew I needed it.
I wanted a broken Gen 3 Prius because I wanted to learn all about the issues they have other than the standard hybrid battery failure. Was I ever in for a surprise? I quickly knew that what the previous owner had tried to do was cover up the problem's root issue by masking it with band-aid solutions.
In this article, I want to cover the infamous "cold start knock," and my take on how it occurs and how it can be dealt with. I will talk about the intake manifold; however, that is not my sole focus for this article.
How Does Cold Start Knock Occur?
Not surprisingly, many people out on the internet tell you cold start knock comes from the oil build up in your intake manifold. I am sorry, but this is probably the least accurate statement I have ever heard. The amount of oil that generally builds up inside an intake manifold is maybe 2 to 4 tablespoons at best.
That amount of oil is not enough to, 1, cause a large plume of mist into the combustion chamber and cause a misfire, and 2, cause much if any damage at all. Oil in the intake on a higher mileage Gen 3 Prius is relatively standard. It is not the cause of your cold start knock, but I know what is.
Cold start knock happens because you have a misfire in at least one cylinder. It happens because something is interfering with your air/fuel mixture. Thus, your cylinder is "misfiring" and causes a "knocking" noise that is very audible.
Engine Coolant - A Prime Suspect
If you have not heard that your generation 3 Prius is susceptible to head gasket failure, you may want to consider reading up on it here. The 1.8L 2ZR-FXE all-aluminum engine that Toyota put in these cars is known to have head gasket failure.
When a head gasket fails on this engine, typically, the coolant seeps into the combustion chamber on one or more cylinders. Remember, what happens when something foreign gets in the way of your proper air/fuel mixture? You got it, that is a misfire.
The engine tries to dispel the excess liquid out of the cylinder when it starts. Because there are excessive amounts of fluid, the firing of the cylinder does not happen. This action creates a "knocking" noise and will stop after a few seconds of run time once the engine has run the coolant out.
To know if you have coolant or head gasket issue is causing your cold start knock, check your expansion tank level and see if your Prius has been loosing coolant. If you have, it is time for a new Prius or replaces the head gasket.
If you want to know more about head gasket replacement, check out another story here.
The EGR System - Secondary Suspect
No coolant is missing, but you still have a cold start knock on your Gen 3. No worries, there is a second solution that can also cause the problem.
Your EGR or exhaust gas recirculation system can be at fault. The issue lies within another couple of intertwined problems that the EGR valve itself can have. It is not the most straightforward fix, but the dealer may help you out with this one.
There are a few different issues that the EGR can have. One is that the pintle itself can become physically stuck during operation and cause the valve to hang open. If the valve is open during startup, the gasses will bleed back into the intake and distort the air/fuel mixture, thus causing a misfire.
Second, the EGR valve logic can be messed up. The valve will hang open when it should be closed, and the knocking will occur. Both of these issues focus on the pintle of the valve in the incorrect position. One is mechanical, and one is computer controlled.
The Toyota repair for a faulty EGR is to replace the EGR valve and do a software update. I have also run into a strange scenario where the stepper motor of the valve was faulty. The trouble code P0410, EGR Insufficient flow, kept appearing. I hooked up a lab scope to the valve and saw that the motor was sending an erratic signal.
I would occasionally get a cold start knock while the code was present. After replacing the motor and not the valve, the issue cleared, and the cold start knock never returned.
The Intake Manifold - The Least Likely Culprit
The intake manifold design was changed slightly in 2012. A complete intake manifold redesign was done for Generation 4, but we are not worried about it now.
The "newer" style intake manifold had slightly decreased in air volume flow. It did nothing to help with cold start knock or oil pooling.
Two things can cause your cold-start knock. One is engine coolant, and the other is your EGR valve. You can have both issues and not know it until you start doing some investigating.
The oil in your intake is not going to cause a misfire, and replacing your intake manifold with an "update" to solve your cold start knock problem is nothing more than a waste of time and money.
Are you still having issues? Let me know! I have helped many people with all sorts of problems with their Prius'. Find me on Twitter or Facebook.
Have a great day, everyone. I look forward to seeing you in the next story. What Is The Lifespan Of A Lithium-Ion Toyota Prius Hybrid Battery Pack?
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Peter Neilson is an automotive consultant specializing in electric cars and hybrid battery technologies. He holds a Bachelor of Science in Automotive Service Technology from Weber State University. Peter is also an Instructor of Automotive Technology at Columbia Basin College. Peter can be reached on Linkedin and you can tweet him at The_hybrid_guy on Twitter. Find his page on Facebook at Certified Auto Consulting. Read more of Peter's stories at Toyota news coverage on Torque News. Search Toyota Prius Torque News for more in depth Prius coverage from our reporters.