2010 Toyota Prius Engine bay
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Does A Redesigned Gen 3 Prius Intake Manifold Help With Cold Start Knock?

If you own a generation 3 Toyota Prius and have experienced a cold start "knock" it can be troubling. Learn what can cause the issue and how it can be resolved.

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.

Empty Toyota Prius expansion tank

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.

Toyota Prius engine oil coolant mixture

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?

Watch this Toyota Prius truck with a cute little bed and click to subscribe to Torque News Youtube for daily automotive news analysis.

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.

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Hi Peter! Is it actually a step motor or is it a solenoid attached to the EGR valve? I thought it was more of a solenoid… AnyWho, I am still going to ask your help to work on the 2010 wth the O P0410 soon! Should the first thing I do to it is the motor replacement? You said you had a good one? Thank you! William
This post has way to many issues with it. Just a few-- The title doesn't have anything to do with the article You mention the intake manifold getting oil has nothing to do with the cold start, but if one were to see oil in the intake they should understand to continue to read. I mean if they wanted to figure out the issue.
Hi Peter. I have a strange problem. Mileage is 125000 miles. Very nice oil. Not black or seared. Does not take and oil or coolant. After driving a lot. Started with cold start knock/misfires. I have.Changed./replaced. New Coils on every cylinder. New denso original plugs. Cleaned intake manifold. Throttle house. New gaskets. Maf and map. Egr and egr cooler. Replaced to other not new but ok and cleaned of course. It was working nice but still not always perfect . To see and understand better I got a scanner and it occasionally sends cylinder 1 misfires. It has never showed any engine warning signs at all.. Still it sometimes could cold or warm engine misfire on cylinder 1. I replaced the fuel injector after cleaning them first still no difference. After replacing the fuel injector it was even better. Still. After standing a day or to it can send misfires on cylinder 1. I have noticed.. if I press and accelerate hard and drive it hard for a while...it can send misfires just when the engine is about to stop. Its like it cannot decide what to do?.. Have you any suggestions?..
How is your coolant loss? It looks like cylinder 1 fails pretty often on the hg for these. Mine at 250k, no noticeable oil use, I believe is about to need a head gasket because of this cylinder. I got a 80 dollar cylinder scope tool from harbor freight and put it down in the cylinder it's a little cleaner than the rest, started misfiring on cold and warm restarts. I cleaned the map/maf, egr system/manifold, plugs, pcv replaced etc.. lol I'm certain I'm in for a HG. I'm going to do the injectors at the same time. Have your cylinder head replaced if you do head gasket. or at least resurfaced since you are only at 125k. - All of the best to you.
I put straight edge on both cylinder head and engine block and the wrapage was within the spec so I only lightly sanded both surfaces with sand block. Make sure you put straight edge on middle and also from corner to corner. Good Luck!
I have done some research on generation lll head gasket failure. I have not found any straight answers! Some say it's bad HG design/material, some say EGR failure, some say oil in an intake! I have replaced mine with Fel Pro about 5K ago with no problem so far. I am getting P0410 for the last 100K. I have cleaned the lines and replaced the motor none have made a difference. The best fix I done is to tape the light!
Hi I had this on my Toyota prius gym 3 mine comes and goes now and again but mines been in the Toyota cause it adds a recall on the software but if I don’t use my car for three days then I’ll get that noise turn it on and turn it off and it’s okay and I know it’s my Oil keeps going as well I have to keep topping it up my nose done 215,000 miles and it’s a recon hybrid battery put in it when one was making these noises I was gonna replace the engine Till I read online
A third possibility for EGR related misfiring is clogged EGR passages in the intake manifold. My 2010 Prius got so bad I was afraid to run the engine, it was about to jump out of its mounts. Trouble codes were all 0300s pointing to ignition. I replaced the coils and the plugs to no effect. I never got a EGR related code. I finally removed the intake manifold and found all four EGR passages clogged with a black and brown carbon/synthetic oil tar mixture that was quite difficult to remove. Removal did the trick, however, and the Prius is running fine now. So there's your third possibility for EGR related misfiring.
Hi jay if all the holes are blocked up and there is inlet manifold then if you got something small like an old Longneedle you should be able to get out the spray WD-40 and also if you got brake cleaner I should come out eventually I hope that helps you also check the EGR valve and the other thing what’s offer that will connect into the resource can’t member what is called ok
I don't know how these two things (misfires & blocked EGR passages in intake) are related!? If you have that much carbon build up in the intake you must have so much build up elsewhere in EGR system too. Taking out EGR not very easy job compared to the intake.
I have had this issue but it was caused by EGR Carbon Build up, and carbon build up restricting air flow in intake manifold. That said, I replaced my injectors as well.