EGR/Intake Manifold Cleaning

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Dave1UK, Aug 17, 2026 at 6:11 AM.

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  1. Dave1UK

    Dave1UK Junior Member

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    So I haven't had a chance do the EGR/intake cleaning on my car yet but will get it done in the next 2 days, since owning the car (nearly 2 years now) have seen numerous YouTube videos, read about it here on the forums but just after a bit more info to ensure the process goes as smoothly as possible. Thankfully not experiencing any issues, but want to get it done as replaced the hybrid battery last week, so now the car needs to last as long as possible!

    I already own a spare EGR pipe/valve/cooler set as well as spare intake manifold, bought this when I got the car. Planning to clean the spare EGR pipe and cooler (it isn't too dirty) today so I can have it ready for swapping it out. I think I will clean the cars original intake manifold and EGR valve rather than using the spare, as not sure if this spare valve operates at all, these are used spare parts not new. Got new rubber gaskets ready, got a new PCV valve and also got fresh new coolant ready for the system (haven't changed it yet under my ownership). Will also swap the spark plugs whilst at it.

    My main queries are about what sort of cleaning chemicals everyone uses/what to stay away from etc.
    • The EGR pipe should be easy enough to clean via brake/air intake (carb) cleaner along with bore brush?
    • As there is an electrical component with the EGR valve, I'm assuming go gentle on that? Bore brush/wire brush and air intake (carb) cleaner only? Also how can I test the operation of it?
    • For the EGR cooler I'm planning to soak it in oven cleaner, wash it out, repeat if necessary and then take it to a car wash for high pressure clean. Any advice here, leave it overnight? Few hours only? Neutralise the oven cleaner with vinegar?
    • For the intake manifold, I have only seen people use brake/air intake (carb) cleaner, I am assuming harsher chemicals are not necessary? Also for the small port bores, how careful does one need to be cleaning them out, not to damage the plastic itself or is it pretty durable? Should one pressure wash this as well or is it not needed?
    Appreciate any guidance.

    Thanks.
     
    #1 Dave1UK, Aug 17, 2026 at 6:11 AM
  2. ChapmanF

    ChapmanF Senior Member

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    If you haven't begun yet, this is the best time to take a 'before' reading of the EGR flow test result and save it for later:

    EGR flow data

    Many people forget that step before they start the job, and then get to the end with no comparison they can make to see what their work actually accomplished.

    Edit: by the way, while that post shows Techstream, any scan tool that can show "mode 6" monitor test results can show that.
     
    #2 ChapmanF, Aug 17, 2026 at 9:44 AM
    Last edited: Aug 17, 2026 at 9:50 AM
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  3. Dave1UK

    Dave1UK Junior Member

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    THANKS! I had this in the back of my mind a few days ago but completely forgot. As I was also checking values for my MAF sensor (if it may need cleaning as well whilst I'm at it).
     
    #3 Dave1UK, Aug 17, 2026 at 9:50 AM
  4. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Everything but the cooler can be cleaned with brake cleaner, brushes and rags. Caustic cleaners are only safe for stainless steel, namely the EGR pipe and cooler, but only the cooler warrants caustic use.

    Not having spare components, my strategy is to get the cooler off, corked*, filled with a caustic solution, and stood up in a corner of laundry sink as early as possible in the process. Let it sit about an hour, rinse, inspect**, and repeat till clean, or as close as you can get it. I’d cap the sessions at 5 max, any more won’t help much I think.

    My caustic has been powdered Oxi-Clean so far (twice). If and when there’s a next time I’ll try KOH (Potassium Hydroxide) solution, probably 30% by weight. It’s a common component of soap making, still obtainable. NaOH (Sodium Hydroxide, aka lye) sales are restricted due to its use in making methamphetamine. Oven cleaner contains lye, albeit relatively dilute, and its a somewhat awkward/expensive way to obtain caustic. A generous sized bottle of KOH is inexpensive. You’ve got to be careful with though, follow safety protocols.

    * ID of smaller end is roughly 21 mm; I believe suitable for #4 stopper

    ** Be fastidious when inspecting for degree of clean, any residual carbon may be trivial for flow, but still is detrimental to cooling. A point light source inside the entrance, not glaring on the end flange, is good.


    if you haven’t already, have a look through the relevant links in my signature (on a phone turn it landscape to see signatures)
     
    #4 Mendel Leisk, Aug 17, 2026 at 10:02 AM
    Last edited: Aug 17, 2026 at 10:08 AM
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  5. Mendel Leisk

    Mendel Leisk MMX GEN III

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    #5 Mendel Leisk, Aug 17, 2026 at 10:15 AM
  6. Dave1UK

    Dave1UK Junior Member

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    Perfect, I already have some oven cleaner on the way today from Amazon, so will start with that. We don't have oxi-clean here in the UK, although a comparative product seems to be Vanish Oxi Action here? Although ingredients wise it contains only about 30% Oxygen-based bleaching agents, oxi-clean is about 50-60%?

    Is the active ingredient that helps breaks down the build up hydrogen peroxide or the combination of the washing powder with it? As I believe I have some hydrogen peroxide (liquid form) at home.

    Thanks.
     
    #6 Dave1UK, Aug 17, 2026 at 10:23 AM
  7. Mendel Leisk

    Mendel Leisk MMX GEN III

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    I'd never really researched how Oxi-Clean works, or even what it's active ingredient was. Google AI info, an interesting read:

    https://share.google/aimode/IgvkPvSDSMaZpl86j

    That said, I'm going to try KOH (30% by weight) solution if-and-when.
     
    #7 Mendel Leisk, Aug 17, 2026 at 11:36 AM
  8. CR94

    CR94 Senior Member

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    Wary of unseen water or other liquids getting trapped in those manifold EGR passages, I used only mechanical methods of clearing them, then shook the manifold in various orientations and vacuumed from both ends to remove any loose particles. That may not clean it perfectly, but apparently was good enough. I should unclog the manifold again soonish (having already added >50k miles since I did it).

    I'm still getting good flow test results, so haven't bothered with the cooler (heat exchanger). It will clog a lot quicker if you're burning much oil.
     
    #8 CR94, Aug 17, 2026 at 4:18 PM
  9. Dave1UK

    Dave1UK Junior Member

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    I'd be using a heat gun/hair dryer on low heat for an hour or more and leaving it outside to dry before hooking it back up.

    I also have a spare damaged intake manifold that I have been thinking of cutting open just to actually see how much space there is internally and the "map" of the passages, always helps to visualise what you're dealing with.
     
    #9 Dave1UK, Aug 18, 2026 at 3:33 AM
  10. Mendel Leisk

    Mendel Leisk MMX GEN III

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    For sure don’t use caustic cleaners on the intake manifold. I learned the hard way: the metal embeds are not stainless steel, will start getting crusty. Brake cleaner will disperse very fast, doubly so with brushes/rags and compressed air.
     
    #10 Mendel Leisk, Aug 18, 2026 at 10:23 AM
  11. Dave1UK

    Dave1UK Junior Member

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    Just to clarify is the sensor/name "Mode 6" on OBDII scanners? As I had a look yesterday but looked for "EGR" instead and don't think those are the correct ones (screenshot included). They mostly showed 0 whilst driving.
     

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    #11 Dave1UK, Aug 19, 2026 at 4:04 AM
  12. Dave1UK

    Dave1UK Junior Member

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    I cleaned the EGR cooler replacement that I had, I guess it wasn't that gunked up to begin with, it looked blocked visually. I literally blocked the holes and left oven cleaner (gel type) overnight, washed it out with a normal garden hose in the morning and it's sparkling clean. That's all it took. I don't have before photos unfortunately but will do the same with the one I take off my car and take some.

    Also realised that the EGR cooler inside the "honeycombes" there are small spikes. I have read a lot about people putting things like coat hangers and the like into them to free them up, surely that isn't a good idea? As you're going to bend/damage those spikes - whatever they're for.
     
    #12 Dave1UK, Aug 19, 2026 at 4:16 AM
    Last edited: Aug 19, 2026 at 4:23 AM
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  13. ChapmanF

    ChapmanF Senior Member

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    "Mode 6" is the technical name of a different way the scan tool interacts with a car, where it sends a query that has a value 6 indicating the query type, and the car responds with the results of various automatic self-tests that the car has conducted on its own in the recent past. You usually find this on your scan tool in some totally separate menu place from where you would be to look at sensors or trouble codes.

    The thread I linked to above, if you scroll through later posts, shows a lot of examples of people pulling up the EGR self-test results on a variety of different scan tools and apps.
     
    #13 ChapmanF, Aug 19, 2026 at 11:05 AM
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