I am a Gen 2 and Gen 3 Prius veteran who has cleaned out his Gen 3 EGR loop including unplugging the blasted heat exchanger. I have friends and relatives with knocking Gen 3s that have aged to the 200k mile point. I've tried to recount to them stories I read in this forum about how a clogged EGR loop causes the collection of coffee-colored wet sludge in the throttle body sump and the space above an intake valve. However, several bright listeners push back and ask me to clarify how a plugged EGR loop causes the formation of watery sludge in the intake manifold. They understand and agree that watery sludge in the tight space above the piston can cause head gasket failure, but they don't understand how a non-flowing EGR loop prompts formation wet sludge. They say "a clogged EGR loop should cause emissions to rise but where does the water come from before the head gasket fails?". I confess they that have a point and I really don't fully understand it. Can someone remind me of the succession of events that results in head gasket failure? Thanks from Indianapolis.
no one really knows, just a lot of guessing and disagreement. piston s and rings are a problem up to mid 2014, 10k oil changes are cited, oil viscosity is cited, and egr is cited. who knows? toyota maybe, but they're not admitting anything.
I think at least a couple different stories have been glommed together there, which is why you're having trouble finding 'the' explanation. As for the coffee-colored wet sludge beneath the throttle body, you find that in every Prius ever driven, starting with the first generation in 2001. The first two generations did not have EGR systems*, but still got the coffee sludge. I don't think anyone is seriously trying to pin the coffee sludge on the EGR. The intake valves close well after bottom dead center, when the piston is moving back up on the compression stroke, and that's what gives the engine its Atkinson-cycle characteristics. That pushes some fuel-laden charge back into the manifold. The glop down there may be a mix of that fuel, and some moisture, and some oil that may arrive through the PCV system. Some people worry about the small puddle of glop at the bottom of the manifold, or add oil catch cans hoping to reduce it. One thing I can say, show me anybody who has driven a Prius for n00,000 miles, and I'll show you someone who drove it for n00,000 miles with a small puddle of glop in the manifold. There have been a few posts where somebody thought enough of that glop could be slurped into the cylinder to cause a problem. In one of those threads, I posted a simple cross-section of the engine showing the shape of the manifold and how high the glop would have to get slurped. I'm having trouble finding that thread now and don't feel like drawing the picture again.** The completely different story that gets mixed in with that is the story of how hydrolock, with incompressible fluid in the cylinder, can lead to dramatic engine failure. That really happens. The incompressible fluid is usually coolant, and it gets there through a head gasket that already has failed. So that story has three chapters: chapter one where the head gasket fails for some reason, chapter two where there are some rough engine start-ups because small amounts of coolant have leaked in while it was stopped, not enough to cause hydrolock but enough to cause a rough start, and chapter three where enough coolant has leaked in to stop the piston dead before TDC, and some other part of the engine goes out through the block. Note that's a whole different story from anything about the glop under the throttle body. As for chapter one, what caused the head gasket to fail in the first place, nobody has clearly shown a single culprit. There are some members (or one very convinced member, anyway) convinced it has something to do with EGR. There've been other suggestions about too-infrequent coolant changes, where the coolant's anticorrosion additives get depleted, and the coolant eats away at the gasket sealing surfaces. Others have speculated about the extra thermal cycling seen by an engine in hybrid service. So chapter one doesn't have a very clear cause identified. The parts of the story that we do know are, as soon as chapter one happens (for whatever reason), we know why chapters two (and three, if it's not fixed in time) will follow. * All Prius generations have variable intake valve timing and, on the advance side, it can open the intakes while the piston is still moving upward on the exhaust stroke ... up to 18° before the top of the stroke (for gen 1 and gen 2, I don't have the gen 3 or 4 figure handy***). That's been referred to as a form of "internal EGR", but it can't be very significant; cos(18°) is more than 0.95, so there's no more than 5% of the exhaust stroke volume left at that point, and the exhaust valves are still open too, so it's not like it's all going to go in the intake, plus the exhaust was initially under high pressure, so much more than 95% of it has already gone explosively out the exhaust valves by that point. Also, under the operating conditions where maximum valve advance is used, it may be that the tuned resonance of the intake is pressurizing the intake charge at that point, so the brief overlap of intake and exhaust valves both open may have more of an effect of scavenging the last of the exhaust out of the cylinder with inflow from the intake, than of backing exhaust into the intake. ** Ok, it came up in another thread today. I'm still not finding that older thread or the picture I drew then, so I went and drew it again anyway. The old one was a better, cross-section diagram, but this gives the idea: *** Oh, look, I do have it, it's 29° for Gen 3.
In my opinion, it doesn't, and that normal oily "sludge" isn't itself as big a problem as some people imagine. Any water in the manifold is condensed from PCV fumes, (normal) EGR flow, and possibly combustion products regurgitated through the intake valve. Occasional mysterious rough starts tentatively attributed to water in the manifold were an issue early in the 3rd-generation life-cycle, before head-gasket problems showed up. Very good summary by ChapmanF above, incidentally.
My Gen 1 started having those around 198,000 miles. Whole different engine, that. No EGR. Never did find out what the cause was, but I drove it 40,000 more miles with never any coolant usage or any sign it had anything to do with the head gasket. What finally stopped me driving it was an eastbound Dodge Dakota.
Thanks to all of you for your helpful comments. Don't know how Indiana cultivates so many steely-eyed Prius partisans.
After re-reading ChapmanF's thorough reply and discussing it with another Gen 3 Prius owner, I summarize/internalize what Chapman wrote like this: 1. The early plugging of the EGR loop is a standalone problem that likely is unrelated to head gasket failure. 2. Evidence of "coffee sludge oil/water mix" in the lowest bend in the intake manifold is present on a lot of Prius motors (not just Gen 3) and occurs well before the start of the cold morning rattle phenomenon. Depending on its severity the wet sludge **could** be evidence of a head gasket leak or simply be the normal result of a functioning PCV valve and condensing crankcase vapors. 3. Because the intake manifold's bottom bend below the throttle body is, altitude-wise, well below the intake ports, the likelihood is small that this pool of sludge gets ingested in the motor. 4. However, a minor head gasket water leak eventually worsens over time to create rattling cold morning starts due to water-laden charge in the cylinder. High cylinder pressures associated with the cold morning rattling accelerates a head gasket leak. Game over occurs when a cylinder hydrolocks or the engine overheats due to loss of coolant. Hope this is an accurate summary. Thanks for your response, ChapmanF. Forum moderators should make Chapman's comments a stickie.
The source of liquid pooling at bottom of intake got there via PCV. And who know how much more doesn’t tarry, passes right through, combusts. And that might cause the EGR to carbon-up faster.
Toyota is not going to acknowledge design defects related to high mile head gasket fails because the remedy requires a replacement engine. A dealer will almost never do a head gasket replacement because it is not a reliable long term fix. Most customers will trade it and the dealer will send the car to action. My number one cause of egr clogs are the low tension piston rings that allow excessive blowby. This is aggravated by the egr intake being placed ahead of the cat rather than behind as in the gen4. A gen3 has a smaller and easier to clog egr cooler. All of these egr factors are difficult to retrofit with a better design. Perhaps it is why the crowdfunded marketers on this board only sell lift kits and replacement battery cells. But is the sad state of our gen3 egr design responsible for hg fails? Highly unlikely and not supported by Toyota Master mechanics or most anyone outside of this forum. Bad rings are acknowledged by Toyota and were replaced free under the powertrain warranty when oil consumption hit a quart every 1200 miles. Perhaps a smoking gun (tailpipe?) but not likely the whole issue. The real issues are design based and not eliminated by frequent egr passage cleanings. What about thermal cycling? It is key difference between Corolla engines with the same engine and their stop start Prius brothers. Yes, gen3s have serious thermal cycling impacting the hg region and cylinders. Especially in city driving. Significant coolant system and block redesigns were incorporated by 2016 gen4s. Thermal cycling is a smoking gun but the solution can not be replicated in a gen3 Prius engine. When your hg fails and you want another reliable 100k miles out of your Prius, have a guaranteed rebuilt engine installed and change the oil every 5k miles or sooner. Minimize thermal cycling especially long Ready state idles.
If "excessive blowby" is inevitable, why is my PCV fresh-air intake hose clean and dry inside, not oily---which it would be if blowby rate exceeds the flow rate through the PCV valve? Many engines with EGR of every make an model prior to the 4th generation Prius also take their exhaust gas from ahead of the catalytic converter. Thermal cycling is likely to be more extreme in seriously hilly areas than in city driving.
"marketers on this board only sell lift kits and replacement battery cells." And the availability rebuilt 2ZR-FXE motors in the commercial market is terrible. Customary sources are out of them, which could be due to the arduous rebuild task. Dealer parts department says a short block is available from Toyota ($1800) but no long block. The rest of this vehicle is amazingly durable. Like the hundreds of decommissioned jets parked in Tucson's aircraft boneyard, it will bring tears to my eyes to see rows of otherwise serviceable Gen 3 Priuses at junk yards with less than a deuce on the their odometers--in an era with $6 a gallon gasoline. Clearly Toyota made a lot of errors in the 2ZR-FXE, but preventing them would have required tens of millions of dollars of pre-production life testing on hand-built prototypes. I design circuit boards during the day and I know how hard this is. Thanks again for all of the great postings. Jim Olson Indianapolis, IN US
" 2 Evidence of "coffee sludge oil/water mix" in the lowest bend in the intake manifold is present on a lot of Prius motors (not just Gen 3)" That's the result of the PCV system doing it's job but on an engine that stops and starts dozens of times per trip. The system scavenges blowby gases (hydrocarbons, exhaust byproducts and water vapor) and pulls them into the intake to be fed into the cylinders again. On a normal engine that runs "all the time", all those vapors stay moving in the intake. It's only when the engine is stopped that things settle out. Since that happens only a few times per day (and the engine is quite hot when it does), it can take many thousands of miles for that vapor to build up into a noticeable coating. A Prius has the engine stop and start many times per trip. Each time air stops moving in the intake so vapors settle out. The intake (esp the gen3 plastic unit) likely isn't as hot as some engines - so those condensed vapors don't "bake" into a solid coating. So the "spooge" builds up into a latte. Posted via the PriusChat mobile app.
That's my take on it. There's usually a little pool down there at the bottom of the manifold, minding its own business, and that's just how a Prius is. It's made out of volatile stuff, so sometimes some of the stuff evaporates and gets sucked in with the air and harmlessly burned. And some other volatile stuff maybe comes back out and condenses into liquid in that little pool, so it stays there and minds its own business. The manifold is tall enough that whenever I see talk about that little pool getting slurped into a cylinder as liquid and and causing any problem, my reaction is a resounding "".
Probably so. I can't recall anyone reporting absence of that buildup on a Prius with significant mileage, including ones with no symptoms. If you drive it long enough, the amount will reach some equilibrium level, meaning further entrained oil mist from PCV is effectively sucked into the cylinder and combusted.
