Gutting pre-cats may not prevent engine failures

[Anonymous]

Well, I just had a read of a few results from googling "toyota vvti explained" and looks like Tem is right and I'm wrong (not a first  s;) s;)  ).

I thought that the VVTi operated more like a variable cam in that it had one or two different profiles but it doesn't. I've learnt something today   s8) s8)  

All we need now is for Tem to sift through his memories and remember all this bore and stroke guff because I'd like to know why my engine seems to open up around 4k revs  s;) s;)

This could turn out rather interesting if there is a more widespread problem across VVTi engines in general. Would the resonance be transmitted down the manifold and aid the breakup of the precats as well? Could be that the broken pre-cats are a sympton of prolonged driving in the resonance band? Be interesting to know what happens to the engine internals in this resonance band too.

[Anonymous]

All we need now is for Tem to sift through his memories and remember all this bore and stroke guff because I'd like to know why my engine seems to open up around 4k revs  s;) s;)

that's because your engine has reached peak torque, so of course its going to feel like the car is pulling harder

[Anonymous]

But if it's reached peak torque at 4k, shouldn't the torque then decrease as the revs increase? Or is it flat from 4k to around 6k as mine seems to go a bit mental from 4-6k?

[aaronjb]

But if it's reached peak torque at 4k, shouldn't the torque then decrease as the revs increase? Or is it flat from 4k to around 6k as mine seems to go a bit mental from 4-6k?

I seem to remember a dyno chart showing the torque being absolutely (near as damnit) flat between 4 and 6k - fairly sure it was a chart Tem posted, but I could be imagining that bit..

[Tem]

All we need now is for Tem to sift through his memories and remember all this bore and stroke guff because I'd like to know why my engine seems to open up around 4k revs  s;) s;)

It was more of a "didn't understand it" than "forgot it"  s:oops: :oops: s:oops:

I seem to remember a dyno chart showing the torque being absolutely (near as damnit) flat between 4 and 6k - fairly sure it was a chart Tem posted, but I could be imagining that bit..

This one?
 m http://koti.mbnet.fi/temmeke/tmp/dyno.jpg m
(dyno of my stock engine about 6 months before it blew)

[aaronjb]

This one?
 m http://koti.mbnet.fi/temmeke/tmp/dyno.jpg m
(dyno of my stock engine about 6 months before it blew)

That looks familiar, so maybe that is the one I was thinking of, yep  s:) s:)

[Anonymous]

Aaah, that would explain a lot. Thanks guys  s;) s;)    s8) s8)  

I'll get back to trying to understand things I'm a bit more familiar with   s:oops: :oops: s:oops:

[Anonymous]

I don't know what I was thinking (dyno idea).  After further thought, I don't think a problem (wear caused by vibration) would show up on a dyno at RPM that caused the wear.  It would have to be a small thing that took a long time to do damage, so you wouldn't hear or feel it at any one time unless you had sensitive equipment.  Awaiting GSB's test results.

[Anonymous]

Awaiting GSB's test results.

Me too, with great interest.  I share his scepticism.

[Anonymous]

Awaiting GSB's test results.

Me too, with great interest.  I share his scepticism.

As do I. If only so it means I didn't spend a whole saturday beating up my exhaust.   s:wink: :wink: s:wink:

[Anonymous]

What I have heard (from DaSpyda/Don on SC), to the best of my recollection, is that Toyota had some sort of miscommunication between two groups of engineers and this resulted in inappropriate rings, which they later fixed (change went into 2003 and later cars).  COULD SOMEONE VERIFY?

If rings wear too much and/or warp due to running lean or anything else that causes excess heat, oil will get by the rings.

The engine employs valve overlap for emissions reasons.  Sport Compact Car speculated that this was what caused some NISSAN engines to have catastrophic failures.  I can't recall if they have something like precats.

Hot oil on precat material (ceramic and ?) could cause them to deteriorate.  THIS COULD BE TESTED.

[Anonymous]

(continued)

Ceramic particles could get drawn into engine due to valve overlap. Ceramic is hard and could score cylinder walls, causing further oil to get by rings.  At the very least, precats getting clogged precedes serious engine problems.  There would be more heat in that area as well as increased backpressure due to clogging of precats/main cat.

I have not read of any cat-astrophic problems on 2003 and up engines, which would give some weight to the idea that the original problem was the rings.  Perhaps there is another possible cause of failures, so I shouldn't say THE original problem.  It could be that removing precats would be unnecessary on 2003 and up engines.  It could also be that the amount of wear in cylinders would "never" cause a catastrophic problem if nobody had precats.

The mystery continues.

[heathstimpson]

What I have heard (from DaSpyda/Don on SC), to the best of my recollection, is that Toyota had some sort of miscommunication between two groups of engineers and this resulted in inappropriate rings, which they later fixed (change went into 2003 and later cars).  COULD SOMEONE VERIFY?

MR Mikes September 2003 2 had an engine failure  s:? :? s:?

[Anonymous]

I was under the impression that better oil control rings were fitted to the pistons of all 2002 roadsters onwards!?!

See GSB excellent post here!  s:wink: :wink: s:wink:

[Anonymous]

Well, so much for the rings being the magic bullet.  An '03?  That stinks.

[MRMike]

What I have heard (from DaSpyda/Don on SC), to the best of my recollection, is that Toyota had some sort of miscommunication between two groups of engineers and this resulted in inappropriate rings, which they later fixed (change went into 2003 and later cars).  COULD SOMEONE VERIFY?

MR Mikes September 2003 2 had an engine failure  s:? :? s:?

The engine never failed..per se.  The pre-cats were changed, and the VVTI mechanism seemingly wasn't operating, which was a result of the car consuming Oil. I still subscribe to Mark and Steve's initial theory on the fuel rail.

[Anonymous]

I just went through the sticky topic on spyderchat and came up with the following numbers (with a few possible mistakes)

2000 model year: 32 reported cases
2001 model year: 22
2002 model year: 2

Both of the 2002 people said the build date was in late 2001.

One person with a 2003 car had a main cat replaced, but not the engine.

No cat-astrophies for 2003 or 2004

So it could be the rings, right?

[Anonymous]

I just did this.

[heathstimpson]

I just did this.

Some low mileages there  s:shock: :shock: s:shock:    s:? :? s:?

[Tem]

I just did this.

 m http://www.spydermagazine.com/files/eng ... _miles.jpg m

How do we read this...? At first I thought the X-scale is tmiles and Y-scale blown engines, but that would equal to several thousand blown engines and there's no such info I suppose...?

If Y-scale is tmiles, how can there be several bars for 40tmiles...

I'm confused  s:? :? s:?


Edit: Uh...nevermind...X-scale is individual cases and Y-scale their mileage, right?  s:oops: :oops: s:oops:

[Anonymous]

Right on the last sentence.  Sorry for the confusion.

[Anonymous]

Interesting that this resonance problem occurs around the same rpm for the valve timing change.

Valve timing change occurs from idle to rev limit  s;) s;)
(that's why it's called variable)

I dont know where this mis-conception comes from, but VVT-i is an on-off switch - the only reason it is called 'variable' is because there is a change in the valve timing. The 'i' bit is because the timing change occurs at different points in the rev range depending on the current demands on the engine.

Take a look at the workshop manual for the engine - the valve control mechanism is controlled by an on/off solonoid - there are absolutely NO analogue devices present in the control system.

Now if it were called :-

C onstantly
V ariable
V alve
T iming with
I nteligence

we would be talking a whole different ball game.

Mmmmmmmhhhh.... I don't think so. The VVT-i is a Variable Valve Timing - intelligent system. OK, there's no analogue ECU (it's digital) but it controls the valve openning and closing position (advance, retard, hold). This position is variable (not just on/off)...

Just to quote some explanations (from Toyota's "New Car Features"):

The camshaft timing oil control valve selects the path to the VVT-i controller according to the advance, retard or hold signal from the engine ECU.

In proportion to engine speed, intake air volume, throttle position and water temperature, the engine ECU calculates an optimal valve timing under each driving condition and control the camshaft timing oil control valve. In addition, engine ECU uses signal from the camshaft position sensor and the crankshaft position sensor to detect the actual valve timing, thus performing the feedback control to achieve the target valve timing.

I think you are getting a bit confused with the VVTL-i system where you actually have two intake camshafts (like in Honda's VTEC) instead of just the one. With that system, the engine switches to the high-rev high-volume intake mode. I believe this happens above 6000rpm on the 2ZZ-GE engine.

[Anonymous]

I am a little bit sceptical about the claim that the engine could suffer excessive wear from a resonant frequency in the crankshaft. Although this is something common in light aircraft engines (that run at constant rpm's for long periods of time), I doubt the harmonic frequency quality would be low enough (spread wide enough) to be damaging the engine. (Engine rpm's in a car are hardly perfectly steady, even on a motorway.) Besides, it's something easy to fix and I cannot see how the engineers at Toyota could have missed something that simple.

I believe that many engine failures nowadays are due to oil starvation (low oil levels) which has been aggravated by the extended service intervals (and many people who don't check and top up the oil regularly enough)... I also heard of many cases in the US (where manual gearboxes are a bit of a novelty) where engines were destroyed (or had serious valve damage) by accidently overreving the engine (putting in 3rd instead of 5th or 2nd instead of 4th, etc...  s:? :? s:?   )

[Anonymous]

I also heard of many cases in the US (where manual gearboxes are a bit of a novelty) where engines were destroyed (or had serious valve damage) by accidently overreving the engine (putting in 3rd instead of 5th or 2nd instead of 4th, etc...  s:? :? s:?   )

that's been a problem with mainly the 2zz-ge, 6-speed equipped vehicles......as far as i know, this has not been a major issue with the mr2......(but then again, i primarily associate with enthusiasts)


the biggest question i have is with the model year trends - this problem as been almost exclusively associated with earlier cars.....the failures have not been tied to mileage, so we can sort of scratch that off....this lends credence to the various theories that say this is reulting from a manufacturing defect, but...and this is a big but  we've seen numerous replacement engines fail as well (in some cases, owners losing two or three engines on the same car)....if this was a manuf. defect, and the problem has been fixed, wouldn't it be safe to assume that these replacement motors are also free of defect?

i guess what i'm asking is - though we haven't seen many motors pop on '03 or  '04s, why are replacement motors built "after the 'fix'" going boom?

kev

[Anonymous]

There's definitely a few design issues in the earlier MR2s. I noticed that the lattice structure in the pre-cats (or warm-up cats) fitted on the earlier models ('00-'01) is definitely finer and more fragile than that fitted on later models. So I reckon they are more likely to desintegrate and cause engine failure.

IIRC many of the cases of new replacement engines failing shortly after being installed could be attributed to a blocked exhaust line (i.e. pre-cat debris blocking the entrance of the main cat, causing massive back pressure and quickly warping and damaging the new engine.)

If the first engine was damaged (e.g. because of letting the engine run out of oil), it is possible that the pre-cats got shot too. So if the engine is rebuilt (or replaced) but the exhaust line isn't cleared, this could explain why a new engine breaks shortly after being fitted.

I've always taken care of my car (an '01 model), always used the best oils and checked the oil level, etc. Despite all that, the pre-cats were toast after less than 30000m. I was less than impressed!