Valve clearance checks

[Fishboy]

Does the IS250 require valve clearance checks or are they hydraulically actuated? My motor seems fine, quiet on tickover still smooth as butter when driving, lovely.

[Verbout]

It's 24 valve VVT-i isn't it?

Are you getting it mixed up with a Morris Minor A Series engine? ;)

[Fishboy]

I'm the next generation up, I had a mini 1000 which I think had the same pushrod engine? Out with the feeler gauges with the simple screw and locknut adjustment, even I could manage that job as a 17 year old apprentice! I wondered if Lexus have the bucket and shim type adjustment as do a number of Hondas and the same as my Triumph motorbike.

[steve2006]

More like the A series engine on these as the valve clearance appears to be mechanically adjustable going from the part description and the lack of a pressurised oil feed.

I hope they don't wear out being priced at £70.00 each!

Look at the part commencing 13750 on the exploded diagram described as "Adjuster- Valve Assy"

 

[johnatg]

Come on chaps - almost all modern engines have hydraulic tappets (Japanese manufacturers call them Lash Adjusters). Including IS250 engines.

[steve2006]

Cheers John I am enlightened 

Probably explains the cost!

I saw something about lash adjuster and quite wrongly assumed it was something to do with the chain tensioner....ooops!

So basically the only maintainance required is regular oil changes of the correct specification to keep them running sweet.

For my error I will accept 20 lashes with a shoe lace and stick to my shims in the V8.

[johnatg]

Shims are a whole lot cheaper - just think - 24 lash adjusters @ £70 each! Crazy!

[Verbout]

4 hours ago, johnatg said:

Come on chaps - almost all modern engines have hydraulic tappets (Japanese manufacturers call them Lash Adjusters). Including IS250 engines.

I thought from my mentioning of the VVT-i it was obvious tappet adjustment was Jurassic.

[johnatg]

Not sure I understand the relevance of that, even with the lol!

VVT and tappet adjustment have nothing to do with one another - VVT changes the valve timing by changing the relative angles of the camshafts (usually just the intakes) (at the drive end - like a sort of variable clutch) and the crankshaft. Tappets are located between the rockers and the valves to get the correct amount of free play during installation ('grinding-in') or during running - the valve length changes as the engine warms up. With screw and locknut or bucket and shim tappet adjustment there is a compromise - with hydraulic tappets they are slightly flexible, being cushioned with high pressure oil. Incidentally they don't need a high pressure oil feed - they pressurise themselves when immersed in oil due to the squidging (tech term! - as in step 3 in my attachment - that's to get it started but it happens automatically when the engine is running) up and down with the valve (in the tappet) and very fine clearances for oil to escape. Sort of hydraulic lock with a relief channel.

Oh - VVT-i - the i is just the fuel injection.

And another 'incidentally' - the proper operation of hydraulic tappets is very dependent on having fresh clean oil. Old oil deposits gum in the tappets and makes them noisy on start up (and all the time if they get really bad) because they can't adjust themselves quickly enough. And if you get them really gummed up they are pretty much impossible to clean - see the comment about 24 x £70! Change the oil at least twice as often as Lexus recommend if you want your engine to last!

[Vintagesixtysix]

John, what's your thoughts on oil flush on a 2006 manual 85K with not 100% service history ( before I had it ).....probably never had a flush in it life. I've been dropping the oil on average every 7k ish for past few years over last 40k.

I don't have too much chatter/ rattle at start up.

Do you think good idea.....well overdue or could cause an issue if gunk build up on tappets?

Phil

Sent from my Iphone using Lexus OC

[johnatg]

I think it's always a good idea to flush (with flush additive) at every oil change. There are reservations if the engine is really coked up but that's not likely if the oil has been changed regularly within the standard 10K service interval (and the correct fully synthetic oil has been used).

BTW - a couple more 'incidentally' items to expand on my previous post.

1) Although VVT is often applied to intake side only, I might have mentioned that IS250 engines have VVT on both intake and exhaust camshafts - hence the valve timing gears on each camshaft in Steve's post above.

2) IS250 engines have an interesting valve operation system. Often OHC engines have bucket and shim tappet adjustment - the bucket simply sits on top of the valve and there's a shim between the camshaft and the bucket. These work well but are a pig to adjust and especially would be with the confined access to some of the valves in the IS250 head. Instead IS250s have the valve offset from the camshaft and the lash adjuster (tappet) is at the opposite end of a rocker. The camshaft bears on the centre of the rocker like so: (there's no rocker shaft - the rocker pivots about the lash adjuster head and is not positively attached anywhere)

[johnatg]

Ah - they're not really shown very well in Steve's diagram (which isn't of a IS250 engine). Here are the valve/camshaft timing gears, etc:

[Shahpor]

Excellent info John.

I am a little confused though.  Does the Lash adjuster use oil to vary the lift?  Does it also adjust valve opening times?  Lastly, if the rocker arm is not attached, how is it held in place?

Thanks.

[johnatg]

The lash adjuster doesn't vary the lift - there is no lift variation in IS250 engines (or in the vast majority of internal combustion engines generally. To vary lift you need a different camshaft or shafts, with reprofiled (ie higher) lobes) - there are other ways of doing it but beyond our scope here - and they are very complex and expensive). And it doesn't adjust valve opening times (vary the valve timing - that job is done by the timing gears on the front of the camshafts in the diagram).

The rocker is held in place by the fact that it has little cups in the ends which take the lash adjuster tip and the valve end. Then the cam lobe sits on top of it, exerting no pressure when the valve is closed but considerable pressure when the valve is open. Even when the valve is closed the rocker arm can't move out of place because of the presence of the cam.

The lash adjuster only provides a little cushioning between the valve and the cam. When under pressure it is like a cylinder full of oil - effectively solid and incompressible - the one way spring and ball valve closes. But it has a tiny leak facility through a very small channel. When not under pressure it can refill with oil (it sits in a recess full of oil and the one-way spring and ball valve relaxes). All this happens very quickly of course - multiple times a second - so that the lash adjuster finds its own length and this can vary depending on the length of the valve. The valve length varies with manufacturing tolerances, fitting (grinding-in) and, during service, temperature. The high pressure chamber shown a few posts back is only at high pressure then the lash adjuster is pushed down by the cam.

If you didn't have a lash adjuster and the tappet was absolutely fixed length or the cam bore directly on the valve, there would be varying clearances between valve and cam due to the factors above (tolerances, temperature etc) and the valve gear would be noisy or the valves would not close properly and burn the seats and the whole shooting match would be impossible to adjust. The hydraulic lash adjuster does the adjustment automatically.

[Shahpor]

Of course, what I am I talking about   Sorry, I am not normally this silly, I just got a little confused.

If it wouldn't confuse the thread, I would remove that post immediately!

Not sure where I got the idea of variable lift.  The only engine that I know of that does that is Honda's V-TEC, and that is a different cam lobe.

Again, somewhat embarrassingly, the timing adjustment gear is clearly marked on the diagram you posted. 

Believe it or not, I do know how a hydraulic tappet works, contrary to what you might have read!  It makes perfect sense now.

It is a rather novel system and I like the fact that it is self adjusting to compensate for wear.  Certainly beats the eccentric lobes the rocker arms had on my old BMW!  Even using a feeler gauge, they would be quite noisy at the best of times and needed adjusting on a regular basis.

Thanks for taking the time to explain it for me John.