Conditions that lead to bent valves

[rtd111]

94 SC400 185K miles (Non interference engine)

Sorry could not find much in this forum or the web about this so apologies in case I missed a similar case.

My mechanic just told me that I have bent valves. Before I go into the history, a simple question:

- what are the conditions, other than perhaps a timing belt issue, under which valves get bent? Any and all ideas welcome (goes without saying, but theories need to be backed by mechanically defensive reasoning and data :-) ).

Details:

1. Car was parked for over two weeks (occasional use for 1-2 months prior to that - driven less than 1000 miles in this time frame). No engine / mechanical problems with the car before this.
2. On starting, heard a clunky, metal sound and car would not start
3. I was not the one that experienced this. My mistake was not cranking the engine when I got to the car (to limit damage) and started the following ....
4. I knew the car was overdue for a timing belt (last change was at 75K miles). I had even contemplated ordering TB parts a week prior to this incident (I did the TB work on my LS400 a couple years ago)
5. I assumed it is the timing belt, so started disassembly while I ordered the parts
6. I saw that the timing belt was ok (not broken, but it had quite a bit of wear). I did not think about checking slippage or such. I just assumed since it hadn't split, it wasn't the issue that caused the car to not start (default action of the non-interference engine).
7. So I figured that something else could have gone wrong. After some research and discussing with the person that experienced (2), I suspected that it might be the starter. So I started disassembling to get to it (you can read in this forum or on the Internet what a massive PITA that is).
8. Well, this work ran over 3-4 weeks because I work away from where the car was, could only do this on the weekend, and I was soon to leave on a 25-day international trip
9. So I reached out to a mechanic that we've had good experience with (on a non-Lexus) and towed the car to him. (BTW, I had meticulously organized the disassembly which he greatly appreciated but assured me that even if I had taken the parts in a box, he can handle it, given he was doing this for 35+ years. He also knew some of the nuances of the car, saying he owned a Lexus himself. Basically, he gave me confidence).
10. While I was away, he put together the new starter, reassembled the manifold, etc etc as well as the timing belt/waterpump/rotors/plugs/etc. However he reported that the car would not start
11. He said he did a compression test and 3 of the cylinders came up low, and he assessed that the values are bent. He couldn't tell me what would have caused this.

So,

- can you tell if this was this the problem in the first place?

- OR, was the timing belt not done right causing a timing issue leading to the valves to be bent? is it even possible with a non-interference engine?

The cost to do the valves is quoted at $2K. (The starter, TB and related labor was $800 -- reasonable I believe.) 

Is it worth doing the valves or is it time to junk this beauty?

Thanks in advance for any feedback or guidance.

 

[TrevC]

Low compression on three of the cylinders would almost certainly confirm bent valves or a problem with pistons/bores.

The belt may have shredded a tooth or two at the crankshaft pulley which to all intentions looks like the belt is still intact but this is common on engines when starting up.

Good luck with it all and keep us informed of the progress.

Cheers  Trevor

[rtd111]

9 hours ago, Trevor Catt said:

Low compression on three of the cylinders would almost certainly confirm bent valves or a problem with pistons/bores.

The belt may have shredded a tooth or two at the crankshaft pulley which to all intentions looks like the belt is still intact but this is common on engines when starting up.

Good luck with it all and keep us informed of the progress.

Cheers  Trevor

Thanks for the response.

I thought that this non interference engine is protected from all timing belt and related "mishaps". Not the case then? 

I understand that the engine can give out without help from the other subsystems, but given how strong these engines are, given low mileage, given the car has been well maintained, and given it's never been driven harshly (at least not for the last 120K miles I've owned it), I am a bit surprised at this bent valves situation. Of course, it could be luck of the draw. My 91 LS400 is at 240K miles and still drives like a dream (but has recently developed engine oil leaks - I'll save that for another day).

 

[TrevC]

I would ask them to perform a cylinder leak down test which would confirm if the air is escaping past the rings, valves or through the coolant