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#680753 - 02/12/13 06:00 AM Timing belt broke: best way to test valves?
Soul Man Offline
Junior Member

Registered: 02/12/13
Posts: 2
2000 Xterra, V6. Timing belt broke driving 50 mph. Engine turns over. (Belt only has 60k on it)
If my valves are bent, I need to scrap the truck. What is quickest, cheapest way to test? I don't want to replace the belt and then test it, don't want to take the time to pull the cover so I can rotate cams to do leak-down or pull the head covers to check valve clearance.
Thinking about renting a bore scope to look in cylinders for evidience of valve hit on pistons? (If I can find a scope to rent).
I did a compression test on 5 cylinders (rear driver's plug is SOOOO hard to get out, even after I took hood off, using mirrors etc) and 45 psi on 4 cylinders (one has 60). I figure some/most(?) should have the valves closed at any given time, so all of them being low would mean certain valve damage. Is this line of thinking corect? (compression gauge is a Snap-On, so should be accurate).

Any other ideas on confirming if my valves are shot??? I'm pretty confident they would be, but have read via my searching of people who had belt break while drving and only had to replace belt with no head work, so I figure it is possible....

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#680756 - 03/12/13 02:58 PM Re: Timing belt broke: best way to test valves? [Re: Soul Man]
e207 Offline
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Registered: 28/06/09
Posts: 549
Loc: Ashland, PA U.S.A.
It IS an interference engine, it means it CAN happen, Different situations will make a difference of weather or not it will happen. so yes there is a good chance of damage, but there have been many people who have lucked out without getting damage. Bore scope may help, not sure if you would be able to see all good enough to be sure though. I would think you would have to pull enough off to at least turn the cams so that you know when the valves are closed on a cylinder. even though the belt is broke, the position of the cam's would still keep some valves open.

I've never had to deal with this situation so cannot give you confident info, just going by what i've read on these sites. I'm sure someone who knows a lot more than me will chime in and help out more than I can. but just wanted to try and give you something to think about til then. good luck.
_________________________
2002 SE/SC R.I.P. March 2015
Bielstien's
Shrock Sliders
Reman'd engine installed 5/16/09 at 87,894 miles

Captain 38-90
Member of Fire District-38 Station-1 since 1991.
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http://www.pioneertunnel.com/home.shtml

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#680759 - 03/12/13 05:50 PM Re: Timing belt broke: best way to test valves? [Re: e207]
Xwrench Offline
Member

Registered: 08/06/13
Posts: 21
Loc: Mount Laurel, NJ
Several issues with regards to the compression test:

All sparkplugs MUST be out.

The throttle plate MUST be wide open.

AND-- you need to have a battery charger connected for consistency.

The test isn't really valid with 1 plug still in.

Forget the bore scope, in the time it takes you to go get one, you can have that last plug out.

Happy to help, I am a mechanic with 30-years experience.

Brad

btw, where are you located?

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#680789 - 18/12/13 01:11 PM Re: Timing belt broke: best way to test valves? [Re: Xwrench]
Soul Man Offline
Junior Member

Registered: 02/12/13
Posts: 2
UPDATE:

1.) 2000 V6 Xterra is NOT an interference engine (as best I can tell)
2.) Compression test was silly idea, since if intake valves can't open/close no air can get in so won't build compression. My thinking was that some cylinders would have all valves closed and I'd get compression, but no working intake means that isn't valid...

Cams are VERY hard to rotate with any pressure on valves, so they tend to wind up in an all-closed position: good news for anyone who breaks a timing belt means VERY good chance cam will quickly stop with all valves closed (ie. mechanically unlikely that it will come to rest with valves open).

Put #1 at TDC using borescope to make absolutely sure at absolute TDC, then rotated cam and could easily rotate cam through several rotations (using a socket). Does not appear that valves touch piston (at least nothing that prevented me from rotating cam 360 deg).

borescope was helpful in that I could confirm (can see both valves using the angle mirror on the scope) that all vavles were closed, so that made me feel more confident in taking the time to pull the timing cover. Also, didn't see any signs of valve hit on the pistons, but piston was surprisingly hard to see (could not see all of the piston surface)

Anyway, if you break your timing belt there is a very good chance your valves survived, and IMHO it would be worth replacing the timing belt and see if it runs (mine runs great).

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#680790 - 18/12/13 04:39 PM Re: Timing belt broke: best way to test valves? [Re: Soul Man]
e207 Offline
Member
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Registered: 28/06/09
Posts: 549
Loc: Ashland, PA U.S.A.
as far as I've heard, ALL 3.3 Nissan Xterra Engines are an interference engine.

as far as the valves all closing at the same time? how is that possible for all of them to be closed at the same time when the cam lobes are designed to make sure some are closed and some are open as needed, I dont really think there is a position that the cam could be in that all valves would be closed at the same time. yes for some cylinders, but not for all of them.

valves may not make contact when turning slowly like that. the interference engine means that the valves CAN make contact, not saying for sure they will. when things let loose at speed, higher rpms it would be possible for the valves to make contact.


Edited by e207 (18/12/13 04:41 PM)
_________________________
2002 SE/SC R.I.P. March 2015
Bielstien's
Shrock Sliders
Reman'd engine installed 5/16/09 at 87,894 miles

Captain 38-90
Member of Fire District-38 Station-1 since 1991.
https://www.facebook.com/groups/172465012765747/
http://www.pioneertunnel.com/home.shtml

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