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Bent Valves Q.


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6 replies to this topic

#1 markjw

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Posted 24 November 2011 - 10:55 PM

I've been reading the archives about bent valves on interference engines due to incorrectly installing timing belt.

I have come up with conflicting information. Specifically, my question is this.

If valves are bent because of a botched timing belt install, will the cylinder show No compression, meaning absolutely Zero, or will it possibly show Low compression in relation to how bad the valve is bent? Put another way, if one cyl. is showing 125 psi and everything else is showing 185, could that be a bent valve?

#2 Fairtax4me

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Posted 25 November 2011 - 12:27 AM

A bent valve will more than likely show 0 compression. The usual cause for low compression after a timing belt install is incorrect timing. Being wrong by just one tooth changes the timing of the closing of the valve in relation to the position of the piston during the compression stroke, which means there may be less air in the cylinder to compress.

But the fact that its only low in one cylinder does put up a red flag. A leak down test would tell if the valves in that cylinder are bent.
What engine are we working on? Year? Mileage? Reason for timing work?

#3 kidashley25

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Posted 25 November 2011 - 12:52 AM

One of the most common reasons for a valve is bent or broken chain could cause belt.Probable He has no power and will probably not be enough to drive, but the engine is still running.

Replace bent valves and pistons control and head injuries. Replace defective parts and repair if necessary.There are few Subaru repair guides you can see for more info about bent valves and how to replace them.


Edited by kidashley25, 06 September 2013 - 01:07 AM.


#4 markjw

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Posted 25 November 2011 - 01:01 AM

Its a 1997 Outback. ej25 DOHC

I bought this car a week ago. It has sat for a couple yrs because the PO couldn't get it to 'run right' after he installed a New timing belt. The belt was changed out for preventative maintenance rather than breaking while the car was under way.

After trailering the car home, I was able to start it in just a few minutes. It ran like crap. I removed the timing belt without first checking to see if it was even installed correctly. oops!

I researched the installation proceedure, re installed the belt and it ran Much better. It still runs rough, but with no 'tune up' effort and two yr old gas, it ran Ok.

So, I was super busy today but I was able to sneak out early this morning to mess with the car for a minute. I managed to destroy the plug wires while trying to remove them, I removed all four plugs (NGK), propped open the intake and do half of a compression test. I got 125 out of the #1 hole and 185 out of the #2 hole. I ran out of time. I had to walk away from it at that point so I really don't know what 3 and 4 are. (I can't believe how inaccessible the plugs are in this engine/engine bay.) I did this with a remote start switch so the ign was never on. And, this is my first new model Subaru. Everything else I have ever owned is early Legacy's and older.

So, my concern is that since this is an interference engine, the valves were bent during the initial startup a couple yrs ago after the timing belt install. That prompted me to search for symptoms of bent valves. Some say a bent valve will result in Zero compression, others say it will be Low compression.

I will finish the compression test first thing tomorrow. I'll do a wet and dry test. Maybe I installed the belt wrong, too. Maybe I am a tooth or two off.

#5 GeneralDisorder

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Posted 25 November 2011 - 01:33 AM

It entirely depends on how badly the valve is bent - I have seen burned exhaust valves still register 40 psi on a gauge. But when you get readings that low (well below 100 psi) then you need to start pulling heads off and investigating.

It's very possible that a valve is only bent enough to not seal properly and cause a reduction in cylinder pressure.

You need to run a leak-down test to be sure. If it's valves that are leaking the test will tell you definitively where the damage is.

The interferance on these engines is between the valves - not between the piston and the valves. When a belt goes or is installed incorrectly it can cause the intake valves to hit to the exhaust valves.

GD

#6 Fairtax4me

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Posted 25 November 2011 - 01:39 AM

Yeah the DOHC heads are a pain to get the sprockets lined up from what I understand. Check the cam sprockets on the number one side again just to make sure.

#7 1-3-2-4

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Posted 17 June 2013 - 06:20 PM

It entirely depends on how badly the valve is bent - I have seen burned exhaust valves still register 40 psi on a gauge. But when you get readings that low (well below 100 psi) then you need to start pulling heads off and investigating.

It's very possible that a valve is only bent enough to not seal properly and cause a reduction in cylinder pressure.

You need to run a leak-down test to be sure. If it's valves that are leaking the test will tell you definitively where the damage is.

The interferance on these engines is between the valves - not between the piston and the valves. When a belt goes or is installed incorrectly it can cause the intake valves to hit to the exhaust valves.

GD

GD this is true? I always thought it was valve to piston.. Mine went and while I forgot to take a picture it was way off on the drivers side.. like 11 teeth off CW and the exhaust was maybe 7 off CCW, I never heard any sound when I lost power, when I reset the cams and started it up the idle was really rough as in everything shaking, it would idle but  certain throttle inputs if you are too smooth could cause it to stall or slowing down to a stop can cause the idle to dip to low 500's then come back to 750.

 

Having said that.. would the valve guides be ok?






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