While rebuilding my valves, I noticed that 4 out of 6 exhaust valves on one side of the V6 engine were leaking after lapping and I had to relap them. I determined that by doing a water test. I am guessing they were leaking even before (IOW that the leaking wasn't a result of a poor lapping job) which sounds tenable because the exhaust ports looked like a mess before I cleaned them. The fact that none of the intake valves leaked led me to a conclusion that exhaust valves take more heat so they are more likely to need readjustment.

But what are some effects of leaky exhaust valves? E.g. I underwent the complete head gasket and valve rebuild procedure because my engine was overheating and the head gaskets got fried. My speculation was that was because of a stuck thermostat that caused overheating. But could it have been due to leaky exhaust valves?

  • If they are still leaking after lapping, you need to have a valve job done on your head(s). Exhaust valve leak would not have caused the overheating issue. Commented Jul 27, 2015 at 17:14
  • they are not leaking after lapping and i am familiar with what you are saying, my question is what happens when they leak over time, independent of servicing them, let's say 5 years down the road
    – amphibient
    Commented Jul 27, 2015 at 17:19
  • Overheating and someone told you it was the thermostat. 10 out of 10 times its not the thermostat. That's a good way to drive off any help. I literally have nothing to say. If I did you would believe the guy that told you that instead. Commented Jul 28, 2015 at 2:02

4 Answers 4


Under normal use (non-performance type use), the valves, both intake and exhaust, should last the life of the engine. If and when they do leak, it will not take long for the valve to become a burnt valve. In your case, I doubt they were leaking prior to pulling everything apart. If the exhaust valves had leaked, they would have been fried. If valves do not seat correctly, they will not transmit heat through the head as they are supposed to and will become very hot, thus causing the "burnt valve" syndrome. It wouldn't take the engine 5-years for this to occur, but rather months if not weeks for it to actually start causing you issues (if not less time).

Even if the exhaust valves were leaking, this would not have caused the overheating issue you were experiencing. Valves transmit heat into the heads through proper seating. If they aren't seating correctly, you'd have experienced less heat traveling into the heads through valve transfer. As well, you'd been seeing lower compression in the affected cylinders due to bleed off during the compression stroke (slight bleed off until a valve gets very burned, but bleed off none the less). Due to this loss of dynamic compression, you'd have not been seeing as good of power coming from that cylinder, as well as a reduction in heat. This reduction would not have been great, but it would still be less than optimal.

During normal use, valves spin as they are used (not spin as in a top would spin, but they do rotate). This actions provides that they do not sit still, helps clean off of the seats and faces, which helps it to continue to seat throughout its lifespan.

I don't think your exhaust valves leaked due to a poor lap job, but probably an incomplete lap job after you mixed your valves up. I'm sure if they were put into the same holes they came out of, your valves would have had no issues what-so-ever. When you clean up your valves during the rebuild process, even though you may keep the valves in the same positions, lapping is not a bad idea to promote better sealing.

  • i didn't mix my valves up
    – amphibient
    Commented Jul 27, 2015 at 17:45
  • That is not what you implied in this question. Sorry if I misunderstood. Commented Jul 27, 2015 at 17:47
  • Regardless of how they got to whatever state they are in, if they are still leaking after a good lapping, the heads need to be sent to the machine shop to have a valve job done. Commented Jul 27, 2015 at 17:58
  • that question was more hypothetical. i did keep track of the valve positions. one mistake i did make was that i didn't track valve lifter positions because i didn't know it was important but now it's too late and I will have to measure valve lash between each valve and cams before redistributing
    – amphibient
    Commented Jul 27, 2015 at 18:02

A leaky exhaust valve means the valve is not closing properly, failed compression stroke and a burnt valve. In some engine types, this burnt valve and the piston can hit bending the valve and causing major engine damage.

If you suspected the thermostat, you should remove it and you can drive the car. Just warm up the engine first as it will take longer to reach operating temp. You can determine if this is the issue in one drive.

Note. The thermostat is more important in colder climates for warming up your engine.

If the thermostat was the issue, the engine will no longer overheat once removed. If the thermostat was good, the engine will still overheat as the problem lies elsewhere. Removing the thermostat is much much easier than pulling the head.

Obviously, a major overheat usually means failed head gasket, warped head etc. But overheating means inspect the cooling system and look for leaks or loss of coolant.Find the source of the overheat is important. Check oil level and colour also. Waterpump, Thermostat, Radiator, Coolant, Radiator Hoses.

Note, things don't last forever. May be better off changing the waterpump, radiator hoses, thermostat if you have given the system a proper flush. The radiator can be tested out of the car to know if it needs replacing.

I troubleshooted an overheat in my turbodiesel several times with several issues over the years. Lots of fun and games. On all occasssions, it was after the event and modern diesels are not forgiving to an overheat. Preventative maintenance,regular servicing and hindsight.

Hope that helps.


Overheating... I agree has nothing to do with the components that do the "breathing" of the vehicle. Thermostat is easy to diagnose but cheaper (time vs $) to just replace and verify diagnosis. If the water pump works and the radiator is in solid condition my concerns would be head-gasket (warping of head due to heat) and clogged water jackets. A proper flush 9/10 will eliminate the clogged jackets.


Lapping can be difficult, even the pros don't get it right the first time. You fixed it right? A lot of guys have to go and re=lap a vale or two. As long as it is fixed now and is fine your did a GOOD job. Live and Learn right?

I do not think your overheating problem was due to leaking exhaust manifold nor do I think exhaust valves are causing your overheating problem. The car is overheating due to cooling system problems. Usually the radiator gets rusty over time, this rust breaks off and gets clogged in the thermostat. This causes the thermostat to overheat and fail. The thermostat will only open at a higher temperature each time. This is called spiraling and is a sign that the thermostat is failing. Orange or red colored water is a sign of rust.

I recommend to do a cooling system flush/ radiator flush. Follow instructions, sometimes a second application is needed. Then I recommend to add a cooling/radiator conditioner sold close to the flush. This is a chemical that does not need to be flushed and is safe to use with your antifreeze mix. It will take a couple weeks to lubricate and condition all the rusty spots in the cooling system etc. The condition usually says it stops leaks too, so if you have a seal starting to leak or a pin hole in the radiator from flushing it will seal it for you and extend the life of your vehicle many years.

P.S. If your valves are going bad it is usually due to warping caused by overheating. You should have bought all new valves, you got lucky the intake valves were not warped as bad as the exhaust.
I would assume the intakes did not warp because they are exposed to cool air from the intake etc while exhaust valves are only exposed to hot air escaping the cylinder. This could be the reason they warped and not the intake valves.

I hope this helps...

  • 1
    While this is decent information about overheating, it really doesn't answer the question at hand. Please reread the question and see if you can improve upon your answer. Commented Jul 27, 2015 at 17:51
  • Sorry forgot to write the first part. Thanks for pointing it out to me. Commented Jul 27, 2015 at 18:49

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