2nd burnt exhaust valve in under 2 years! Help!!

[twirken]

Just found out I have yet another burnt exhaust valve on my 4.0 V8 in under 2 years.

Both heads were replaced with rebuilt ones in June of 2016 when cylinder #2 valve burned up.

Now it is cylinder #1.

Could really use some help diagnosing why this is happening.

Have always ran 89 octane fuel in this truck for past 15 plus years so not sure why that would cause this.

Am I looking at to much fuel being injected? If so how to test that?

What are causes for this happening on these motors?

Thanks for the help!

 

[robertf]

so its always the front edge of the front cylinders?

low coolant level combined with hot spots from preignition leading to a runaway temperature spike?

 

[twirken]

Yes they have both been on the front cylinders. First time was #2 and now #1

Coolant level was a little low when this happened but temp gauge never rose above normal.

 

[discostew]

I think the valve guides are too tight. We used to run a 7 micron larger reamer thru the exhaust guides. If they didn't do that when they did your valve job that could be your problem.

 

[ezzzzzzz]

Running lean or crappy fuel could lead to this but I think you'd have seen other symptoms like poor idle or acceleration too. Seems this would possibly lead to a misfire code in the ecu too due to changes in crankshaft rotational speeds. In your case, not knowing what constituted a rebuild no one can say for sure. Maybe they knurled the guides to tighten them up? Maybe the valve faces and seats weren't mated properly? It sure sounds like whatever they did in the rebuild is the source of your problem.

 

[lrover94]

What he said. Lean run condition. Did you replace injectors?

 

[twirken]

Thanks for all the insight. I am having the head checked out as we speak. Then I should know if it was a mechanical fail with the valve, seat, or guide. Then I can start at looking at fuel etc... leaning toward the fuel at this point.

I did not notice anything wrong both times this happened. I was just on the highway and then all of a sudden was running like crap and then when I slowed down really noticed rough idle etc...

Same issue both times. The truck does burn a bit of oil and before the valve issue on #2 I had a fouled plug but no fouling since rebuilt heads went on.

So what is the best way to check injectors to rule out the running lean possibility? Could the ECU be acting up as well?

Frustrating problem as there are so many possible reasons.

Thanks!

 

[ezzzzzzz]

For what it's worth, I have driven rovers for 25 years. Only on rare occasion do I run premium fuel. When petrol was upwards of $4@gal I ran regular. I've never had engine problems related to low octane fuel.