icanfixall
Official GMM hand model
This pic of the intake valve is clearly recessed too deep in the head. I'm surprised the valve spring was able to close the intake valve...
TECH 101 engine chuff/ valve breaks POST MORTEM
- Thread starter riotwarrior
- Start date
gdhillon
Full Access Member
A year eh? crazy I wonder how long Al's truck woulda ran with this issue
idiabuse
Full Access Member
Classic case of RUN TO FAILURE. That engine had been bad for years before it killed itself.
Javier
Javier
gdhillon
Full Access Member
But it was still driveable, wasn't it?
The answer to your question is in the video
http://youtu.be/B9Ui7Fp8QYw
and then I said this a few posts later...
Guide wear is something that comes with either MILEAGE or with TIME/HOURS of engine operation, this truck was moderately low miles, when I got it 197000 miles original however one caveat, it was used as a service truck for a logging company and IMHO sat idling away for hours at a time in the bush.
This continuous idling will wear items out as well.
The thrust of the rocker against the valve induces a sideward thrust that wears out guides in time or miles as mentioned.
As hot exhaust gasses pass over an open valve that super heats the edge of the valve and when it closes it transfers heat to head. This is one of the reasons HI EGT can have a negative effect on these engines. IT can and IT WILL destroy pistons and valves in short order!
The crack came from heating and cooling and contacting the valve seat in an uneven manner causing hot and cool spots on the valve making it more susceptible to cracking or other damage.
As the guide it self wears and more exhaust passes up the stem, there is more and more soot causing the oil to become abrasive and wear the guide faster exacerbating, the pre-existing condition and making matters worse.
Had I been able to get a set of heads done valve grind and so forth in time I could have saved this engine, and it would have been a very good runner based on the compression values I achieved with my tests.
In essence the cracks came after the guide wore out not before the guide wore out.
Now does that answer your question?
If not I'm not sure what more I can say. Bottom line I changed oil regular, I'm second owner of truck, it was a logging service truck and well lets say spent a great deal of time idling away at log landings etc.
Watch the video, read the entire thread and you will pick up lots of great information from several members
Al
My old 7.3 I had, had a chuff out the exhaust. I did an compression test on it, all 8 came back good 440 plus. I decided to pull the heads to rebuild them since the engine had 310,000 kms. All the exh valve guides were as bad as riots like BAD. I was told by the rebuilder that knurling was a bad thing, and that replacing the guides was the way to go. A compression test sometimes wont tell you if your chuff is caused by bad guides. I put the engine back together and the chuff was gone. When I rebuilt my 93 turbo engine the intake guides were as bad as the exh guides so both of them can wear just as bad.
gdhillon
Full Access Member
Where in Canada are you from?
Good post BigDiesel. I have an intake chuff. It is quite noticeable with the lid of the air cleaner partly lifted. Compression test a few months back showed good compression on all 8 (440-455, 475 high). I suspect it is going to look like Al's right bank when I pull my valve covers. 
Knurling a guide is a reasonable way to get stock level performance out of a head with slightly worn guides. For ones as bad as mine, there is no way to knurl them as they are way to far gone and replacement is the ONLY option.
Knurling works and works well for the most part, however in this situation with engines like ours I'd not think that it would be that good an option even slightly worn guides.
I'm not sure why these heads wear as they do perhaps bad valve geometry I don't know but I'm wondering if a nice bronze guide may offer better performance instead of cast guides....Anyone have any ideas on that?
Al
Alan Slingsby
Full Access Member
This is a great thread,lots of learning go'n on for me!! Thanks!
idi_econoline
No Bed For Me!
Same here, Al. Had no idea about the whole topic until seeing the revived "Chuff" thread a few months back. Have been driving my beast gently since then, but your experience has convinced me to park it until my long-procrastinated compression test and engine pull.
The screwy part about my heads is that the chuff started AFTER having my heads fully redone by a regionally-reputable head shop. Guides should have been replaced at that time.... stay tuned.
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