So you think running on is OK !!

SKiddell

Well-Known Forum User
Whilst striping a head down yesterday from a well choked and coked up engine that had typical "run on" characteristics.

See the “pitting” damage all around the chamber and the seat recess, all done by the untimed detonation of the fuel/oil mixture self-igniting (dieseling) probably due to hot spots.

 

zNathan

Well-Known Forum User
Hi Steve, I'm a noob when it comes to the technical stuff. Have just read up on 'running on' (or dieseling??). That photo looks scary!

Is it possible to get the same damage as this whilst the engine is running normally? Sorry if this is a stupid question...
 

SKiddell

Well-Known Forum User
You could get similar(ish) damage through mild detonation, detonation and "run on" have similar crossover areas, both are a result of uncontrolled ignition of a flammable mixture outside of the ignition cycle, detonation though is ultimately more destructive as cylinder pressure rises to huge unsupportable levels due to multiple shock waves produced by an explosion rather than a controlled burn.

The clue with this engine was the oil contamination in the chamber, add this to a little fuel and it cokes up into a sooty mess which then creates hotspots which when the engine is turned off ignites any available fuel/oil….hence the term dieseling




But to answer your question, a well maintained/well adjusted stock(ish) L series shouldnt incur damage of this nature
 

richiep

Club Member
Hmm. Interesting. Considering my car's propensity for running on (I gently stall it out before it gets a chance) I shall be having a close look at the chambers when I pull the head this winter (it needs a new gasket and the head checking for straightness).

What sort of mileage had that engine got on it Steve?
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
Yes that is shocking Steve, was it on all cylinders or just one?

I'm not saying running-on is desirable but as you know and documented on other Threads my engine ran-on for about 12 years (no not continuously :rofl:) without any sign of damage (I did have the head off after about 10 years and it was fine). It 'ticked-over' slowly too without any sign of detonation. I'm guessing that your post was aimed at me and thank you because this is shocking and I'll now consider a valve on the inlet manifold of my blue car which also runs-on.

Are you 100% sure that the damage was caused by the engine running-on? To be honest that engine looks like it's been running on it's oil too, no wonder it was dieseling. The exhaust valve seat is incredibly narrow too isn't it?
 

SKiddell

Well-Known Forum User
Richie, the engine was of a galactic mileage variety


No Rob....Its not aimed at you in particular, the thread the other day highlighted some myths and hyperbole which prompted me to think that it would be a good subject for another technical thread which is my area on the forum.

"running on" is a symptom not a cause and as such there can be many causes, this one was in part oil related and has been used for illustrative purposes.

But in no way is ANY form of running on a "good" thing as "running on" is when the ignition is off and remaining available fuel is being ignited by other means and as such is not at a desired point in the crank cycle.........thats why when an engine "runs on" you get that horrible bottom end death rattle meaning your bearings are getting hammered as well.

without any sign of damage (I did have the head off after about 10 years and it was fine)
as mentioned the damage can be elsewhere such as the bottom end, piston lands and rings, bores etc
Some of this early damage is difficult to spot particularly by the untrained eye.
 

richiep

Club Member
Checking carbs/fuelling might be another factor. Apparently, carbs tend to be more prone to contributing to running on as the fuel supply isn't shut off with the immediacy that injectors are. The vacuum can still pull excess fuel/air in after the key is turned off, which then detonates on contact with hot spots.
 

Stockdale

Club Member
Whilst striping a head down yesterday from a well choked and coked up engine that had typical "run on" characteristics.

See the “pitting” damage all around the chamber and the seat recess, all done by the untimed detonation of the fuel/oil mixture self-igniting (dieseling) probably due to hot spots.


In my many years I have come across pitting similar to the image. It was usually due to excessive and prolonged engine revolutions at or near to the maximum recommended. Don't suppose you have been pushing the old girl a tad too much?
 

pmac

Well-Known Forum User
Yes that is shocking Steve, was it on all cylinders or just one?

It only occurs normally on the back (no 6) cylinder.
This is the 3d or 4th head I have seen with the same symptoms.
These long L6 heads do not cool so well towards the rear so excessive temps can be seen. This adds to the risk of "det".
The waterways get clogged up over time with corrosion sludge and sometimes old casting sand coming loose, thus reducing flow in the head particularly at the rear.
It is one of the reasons why Skiddell, Z bloke and myself go to the trouble and expense of having blocks and heads hot Caustic dipped or acid dippped before building a quality engine to ensure we maximise water flow/cooling.
We were fortunate at one stage in being part of an experiment involving multiple sectioning of a head to try and understand the "flow problem"

We have highligted this issue as good unmolested undamaged heads are becoming harder to source.
The head that Steve pictured is beyond economic repair at current values.
Hoever the desirable N42 we have with a similar but less severe issue has been put to one side till I can get our machinist to repair it

What sort of mileage had that engine got on it Steve?

I only acquired the head in a job lot purchase there was no block/bottom end.
Why not ... well given the severity of head damage one could assume with some confidence that the det destroyed the piston crown with catastophic results for all the rest of it. Speculation of course but very possible.

So if you are having uncontrolled explosions inside your engine its not good and needs fixing while its still fixable:D
 

morbias

Well-Known Forum User
The head on the engine that was in my car when I got it had the same sort of damage to cylinder 6! Some of the valves in the other cylinders were beaten up something horrible too.

IMAG0276.jpg
 

SKiddell

Well-Known Forum User
In my many years I have come across pitting similar to the image. It was usually due to excessive and prolonged engine revolutions at or near to the maximum recommended. Don't suppose you have been pushing the old girl a tad too much?


Prolonged high rpms would do damage to piston lands, bores, bearings and valve train, not the chamber, if your seeing chamber damage at high rpms its due to poor mixture and/or ignition timing .......cause does not equal correlation

No, my all steel engine is built to rev out in excess of 9K and cost more than most peoples cars, this is from a stock daily driver.

A well manitained stock L6 will sit at the redline for a lot longer than you think, by design its extreamly well balanced
 
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