My M715 has the same problem, sometimes it diesels for a second or two,
sometimes unless I stall it out it will go on longer. I tried tuning it up
according to the manual but did not seem to solve the proble. I've been living
with it for 5 years and while its not good I have so many other things like
clutch, oil seals etc that I 'm just going to do a rebuild sometime in the
future. One thing that did seem to make a difference was I had to tow a couple
of M37s last month and to keep my engine knock down I put premium gas 91 to 93
octane. Much to my surprise the darn thing did not diesel the whole week. Did
I burn out all the carbon running it hard? don't know. Thank you for the other
tips,
Tom B....
______________________________ Reply Separator _________________________________
Subject: Re: [MV] Need NAPA part number for M715 part. ctx, also advi
Author: "Richard Notton" <Richard@fv623.demon.co.uk> at SMTPLink-TECOM
Date: 9/29/98 8:07 PM
-----Original Message-----
From: Rick Green <rgreen@mail.datasys.net>
To: mil-veh@skylee.com <mil-veh@skylee.com>
Date: 28 September 1998 00:31
Subject: [MV] Need NAPA part number for M715 part. ctx, also advice
needed.
Hi,
> The other question is:
>
> When I turn off the truck, it wants to diesel for about two seconds.
>Also if I try to turn the truck on with the switch in the off position,
>it tries to diesel. What would cause this, if valves are causing it,
>great, I can fix that. Anybody ever had this happen to you, what did
>you do to fix it??
>
I'm not familiar with this particular engine but dieseling is caused by
small over-heated spots in the combustion chambers, usually an excessive
carbon build-up.
The carbon on poorly cooled spots in the head, typically sharp
protuberances or casting lumps, become incandescent and act like a
glow-plug, the effect can be exacerbated by weak idle mixture,
mis-timing or choked water channels as all, or any of these will make
the combustion spaces hotter than designed.
Badly fouled or plugs of the wrong grade can also lead to the effect in
the same manner.
Some engines are more prone than others owing to combustion space design
and the final crop of pre-injection but lean burn car engines here were
so prone to it from new that a solenoid valve was included to open the
manifold to air at switch off to ensure the engine stopped, I suspect
some US cars were the same, with the advent of electronic systems,
injection is simply inhibited at switch off.
Excessive compression ratio and low grade fuel is another cause but
highly unlikely in an older military engine although a heavy carbon
build-up will raise the compression ratio some.
If you are unsure of the history of the engine a decarbonisation would
be in order anyway with an accurate re-setting of the ignition timing
and I note another post suggested a tune-up helps which suggests
mis-setting of the idle mixture and spark is a likely culprit.
Oil consumption needs to be considered too as a worn engine with
ineffective inlet stem seals will quickly become carbon fouled again and
the symptom will return.
It is not advisable to assume todays criteria apply to the older gas
engines, they required comparatively frequent servicing, adjustment and
regular de-carbonisation owing to the designed-in oil consumption.
Regards,
Richard
(Southampton UK)
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