They've got CSI Las Vegas and CSI Miami, now there's CSI Felton.
I'm fairly certain that the ultimate root cause is predetonation. On
this incarnation of the motor, I've tried several combinations of
carburettion and timing, and the way that I'd find that they didn't
work was that I'd hear that all to familiar sound like bbs rattling
around in my cylinder. Judging by the condition of cylinder #3, that
sound may not have always come from gases burning too fast.
http://red4est.red4est.com/jasmine/postmortem030608/pict1555N.jpg
Since I got the crank polished, after blowing an oil seal, oil
pressure on this motor has never been quite what I'd like. At the
start it would only hit the cutoff valve of 75 psi when cold, but it
would generally run at 65psi at speed.
The pressure has been dropping for a while, a little bit at time. Up
until the last weekend I drove it, I'd usually see at least 45-50 PSI
once the engine was up to speed, even if it was hot. Over the course
of my last weekend (at Infineon nee' Sears Point) it got to the point
that at speed, I'd only see 25 psi. Despite having had a lot of fun
beating up on an X1/9 in the open track group, I stopped driving
Jasmine in the fast sessions and I suckered ^h^h^h^h^h^h talked my
friend Jane into letting me drive her car during lead/follows with my
students. By the way, a 150 hp Del Sol powered 1989 Civic Wagon can be
a rather fun car on the track. It should be very amusing to see what
that car would do with better than a stock suspension on better than
stock tires. But, as I"m so good at doing, I digress.
The other symptom, which I now realize was significant, was that over
the last few weeks I had been driving my car, the blowby had increased
dramatically.
My forensic analysis is thus:
Testing various carb needles, and ignition timings caused some
predetonation. Since cylinder 3 is one of the middle cylinders, it's
better insulated. than 1 and 4. Since it is in the back it gets a
little less cool air to the block than #2. Therefore, it took the
worst damage from the predetonation.
Since I was pretty careful about backing off at the first sign of
predetonation, I only did a little bit of damage to the edge of the
piston. Therefore, the damage was not immediately obvious. Since I was
experimenting with a carb needle leaner at the bottom end (BBC) than
the one I had previously figured out (BCV) trying for better mileage,
I may have gotten things hot then, and finished the job of causing
damage. Either that, or it was entirely coincidental that the damage
manifested itself after playing with the BBC needle.
Whatever the cause, the damage to the edge of the piston finally
reached the rings. (or maybe the predetonation busted the rings and it
just took them that long to work their way out). Eventually, little
bits of metal found their way both to the top of the piston as in the
picture already shown and to the oil pan:
http://red4est.red4est.com/jasmine/postmortem030608/pict1558N.jpg
http://red4est.red4est.com/jasmine/postmortem030608/pict1565N.jpg
Fortunately, the cylinder head seems to have escaped unscathed:
http://red4est.red4est.com/jasmine/postmortem030608/pict1566N.jpg
Enough metal bits however, were small enough to get through the screen
for the oil pump. They were not small enough, however, to get through
number 5 main bearing (isn't that the one that pressurizes the crank?)
and caused exessive wear:
http://red4est.red4est.com/jasmine/postmortem030608/pict1577N.jpg
http://red4est.red4est.com/jasmine/postmortem030608/pict1578N.jpg
http://red4est.red4est.com/jasmine/postmortem030608/pict1579N.jpg
http://red4est.red4est.com/jasmine/postmortem030608/pict1580N.jpg
Hence the low oil pressure.
My full set of pictures are at:
http://red4est.red4est.com/jasmine/postmortem030608/
click on the thumbnail to see the 800x600 picture
If you have a morbid curiousity the 1600 x1200 shots are at:
http://red4est.red4est.com/jasmine/postmortem030608/fullres/
Tomorrow, I need to take the various pieces to the machinist to see if
I need to buy a new piston, or if I need to buy a Miata to abuse while
I come up with the money to build a whole new motor. Cylinder #3
doesn't look too horribly damaged, and I'm hoping beyond all reason
that I'll be lucky and will be able to get by with what I've got. I
also have the irrational hope that when venolia made my last set of
pistons, they recorded the weight that they were balanced at and that
they can make me a set of five (or six), as I know they won't make
just one, so that I can simply put one in without having to disturb my
three undamaged pistons.
Or am I being unduly optimistic and ought to pull all 4 pistons to
check for damage to the rings?
How do I tell if the oil pump has been damaged?
What is the wisdom of the list regarding making the high flow
modification to the oil pump per the British Leyland special tuning
book?
--
Girling is not a verb.
lrc@red4est.com http://www.red4est.com/lrc
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