As soon as it fires up again I‘ll prepare to sell it and rely on your E in the future when I begin to miss it (which will happen sooner than later…)
And I won’t ever see how fast it goes. I bet it would take it as it did with the previously installed ‚repairs‘. But I don‘t want to ask you to tow me again and go through it all the third time.
Also, without all your help nothing would have been possible, especially not that easily. I wouldn’t have enjoyed it half as much - that being said, the XK is a great engine and I don’t regret a second I spent on or driving the XJ.
**
How did you set it static, David - and to which value?
Nominally; apart from ignition by plug spark, as per ign timing - there is glowing soot, or plain high compression ‘diesel’ ignition. And with enough carbon build-up, compression may be very high indeed - have you checked that lately…?
I didn’t touch timing, although I had to install a new hall sensor, or pick up. I presumed that timing wouldn’t change and as it ran after all the work done it can’t be wrong (maybe something got stuck? Advance?). I did just move it to (I think) 6° where it sparked so I was sure it didn’t move far if at all.
As the head was off there should be less carbon than before.
I‘ll reset static timing which will be the fourth time I‘ve done that successfully on a car and have a look inside the distributor in a week.
If nothing helps I will look at the cam timing and if something moves further I‘ll get it perfect. But I‘m still convinced that it should fire right up.
Also, it couldn’t diesel if the engine was dead cold, could it?
**
Well, David - if the compression is high enough…
Sort of thoughts about running backwards. Normal compression pressure gives a force of some 1400 lbs on the piston - overcome by the starter and engine inertia. If ignition is too early, combustion pressure is added - and, with ign key released, may stop the piston/engine before TDC.
The built-up pressure then drives the piston down - turning the engine backwards. And since the cams and valve overlap is fairly symmetric, the engine is not prohibitively biased for turning one direction the engine may go on running backwards Admittedly not efficiently, the ignition will be late, past TDC…?
I doubt if your case is plain ‘dieseling’ - but as compression and combustion pressure combine with early ignition; temperature increase may cause spontaneous combustion of remaining petrol. Which normally is noticed as ‘pinking’, but engine inertia will still drive the piston past TDC for nor mal running direction.
However at the slow cranking speed; inertia in itself may be insufficient to do this when starter is not helping. And the engine may run backwards as mentioned above…
I can almost believe in that beautiful theory myself…
At least enough to suggest a compression test. And indeed verify ignition timing; the specs for static timing, engine stationary, is seldom given. However 6 deg doesn’t sound excessive. Just for the record; have you checked that the TDC mark is correct - at piston TDC…?
My experience with backward running engines is with model aircraft. Which an backwards at e drop of a hat…
Frank
xj6 85 Sov Europe (UK/NZ)
**
‘Soot’ ignition, glow plug, is improbable with a cold engine
Thank you, Frank, that is my theory as well. A four-stroke can basically run backwards on the same valve timing.
Timing is then 6° after tdc.
It needs some amount of combustible fumes in the exhaust to run on, and something will find its way in from the injectors. It shouldn’t take too much to reverse direction on the last compression stroke, or even just to fire that mixture (Especially if timing was too advanced). This would then run. I just wonder why it wouldn’t fire in the right direction.
Engines will always stop halfway through a compression stroke and might well reverse a few degrees, just not much. If I retarded the timing it might go better, and then I can keep advancing it until it runs well, but this situation should not be happening on a cold engine!
I haven’t moved the timing since it ran before but maybe the advance got stuck, or I did do something stupid and forgot. Yes, the harmonic balancer is right on, one of the first things I verified!
I must have found the timing value in the workshop manual (which gives the procedure) or on the hood. I know federal models differ, etc.
Model airplanes run backwards happily all the time (mind the prop…), but they are two strokes which don’t mind crank direction at all and tend to have glow plugs. Timing cannot be wrong with them as the platinum decides when the mixture ignites.
Saturday will at least produce a video.
Until then, I‘ll have to stop thinking about it and concentrate on other things.
Being away from numerous issues that keep me up at night without any way to solve them is immensely frustrating… so, Saturday will show. At least the Jaguar is a patient subject. The battery is charged and waiting. I will do a compression - and when it runs, a leak down test just so I have the data.
I like your repair quite a lot, (considering the alternatives), and have read that short stud engines are “favored” by performance builders anyway, and not heard the short stud versions were prone to head gasket failure ?
In theory, so long as the head and block are reasonably true, it would seem to me that their would be no significant loss of clamping pressure with your repair, (engineers, or others correct me if I am wrong)
Thank you. I read that the block deck was prone to warp a little (I doubt that), and the long studs are of course anchored down in the block where the stresses are as “the crank pushes against the head”. Longer studs are better because you can have more tension on them, and it seems that you want tension. Also, the block face is not very thick; seven or eight threads UNF maybe. Anyways, the reason for long studs seemed to be the head gasket, and most modern engines are built that way even if it means more cost in machining and material.
I also think it will not matter much but you bet that I don’t want to find out. The two studs generally held up well for years, but the one with the peened in insert (my work) had very soft threads and eventually didn’t take torque when I retorqued the head a few months ago because I did suspect something. That will not happen with my studs, they hold torque and have more meat.
Notably, @Wiggles claims it won’t hold long especially in slotted blocks.
all good points, I guess if there is less meat around the top of the stud on the the later block, that would be bad, also, best practice with inserts and studs would be ideal
I have a couple of short stud 4.2 blocks
I get the notion, that if Jaguar redesigned the studs to improve the head/block interface,
that over the very long term, that has proven to be an engineering fail, given the repair problems with the long studs, creating a significant problem to try and improve something that wasnt (as far as I know) a problem anyway. The gasket/block/head mismatch is a problem
This is tapped M14. M16 1.0 (fine) is another, better alternative with inserts that provide M10 inside threads. Maybe you can compare; I bet the short stud heads have way more threading. You can see the slot that is certain to expand a little… The engineering fail is the coolant surrounding the studs that connect an iron block with an aluminium head. With the change from short- to long studs in itself they did nothing wrong in my opinion. And after all, they do hold up for tens of years and longer with good service. I couldn’t educate you, but the AJ6engineering website has a section on it.
Some other changes were clearly improvements - for instance the much revised long stud block gave a more direct load path between head and main bearings, provided more secure clamping pressure on the head gasket and eliminated the bore distortion which had become apparent from stretching the block to 4.2 litres. (This lesson was not forgotten when the AJ6 block was designed - the head clamping stresses being taken down via heavy ribbing in the outer wall of the structure). From jagweb.com
**
There is no loss of clamping pressure as such, Tony - it is decided by the appropriate torque.
Bear in mind that torque is a function of friction between nut, thread and head - which may vary with nut size for the same clamping force.
Torque is not clamping pressure - but the short stud blocks were designed to ensure that the force acting on the head did not cause failure of the block itself. Transferring load to the bottom of the block, using long studs, eliminated this - but did, as you say, cause other problems…
I’m not sure why Jaguar changed the design - it may relate to revamping of the cooling jacket? Or indeed if torque specs were different - what is specific in the manual is that re-torqueing head was a scheduled maintenance operation with short studs…
Head studs are designed to stretch. The longer the stud, the more tension you can apply to it and the more power is in that stretching. When the engine heats up and cools down, you have more material to compensate; the clamping pressure would be the same but the long studs have more reserves besides being better statically and not warping the deck or, as my article said, the bores even.
**
I think some very specific conditions may coincide precisely for this to happen, David. And still; in my 23 years with my Jaguar - I have never even been remotely close to such an event…
Somehow, there must be something in you set-up that promotes it. Your 6 deg after(!) TDC (ATDC) cannot be right in any case; no specs for any market or versions of the xk engine is listed with ATDC ignition. Are you sure of your actual setting…?
Static advance is usually only given for points ignition as it’s easy to set - EI static setting is more complicated and uncertain…
I cannot see your stud modifications play in any part - and while ‘glowing embers’ in the cylinders do funny things; it’s irrelevant fro a cold engine. Compression may be involved, as said - and is easily checked. More exotic; ignition sequence may be involved, and should be checked - though faulty it would certainly show up in normal running. HT cross firing is a possibility, though it would likely have to be intermittent to cause occasional backwards running…
Once starting up backwards engine would run - like if the starter spins backwards. Which is too remote to even consider…
Certainly not!! It must read ‚BTDC‘. I am certain that it is set to spec. But I will look into it.
Sorry about that.
The manual explains it for the EI, and timing can be altered a few degrees with no ill effects…
As it ran before (here is proof: https://youtu.be/SRC9TGFZXsI), mixed up wires are out of question. The rest might not be out of question. Well, the starter works and the spark wires are fairly new. Besides compression taking place, which it does: what could happen?
David
PS.: here is a reverse-running motor. https://youtu.be/rbwFgi8aDPo
There are videos of a civic, and diesels doing it.
**
Somehow; the engine must stop before TDC, before it can run backwards, David - and ‘compression’, in its widest meaning, is a force to be reckoned with? While driven by the starter, this cannot happen - but there is a brief moment of opportunity at the instant the key is released…
Digging deeper; hydraulic lock due to coolant in the cylinder may stop the engine - but it is a bit remote, and will not in itself cause backwards running…
What was the static specs given - the EI procedure in itself is insufficient - and ignition timings is rather important for engine performance…
It is the same ignition setting that ran repeatedly the day before.
It’s the same fuel and - tank, the same wiring, the same AFM and ECU with nothing altered. All I changed was that I installed the fan and its clutch, put the air filter in place, put shrink tube on the inner head lamp wiring and fixed the main power line to the lights that tore off when the head was replaced. I could remove the air filter and hope for a difference, otherwise not a single thing I can see giving trouble!
Yet, don’t trust the mechanic before you, yourself, and what you already fixed. I‘ll do one attempt and then I‘ll likely dismantle everything.
I know what hydraulic lock is like and it is not happening. I don’t believe it leaked anything. An engine almost always stops on a compression stroke before TDC. When the key is released, this can force the piston back down. If the mixture is fired on the reverse compression stroke, it might be likely to fire the next cylinder as well etc. and the engine runs reversed and retarded until the fuel is gone.
As I had to crank a little this might be the cause; but then again it happened twice.
Maybe there are still some remains of ATF in the motor and exhaust and they make it run differently. Who knows. Maybe I‘ll find time tomorrow, but I‘d have to be very lucky to get to it before Sunday.
**
That it happens once may be some freak coincidence, David - that it happened twice is suspicious. And if it happens again - something is definitely amiss…
Compression…?
Runing backwards is sort of harmless, though the sudden change in rotation direction places heavy loads on the engine…
No matter, don’t let my inquisitiveness interfere with problems that are more important…
No way! Either the chain got loose (hope not, and it was tight) or the distributor is wrong and it wasn’t when it ran. Could reversing the cap be possible? I don’t know. I can’t wait to find out.
All I‘m worried about is the tensioner or the cause being valve timing with consequences. But I cannot see how my valve timing would be off… although it was changed a little from how it was.
My next door friend as a teen had a two-stroke BSA that ran backwards if you just jabbed at the kick starter and ran forwards if you push-started it.
Dieseling is easy enough to explain but someone needs to convince me how an XK engine can run backwards if the exhaust is sucking air and the intake is blowing mixture.
Irrespective of rotational direction; the pistons move up and down to suck from the inlet and blow out through the exhaust manifold…
Which in itself doesn’t explain how it can be induced to run backwards…
Apart from that, engine output would be dismal for driving. And with one gear forward and 3 in backwards, the driving experience would be ‘unsatisfactory’ - and as the water pump and fan are not bi-directional, it would quickly overheat. But the alternator and electrics will work normally…
**
Reversing the dist cap would require some effort, David - and would in itself have no directional effect. The ignition would still be directed by the rotor - which would point, correctly, to the cylinder ready to fire…
Nor do I see any increased tendency to induce reverse running with valve timing off. However, excessive diversion from proper valve timing would simply bend the valves.
The backwards running must relate, somehow, to compression and ‘when’ the mixture is ignited…?
