I am having an XJ6 Series 3 4.2 liter engine overhauled. My mother was the original owner, so I know it’s service History is solid. The mechanic said the engine looks good, even though it has 250k miles. What’s the maximum number of miles someone has put on this engine? Will it just keep going with proper attention?

 

Most of the issues with XK engine are confined to the cylinder head and head gasket.

As some of my American friends are quick to say: 'If it ain't broke, don't fix it.' If the car is running good, enjoy driving it.

 

Good maintenance takes many an engine of any specie many miles.


I'm with NBCat!


Carl

 

Engines seem to last longer now than they used to in the Sixties. Better quality oils and all that emission equipment causes the motor to heat up quicker and run at a constant higher temperature, this keeps the oil cleaner and carbon deposits down. I think that good servicing plays a part also. As cars depreciate in value, subsequent owners are more likely to let good maintenance practices slip, to he detriment to the vehicle.

Was your point that the motor was in the process of being rebuilt and that the mechanic was remarking that the components were in good shape?

On a running engine you can monitor the oil pressure, take a compression check of the cylinders, and keep a record of oil consumption. This will give you a good idea of the health of the motor. You could have an oil analysis done and that can tell you about the presence of bearing metal or other contaminants in the oil. I agree if it isn't smoking or overheating and oil pressure is up, it's good to go.

 

My best was nearly 300,000 miles done after an initial 125,000 miles done by the initial owner but on my avatar Mk1, not on a 4.2L XJ6. When miked prior to rebuild at 125,000 all crank journals and big end pins and all camshaft journals were found to be unmarked and still within factory tolerance for new. Indeed the only wear of any significance was about 2 thou of taper in cylinder bores and resulting worn rings.

I agree with previous advice . . . and especially with a known family endorsed service history . . . continue the good work, keep it in top condition, and enjoy!!!

Best wishes,

Ken

 

The mechanic checked cylinder pressure before the overhaul and said one piston was at 30 percent. The rest were at 100. He did the overhaul, including changing the oil pump and exhaust valves and said the engine looked good in general. No rebuild was required. We will probably start it up this week. Is it reasonable to expect another 100k from this engine? I originally thought about putting a new V8 in, but the mechanic said it was best to keep it original and said the XK was built for endurance.

 

Given that it's in good shape now it's reasonable to expect another 100k miles of service, and reasonable to expect that the engine will might well start showing the signs of wear along the way.... such as oil consumption and less oil pressure.

However, reasonable expectations are obviously not the same as absolute assurances . At 250k miles the engine doesn't owe you a dime and 350k miles is an very long service life no matter how you slice it.

Cheers
DD

 

Worse comes to worst, I could have the block bored out and put in some oversized pistons, right?

 

Unless the cylinders are severely tapered or scored, you should be able to reuse the original pistons with new rings.

As stated earlier, most of the issues with the XK engine are confined to the cylinder head such as dropped valve seats and tappet bucket movement.

If the engine has been overheated, the head may be warped.

 

Exactly . . . but, with respect, a few further checks are warranted before any engine pull. If one cylinder is reading only 30% and all others are strong at 100%, repeat comp test (all plugs out) but having added 30ml oil to offending cylinder. Any significant improvement on that 30% figure would indicate broken piston ring(s) rather than valve/seat/guide issues. I am guessing this is most likely cause, unless you have related coolant issues that would point to a head gasket failure . . . common on many 4.2s.

Next, in these days of cheap borescopes (yet another app for your phone?), slip one down the spark plug hole after first sending that piston to BDC. Inspect that bore top to bottom around full 360. Any significant bore damage from broken piston ring(s) will be apparent.

Many an otherwise great Jaguar 6 with your symptoms can be easily brought back to top strength with little more than a top end overhaul, valve lap and new gaskets . . . or, progressing deeper with an engine pull, fitting a set of new, correctly gapped piston rings. The costly part of that fix . . . or if DIY, the time consuming part . . . is the engine R&R.

Best wishes,

Ken

 

What year is the car ?
I ask because up to the early 80s the 4.2 long stud engine which came in in the 70s were very prone to cracking between the bores. These cracks extend down to the water passage between the bores. Over time the metal sinks, having lost the bridge strength, the head gasket blows and combustion chamber gases get into the cracks and pressurise the coolant jacket, forcing out coolant and overheating the engine. It is no good just replacing the head gasket as the syndrome will occur again within very few miles. This happened to me in the late 80s, and was cured by rebuilding the engine around an uncracked block. Of course your engine with one cylinder down on pressure may not have cracking, but one cylinder down quite drastically means something is wrong so the head really needs to come off for an inspection. This is when you will find out if there are any cracks.

An engine that has got to 1/4 million miles could well have no cracks at all, there was a small minority of engines that never suffered for some reason or other. If the car is from about 1981 the block is likely to be of the slotted type, a redesign that was intended to stop the cracking. It worked, but even so, the engines did still tend to blow headgaskets at around 80-90k miles. Fix it and you get another 80k ! It will be immediately obvious if you've got a slotted block because the slots, (for coolant transfer), are a very prominent feature between each cylinder bore.

If you can tell us more about what work you intend to do in this overhaul, more advice can be given. I've had an XK down to a bare block and rebuilt it up to a complete engine. An engine on 250k miles will have had to have some significant work done on it over the years. Has your mother kept all the service records and invoices ?

 

I would expect most straight sixes, whether british, german, or american, to last between 300-400k miles before they start getting low enough in compression to warrant a rebuild.

To comment on what someone else mentioned about modern engines lasting from lubricants and materials, one of the biggest reasons old engines didn't last more than 100k or so is from carburetors. Fuel was not always mixed evenly (not as big a deal with SUs) and you ended up with "cylinder wash" where the excess fuel in the cylinder would rinse the oil from the cylinder walls and cause extra wear.

So realistically a fuel injected 4.2 should see a lot less wear than a carbureted one. That being said, its still a straight six and they are known for eating up miles.