S2 engine into S3: issues?

I’ve been asking a lot of questions lately, I hope I haven’t used up my fair share.

A friend has a 1978 XJ6 with fuel injection, and in a run of bad luck, had the engine start knocking badly just after paying large to have his BW65 rebuilt.

I have a good S3 engine I offered him, but I’m not sure about a couple of things.

Can I leave the intake complete from the S3 and use it in his car? The fuel rail is a different shape, and maybe the ECU won’t like it? If I have to put his intake on the later engine, that’s not too big a job when the engine’s out, but it would be faster just to swap it whole.

The transmission shift cable linkage appears the same. The fuel return line is a different shape, but that’s not a big problem.

Does anyone see an issue doing this swap?

As an aside, this friend works for a company that maintains lake freighters that run up the Great Lakes near my home. He got me a tour of a ship while in winter dock, when they do annual work. Pictures attached.



I would use the more modern fuel injection pipes. The big LOOP fuel system was replaced by the later one and is easier to work on as well as fewer potential leaks.


The Ser III engine might have the later style cooling system plumbing: different water pump, thermostat housing (or perhaps entire water rail?), some hose differences, different coolant tank, etc.

I don’t think it matters too much if you choose early or late style…but you’d probably want to all newer or all older rather than trying to meld the two types together


If the complete S3 engine is transplanted, Robert, there should be no issues. It’s swapping parts between engines that may be an issue…

I assume he will be using his original ECU to run the S3 engine - which it will. As far as I know, the relevant sensors are the same and there should be no problems connecting the other electrics, like S2 injectors to the S3 engine…?

The BW66 in the S3 is externally more or less identical to the S2 BW65, and the controls are the same - so the latter should bolt directly to the S3 engine.

xj6 85 Sov Europe (UK/NZ)

Someone mentioned that the AFM location is different with the idle air adjustment more accessible on the S2. Is that correct?
The manifold should be the same, so that won’t be a problem hopefully.


your friend’s 78 FI SII car is an unknown beast in this part of the world, so I know even less about the FI used on these cars than about the FI used on SIII cars.

Maybe that’s why I find Doug’s advice so convincing - if I don’t really know what’s ahead of me, I’d rather stick with what I know works well. And that is either the entire old system that has been running well except for that bearing damage or the entire new system. The latter choice is probably even better as the SII head has smaller valves and maybe the SII FI was adjusted to the smaller valves while the younger SIII FI was designed together with the new head.

Good luck - and nice pictures! Thanks for sharing


75 XJ6L 4.2 auto (UK spec)

Thank you all for responding.

We have my wife’s Dad here from Wales - he’s an HVAC guy - and I conned him into helping me pull the good motor.

There are a lot more connections than on the carb models I’m used to.

Here’s a picture of a piston from the freighter

More to come…



The piston is larger than the whole XK engine. Very impressive!

Hello folks,

I’m half way through swapping a S3 XJ6 engine into a S2 car.
The S2 car had the big heavy cast flywheel/ring gear, and it has tooth damage, so I don’t want to use it.

Can I use a S3 flexplate? It seems like it would fit, everything I can measure appears right.
The parts book says the heavy one for was for BW12 and 65. I will be retaining the 65 from the S2 as it was rebuilt.
Am I missing something? Why did Jag change anyway?
I see Moss Motors lists one light weight flexplate for all series.

I’d like some assurance, as it would mean engine out to correct if I get it wrong.

I do not have a specific answer. But, things to consider.

  1. Tooth count. to match the pinion on the starter.

  2. Are the engines internaly or externaly balanced.


Carl has a very valid point, Robert; your picture seems to show different pitch (and diameter?) on the two ring gears - which would mismate with the starter pinion unless properly paired off…

The flexplate is part of the inertial mass in a ‘flywheel’ action. Generally; lightweight items are fitted for quicker engine response - at some possible cost to engine smoothness at low revs.

Whether Jaguar changed items you mention for this or other reasons is not known (I think!). But the S3 engine itself should mate with the BW 65 without problems - there is little difference between the 65 and the BW 66 usually fitted to the S3 engine…

xj6 85 Sov Europe (UK/NZ)

Hello everyone,

I’m digging up this thread again.

I now have the S3 engine in the S2 car, and there seems to be no trouble with using the later light weight ring gear type flywheel in place of the damaged heavy thick type. What a relief. This also the first time I removed the engine only, leaving the trans in place.

The engine spins over fine, BUT…no spark.

Both engine’s ran before removal. The '78 was knocking and smoking something terrible. There’s oil in coolant too. The '85 has low miles, although it looks dirty, and ran perfectly before removing it.

I used DD’s No Start guide, and everything checks out until his guide says “bad amp or pick up coil in dizzy”

As the engine was running fine, and I didn’t disturb any wiring, I don’t think the amp or pick up could be faulty. I haven’t tried swapping them out. When pulling the motor, I unplugged the engine harness at the back of the coolant rail, and at the front, down by the resistor pack, and left the harness in place.

The only difference I can see between the ignition systems is that the S2 had no ballast resistor, and the S3 does.

So, I have power from the white wire to the + coil. Power goes through ballast. Power on NEG terminal too.

Any ideas please?

When I get this job done, I’ll list all the differences that had to be dealt with to do this S3 into S2 swap.


I’m not so sure.

As far as I know a Series II will have the Lucas OPUS ignition. A Series III will have Lucas CEI ignition (with a few exceptions found on very early cars which wouldn’t include your 1985 engine). The two systems are quite different in operation.

I’m not sure what checklist you are referring to but, whatever it is, it wouldn’t include anything relevant to the OPUS system…because I don’t know anything about 'em :slight_smile:

Before getting too deep into weeds let’s clarify which ignition system you are using.

If you’re using the CEI from the '85 engine you should have an ignition amplifier bolted to the front of the intake manifold. Black box, about 3" x 4".


Wow, that was fast! Thanks for taking time on your weekend.

Both engines have the AB14 Amp, and both use a pick up coil in the distributor.

Wiring and plugs all seem the same.


Oops, forgot



I’ll be darned. Not what I expected. But that means nothing :slight_smile:

Here’s a diag tree for the CEI system, almost verbatim from the ROM

The test assumes a fully charged battery

  1. Measure voltage at coil “+” terminal with key “on”. It should be within
    one volt of battery voltage. If not suspect a problem with the wiring to the
    ignition switch, or the switch itself.

  2. Measure voltage at the coil “-” terminal. Result should be the same as at
    the “+” terminal. If Ok, go to step 3. If not…
    Disconnect the wire from the amplifier from the “-” post of the coil and
    measure voltage again. Less than 2 volts means the coil is faulty. More than
    2 volts means the amplifier is faulty.

  3. Disconnect distributor pickup coil from the amplifier (this is the
    harness from the distributor that plugs into the amp). Measure resistance
    across the terminals. It should be 2.2k to 4.8k ohms. If Ok, go to step 4.
    If not, replace the pickup.

  4. Reconnect the pickup to the amplifier. Measure voltage at coil “-” post
    while cranking engine. The voltage should drop. If OK, go to step 5. If
    not, the amplifier is faulty.

  5. Check distributor cap and wires, distributor rotor arm, spark plugs, coil

Note that with your ballast in place you won’t have 12v to the coil “+” post. Remove the ballast to run the diagnosis. If you have a mind to you can toss it into your junk drawer and run without it

Post back if no joy


Connect a test lamp between coil neg and ground, Robert - to test amplifier…

With ign ‘on’ the lamp should be lit - confirming power through coil and the no ground through ign amplifier. In ‘crank’ the lamps should flicker - indicating the amp is making and breaking coil ground. The coil fires when coil ground goes ‘break’…

(this is the same test as voltage drop on coil neg while cranking, but more ‘visual’)

The test is not foolproof, but if the test fails, no flickering, the ign am is not operating. If the lamp goes out at any; coil power is lost - or the coil neg somehow shorts to ground…

The ign amp also operates the ECU for injection. Are the injectors clicking in ‘crank’?

If the pick-up coil test, as mentioned by Doug, is ‘good’; the pick-up alignment and gap should be checked - even if there is no indication that the move has interfered with it…

You could also swop the distributors…?

And, for the record - how do you test ingnition…?

xj6 85 Sov Europe (UK/NZ)


When pulling the plug from dist to amp, I noticed the contacts rusty. A clean and lube, and
she fired!

Funny that this engine was just running before it was pulled. I guess it got disturbed somehow.

Thanks to all, now to finish putting this back together.

I’ve copied this thread and put it in my manuals. I never did learn how to test these.



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