Mystery misfire

[JerryL770]

I'm helping a friend with a 3.8l S-type. Now I know this is not an E-type but if I ask the question on another forum, I will be lucky to get useful response this year.

He got the car knowing it had a misfire and compression was down on the odd cylinder. So the head came off when it was found to have the odd bent valve and for some reason, oversize valves. The head was sent for skimming and was fitted with new valves, to replace the odd bent one. Everything was checked over, valve guides were fine and all bores in the block are good.

The engine was rebuilt and started up OK but there was still a misfire.

Plug leads, coil, distributor (normal a 123 made no difference), distributor cap and plugs were all changed to no avail. The carbs were removed and checked and all OK, both pistons rise n drop OK, but it continued to misfire on the rear cylinder, even when a plug from a firing cylinder was swapped to the rear. A plug on the lead to that cylinder laid on the head with the engine running, sparked OK. Temperature measurement of the exhaust manifolds showed a 100°C cooler outlet on rear cylinder. Compression pressures consistent at ~170psi on all cylinders.

The head was removed again (no head gasket blow by) and it was clear that the rear cylinder was not firing at all; the combustion chamber was clean aluminium whilst all the others were now blackened - not heavily, just as much as you would expect to find.

Checked the camshafts, no mis-timing of valve openings.

Any ideas?? Advice appreciated, thanks.

[mgcjag]

Hi Jerome...well you said you have compression....and you have a spark.....so just fuel needed..... without letting the engine start does the plug get wet...Another thing to check is the spark for that cylynder fireing at the correct point on the compression stroke..Steve

[christopher storey]

It may seem a silly suggestion after all the work which has been done, but is the inlet cam , and its associated tappet , on the rear cylinder in good order ? I had this very problem on a Rover 2000, which turned out to be caused by the tappet having had a depression worn in it so that the inlet valve was hardly opening at all. Similarly, check the inlet manifold for a blockage affecting only the rear cylinder

I should say that normal compression readings rather militate against either of these faults, but something has to be causing no mixture to reach that cylinder

[JerryL770]

Thanks for the responses, much appreciated.

Well, the inlet manifold is clear and of course the 2 cylinders in front get fed by the same carb and they run OK it seems.

Spark timing on that cylinder could be a possibility but having tried a different dizzy it would seem not. Plugs and leads have all been swapped all over the place and it makes no difference - that cylinder keeps refusing to fire

[mgcjag]

Is the camshaft actually opening the inlet valve...Steve

[JerryL770]

Is the camshaft actually opening the inlet valve...Steve

The tappet goes up n down freely and on comparing the cams with another set, the lobes are all in the same position - certainly not a visible difference which could possibly cause the problem.

Although it had the steel shim type head gasket, there's no visible leakage either. We will try to get the composite gasket when replacing the head but these have become very difficult to find and if you do find a set, it costs about £198 !!!

[christopher storey]

If you have normal compression readings then it cannot be a head gasket problem . I am not familiar with the manifold for a B type head , but the fact that cylinders 2 and 3 ( numbering from the rear ) are working normally does not mean that something is not preventing mixture reaching cylinder 1 . Has the manifold gasket been checked carefully ? a failure of that gasket near cylinder 1 could result in air being drawn into the cylinder rather than mixture . Also, it may be worth checking that the pipework for the "hisser" cold start system which goes IIRC to at least 4 points at the base of the manifold has not come adrift , as if this happened near the rear of the engine it could conceivably produce this result by letting air in

[ralphr1780]

If you see the spark when the plug is laid on the head, that confirms well that the plug is ok and getting the coil discharge, but it does not imply that the plug is sparking when it is fitted in the cylinder hole.
Could it simply be that this plug is not getting proper ground?

[JerryL770]

If you have normal compression readings then it cannot be a head gasket problem . I am not familiar with the manifold for a B type head , but the fact that cylinders 2 and 3 ( numbering from the rear ) are working normally does not mean that something is not preventing mixture reaching cylinder 1 . Has the manifold gasket been checked carefully ? a failure of that gasket near cylinder 1 could result in air being drawn into the cylinder rather than mixture . Also, it may be worth checking that the pipework for the "hisser" cold start system which goes IIRC to at least 4 points at the base of the manifold has not come adrift , as if this happened near the rear of the engine it could conceivably produce this result by letting air in

Those are some possibilities to check further, thanks.

If you see the spark when the plug is laid on the head, that confirms well that the plug is ok and getting the coil discharge, but it does not imply that the plug is sparking when it is fitted in the cylinder hole.
Could it simply be that this plug is not getting proper ground?

We thought of that one but cannot see how the plug could not earth when screwed in.

We've not tried a ColourTune plug which would give a good indication - 'cos we don't have one

[cactusman]

Another thing to check may be the plug cap. If you have a resistance cap then check the resistance between the lead and the bit that pops on to the plug itself is no more than about 10,000 ohms. If the resistance fails high or open circuit you may see a small spark in open air but at the high pressures in the cylinder it won't spark....sadly the repro champion resistance plug caps are prone to the resistance failing..I have had two pack up...

[ralphr1780]

We thought of that one but cannot see how the plug could not earth when screwed in.

Jerome, if the plug seat in the head has some cracks, often hardly visible to the naked eye, you lose the spark.
With an ohmmeter set to the lowest scale check the resistance between the spark plug base and the head.
I had exactly the same symptoms years ago on an Alfa, drove me and fellow mechanic crazy till we nailed it by grounding the plug with a battery jump cable.
Other piste would be...the dizzy out by 180 degrees.
Hope it works out for you.

[Series1 Stu]

If the plug is getting wet then you have fuel. If the plug sparks against the head when cranking then you have electricity. If the timing is right and the valves are actually opening and closing as they should then the only suggestion I can make on top of all that is that the plug is breaking down under compression, which I have experienced before. Try changing that one plug.

Regards

[abowie]

If all above is correct, you have a single cylinder that

• is getting spark and the plug works

• is connected to a carb that is providing fuel to 2 other cylinders successfully

• has normal compression

To me the only possible explanation is that for some reason the inlet valve isn't opening to allow fuel in. Which seems pretty far fetched.

I would, as above, recommend ensuring that the plug is not the culprit. Just swap it with one of the others that you know works.

[abowie]

When you find out what is wrong can you let us all know please?

[JerryL770]

When you find out what is wrong can you let us all know please?

I will indeed.

Thanks for the further suggestions.

The head has been off and thoroughly checked over including for leaks from the combustion chamber, inlet and exhaust ports (by filling each with fluid). No leaks. The inlet manifold has been swapped for another.

Can't see any cracks round the plug seat but the resistance of the plug earthing has not been checked yet. If the misfire persists, it will be. The head is currently being re-installed but I can't see it again for a week.

With regards to swapping plugs and leads and dizzy, you can be sure all that has been tried without success.

It's probably going to be a case of put it back and all works OK and we have no idea what caused the little difficulty in the first place

I will let you know in due course.

[Hugo]

We thought of that one but cannot see how the plug could not earth when screwed in.

Jerome, if the plug seat in the head has some cracks, often hardly visible to the naked eye, you lose the spark.
With an ohmmeter set to the lowest scale check the resistance between the spark plug base and the head.

You don't need a good ground - or any ground at all - on the HT side. The spark will jump about half an inch to find one at atmospheric pressure.

[Hugo]

That's a weird one. Have you tried running the engine at night to see if you can see any electricity leaking out anywhere? I'd be tempted to run a separate HT lead to the rear cylinder, away from all the others, just in case the spark is being led astray en route where the leads are all bundled together.
How did the valve get bent in the first place?

[JerryL770]

That's a weird one. Have you tried running the engine at night to see if you can see any electricity leaking out anywhere? I'd be tempted to run a separate HT lead to the rear cylinder, away from all the others, just in case the spark is being led astray en route where the leads are all bundled together.
Yep, no loss of sparks!

How did the valve get bent in the first place?

Who knows, the car came from the other side of the channel.

The head has gone back on and the problem remains

[mgcjag]

You didnt really answer if the inlet valve actually opens.....you said that the tappet goes up and down.....that dosnt mean that the valve opens

[Hugo]

Here's a silly question - I presume you have checked that the piston is actually going up & down? It hasn't come unglued at the bottom end has it? I've known exactly that happen to a clapped-out Gardner diesel engine once - took them ages to trace it - new injector, head off, etc etc. Then they found it had thrown a big-end & the con-rod had got wedged up in the block.