1979 leyland Sherpa 230 1.7l with an "o series" engine.

I've an overheating problem for a while now and still trying to get to the bottom of it. The cooling system and heater matrix are all OK and the thermostat works.

First of all I did a power balance test and noticed the revs of the engine did not drop much on removing cylinder 1 spark plug where removing the others - there was a discernible loss of power. With the aid of a color tune I've discovered that No.1 cylinder is running leaner than the other 3. I can see that the flame is blue and won't get flecks of orange even if I richen the mixture. It would suggest an air leak of some sort but on spraying gaskets with propane I only get a 1% increase in revs. I personally think that's not enough of an leak to lean out the mix to that degree.?.

I'm starting to think internal air leak from inside engine like through valve guides or something but can't find any info confirming this will happen on Google. What do you think - any ideas?

As a leaner mix runs hotter and the said cylinder is the nearest 1 to both the thermostat and temperature sensor do you think that this could be enough to contribute to an overheating problem

  • Is it a carburated engine? If so, can you show a picture of above the engine, so we can see how the manifold/carb setup is arranged? IMO. air leaks are easier found by spraying soap. If it starts bubbling or disappearing, you know there's an airleak at that location.
    – Bart
    Commented Feb 15, 2018 at 18:01
  • it's a carbureted engine and the manifold is both for inlet and exhaust. I cant take a photo right now but follow this link and there is roughly the same engine. en.wikipedia.org/wiki/BL_O-Series_engine Commented Feb 15, 2018 at 23:32
  • I was under the impression that fairy liquid (bubbles) was good for a leak "out" but never considered it would be good for a vacuum loss. I'll give that a go. But again - it's a question of degree, no? That was a sherpa 230 not 23 by the way - all I have to do is hit return and and comment is posted. - hence 3 comments in succession! Thanks for your interest, Michael Commented Feb 15, 2018 at 23:42

2 Answers 2


You're just not getting good air flow. Have you thoroughly checked the condition of the valve train: worn camshaft lobes? loose timing chain? weak valve springs? improper tappet clearance? Is there any blockage of the exhaust system (emission controls, such as a collapsed catalytic converter, or damaged exhaust pipes, carbon buildup in the exhaust manifold, etc.)?

The compression of 150 psi is a bit high, after testing many engines over the decades. A high reading suggests a lack of exhaust valve opening or some sort of exhaust back pressure.

  • I think you may have already answered your own question: "revs of the engine did not drop much on removing cylinder 1 spark plug." Time for a compression check.
    – Carguy
    Commented Feb 16, 2018 at 2:07
  • Thanks......I have done a compression test on cylinders 1,3,4 got 152,148,150 (couldn't test 2 as tester wouldn't fit under dizzy) . Had there been a compression loss on this cylinder would that cause it to run leaner than the others? Commented Feb 16, 2018 at 14:27
  • I tried the fairy liquid test today and couldn't see bubbles disappearing however the revs did rise up again - I guess the water acts as a temporary seal. @Carguy - I don't really understand how I've answered my own question. ?. Commented Feb 17, 2018 at 0:20
  • Great, no compression loss! If there was a drop in compression on #1, it could "lean" the mixture because not all of the fuel would burn.
    – Carguy
    Commented Feb 17, 2018 at 3:48
  • Thanks for that Carguy. My reply was 570 too long for a "comment" so it seems I cant engage in conversation or give a proper update? No blue smoke, swapped spark (no.1) with new (they're all quite new anyhow) New leads, rotor and coil and I swapped cap for my old 1 that was replaced just for a service. There shouldn't be too much of a carbon build up as cylinder has been removed and cleaned. Sparks are great when I remove and ground (17mm). Yes cooling system has had an industrial detergent for about 300miles - heater matrix ok. Commented Feb 18, 2018 at 13:02

On second thought, there should be no increase in revs when you apply propane. An intake manifold air leak is therefore most likely.

I've never heard of a worn valve guide seal causing an internal air leak, but I'm no expert. It would seem that if a valve guide were the issue, you would see blue, burning oil smoke coming from the tailpipe.

I did once read that carbon buildup on the piston head can cause a lean mixture. With each compression stroke the carbon absorbs and traps the gaseous fumes, then releases them upon the exhaust stroke. It acts like a carbon filter. Another internal possibility is that carbon buildup exists on the intake valve stem, which causes the intake stream to swirl in a way that interferes with complete combustion.

It may be obvious, but have you tried replacing the spark plug on cylinder #1? What is the ohm reading of the spark plug wire? Connect a timing light to the spark plug wire at idle to verify there are no skips in the ignition.

You mentioned that the manifold is both for intake and exhaust. Is it possible the manifold is cracked, allowing some exhaust gas to bleed into the intake?

Finally, I don't think the heat of a lean cylinder would affect the thermostat. The metal block is extremely effective at distributing heat. Have you tried removing the inlet and outlet hoses from the block and flush the block with water mixed with dishwasher detergent to remove scale? You can do the same with the radiator and hear core . . . you'd be amazed the crude that comes out of a seemingly clean system!

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