Cylinder 1 seems to be running hotter than 2,3,4

Question

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

Answer 1

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.

Answer 2

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!