Automobile Vacuum gauge
September's homepage diagnosed poor engine starting as being due to low engine vacuum. Insufficient air was being pulled through the traditional oil bath air filter found on Series Land Rovers. The problem was solved by disconnecting the oil bath filter and fitting a K&N air filter. So this month we set about diagnosing the cause for this reduced engine vacuum.
The engine manifold vacuum is created between the cylinders and the carburettor when a cylinder moves downwards to draw in air/fuel mixture. The only route that it should be possible to draw air in is via the carburettor. If other routes are available these will cause additional air to enter. The vacuum will then be less effective in pulling in the air/fuel mixture that is required for the engine to function most efficiently. So lets consider these sources of possible air "leaks". The vehicle under consideration is a 1961 LWB 2286cc petrol-engined Series Land Rover fitted with a Zenith carburettor, but the principle is the same for any equivalent arrangement.
At this point it is important to remember that, in general, a particular observed problem may not be the result of a particular fault but the sum total of a collection of faults. So if we are searching for air "leaks" then finding one should not distract us from looking for other "leaks" that could also tribute to the problem.
The vacuum pipe that leads from the carburettor to the distributor (or if servo brakes are fitted, then the brake servo pipe) is the first item to check in the quest for loss of vacuum. It is of rubber type but there is no sign of it being perished or split and the end connections appear to be still air tight.
Pistons and rings:
If the cylinder bores are worn, then it will be easier for air from the oil sump to brush past the cylinder as it descends. This would result in less air/fuel mixture being drawn into the combustion chamber and so less vacuum being created in the inlet manifold. The engine was rebuilt only 5,000 miles ago, so wear should not be present. No blue/grey smoke is being emitted from the exhaust when the engine is under load. Nevertheless, a simple measurement of the cylinder compression, using an inexpressive compression tester, is done as a check. Compression on all 4 cylinders is 200psi, which is fine for the ACR 9:1 cylinderhead fitted. For the previous 7:1 cylinderhead then 150psi would have been OK.
The job of the cylinderhead gasket is to seal off the cylinders from each other. If the gasket becomes damaged, then compression from one cylinder can leak into another and affect the manifold vacuum. Most Series Land Rover engine gasket kits include a copper based gasket but these are more prone to leaks than the compressed material type. A compression test can often identify which two cylinders are linked via a damaged gasket (lower pressure seen on two adjacent cylinders). The earlier test confirmed that all pressures were uniform and high so no problem there.
Much publicity has been given to highlight the problem of unleaded fuel causing damage to cylinderhead valve seats. Most Series Land Rover owners have now either converted their vehicle's cylinderhead to run on unleaded fuel or are using fuel additives to combat the effect. If the inlet valve does not seal properly then combustion mixture can be forced past the valve and reduce inlet manifold vacuum.
It's important for the valves to open and close at exactly the correct time to maintain adequate compression and sufficient inlet manifold vacuum. As noted earlier, the engine was re-built recently and if this was done incorrectly, it could cause the problem. But that is unlikely to be the case. Even the timing chain was renewed, as this can stretch and affect the accuracy of the valve timing. So, simply checking the valve clearances under the rocker cover and finding them to be correct, was sufficient to confirm that the problem did not lie with valve timing.
Efficient removal of exhaust gases is an essential part of good combustion. If the silencer is partially blocked due to corrosion debris building up inside, then this will cause some back pressure in the system, leading to reduction in engine vacuum. Silencers cannot be cleaned out, so replacement is the only option. A basic confirmation of internal silencer corrosion
is often gained by removing the silencer and shaking it to see if debris is moving around inside. A stainless steel silencer was fitted recently so this is not an option for the cause of the problem.
It's important that the inlet and exhaust manifolds have an airtight fit to the cylinderhead. Exhaust manifolds on Series Land Rovers can warp and twist over time, making a good seal difficult to obtain. It is sometimes easier to get a good seal when the inlet and exhaust manifolds are tightened up individually first to the cylinderhead and then the two manifolds bolted to each other.
This appeared to be the cause of the vacuum problem, as removal and re-connection of the manifolds with a new gasket raised the vacuum significantly and engine performance markedly improved.
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