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Matt
Matt, Engineer
Category: Jaguar
Satisfied Customers: 20592
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Wondering if you could help me. Im working on a 2002 S type

Customer Question

Hi my name is Eddie
Was wondering if you could help me.
Im working on a 2002 S type Jag 4.0 v8
and would like to know what the compression should be?
Have rough idle issue.
Spark plug tubes where full of oil which i thought was the problem
Submitted: 11 months ago.
Category: Jaguar
Expert:  Matt replied 11 months ago.

Hello

compression pressures on these should be about 160 psi to 180 psi on all cylinders

if this is OK then I'd replace the plugs and also check for an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

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