18 replies to this topic

[dklawson]

Can I just ask, with the compression test, do I have to disable fuel as well as the coil? Or can I just plug in and go? It's a standard 998.

Bring the engine to operating temperature then switch off.
Mark and remove all the spark plugs so the engine will turn over easier.
Have an assistant press and hold the gas pedal to the floor.
While you monitor the compression gauge, have the assistant turn the engine over using the starter.
Keep cranking the engine until the compression gauge no longer climbs higher.
Record the numbers and repeat on all 4 cylinders.

With the plugs out, the only cylinder that will draw in any fuel is the one you are compression testing so you don't need to do anything to disable the fuel system.

[lewBlew]

Can I just ask, with the compression test, do I have to disable fuel as well as the coil? Or can I just plug in and go? It's a standard 998.

Bring the engine to operating temperature then switch off.
Mark and remove all the spark plugs so the engine will turn over easier.
Have an assistant press and hold the gas pedal to the floor.
While you monitor the compression gauge, have the assistant turn the engine over using the starter.
Keep cranking the engine until the compression gauge no longer climbs higher.
Record the numbers and repeat on all 4 cylinders.

With the plugs out, the only cylinder that will draw in any fuel is the one you are compression testing so you don't need to do anything to disable the fuel system.

Thanks a lot for clarifying.

He also mentioned it sounded like I had a slight blow at the exhaust manifold. I don't see why that would prevent one cylinder running though, but I suppose it's something I can fix if/when I take the head off.

[dklawson]

There is another test I really like to recommend if you can either borrow or improvise tooling for it. Try a leak-down test. You can buy the leak-down tool, make your own, or truly improvise just to get a feel for what is going on.

In a leak-down test you thread an airline adapter into the cylinder head using a spark plug hole.
Remove the radiator cap and the oil filler cap.
With the piston for the cylinder you are measuring at top-dead-center with both valves closed, you put the car in gear with the parking brake on and supply 100 PSI compressed air to the cylinder.
If you have the leak-down tool there is a pressure gauge on each side of a 1mm orifice hole inline with 100 PSI air supplied to the cylinder.
You read the "downstream" (cylinder side) gauge on the tool and compare its PSI reading to the 100 PSI supply. That gives you a crude percentage. Ideally the leakage would be close to zero meaning 100 PSI would be on the second gauge... matching the 100 PSI before the orifice. That never happens.
In the real world the downstream value may be 20% lower for a road engine with mileage. When you get below 80 PSI downstream you need to start looking for problems. Looking for the problems is easy.

Look in the radiator. Do you see bubbles coming up after a minute or so? You either have a blown head gasket or a crack in either the head or block.
Listen to the car's tailpipe. Do you hear air coming out in volume? You may have a damaged or worn exhaust valve.
Listen to the carburetor throat. Do you hear air coming out in volume? You may have a damaged or worn intake valve.
Listen to the valve cover. Do you hear air coming out in volume? You have blow-by around the rings.
To listen to the various areas hold a length of tubing/hose to your ear with the other end of the tube held in the area you are listening to.

If you can't find or borrow a leak-down tester you can build your own... there are plans on the web.
If you don't have the money to buy or build your own tester, you can just make or otherwise adapt an air line to screw into the spark plug hole. You won't know the percent leakage but you will be able to look and listen for leakage.

Of course... if you have no source for compressed air... the whole leak-down test is not an option for you. However, if you have a source for compressed air the leak-down test will tell you where there are problems and it can find problems a compression test will miss. For example, on a non-Mini car I was working on I had a running problem. A compression test showed low but uniform compression around 100 PSI. A leak-down test immediately found one cylinder leaking into the cooling jacket, two with exhaust leaks, and the third very so-so with blow-by. The engine was in need of major help but the compression test only indicated it was tired.

[lewBlew]

Thanks for the info. I will get it tested. However something seems to have changed since Friday as just now there was a noticeable dip in engine noise when I removed plug no. 2. Which is good, but I'm still going to get each cyl tested just to be sure, and apparently the exhaust manifold may be on the way out. There's other jobs to do while I'm at it too.