Quick one. How? Hoping to do today. Cold engine, all spark plugs removed, supply to the coil disconnected. Fit guage and get the Mrs to crank it. Or should the engine be hot, but then thought I might damage the spark plug sockets when taking the plugs out as they'd be tighter.
Real can of worms this if you read up on it. I do it hot as it's more representative of engine conditions, though I think what you're looking for is inconsistencies between cylinders. Some folks are saying the results are likely to be higher done cold which might offer false hope on a worn engine. I've never had any difficulty getting the plugs out of a warm engine. edit: Tom Wilson agrees with warming the engine up, but Bentley doesn't mention it.
Hot is probably better but cold will do and by the time you have stopped the engine, removed the plug leads, taken the plugs out and connected the pressure gauge the engine is no longer at normal running temperature. As you say, you’re looking for differences between cylinders as much as a particular pressure. And I doubt if the pressure would be higher done cold, on a hot engine the piston and rings have expanded and there are smaller ring gaps. Do it hot in the winter so your hands don’t get cold and cold in the summer so you don’t burn yourself.
If its blowing past the piston hot or cold wont make a lot of difference. But hot will find valves with overtight adjusters. So will a feeler gauge.
Worth taking absolute pressure readings with a pinch of salt, IMHO. Some of the cheaper gauges aren’t that accurate - I’ve seen variation of 10-20psi across different gauges. Ideally done hot (or warm), throttle plates open so engine can breathe.
I did a trip to Paris a couple of weeks back and ended up nailing it most of the way back trying to catch the ferry. The engine sounded different when I got back, but hard to tell as noisy at the best of times. Looks ok, did it cold and got No. 1,2 & 4 = 130 psi No.3 = 140 psi Cold oil thicker oil may seal better, did No. 3 last so that's may had more oil. Found I've an exhaust manifold leak of No.3 cylinder and my low oil pressure light at low revs was because I'd swapped the generator and oil wires at the back of the dash last year! Everytime the revs dropped. Loving the van again now the engine is ok - subaru conversion thoughts put back on the shelf.
