compression testing

[Rangie Motivated]

There seems to be some disagreement on which way a compression test should be done.

Are you meant to take all the spark plugs out then do the test.
Take one plug out at a time,test then replace before moving on to the next plug.
Or doesnt it matter.

I had always thought it was one plug at a time otherwise there is no load on the other cylinders which as far as I can see would give an incorrect reading.

Looking forward to the answers

[GRIMACE]

I have always done one spark plug at a time. and do one test dry and one test with a squirt of oil.

[Cossie]

Take them all out, that way if the head gasket is gone between 2 cylinders it will be much more obvious. Theres no reason to do them 1 at a time.

[mkpatrol]

This is how I was taught at tafe:
Remove all the plugs
Connect guage
Disconnect ignition so you dont blow things up/ignight any fuel that may be lying around.
Press throttle to full & wind engine for 3-4 turns
If a cylinder is low, place a squirt in clyninder, wind for a coyple of revs & then reconnect guage to check again.

[Struth]

The engine needs to be warmed up too, or ran first.

If it hasn't run for a while the lifter wont work correctly and will give you low readings.

Cheers

[BEE-JAY]

There is a specified RPM at which the test should be preformed, so plugs in or out shouldn't matter as long as you turn the engine at the correct speed.

[Rangie Motivated]

I'm not worried about the RPM at this stage, I'll just be happy to get some kind of reading at all. Trying to find out why this thing has no power on take off or in 5th on the open road, and struggles to pull a trailor. I'm having a bitch of a time finding a replacement motor 3.5 or 3.9.

I have an 87 RR with 3.5 and a disco with 3.9, but this 84 swollows like a professional, it uses more petrol than the other 2 combined and thats only going 1/2 the distance.

[love ke70]

i know its totally not to the topic title, but being pig slow and chewing juice etc, it isnt dragging a brake/handbrake is it?

[DamTriton]

i know its totally not to the topic title, but being pig slow and chewing juice etc, it isnt dragging a brake/handbrake is it?

x2

I would be wondering what it is that makes you think it's the compression causing the problem.

Generally if it was the compression you would first have mentioned about the engine missing, or oil in coolant/coolant in oil.

Would be more useful if we knew more information

oil color/consumption
coolant color/consumption
missing/uneven idle/running
sudden or gradual onset
color of plugs when removed
recent history (overheated?)
unusual noises

ignition system checked? (leads/plugs/distributor if fitted)
fuel system checked (leaky hoses, blocked jets in carby if fitted)

[-Scott-]

'84 RR? Twin carbs? If so, while doing the compression test, do the WOT test as mkpatrol describes, but repeat it with the throttle closed. Watch the gauge as you crank, and count the engine revolutions to max reading - on my daily driver it could take 12-15 revs.

If you have an issue with throttles or leakage into your manifold it MAY show up this way.

[chimpboy]

No comment on whether poor compression is likely to be causing your problems or not, but afaik the according-to-hoyle way to do a compression test is to pull all the plugs so the starter motor can crank the engine without getting as stressed.

First do it dry, then if there is a sign of poor compression, put a teaspoon or so of oil into that spark plug hole and run it again. If compression improves drastically with the oil, suspect rings - the oil you added must be helping them seal. If not suspect valves etc.

What you're looking for is not an absolute reading so much as a significantly low reading for some cylinders compared to the others.

That is just what I was taugh, YMMV as they say