How to do a compression check?

[charlg]

I am getting ready to do my own compression check for the first time. What are the correct steps on a 350 Mag MPI?

 

[northern]

Not sure if this correct. I do the following:
Remove the ignition wire from the coil to the distributor
Turn on blower and leave it on
Get every one off boat except person reading gauge
Turn off all breakers except those that allow you to turn over engine
Put out some fire extingushers
Check compression when
Engine cold
Engine cold with oil squirted into the cylenders
Engine warm
Engine warm with oil squirted into the cylenders
I have a NAPA compression tester that screws into the spark plug hole. Think it as $40
If all readings are within 10% I feel good. Especially when engine warm. So far this has been the case.

 

[psubill78]

Check compression when
Engine cold
Engine cold with oil squirted into the cylenders
Engine warm
Engine warm with oil squirted into the cylenders

I just had a compression test done as part of a survey, and they didn't do it 4 times... they did it just a single time, engine warm. No squiring of oil that I saw/noticed.

Why would you run the test 4 different times?

 

[Bucit]

Why would you run the test 4 different times?

It allows you to drink a few more beers!

 

[JediJD]

Reader's digest version is that a wet reading is done to check rings. Oil in the cylinder will seal leaking rings. Very often a test will be run once, when cold, and if the numbers are within 10% of each other and within a practical expectation range of results, wet readings won't even be done. If you have some cylinders that are off, wet readings tell you if it is a rings problem (numbers improve) or valve problems (they don't).

 

[psubill78]

Bucit - I prefer my beers while not hairpinned against a warm engine block

JediJT - great explanation. So if everything looks good on a cold and/or warm test - no need to do the 'wet' test. Makes sense. My sea trial survey numbers were all 160-165 - so we didn't proceed further.

 

[charlg]

So, you can test with the engine cold? Do I need to shut off the fuel supply?

 

[Searaybio270]

So, you can test with the engine cold? Do I need to shut off the fuel supply?

I think that is correct. I'll ask my mechanic today when I am at my boat and write back later.

Beer should be cold unless you are boating in Germany then room temp is good.

 

[300SR]

So, you can test with the engine cold? Do I need to shut off the fuel supply?

Noooo warm!

 

[wish2fish]

Not sure if this correct. I do the following:
Remove the ignition wire from the coil to the distributor
Turn on blower and leave it on
Get every one off boat except person reading gauge
Turn off all breakers except those that allow you to turn over engine
Put out some fire extingushers
Check compression when
Engine cold
Engine cold with oil squirted into the cylenders
Engine warm
Engine warm with oil squirted into the cylenders
I have a NAPA compression tester that screws into the spark plug hole. Think it as $40
If all readings are within 10% I feel good. Especially when engine warm. So far this has been the case.

can you explain the reason for the bolded areas of your comments?

 

[ardeprint]

Keep it simple:

Disconnect coil cable at coil.
You in bilge with engine.
Buddy at helm to turn engine over
Test each cylinder using a compression gauge and have freind turn engine over three cycles.

LOG EACH READING ON PAPER. If numbers are close to each other (150, 150, 145, 150, 145, 150, 150, 145). Then drink beer!!!

If you have numbers like (140, 150, 125, 150, 100, 90, 150, 90, etc). Then do wet runs and call a mechanic.

 

[SeaRay]

Another note worth mentioning for help in diagnosing.....If you get 2 low readings on cylinders that are side by side (and adding oil doesn't help) you probably have a head gasket leak between the cylinders.

There are also wired remote starters so you can do this by yourself while you are conveniently sandwiched in the bilge

 

[300SR]

Keep it simple:

Disconnect coil cable at coil.
You in bilge with engine.
Buddy at helm to turn engine over
Test each cylinder using a compression gauge and have freind turn engine over three cycles.

LOG EACH READING ON PAPER. If numbers are close to each other (150, 150, 145, 150, 145, 150, 150, 145). Then drink beer!!!

If you have numbers like (140, 150, 125, 150, 100, 90, 150, 90, etc). Then do wet runs and call a mechanic.

Don't you turn the engine over 4 times? 4 cycle engine ?

 

[northern]

I always turn the engine over until you get a constant reading. Never counted the revolutions. It normally takes a couple of seconds before I got a max reading.
Oil I was told would give you an indication of worn rings. Cold then hot would give you an indication if a ring was brocken and the heat had expanded it to seal the break. If the oil increases compression by more than 10+% I was told you probably had head problems most likely valves.
The main thing you are looking for is uniform readings. Good point that if two are low and beside each other head gasket.
A real mechanic probably knows what to look for and has his own test methods.
If you do compression test once a year under the same conditions (one test either cold or hot and no oil) over time you see how your engine is doing.

 

[ardeprint]

Don't you turn the engine over 4 times? 4 cycle engine ?

I normally do 3 revolution, but 4 will work just as good. Too many revs will flood cylinders, which is why I do three. I dont think one more will hurt.

 

[retromode]

I just did my compression test yesterday on my 5.7, though mine is carbed. How I do mine is to shut off the fuel at the tank and start it up. I let it idol for about 5 minutes to where it stalls. This will run most of the fuel from the carb bowl. Also, this gets the engine warm, but not blazing hot. On my 89 220DA, like many boats, taking out the plugs is a bit of a contortioning.

I NEVER work on a boat alone. If something happens and your down below you could be stuck all day, especially using a remote starter. Fire suppression is a must. Make sure you are open to the outside. Don't try this with the camper back on because it's raining etc (believe me I've seen someone try it). There will be fumes no matter how many precautions you take. Kill power to anything not needed at the breakers. Also, with the plugs out there shouldn't be that much battery drain, so you shouldn't need a charger. If you do need a charger mid-test, do not connect it to the post or turn it on until after the fumes have cleared.

The setup details for your MPI are probably year specific which could include disabling the Fuel Pump and other precautions, but the mechanical should be the same. I pull the coil wire and remove from the distro cap & the coil so it doesn’t have any cross spark. Though with no plugs it wouldn’t do anything, but might get a stray shock.

In advance I make eight labels 1-8 to mark the plug wires as I take them off. I actually mark both ends so I can see them on the distro cap for reference. Saves time and it’s easy to cross a wire. Your main reference should be your intake, as all the cylinders are noted with the firing order.

Then I remove all the spark plugs setting them aside in order they were removed to do any CSI work on later.

I then slowly move the throttle (not in gear of course) full forward to WOT. This lets the motor breath while turning it over. I do leave the spark arrester on so no raw fuel spits out.

On my compression gauge I clean the threads with a towel sprayed lightly with WD-40 and repeat with each cylinder. Insert into the threaded recess and turn slowly! Do not force and keep one hand as a bushing to keep the hose aligned as straight as possible as you turn with the other hand. You do not want to cross thread.

Make sure you communicate with a sober assistant that can actually focus for more than 5 minutes without their ADHD meds. I use the ignition key as a tagout/lockout point. I hold the keys on me as I insert the compression gauge into the cylinder head. This keeps any accidental starting while you’re in the pinch point area. Not a time for cooler jockeys.

When complete on the first cylinder (I do one bank at a time) and all equipment, towels etc are out of the motor area I hand the keys back over to my assistant. I like to observe the gauge up close and also to listen for any odd noises. I have my boat log book and I note what the compression is at each cylinder. Specs for my engine is 160 psi. Make sure you complete all cylinders before you panic about a bad reading. Repeat cylinders in question for peace of mind.

You can tell when you have turned the motor over enough because your compression gauge will just max out and not climb any further, prob after 4-5 revs. If you have a good gauge it should not bleed down after removing the cylinder, though I note the max psi immediately while in the cylinder. Always remember to press the small pressure release after each cylinder.

So if I would of found any strange pressure readings then I would do the wet test and research further. Remember to move slowly in the engine bay so your big feet don’t damage wires and fuel lines. It’s not a race!

While your climbing around down there you might as well replace the plugs, wires etc. Spend the time to check major wire connections like on the starter and ground connections on the block for corrosion. I always take a full set of sockets, small tooth brush size wire brush, and contact paste.

Plan to spend some time with this project and don’t expect to have it knocked out in an hour. Probably the most important yearly test you can do on any motor, marine or land.

 

[Pack66Dad]

I watched my mechanic do mine. It looked pretty easy, pull a plug, put in a gizmo with a gauge in the hole, turn over the engine (he hooked up a switch to do it while he was back by the engine), write down what the gauge said, replace the plug and repeat..... 16 times.

 

[AKBASSKING]

Not sure if this correct. I do the following:
Remove the ignition wire from the coil to the distributor
Turn on blower and leave it on
Get every one off boat except person reading gauge
Turn off all breakers except those that allow you to turn over engine
Put out some fire extingushers
Check compression when
Engine cold
Engine cold with oil squirted into the cylenders
Engine warm
Engine warm with oil squirted into the cylenders
I have a NAPA compression tester that screws into the spark plug hole. Think it as $40
If all readings are within 10% I feel good. Especially when engine warm. So far this has been the case.

I hate to disagree but I have worked on engines most of my life and I have rebuild many.

The safety stuff is right on but:

1. Run engine until at operating temp, shut down disconnect coil wire.
2. Remove all spark plugs and inspect them as you remove them.
3. Place throttle in WOT position or remove cable and wire full open.
4. Start with #1 Cylinder and screw in gage. Have friend turn over engine until the gage stops moving up, normally about 4 revolutions or more. Record reading on paper. This is your "dry" readings.
5. Once you have done all cylinders, it is time to do it again, but wet.
6. Go back to #1 and squirt a little oil (any good motor oil) into cylinder, connect gage, turn over engine. Do this for all cylinders.
7. Compare all readings and if it is more that 20psi differance you MAY have an issue.

Keep these reading for future test to compare.