Compression Test for 4 Cyl

[kkuhia]

I've never done a compression test, but I've read my service manual and also read the last year's worth of posts on compression testing on iboats. I still have a couple of clarification questions. Here is what I've gathered is the correct step-by-step approach with my questions embedded (my engine is a 1987 4 cyl Volvo Penta AQ131A with Volvo 275 outdrive):

1 - Get compression testing gauge (got mine from Shucks for $40)
2 - Remove ALL sparkplugs
3 - Activate KILL switch (by pulling lanyard)
Question: Do I need to ALSO ground the plug wires as many posts say to do or just use the KILL switch?
4 - Screw in compression gauge into cylinder 1
5 - Push in button on throttle control lever (so drive is not engaged) and push lever forward to full throttle (WOT)
6 - Turn engine over (using key/starter) 7 to 9 times
Question: My boat is not in the water, so do I need the muffs with the hose on to do this?
7 - Write down highest compression reading for cyl 1
8 - Remove compression gauge and screw into cyl 2
9 - Turn engine over 7 to 9 times
10 - Write down highest compression reading for cyl 2
11 - Remove compression gauge and screw into cyl 3
12 - Turn engine over 7 to 9 times
13 - Write down highest compression reading for cyl 3
14 - Remove compression gauge and screw into cyl 4
15 - Turn engine over 7 to 9 times
16 - Write down highest compression reading for cyl 4
17 - Compression readings for all cylinders should be within 20 to 30 psi of each other
Question: I've read a couple posts where people have put the compression gauge in too far (and damaged their heads) or too tight (stripped spark plug hole threads), how can avoid doing this (how tight do I have to screw in the guage)?

Please correct any steps that I have wrong, and thanks for the help!

 

[horsefly38425]

Re: Compression Test for 4 Cyl

just remove dist. cap / or take hot/ + wire loose from coil to stop the engine from starting. No need to W_O _T -----No muffs needed / turn mine over only 2 or 3 times . after 6-7 times they will build comp even with a leak /wacth reading on 1st comp stroke 8) and you don't have to put the end of the tester in real thight /just by hand -unless you have the one with the rubber end on it and then you will have too hold it in while you use a remote starter swicth or have help.

 

[tie_one_on]

Re: Compression Test for 4 Cyl

Wait a minute.
You do need to go to W_O_T.
You should turn it over at least 4 - 6 revolutions.

 

[kkuhia]

Re: Compression Test for 4 Cyl

Is the hot wire from the coil just the middle wire coming off the top of the dist cap? Just pull that off instead of the KILL switch?

 

[bjcsc]

Re: Compression Test for 4 Cyl

It's easier just to use the kill switch. If you pull the HT lead it could arc. Using your kill switch is easy, spark free and safe. There is no need to groung spark plug wires. When your kill switch is activated there is no voltage to them. I didn't read anything about running it to operating temperature prior to starting the testing. Do that if you can. As long as you don't use anything other than your fingers to tighten the gauge line in you will be fine. Watch the gauge closely as a cylinder with 150psi on the first compression stroke means something different than one that gets 50psi. on the first and then 150psi. on the next one. Remember you're only going to get one compression stroke for every four revolutions. That's why the procedure calls for 7 to 9. The reason you open the throttle is so that you do not restrict the intake of air during the test. The choke and throttle should be wide open during testing. Good luck.

 

[vipzach]

Re: Compression Test for 4 Cyl

I think it would be smart to have the muffs on. I do not know if a volvo impellar spins like a merc, but anytime the engine is turning over on a merc the impellar is turning and that will destroy a dry impellar. I could be wrong, but that is how I see it.

 

[hullofalottatrouble]

Re: Compression Test for 4 Cyl

vipzach.. never thought abou that myself.. but could be a valid point.. so if you follow the correct warm up procedure you should have the muffs on anyway.. guess it wouldnt hurt to just leave the water running while you test the compression too..

 

[Silvertip]

Re: Compression Test for 4 Cyl

Regarding how far to screw in the tester, look at the threads on the spark plugs. The tester has several adapters. Use the one that matches the spark plug thread depth. Do not use one longer than that even if it does fit in the hole. If you have a long reach plug its ok to use a long reach or short reach adapter.

 

[calwldlif]

Re: Compression Test for 4 Cyl

Are you having performance
issues?
I feel if a person is going to do
comp check,
might aswell go the distance
and put some oil in the cyl and recheck
to get an idea of how the rings are.
Could let you know if a head job(hmmm)
would do ya good?

 

[kkuhia]

Re: Compression Test for 4 Cyl

Thanks for the explanations of why KILL switch and why WOT. Given the water in my oil, I don't want to run the engine before the compression test so that is why I didn't have it in my steps (I did read that you should do that first before you do a compression check).

Sounds like the KILL switch is what I will go with and not worry about grounding plugs or pulling hot wire on distributor cap. I will also match up the adapter to the spark plug thread depth. I will use the muffs as well to be safe.

As to if I'm having performance probs, I'm not sure yet as I have to do quite a few things before I can get this boat in the water, but I wanted to check compression while I'm doing the manifolds and such in case I do need to worry about rings or anything else. Thanks for all the advice! I'll let you know what the results of the test are.

 

[kkuhia]

Re: Compression Test for 4 Cyl

Alright, I'm not sure if I did it right as the readings were higher than I expected, but here is what I got:

Cyl 1: 155
Cyl 2: 165
Cyl 3: 165
Cyl 4: 170

Questions:
1) I just screwed the rubber hose straight off the compression gauge into the spark plug hole, was I suppose to use the adapter that came with it instead?

2) The compression readings went up as the engine cranked. Was I supposed to take the first reading (on first crank) or the final reading on the gauge when the engine stopped cranking?

3) It was dark outside when I did this so I'm planning to try it one more time tomorrow in the daylight, is it alright to leave the spark plugs out overnight if I put the engine cover over the engine (totally clear night prob 65 to 70 degrees outside)?

 

[horsefly38425]

Re: Compression Test for 4 Cyl

just trying too help! that is why you ck reading on the 1st comp. stroke , they will build compression the more times you turn it over /no need to turn 6-7 times, but if you do you may want to use the muffs so you don't damage imp.

 

[bjcsc]

Re: Compression Test for 4 Cyl

The readings you got are good. To answer your questions:
1) You don't have to use the adapters. If you look closely at them you'll notice the thread pitch / diameters are different. If the hose end screwed in, you're fine.

2) Yes you want to watch how it builds up. You don't want to stop and take individual readings on the compression strokes on the same cylinder, but just watch how the needle moves with each time and then make a note of it at the end when you record the final pressure. You can repeat on the same cylinder to doublecheck.

3) Plugs out overnight won't hurt anything.

Based on the numbers you reported it looks like you're OK.

 

[kkuhia]

Re: Compression Test for 4 Cyl

So should I just try to remember what the readings were for each compression stroke and record those after or just look at what the first stroke started at and the reading at the final stroke?

I'm going to do another test today as I want to do it in the daylight and see if there is any water that comes up out of the spark plug holes to see if there is any water on cylinder heads.

 

[bjcsc]

Re: Compression Test for 4 Cyl

Yes, basically. You're interested to know whether or not the compression is instantly there, or builds to the final number you get. You don't really care how it builds up, only if it does. What the actual numbers are as it's building (if it does) doesn't matter. Think of it like whether or not it pumps up. With numbers like you have so far, I doubt you will see anything.