Its been so long since I did a compression test myself, I've forgotten some details, and I’ve never run a comp test on any Volvo. I bought a comp tester today and will attempt to run the test this weekend.

The questions are: Do I take out ALL the spark plugs first, or 1 at a time, putting each one back before the testing next cylinder?? (I will disconnect the coil wire so it wont start.) . Should I lube in some way the threads on the tester? Do I have to be concerned about fuel “pushing out“ when the engine is turning over, or fuel collecting in the cylinders?

I also assume it takes just a few revs with the starter to get the numbers on the tester. Any other suggestions? I’ll look up the “recommended” compression numbers in the book, but I at least hope all cylinders will be pretty equal to each other. .

It really has been a lonnggg time. Thanks.

disconnect all fuel injector harness connectors
disconnect coil to diz cable
remove one plug at a time
test

Agree with all, but: "remove one plug at a time"

If you have a leak between 2 cylinders, the test won't show any change.
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Post Back. That's whats makes this forum work.

Well, I'm an all plugs out person. Test with wide open throttle. Test all twice, if consistant, add a couple of squirts of oil and retest as before. Note, as the engine is slanted, oil can settle to the side so test immediately after you add the oil. Increased numbers after oil test points to rings. If numbers are low suspect valves, but if numbers are low but within 10% likely just fair wear and tear.Two people make the job easier.

I do it the way derek uk suggests.

Something I haven't seen mentioned yet - Volvo incorporated a service socket to crank the starter without activating fuel or ignition systems. (Unless the car has a ballast resistor in the coil supply line, in which case disconnect the coil #1 terminal)

On my 83 it is located behind and above the cyl head protruding from the harness on the firewall there. It's a Blue wire, usually with the final few cm's wrapped in a Red label sleeve. Connecting its female spade terminal to +12V cranks the starter. With a simple setup of wire and temporary contact pushbutton switch you can crank the engine from under the hood while watching the compression test gauge.

Don't just disconnect the coil to distributor high tension wire---use a jump wire to ground the end of the cable---or better yet, just disconnect the + wire at terminal 15 on the coil. -- Dave

"better yet, just disconnect the + wire at terminal 15 on the coil"

Primary + wire on coil 15 will do it, but the green book says to remove the ground side wire on terminal 1. (just sayin')


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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

"Primary + wire on coil 15 will do it, but the green book says to remove the ground side wire on terminal 1. (just sayin')"
I can certainly see the advantage (safety) in removing the - wire at the coil---then there is no live wire dangling while cranking--no chance of a short. Either way you are eliminating the chance of a high tension spark. -- Dave

Grounding the spark plug may be a good precaution. On electronic ignitions with Hall Effect sensors, at least, ungrounded spark plugs can damage expensive components. When working on BMW airheads, it's common practice to ground the spark plug. Not sure if same applies to ignition systems where crank position sensor provides the timing pulse. More knowledgeable BBers may want to weigh in here: Does cranking the engine with spark plug wires loose pose potential damage to ignition system?

Chris in VT, '94 945 w/ 178K, R80RT (x2), R100RS

Been reading "Snowbum's" airhead tech articles, haven't you, Chris?

Me too.