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Guest Message by DevFuse

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Compression Testing Confusion

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3 replies to this topic

#1 Marck


    Certified Subaru Nut

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Posted 03 October 2003 - 09:24 PM

According to the Haynes Manual, I need to:
(1) Warm up the Engine
(2) Remove ALL Spark Plugs
(3) Block the throttle wide open
(4) Disconnect the primary wires from the coil(s)
(5) Hook up the compression gauge and crank engine at least 4 times. Record the highest reading.
(6) Repeat procedure for remaining cylinders
(7) Add three squirts of oil to each cylinder and repeat test

According to my 87 FSM, I need to:
(A) Warm up the Engine
(B) Remove ALL Spark Plugs
© Disconnect the harness connectors for injectors
(D) Fully open the throttle valve
(E) Crank the engine by means of the starter motor and record when pointer on guage is steady
(F) Perform at least two measurements per cylinder

Some of my questions are stupid or basic, but I just want to make sure I get correct readings without screwing anything else up. My questions are:

Do I do 3 or D by holding the gas pedal down or by using my hand to pull the accel cable?

How do I do 4? Which wires are the primary wires? Are they talking about the wire that goes from the distributor cap to the ignition coil or are they talking about the wires that are connected to the neg and pos on the coil or both?

What happens if I skip C? Why do I need to do C? Why doesn't the Haynes Manual say to do C?

Why doesn't the FSM say to do 4?

Does E just mean to crank the engine by turning the key or is there another way to crank the engine?

Do I follow the FSM or the Haynes Manual or what? When I did a compression test in a high school auto class 3 years ago I thought I just removed one spark plug at a time, hooked up the compression guage, and cranked the engine. I am confused :confused: .

#2 baccaruda



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  • SpoVegas, WA

Posted 03 October 2003 - 09:37 PM

i usually ground my coil wire to one of the 3 strut bolts in the tower.. it clips right on.
unplugging the fuel pump under the car would be the easiest way i can think of to cut off the fuel. then try to start the car and let it run until it runs out of gas.. that way the system's empty and depressurized.. then ground the coil wire and remove the plugs.
open the thottle by turning and holding the throttle cable receiver, then stick something in to block it and don't forget about it :D
i think that 4 and C serve the same purpose. both involve unplugging the fuel delivery circuit. i think the coil powers the fuel pump relay? that's why i suggest unplugging the fuel pump as it's simple too.
you can get a remote starter switch that you hook up under the hood.. i've yet to use one so get one with instructions :D
good luck with your test

#3 Supaglu


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Posted 04 October 2003 - 09:17 AM

To do a proper compression test you need to -

1) Disconnect your coil from the ignition circuit - basically to prevent any damage to your ignition module - (stop any sparks) because the leads are not connected to the plugs.

2) Prevent your fuel system from delivering any fuel - either by disconnecting the injector plugs or the fuel pump itself. If you have a catalytic convertor you dont want neat fuel going into the cat.

3) Remove all spark plugs - if your head gasket is blown between two cylinders then you may still get good compression if that plug is left in !.

4) When you crank the engine (turn it over with the key) put your foot on the throttle pedal and hold it to the floor whilst it turns over - and turn it over and listen so that you get the "four cycles" ie - Induction, compresssion, power and exhaust. it needs to be at least four times because the compression stroke is only one of the four cycles ( hence - four stroke engine)

5) The throttle pedal needs to be held open because if you have an inlet valve thats burnt out you will still get some compression because the throttle flap is shut.

6) You should also carry out the compresson test on a cold engine and then compare the readings with the engine hot.

Hope this helps


#4 asavage


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Posted 04 October 2003 - 01:06 PM

If you have to disable fuel feed . . .

My manual says to disconnect the injector at the connector (SPFI, but similar x4 for MPFI), but when I was all done checking compression and got it running again, the ECU had set a code (14: injector) because it sensed the open circuit on powerup.

This doesn't set the CEL (because once you reconnect the injector, it's no longer an immediate problem) but the malfunction code does stay in memory, which can confuse you later when your do a Read Memory check.

So, if you go this route (disconnect injector for compression check), you may want to consider running the Clear Memory procedure to remove the now-hidden code from the ECU:
To clear codes, you have to run the Diagnostic (D-check) and also have the Read Memory connector connected. Warm up the engine to operating temperature, shut it off, connect BOTH the D-check (green) connectors and Read Memory connectors.

Turn on the ignition, wait for fuel pump to stop (about 1.5 seconds), verify that the CEL light comes on, press accelerator to floor, then back off to half throttle for two seconds, then release it completely. Start the engine.

Then drive it constantly above 5 mph for at least 39 seconds (yeah, I know). The CEL is supposed to blink, indicating no codes found -- and the old codes are now cleared. Shut it off, disconnect the D-check (green) connectors and the Read Memory connectors, and you're done.

If, after about two minutes of driving above 5 mph, you get the CEL light again and it's not blinking, the ECU has finished the diagnostic check, and it found another code, which you will have to read and deal with. Read it off the O2 sensor monitor LED on the ECU itself: see also http://usmb.net/repa...e9.39782181.art

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