Testing Compression in an S-Series

[Saturn-Eh!]

doing a compression test on an s-series is easy:

1) remove the fuel pump fuse, and start the car until it stalls. We want to remove any chance of fuel washing down the cylinder walls and skewing the results

2) reach down and unplug the connector(s) on the ignition coil (don't want to get a shocking result do we?)

3) remove sparkplugs

4) install compression tester on cylinder #1 and crank the engine over 10 times with the gas pedal to the floor. Saturn's s-series spec is ten cranks, most other manufacturers specify 3-4.

5) note the maximum reading, release the pressure in the compression tester, and repeat for the other three cylinders.


Saturn's should be between 180-210 psi, with no more than a 10psi difference between any two cylinders.

IE 180 180 180 180 is better than 180 200 205 200

even 165 165 165 165 isn't bad, as long as the cylinder readings are within 10psi of each other.

If you have a low cylinder(s) pour a tsp of engine oil into that cylinder and repeat the test. If the reading increases considerably, then the piston rings are not sealing well. If the reading stays about the same or has a minimal increase, then that cylinder has valve issues.

Note: it helps to have someone crank the engine while you watch the compression gauge. a cylinder that reaches 200psi on 3 cranks is much better than one that takes 10 cranks. watching the gauge while cranking is the only way to determine this...

Don't take compression readings to be gospel, as bad cams/lifters can still return good compression results. A running vacuum test is a good companion to a compression test when determining overall engine condition

[SLeepr]

Great post! Might I add that this should be performed when the engine is warm.

[amazinghl]

If Wolfman is here, he'd say you would want to test the engine COLD.




Saturn-Eh!-

That's funny, "(don't want to get a shocking result do we?)"

Oh yeah Eh, you got PM.

[SLeepr]

I don't know why. Your compression numbers won't be totally accurate. It's even stated in the FSMs that the motor should be WARM........don't burn yourself.

[wolfman]

Originally posted by amazinghl
If Wolfman is here, he'd say you would want to test the engine COLD.

ANY engine will test higher WARM due to the thermal expansion of the pistons/cylinder walls and valves. I test COLD because I want to know what the engine is doing at it's WORST not it's best. (Even full blown AIDS patients have "good" days) Screwing things in and out of warm aluminum spark plug holes is also NEVER a good idea...

[SLeepr]

Originally posted by wolfman
ANY engine will test higher WARM due to the thermal expansion of the pistons/cylinder walls and valves. I test COLD because I want to know what the engine is doing at it's WORST not it's best. (Even full blown AIDS patients have "good" days) Screwing things in and out of warm aluminum spark plug holes is also NEVER a good idea...

From Saturn FSM Book Engine Mechanical, page 22:

Compression Check

1. Start the engine and allow it to reach normal operating temperature.

2, Shut engine off and disconnect the ignition module wiring harness plug.

3. Disconnect spark plug wires and remove all four plugs.

4. Insert compression gage bar kit SA9127E into the spark plug hole.

5. Fully open throttle (TBI and MFI)


IMHO, you aren't getting accurate numbers unless you are collecting information with your car at temp. Saturn's spec numbers represent the motor at temperature and wouldn't give you a reasonable unit of measure if you're gathering the information cold. I want to know what my car is doing when it's running under normal conditions. Once started, it doesn't stay cold long.

One of the very few times I disagree with you , Wolfman.

[Dr. Bob]

Dumb question... if you drive the vehicle to the retailer, how in the heck would the techs be able to do a compression check on a cold engine? Wouldn't the engine still be somewhat warm when they do the test, even if they let the car sit for a few hours?

Perhaps the warm (but not hot) Saturn specification reflects the reality that testing on a truly "cold" engine rarely happens?

From a tech perspective - what does a "cold" engine mean anyway?

[David 93 SL2m]

Here is page 3-50 of the Chilton repair manual for 1991 - 1998 Saturns:

http://home.comcast.net/~David96SC2/...-page-3-50.jpg

In step two of the "Compression Test" section they wrote:

Warm-up the engine to normal operating temperature, then shut the engine off.

And in bold letters they wrote:

Be careful not to crossthread the spark plug hole.

The second paragraph under "Determining Engine Condition" has something interesting about excessive oil consuption:

As a general rule, an engine that uses no more than one quart of oil every 1,000 miles is in good condition. Engines that use one quart or more in less than 1,000 miles should first be checked for oil leaks. If any oil leaks are present, have them fixed before determining how much oil is consumed by the engine, especially if blue smoke is not visible at the tail pipe.

[Saturn-Eh!]

I realize that the Saturn factory manual says "fully warmed up", but I tend to agree with wolfman, aluminum heads do not like plugs being removed when hot, for the same reason that the compression will test higher - thermal expansion.

Having had to repair many spark plug threads both on and off the car (wanna pay for a head gasket because you yanked the plug when it was hot?) I'd have to say that I agree with "warm" but not "fully warmed up"

Keep in mind that the same Saturn factory manual says that 1 quart in 1000 miles is completely normal oil consumption for a brand new s-series engine.

FSM's are a valuable resource, but tech's in the field deal with day to day realities that are not always anticipated or documented by the engineers who write the Factory Manuals.

[SLeepr]

I totally agree. As I stated in my original post, make sure the car is WARM. I wasn't suggesting that you drive the car around the neighboorhood until the fan kicks in. A minute or so is all it takes.

My motor shows compression numbers of 165 165 165 165 when totally cold. These would be slightly out of range if I were to take them seriously. But, with a new motor, I know better. When warm, I get closer to 225 on all 4.

I'm not trying to discount experience whatsoever. Wolfman has made me eat my words a number of times. But, I find that a WARM engine (not hot) provides more reliable and measurable data.

[amazinghl]

Originally posted by SLeepr
From Saturn FSM Book Engine Mechanical, page 22:

If you would open your owner manual to the page about spark plugs, you'll find


"Do not remove the plugs when the engine is hot. Tighten the plus to a torque of 20 ft lb with the engine cold. Damage to teh spark plus thread can result if they are not properly replaced and torqued."

At least that's what my 93 owner manual says.

I know you said "warm" but how "warm" is hot?

The idea of risking to damaging my plugs holes....

EEEWWWWWW, I don't want to even think about it. I'll just need to know the COLD compression, thank you.

[92saturnsl2]

Originally posted by Saturn-Eh!
doing a compression test on an s-series is easy:

1) remove the fuel pump fuse, and start the car until it stalls. We want to remove any chance of fuel washing down the cylinder walls and skewing the results

Assuming the compression test is being done correctly, there is no need to remove the fuel pump fuse, or attempt to halt injector flow.

When the engine is turning less than 400rpm, and the throttle is greater than 75%, the PCM will close the injector(s), and as such, cease fuel flow into the cylinders.

Unless your TPS is screwy, fuel flow is automatically stopped when you try to start the car at WOT.

[sierrap615]

i won't risk it, taking out a fuse isn't that hard. what if the engine started with the tester in?

[92saturnsl2]

It won't start.

The engine needs a spark to ignite the air/fuel mixture. (It won't start without ignition!)

The point of pulling the fuel pump fuse is to cut fuel flow to prevent fuel from washing down the cylinder walls, skewing the results. By keeping the throttle wide open (which you should be doing anyways for the duration of the test) you are effectively cutting fuel flow (closing the injectors) for the duration of the procedure.

Next time you go out to start your car, try it. If it's doing it's job, the car will NOT start at WOT, this is a method the PCM uses to clear a flooded engine.

[SLeepr]

Originally posted by sierrap615
i won't risk it, taking out a fuse isn't that hard. what if the engine started with the tester in?

Anyone remember the movie Christine ?

[Saturn-Eh!]

Originally posted by 92saturnsl2

The point of pulling the fuel pump fuse is to cut fuel flow to prevent fuel from washing down the cylinder walls, skewing the results. By keeping the throttle wide open (which you should be doing anyways for the duration of the test) you are effectively cutting fuel flow (closing the injectors) for the duration of the procedure.

Next time you go out to start your car, try it. If it's doing it's job, the car will NOT start at WOT, this is a method the PCM uses to clear a flooded engine.

I tend to disagree. WOT in any general motors vehicle with no speed input is affectionately refered to as "clear flood" mode, and will reduce the Pulse Width of the injectors by 75% - so you still get fuel. Go try it, put your foot to the floor the next time you start your saturn. If cold, the car will likely burp periodiacally, if warmed up, the car will likely start and go to 3500rpm...

now, let off the gas when going down hill, and the pcm will shut off the injectors entirely...

but for doing compression tests, you still need to disable the fuel in some way (fuse is my favorite).

[92saturnsl2]

I invite you to try it (I have several times).

Clear flood mode in a Saturn is not VSS based, rather RPM referenced. This is applicable for 1991-1999 vehicles. When the RPM's are under 400, and the throttle position sensor indicates WOT, the clear flood mode is activated, pusle width becomes zero to halt flow.

On the newer cars with the Motorolla PCM's I'm not sure if they changed it, as I don't have an FSM for these years, nor do I know much about the PCM.

Cold, warm, hot, doesn't matter I can't get my vehicle to start under any condition w/ the throttle wide open. If I quickly let up the gas, then resume WOT, I can get the engine to fire a couple times, but quickly returns to normal cranking.

I'll be pulling the rail on a friends '94 sometime next week to install some larger injectors, I'll check for fuel flow at WOT at that time.

[SimPlyxTriPPiN]

Originally Posted by Saturn-Eh!

doing a compression test on an s-series is easy:


2) reach down and unplug the connector(s) on the ignition coil (don't want to get a shocking result do we?)

can someone plz re-phrase this on how to do it on my '97 sl2. do i disconnect it where it connect to the spark plug or where it connects to the distributor?

[SLeepr]

distributor????????

[SimPlyxTriPPiN]

haha. sry bout that. when he said ignition coil, i interpreted it as wires. i got it now. thanks anyways bud.