XtremeVette

So Iam gonna get a kit from autozone and do a compression test this weekend...can someone check my procedure and let me know if Iam forgetting anything....Thanks


Engine Compression Test

1. Charge the battery if the battery is not fully charged.

2. Disable the ignition system. Pull Fuses marked IGN1 and ING2?

3. Disable the fuel injection system. Pull Fuses marked INJ1 and INJ2?

4. Remove all the spark plugs. Remove all at the same time? or do you just remove 1 for the cylinder your working on and leave the others in?

5. Turn the ignition to the ON position.

6. Depress the accelerator pedal to position the throttle plate wide open. (do you keep the accelerator pedal down during the cranking or just press it down once?)

7. Start with the compression gauge at zero and crank the engine through four compression strokes, 4 puffs. Do I keep letting it crank until the needle stops moving or just stop after 5-6 seconds?

8. Check the compression for each cylinder. Record the readings.

9. If a cylinder has low compression, inject approximately 15 ml (1 tablespoon) of engine oil into the combustion chamber through the spark plug hole. Recheck the compression and record the reading. Just pour 1 tablespoon of Mobil 1 5W-30 down the hole where the spark plug goes?

10. The minimum compression in any one cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read less than 690 kPa (100 psi). For example, if the highest pressure in any one cylinder is 1 035 kPa (150 psi), the lowest allowable pressure for any other cylinder would be 725 kPa (105 psi). (1 035 x 70% = 725) (150 x 70% = 105).
Normal -- Compression builds up quickly and evenly to the specified compression for each cylinder.

Piston Rings Leaking -- Compression is low on the first stroke. Then compression builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil.

Valves Leaking -- Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you add oil.

If 2 adjacent cylinders have lower than normal compression, and injecting oil into the cylinders does not increase the compression, the cause may be a head gasket leaking between the cylinders.

How to do a leak down test? (I stole this from another guy on the forum)
1.) Get a leak down gauge
2.) Remove all the sparkplugs from the holes
3.) Hook the guage up to your air compressor and inject 100 psi of air into each cylinder at its TDC, you can find tdc by injecting say 20psi and listening for TDC or also its right after the crank gets super hard to turn to super easy. Once at TDC inject 100 psi, look at the 2nd leakdown gauge on the tester. if it says 90 then then that cylinder has 10 % leak down.

My biggest concern is I have never done this before so inexperienced in finding TDC.

Also you need to listen to where it is leaking from. If it is going into the oil pan you have blowbuy IE ring/pison issues. If it is out the intake then your intake valve is fuxored. Likewise if you here air escaping out the exhaust its your exhaust valve. If you see bubbles in you coolant than you have a head gasket issue. Its a really cool test to do especially being able to hear wear the problem lies. Also anything above 10 % is real bad, 7-10% is still bad but ok. 4-7 is pretty damn good. Under 4% and your leakdown % is rivaling race cars. I could be wrong slightly on the above % but basically anything up to 10% is ok with 9 and 10 % being iffy.

CamminC5

Don't waste time doing a compression test. Just do leak down. It will tell you everything you need to know.

Unplug coils at the white connector.
Remove fuel pump fuse and/or relay in the underhood box. You can unplug injectors if it makes you feel better.
Remove all plugs so the motor rotates easy


It's hard to find TDC on these cars since you get easily get to the crank bolt. There's a few ways to do it.

Remove flywheel inspection plate and turn the flywheel by hand (remove spark plugs first). Have a buddy let you know when you're at TDC by using a pick in the spark plug hole or bore scope.

I have used a compression tester hose, removed the Schrader valve, taped a balloon on the end and bumped the starter to right when the balloon stops inflating. It make take a few turns, but you'll get it.

XtremeVette

Makes sense...unfortunately I have never done it and don't have anybody around who can help. Obviously if I did I wouldn't be on here asking for help lol. I know a couple mechanics....both are booked solid for 3 weeks. I have a trip coming up next month and can't wait the 3 weeks.

CamminC5

Trying to consolidate from your other post and just comment in here.

I don't understand the logic on 20psi.

You want the piston up top. It's not hard to hit. I have used a balloon in the past, but now use an automotive borescope (that I picked up O'Reilly) and turn the motor over by bumping the starter.

XtremeVette

I hear ya I actually have a camera with a probe on it can I put that inside the spark plug hole and be able to see top dead center is that the same thing that you do? Can I crank it over by the key?

CamminC5

Same concept. Make sure the camera head isn't sticking into the bore just incase the piston/valves come in contact. Use the key unless you get one of those starter trigger switch things.

XtremeVette

Perfect...thanks man!

CTD

Compression test can be useful, record each cylinder on the first puff. Then continue on counting the balance of puff's I generally go 6 or 7.

That first puff is a good indicator of the seal of your motor. Should one cylinder be quite low on that first puff compared to the others it will leak down poorly.

STL Mike

For the compression test, I thought I only removed the plug for the cylinder I tested. Replace when complete.

The accelerator pedal down is meant to open the throttle body. Unsure if the TB opens electrically on these cars when not running. This is done to remove a restriction on the intake track and needs to be done any time a cylinder is under test.

The oil into the cylinder is done after a low compression reading is found. It's used to check the rings and ring seal. Don't fixate on using a teaspoon/tablespoon of oil, use what you need to lubricate the rings around the cylinder.

There are several good videos on youtube as well. Good luck!

=Mike

Summiz

I've always wanted to be more confident in this procedure as well, I'm interested in how it goes for you XtremeVette.

redzg

For compression, you want all the plugs out and the throttle blade open -- no idea how to achieve that with DBW. Like much the idea of recording and comparing 'first puff' as well as total compression in each cylinder. Always amazes me how many people will chase various performance issues without running a compression check, which tells us so very much.

Georgies

what does first puff mean?

And when you crank the motor, how do you keep the TB open?

C5 Diag

Thread from 2016...gotcha !!