Compression Test

[schuvwj]

Since my F/M Dealer will not do a compression test on my MM, I guess I'll do one myself. I have the tools and new compression gage but I don't know how to disable the engine so it will not start or is there anything I need to do with the plug wires/coils to avoid damage to them or ECM!

Can anyone give me some advice on the proper steps to follow for a compression test of all 8 cylinders?

Thanks!

[BillyGman]

always remove all of the sparkplugs first anyway so that only one cylinder at a time will be making compression(the one being tested). That way there will be less of a load on the starter motor. Usually the thing to do is to find the coil and remove the coil wire in order to disable the spark from even going through the plug wires. But because I've only changed the plugs, thermostat, and rear end gears on my car, I'm not even sure where the coil is located on these cars.

[cyclone03]

If you changed plugs you found the coil(s).
I would say unplug the wires at the top of the coil,no way for it to fire then.

[BillyGman]

so you're saying there's eight coils, uh?

[RF Overlord]

Gman:

Our motor has what is referred to as COP ignition... Coil On Plug...that round black plastic thing with the small wire clipped into the side of it, that you had to remove to change the spark plug, is indeed the coil...each one is triggered separately by the ECM...

[BillyGman]

got it. These new cars and all their computer stuff sometimes has me lost, and then I start to miss working on my old Vette. Tankyou for that explaination.

[Petrograde]

why won't your dealership do a compression check? that seems kinda weird.

[Long Live #3]

I just did this on my car. Here are the steps:

1. Disconnect the fuel pump relay.
2. Take out the spark plug, make sure to blow away any carbon deposits.
3. Hook up the tester to the socket.
4. Crank the engine 4 times and only 4 times. Any more and you'll damage the engine.
5. Record the results.
6. Repeat steps 1-5.

Make sure you have someone else with you. While you're cranking the engine, the other guy can record the readings.

[BillyGman]

Originally posted by Long Live #3
Crank the engine 4 times and only 4 times. Any more and you'll damage the engine.
.

Is this simply what you were told, or do you have a specific reason for that theory? I'm not being sarcastic, but just posing a sincere question because I've never heard of that, nor do I understand what the reason for that would be. If you crank the motor through 4 revolutions, then you're going through the four strokes twice anyway, so how would turning it anymore than that damage the motor? Is there something I'm overlooking? the usual procedure has always been to crank the motor for sevaral revolutions while watching the compression gauge until the PSI reading on the gauge reaches it's highest point.

[Long Live #3]

This is what I've been taught by my Auto's teacher, who is ASE certafied.

[BillyGman]

thanks for your honesty and your reply. I wonder why that would be...... I'm not trying to hijack this thread, but because this is relavent to the original question that was posed, is there anyone else that cares to comment on this? BTW, I know that a "Leakdown" test is more accurate than a standard compression test, however I doubt that most dealers even have a leakdown tester. But because the test is more accurate, that's what the racers use.

[Dennis Reinhart]

this is all good but all cylinders may have a good comp reading but the engine may still have problems. I always do a comp check followed by a cylinder leak down this will confirm any broken rings and the dealer can check all cylinders with the WDS in five minutes, this will determine if any cylinders are weak.

[Long Live #3]

Just goes to show that I'm good for somethin', becides NASCAR. :D

[carfixer]

To expand on what Dennis said, the dealer uses a WDS scanner. It performes a "relative" compression test. It does not require plug removal. It disables the fuel system and measures how much time the starter takes to accelerate past each cylinder while cranking the engine. It does this by measuring time between pulses of the crank sensor. This test doesn't give you a compression number, just if each cylinder has the same compression relative to each other. It gives readings in %. Suppose it shows a certain cylinder is 75% and the others are 100%, just pull a plug and check the compression on one cylinder with a manual guage and do the math.
If a cylinder is found to be weak, a cylinder leak test is the next step to find the cause of compression loss.

[Long Live #3]

And why the dealer won't do it, I don't know.

[schuvwj]

Originally posted by Long Live #3
And why the dealer won't do it, I don't know.

Lone Live, I took my MM into the dealer last Tuesday with the fear that I may have the same problem as the silent recall on the 2003 Cobra’s having soft valve guides resulting in premature wearing causing blue smoking on cold start up, ticking when warm, and possibly poor compression.

Since we share the same valve guides as the 2003 Cobra I wanted the dealer to check it out.

The dealer would not check my compression since the ticking they herd was called NORMAL and the cold smoking was not bad enough to warrant a compression test! They said they would do a compression test for $75.00 if I wanted!

Can you believe it!

I did take my Denso plugs out before taking it to the dealer and noticed that cylinder number 7 was wet and black while the other 7 plugs were normal. I would like to purchase a new Denso but can’t find them around here so I need to call Dennis.

Hey! Thanks for the step by step instructions, this site is great!

[Long Live #3]

Like I said before, I'm good for something besides NASCAR!

[looking97233]

Funny, when I went in complaing of a mis, and kinda rough idle, my SM came out with the WDS(computer that plug into the port on the car) he hooked it up, spent an hour running every test you could ever think of on it, then went for a half hour drive with me to record what is going on. The conclusion: SM said all tests were good, he said the MAF and the left bank O2 sensor were well within spec. but that thier readings were kinda funny, "farther from where they should be than I like" he said. I am waiting on a new MAF and O2 sensor now.

[BillyGman]

like any test w/that WDS scanner thing would leave something to be desired. I highly doubt that it can be as accurate as a standard compression tester or especially a leakdown tester.