This was performed on a 1997 Grand Prix GT N/A 3800 Engine
NOTE: I am NOT a mechanic, just a regular backyard guy…
OK, so my problem has been a rough Idle, engine misfire…
I had checked all the usual suspects, MAF, PCV, Spark plugs & Wires, UIM & LIM Gaskets…
All checked out good… so where to go next…
The only way to find the problem is with a compression test, then a leakdown test on any cylinders that seem suspect..
You will need a compression guage, here is a link to one –
Quick-Connect Compression Tester
You will also need an air compressor – regulated to about 70psi
And regular tools
Here is the process: Cold Engine
Remove all the spark plugs, plug wires (Label Them)
Remove the fuse for the fuel pump.
Open the throttle & use a screwdriver or something to hold it fully open.
Use the compression guage on all 6 cylinders, write down the numbers
(crank the engine for 4 or 5 revolutions)
Depending on the miles of your engine you should see around 150-200psi on the cylinders
If you see one or more with low numbers, they need further testing…
Firing order : 1-6-5-4-3-2
Find top dead center on the compression stroke of the suspect cylinder
Put a small piece of toilet tissue in the spark plug hole, and the one prior to it in the firing order.
Rotate the engine using a wrench on the crank pulley – clockwise
When the paper blow out from the cylinder prior to the one you want, go slowly…
When the paper blows from the cylinder you are testing, stop, stick a screwdriver in the spark plug hole & slowly turn the engine until you feel the piston reach top dead center.
Remove the schraeder valve from the end of the compression tester tube & install the tube in spark plug hole.
Remove radiator cap, oil cap & intake hose from throttle body.
Attach air from the compressor to the compression tester hose.
You will hear air rushing from one of these places
Tail Pipe – bad exhaust valve
Throttle body – bad intake valve
Oil filler tube – bad rings
Bubbling from radiator – Head gasket
From adjacent spark plug hole – Head Gasket…
I will update/edit this post when I have time
I used as much common sense as I could find, and these 2 great resources…
Eric the car guy video
Google searches
My results: I have 2 cylinders 3 & 4 that are both leaking through intake valves – showing cylinder heads need rebuilding
My compression numbers are:
1:135 3:65 5:180
2:175 4:60 6:175
I did a leak down test on cylinders 1, 3 & 4
Cylinder 1 showed a small leak through the intake
Cylinders 3&4 were just rushing out the intake.
