Compression test how to / missing step?

My 2002 Tacoma 3.4 has developed a miss. I have changed spark plugs with no resolution. I've owned mostly diesel 4 cylinders over the last 30 years, so my knowledge of the whole ignition coil type system is limited. I was about to buy all new coils and wires, and the parts house guy says, "Why don't you do a compression test before you throw all that money at it?" An excellent idea, but I've never done a compression test. A video I watched gave a basic procedure of:

1. Remove all plugs.
2. Disconnect the distributor so it doesn't short out (but the 2002 Tacoma doesn't have a distributor).
3. Disconnect injector wiring.
4. Install compression gauge, and roll it over 3 or 4 times.
Anything above 200 is good. Cylinder pressures should not be more than 10 to 15 different between cylinders.

Does this sound like the right procedure?
Since the Tacoma doesn't have a distributor, is there something else I should be disconnecting to avoid damage?

Thanks very much for your help here!

 

So just remove the EFI fuse (within the main engine bay fuse box), then you don't have to worry about disconnecting all the injector wiring.

No need to worry about the plug wires or a distributor (just drape the wires off the side off the motor out of your way) .

And usually you want to crank the motor for about 5-6 seconds or more (until the gauge stops climbing and building any further compression). Ideally you'd also get the motor nice and warm before the test (drive it for a while), then quickly get the plugs out, remove the EFI fuse and do the test.

If you do the test on a completely cold motor, you'll get lower compression readings that aren't really accurate for what it'd develop under normal running conditions.

 

Thank you so much!

 

Just pull the EFI fuse to disable fuel and spark, no need to unhook anything. Hold the throttle wide open while cranking.

Standard compression for the 3.4L is 174psi, minimum is 145psi, maximum difference between cylinders is 15 psi.

 

Excellent info! Thank you!

 

I have never seen over 200 haha, altitude matters as well. We see 130-150 on older V6s like yours.

Let us know.

 

I got the 160s-170s a couple years back when my truck had 280k miles, and that's up in Prescott, AZ at 5300 ft. Pretty pleased with that

 

that’s great we are a little lower than that and always happy to see 160+

 

Is the CEL (check engine light) on?
Do you have a scan tool?

Compression test is a good starting point. If the compression in the lowest cylinder is within 10% of the highest cylinder you are good to go. If the compressions are outside that range IMHO I would check valve clearances. If they are out of spec I would seriously consider taking it to a shop that can do the job. It isnt that it is a difficult job it is getting the adjusting shims.

The ignition system on the 3.4 is what is called a wasted spark ignition system. You will see that there are only 3 coils for a 6 cylinder motor. When each coil fires on the respective compression stroke the companion cylinder that is on the exhaust stroke that spark plug is fired too.

 

Thank you all for the excellent information! I'll let you know when I finish the test.

 

I ended up not having to do a compression test, but I have saved your advice for next time. I found a problem with the plug wires, resolved that, and it runs smooth again!

Thank you all, once again! Your help is very appreciated!