[Solved] Best Way to Disable Ignition When Checking Compression on 3.4 Liter Engine

It's been over 55 years since I last checked the compression on an(y) engine. So-o-o,

What is the easiest and safest way to disable the ignition while checking compression on a 3.4 liter engine? I need to do this to make sure my problem is not compression-related.

 

I’ve never done this either but maybe disconnect the + side of the battery and run a lead from the battery’s positive post through a switch to the + post on the starter after removing the cable that is connected to it to prevent any chance of back feeding. That way no power goes to the ignition but you can turn the starter using the switch.

 

Pull the fuel pump relay so the engine won’t get any fuel. That way it won’t pump fuel into the cylinder and up into your gauge when you crank it. Engine won’t fire and you’ll get a more accurate reading without ruining your compression gauge.

 

Disconnect the primary coil wires. Not the spark plug leads but the wires that supply voltage to the coils. I agree with mrl390.....disable fuel also.

 

Here is a diagram of the fuel pump operation:



As shown in the following diagram, the Circuit Opening Relay is high up behind the instrument panel. I'm a pretty big guy and I don't think I can reach it.



Is this a better spot to disconnect the fuel pump? At least I can get to it (I think, anyway).



Thank you for your help.

 

Most fuel injected vehicles have some kind of fuse or larger relay in the BCM/fuse box for the EFI or fuel pump or sometimes both. Check the diagram for your fuse block to see where it is. Test to make sure you pulled the correct one by turning the ignition to on and listen to whether the fuel pump primes or not. Then try to start the engine if it doesn't start then you cut off the fuel supply and should be good

 

Remove the 7.5 amp ignition fuse from underneath steering wheel.

But, yeah, disabling the fuel as noted above would be good, as well.

 

Pull the EFI fuse and disconnect the main power wires going to the coils. Just recently checked compression on a 3.4 3rd gen runner.

 

The simple way is to disconnect the ECM ground .

You can also unplug the igniter which will also shut off the fuel. No bending or getting on your knees to reach things

To us older people unplugging the igniter is like the coil wire in our days

When your all done just clear all the codes

 

What's the trick to pulling the 15 amp EFI fuse up and out of the fuse box (on the driver's side under the hood)? It appears to be stuck. I'm using a little pick tool pull it up but the fuse won't budge and I'm afraid I might break it if I pull any harder. Is there some kind of latch or lock?

Just how much force is needed to remove those little fuses anyway?

Or alternatively, at what point on the truck is the ECM ground that Wyoming09 referred to?

Or, where is the "igniter" that he mentioned?

Once I get that done, I'm ready to check the compression.

V8 engines were a helluva lot simpler dontcha know...

 

If it hasn't been pulled for a while then it can definitely be hard to pull out. The OEM little puller thing in the cover of the box has always worked for me. Sometimes on other vehicles I've had to use these mini needle nose plier things. But was always able to pull the fuse.

 

Yes, now I remember the extractor. Never thought to turn the cover upside down. Thank you Mr. Cruiserguy.

 

No problemo. Those fuses are almost impossible to remove without that protruding leading edge on those tweezer/tool things

 

I've always just disconnected the coil packs. Disconnecting the crank position sensor won't allow spark either. Sounds like there are many ways to skin this cat

 

The coil packs are out. I was trying to see how to disable the fuel pump so I could crank the engine without fuel squirting all over and making my day...

 

You won't get fuel in your gauge. If the ECU doesn't get all the parameters set to 'run', it's not pumping fuel.

Disconect the injector wiring if still worried.