Ford F150 Forum - Community of Ford Truck Fans - Compression test- which fuses to pull?

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lsbbronco

01-04-2018 03:41 PM

Compression test- which fuses to pull?

About two weeks ago the cel came on with the code p0308, cylinder 8 misfire. I swapped the cop with cylinder 4 and put in new spark plugs. Code came back on yesterday.

So, im gonna do a compression test but wanted to confirm which fuses to pull. I'm thinking #27 for fuel pump and #78 for ignition coil power. Are those the ones?

Anything else besides the spark plugs?

Thanks

Inhot

10-01-2018 08:28 AM

I'm about to do a compression test and have the same question. A little help please?

SD_F1504x4

10-11-2018 12:29 AM

Not sure about fuses. You should be able to unplug all the coils and fuel injectors individually right?

Inhot

10-11-2018 07:32 AM

You cannot disconnect the coils from the wire harness. I pulled the fuse for the fuel pump and the fuse for the ignition. I was then able to perform the compression test.

PerryB

10-11-2018 07:51 AM

Quote:

Originally Posted by Inhot (Post 5944415)

You cannot disconnect the coils from the wire harness.

You most certainly can.

FLXLT

10-11-2018 10:54 AM

Quote:

Originally Posted by Inhot (Post 5944415)

You cannot disconnect the coils from the wire harness.

:bsmeter:

SD_F1504x4

10-11-2018 09:08 PM

Yea, I swapped a coil in after a misfire. You just pull the plug that connects into it. I think I disconnected the fuel injector plug too to check it if I recall correctly.

RLXXI

10-11-2018 09:12 PM

Quote:

Originally Posted by Inhot (Post 5944415)

You cannot disconnect the coils from the wire harness.

With that statement I question your ability to perform a compression test correctly and/or interpret the results.
.

Inhot

10-12-2018 07:12 AM

Yeah, I can see why you would say that. That post should have read: “you cannot disconnect the coil plug from the wire harness.” Of course you can remove the plug from the coil and remove the coil. However I was attemting to relate that the exposed plug that has been removed from the coil is still connected to the harness and if you turn on the ignition the coil plug would be energized. Before attempting the compression test I pulled the ignition fuse to preclude having the coil plug being energized during the test. Sorry for that omission.

PerryB

10-12-2018 10:12 AM

Once the coil is disconnected, the plug is a non-issue. The contact sockets are far enough up inside the connector that they can't touch anything even if face-on to a metal surface, and at a whopping 12vdc they aren't going to be throwing any sparks.

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