Hi...trying to help a friend out with a 2017 focus ecoboost 1.0...86k

Comes up with misfire in cylinder 3. Tried the usual swapping over coils.

New plug in cylinder 3...although didn't have the spacing tool for the plug...looked identical but need to check the gapping.

Tonight I did a compression test with a cheapie tester. Ran car around the town for 15mins to get it warmed up.

the results...1 and 2....200psi. 3 was 175psi. As it was getting dark I stupidly didn't retest to confirm so intend to do it again for him at the weekend...

In general the car runs lovely...no noticeable mis fire...no over heating..no white smoke..no pressure in the coolant tank when cold and in 2 weeks the coolant has sat perfectly on the max although he doesn't do many miles.

One telling issue for me other than the lower psi is that the fault only seems to come up after a 20min plus drive, thus concerning me that a crack is opening up in the cylinder when hot.

Don't know history of the car as he just bought it as a cat N

Haven't tried to change the injector, but don't know that its worth it with the lower psi reading...

Any advice, or further tests to give a go would be great...(will try the "teaspoon of oil" in the cylinder at the weekend if the reading is definitely low.

Thanks for reading...any advice would be really appreciated...

Anything starting on cylinder three on these is a sign of previous oil starvation due to wet belt caviar in the oil pick up pipe.

As you've only just bought the car you won't know the history, so you won't know if the belt has been done or even if the car has been abused.

1.0 Ecobooms are a can of worms

I'd move it on ASAP, like the previous owner did

Thanks youre reply... this is kind of the advice i was expecting.

Just to add, the car wasnt "moved on", it was in an accident a few months ago and bought at a salvage auction. But yes, i know nothing of its history.

I've read that a poor injector will contribute to lower compression...?

As it runs so well with zero coolant loss, is there a chance that a different injector could solve this..?

You can keep throwing parts at it forever, if you want to.

But my advice is:

If there's no MIL, move it on, before it goes bang

Thanks for the replies..."if there's no MIL, move it on"...

There is, cylinder misfire code but could go a couple of days without it coming on again...

Ok, you don't mention MIL is on in the post.

If you've only put a new plug in NO.3 (don't know why you'd do that but..) and are unsure if gapped, swap NO.3 with NO.1 and see if the miss fire fault moves to NO.1

That's all I can suggest other than, turn off the MIL and get rid of it

Apologies for not making that clear... management light and subsequent diagnostic was what highlighted cylinder 3 misfire. Only changed one plug because it was so tight, i didnt want to risk breaking an extra one for nothing... since then managed to remove all 3 to do the compression test, which im going to do again, and then wet test cylinder 3 to see if it rises...I need to do any inexpensive tests i can before giving up on what seems to drive without faul.