P0305 weird

97 4.0
Ok so I had a P0305 Cylinder # 5 misfire code---checked compression on that cylinder-- 95lbs.... sigh... I put a Bosch plug in it and cleared the code and it ran good for 1 day... the code came back today. It only skips slightly at idle... runs good going down the road. Head problems? any way to diagnose? I don't think it's the injector thorwing the code, I think it's the low compression. suggestions???

I would do a leak down test and see what you get. Also what is compression on other cylinders?

Have you checked the cap, rotor and wires yet?

Spec compression for the 4.0 is 120-150, with no more than a 30 psi variation between cylinders.

Did you run the compression test on ALL cylinders? If not, you should as long as you have the gauge. Would be interesting to see all six numbers.

Also can run a "wet" compression test on the suspect cylinder(s) by adding a small amount of oil into the cylinder, then running compression again.

If the reading increases more then 10 psi with the presence of oil in the cylinder, the most likely cause of the original low compression reading was a poor piston ring sealing. This may be an indication that the rings are simply worn. And this would be highly likely on an engine with high miles.

new cap., new rotor, new wires, all other cylinders check between 140 to 160. stuck ring? wonder if if pour some seafoam in the spark plog hole and let it sit overnight it might free a stuck ring? im reaching here.

Pull that injector out and test it

I don't think an injector will give you low compression!

Tuffnuttz said:

I don't think an injector will give you low compression!

Click to expand...

No but he needs to check compression across all six cylinders and see what it is. A leaking injector can cause all sorts of cylinder wall and ring issues if let go too long. thats what I was getting at

I'll try it...