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JeremyB 07-12-2013 11:01 PM

Skipping mustang, out of ideas
So my mustang did what most do with blowing out the spark plug. One of my good friends sold me his stock heads for cheap so I had the heads changed. The Drivers side head was a newly remanned head with almost no miles on it. When it was finished, I had a skip on Cylinder 5 (front drivers side). I figured it was a COP or spark plug since the car had been sitting awhile until I got the money up to change the heads. I changed both out and no difference. It was also getting fuel as the plugs would be covered in fuel when I pulled them out. I checked compression and it is low when running, but will hold compression at TDC(doing a leak-down test). I am pretty sure either it's the cam followers or springs as the motor ran fine before changing the heads before the spark plug blew out, so I am automatically eliminating piston rings. I want to know if there is anything else I can try before pulling the head back off again. :icon_gunhead:



JeremyB 07-12-2013 11:05 PM

forgot to mention the compression was around 50 PSI, other cylinders around 175-185 psi. Did the leakdown test and it leaked down no faster than the other cylinders

FlamingoGT 07-13-2013 08:44 AM

1 minor point: cyl 5 is not the driver front; it would be one of the middle ones.

Are you really sure about the rings? When that plug blew out, some particles could have fallen into the cylinder and damaged the rings, piston, and/or cylinder wall. For as bad as your issue is now, it would have been fairly evident if the cylinder was at least visually inspected before the 'new' heads were put on.

I'm not very up on OHC engines, but what about the cam itself? Would a bad lobe do this?

To get at the root of your problem, most likely the head is going to have to come back off. Good Luck.

BraMas 07-13-2013 09:43 AM


Originally Posted by FlamingoGT (Post 8247377)
1 minor point: cyl 5 is not the driver front; it would be one of the middle ones.

Oh really? please explain how that in not #5 because every other 4.6 owner knows that it is

Stevo86 07-13-2013 10:56 AM

Have you spun the engine over with the valve cover off to see if something is going on in the head with that cylinder?

50 PSI is way low but if it worked fine before and doesn't now either you messed something up or that head is junk.

JeremyB 07-13-2013 11:27 AM

It's not the same cylinder the spark plug blow out of is the bad part. The head is newly remanned so I'm trying to get my friend to talk to the place he ordered it from and see if he can get it replaced. I did rotate the motor with the valve cover off and didn't see anything out of the ordinary. It seems to only not build compression when the motor is running, as in the followers can't keep up when the motor is running or something along those lines. It leads me to believe that maybe it's a weak valve spring

JeremyB 07-17-2013 12:06 PM

any other suggestions other than pulling the head? I had someone say it could be a broken valve spring causing the issue

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