My 1991 F150 5.0L (160K mi.) was running rough at idle and at low RPM. My mechanic tells me I only have 50 psi in the #8 cylinder (150 psi elsewhere), and there's oil present in there too. The mechanic says to take it to in internal engine specialist or get a new engine. Any recommendations on the best course of action? Any ideas on the cost of engine repair vs. replacement?

 

First off take your PCV line off the #8 runner and shift it over to one of the center ports. All that oil vapor from the PCV isn't doing it any good and may have contributed to the 50# reading. putting the line on one of the center ports will even out the vapor to ALL the cylinders.

Run a couple of cans of GM top end cleaner (I know I KNOW) through it. It should (more often than not does....strong stuff) dissolve the carboned up oil on the rings and clean up the cylinders, and MAY bring up the #8 hole.

Put another can in the gas and all of this may keep you from having to get another engine for a while. After 500 miles take another compression reading and see what you have. If no change then take and yank it out and replace it.

Larry

 

My wife's (now late) cousin had an '88 302 go bad on the #8 cylinder, and my '90 302 went bad there too. I don't know specifically what happened to the cousin's engine, but mine blew apart the piston (presumably detonation), same symptoms you described, except I had no compression in that cylinder. Yours may just be starting to come apart.

Larry, what the heck are you talking about with the PCV stuff? In my '90 302 there's no PCV ANYTHING anywhere near the #8 cylinder (driver's side, closest to the firewall).

Jason

 

The PCV hose attaches to the manifold on or near the #8 runner. The #8 hole gets all the oil vapor/crud. Move the hose to the center taps and the vapors are distributed equally.

 

PCV is tucked up against the manifold on #8. Find someone with skinny arms to pull it. Its pressed into the cylinder and held on by an O-Ring.
Try running Seafoam. It'll burn off all of the build up. Your truck will run like new.

 

Sounds like I'm working on borrowed time, but at least I've got a few things to try before I start spending big bucks. Interesting that the #8 cylinder is so commonly the problem. The mechanic told me that I was the second dead #8 he'd checked out that day; the other was in a '92.

I'll have to look at the PCV and see if I can change taps. Might need more help later.

Thanks!

 

I gotcha now. You meant "upper intake manifold" or "intake plenum." That makes a bunch more sense now--I didn't know what you meant by "runner." Do you mean to move it onto the vacuum tree that's in the center of the plenum, then plug the hole back by the firewall? I'm just trying to clarify for the other guy...I'm not going to bother with this on a truck I'm trying to sell.

Jason

 

fords known for the bad 8.

changeing the PCV valve placement is a good thing.

I should do this myself. i can feel my truck stutter occasionally on the 8 cylinder.

and yes, jason, they mean to move it to the center of the cover, instead of in the back.

When its at the back, the 8th cylidner sucks all the stuff in.

There are some pictures on the sight someplace of this, will somebody find them?

 

had the same problem on my 93...all caused because of the pcv gases being dumpedi into number 8 and making it run real hot. Check out my gallery to see how it melted mine.

I didin't want to invest a lot at that time, so I just jacked the engine up, pulled the head, dropped the pan and honed the cylinder and replaced the piston/ring for #8...now it's got the best compression of all of them. Plus I rerouted the pcv to the center of the intake (like ford started doing in 95). problem solved for less than $50...but took me a couple days to do it.

 

cylinder #8...there is also an issue with the way the spark plug wires were run, it had to be done a cetrain way or you will get detonation, and possibly destroy a piston.

 

I can see what dkaska77 meant by having someone with skinny arms pull the PCV out. Its right up against the firewall. PIctures of what to pull and where to put it would be nice.

Will any of the over the counter engine cleaners remove the crap in #8? I can't find any Seafoam or GM Top End cleaner. Maybe at a NAPA store?

 

oh, seafoam is the best. don't be scared cause it has pictures of boats and lawnmowers on the front. run it. best cleaner I've used. seems like the truck breaths better after you run it when it's overdue... O'Reilly's has it. not sure about napa

 

Take a look at this past discussion on Cylinder 8 and PCV:

https://www.ford-trucks.com/forums/s...t=pcv+routing+

I did this re-route last year - hardest part is getting to the PCV!

- rlp 89 F150

 

Great info! Can't wait to try it out. My neighbor is always making noise and burning stuff in his yard so this will give me a chance to get even when the smoke screen starts after the Sea Foam goes in.

 

plug wires need to be seperated! #8 need to run all byitsself do not cross it over or under any other wires. the pcv is another good thing to change too. not only for the grim getting in 8 but if it was to ever leak it wound only lean out that cylinder. ford has a problem with their ignition system which was not brought to their attention until oh about 94 i think. just run that wire back on the valve cover all the way to the back of the engine and around the valve cover and plug it right on the plug. remember do not let it cross another plug wire entirely. I found this out the hard way(the same way you have). your #8 piston is burned out and melted, and no seafoam or top end cleaner will help. You'll just have to trust me on this one, i did do the same thing on my 93 5.0 and it does suck. Is your airbox showing any traces of oil in it? that compression is not gonna come back, once there is none there isn't gonna be none there later. All you do now is ruin your block, and if you was to start now you may just be able to replace piston, and rings and get off cheap. I'd be willing to bet anything the ignition is the problem, sorry for the bad news but, i just was in your shoes about 5-6 months ago and i had no help.