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Old 02-06-2013, 09:01 PM
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SBF - oil fouled plug but good compression

I'm preparing to work on my 87 F-150 with a 5.0 SBF. It seems to run kinda rough especially at idle and kinda acts like it might have one cylinder thats not up to par with the rest of them, but not so bad that its misfiring completely. So far I did a compression test on the drivers side and ALL of the readings were 150 PSI or higher. All the plugs looked good EXCEPT the one in the very back. it was black and shiny with some crud built up on it. When I saw that I was sure it was going to have low compression, but it did not. In fact it was one of two that were actually 165psi.

What can cause this problem? Valve seals/guides?

Also got to find the source of the roughtness, but i suspect i will find low-ish compression on one cylinder. Hoping its a head problem and not a piston/ring problem.

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Old 02-06-2013, 09:37 PM
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Could be a vacuum leak at the bottom of the intake.
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Old 02-07-2013, 03:20 AM
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It could be the valve seals, valve guides or leaking intake gaskets.
You maybe able to use a vacuum gauge and find out more.
The link I'm including you'll notice at the bottom of the page it will have Scenario 1 to 15. By clicking on them it will show what the vacuum gauge will do with a specific engine issue so you'll get a better idea on what the problem is.
How to Use and Interpret a Vacuum Gauge
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Old 02-07-2013, 05:29 AM
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My money would be on valve stem seals. Ford uses the usual umbrella style which eventually get hard and break. Also, many, if not most Ford heads, that I've owned or worked on will show valve guide wear. Neither of which will show up in a compression check. A vacuum gauge will only show at best (worst?) a minor needle tick with a single excessively worn guide on one cylinder. The rest will be worn as well, but combining a split stem seal and guide wear will equal oil usage and smoke and time for a set of heads or a full $$$$ rebuild.
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Old 02-07-2013, 09:38 AM
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The engine is original as far as I can tell, but it is supposed to only have 134k on it. The valve seals could easily be old and leaky being an 87 (it leaks oil from other places as well, main seal, and valve covers) but i really hope its not the valve guides. I already plan to pull the engine in the coming weeks and replace all the seals and gaskets, clean it up, add a new cam as well as headers, replace the timing set, oil pump, lower end bearings (plus turn the crank), and water pump.

It does have a lower end knock at startup and oil pressure issues with thin oil. but i think the new oil pump and bearings will solve that. As long as the pistons and rings are still good that will save me some time and money (neither of which I have very much of at the moment).

It runs pretty good and doesnt smoke any that I ever see. Which is why I am not totally rebuilding the engine. If it smoked or had lower compression which would indicate worn rings, id rebuild it, but as of now I am just fixing whats wrong with it and doing some minor upgrades.
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Old 02-07-2013, 01:03 PM
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If it were me, and I was gonna tear it down that far I would go ahead and hone it, clean it up good then add new rings. With that many miles the rings are going to have wear on them regardless. While they aren't completely worn out at this point it's a safe bet by the time it reaches 200,000 miles they will be. My father has a 98 GMC pickup with the 4.3 liter V6 Why I mention this is when he got it, it had 143,000 miles on it, now the truck has just went over 200,000 miles, has good oil pressure, regular PM has been kept on it, but it needs rings. It's started to use about a quart of oil a month. That's why I say, If your going to take it that far down, plus add some performance parts you may as well give it rings too.
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Old 02-07-2013, 02:14 PM
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also, when you start adding those goodies, the weakest things will show up .
i agree with "D" vision.
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Old 02-07-2013, 03:39 PM
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Yeah I agree too. Its just that much more cost and time that I don't have. I NEED to have this done by next Wednesday but I'm not gonna make it. (I have to start making 250 mile trips [one way] to St. Louis and back in this truck).

I haven't got all the parts yet and I haven't even started taking it apart. Plus the machine shop will take a week and a half to get the work done.

I guess i will drive it the way it is until i can afford to do it right and hope it doesn't blow up in the mean time.
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Old 02-10-2013, 09:00 PM
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Quote:
Originally Posted by DoubleVision View Post
It could be the valve seals, valve guides or leaking intake gaskets.
You maybe able to use a vacuum gauge and find out more.
The link I'm including you'll notice at the bottom of the page it will have Scenario 1 to 15. By clicking on them it will show what the vacuum gauge will do with a specific engine issue so you'll get a better idea on what the problem is.
How to Use and Interpret a Vacuum Gauge
Vacuum gauge revealed 19" of vacuum and very steady. No vibrations or drops.
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Old 02-11-2013, 11:42 AM
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Quote:
Originally Posted by Torque454 View Post
I'm preparing to work on my 87 F-150 with a 5.0 SBF. It seems to run kinda rough especially at idle and kinda acts like it might have one cylinder thats not up to par with the rest of them, but not so bad that its misfiring completely. So far I did a compression test on the drivers side and ALL of the readings were 150 PSI or higher. All the plugs looked good EXCEPT the one in the very back. it was black and shiny with some crud built up on it. When I saw that I was sure it was going to have low compression, but it did not. In fact it was one of two that were actually 165psi.

What can cause this problem? Valve seals/guides?

Also got to find the source of the roughtness, but i suspect i will find low-ish compression on one cylinder. Hoping its a head problem and not a piston/ring problem.
Check and see if the PCV valve vacuum hose connects at or very close to that intake runner. If the valve is bad or there's a lot of blow-by, that'll oil up that plug and combustion chamber pretty good. I'd replace the PCV valve and see if there's an improvement.

A new set of plugs would be nice, but just swapping out the oily one for a different plug will let you know if there's an improvement. Clean the oily plug w/carb cleaner if you have any on hand. Otherwise, using it in a different cylinder should burn the oil off if it's not too bad.
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Old 02-11-2013, 12:51 PM
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Quote:
Originally Posted by cobalt327 View Post
Check and see if the PCV valve vacuum hose connects at or very close to that intake runner. If the valve is bad or there's a lot of blow-by, that'll oil up that plug and combustion chamber pretty good. I'd replace the PCV valve and see if there's an improvement.

A new set of plugs would be nice, but just swapping out the oily one for a different plug will let you know if there's an improvement. Clean the oily plug w/carb cleaner if you have any on hand. Otherwise, using it in a different cylinder should burn the oil off if it's not too bad.
PCV valve connects to the front of the intake and the fouled plug is at the very rear. Ill check the pcv valve but i think its ok. Plugs are only a year old, and that particular one has been replaced one since then when it stopped firing for no reason.

I suspect this cylinder is not firing much or at all and I think its an injector problem (plug is wet, cruddy, and black, but the cylinder has compression and the plug fires when laid on the engine and connected to the plug wire, but the engine doesnt run any smoother or rougher with that plug wire disconnected) so I am working on pulling the intake and injectors to clean them right now.
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