Hi folks

I have a '98 F150 with 4.6L Triton with low compression in cyl # 4 @ approx. 30#; all other cyl are at 140# or so. Would it be a burnt valve, bent valve, cam? Any ideas before I rip it apart?

Any harm if I drive it like that? I have not have it on the road yet.

Thanks

 

Did you do a wet and dry compression test?

 

The unburned fuel from that cylinder will
damage the catalytic convertor
The excess oxygen will likely trigger
a lean code for that bank

Good luck and keep us posted

 

Ok, put oil in it, compression came up to 90 lbs, released the gauge & cranked again and now comes up to 60 lbs. Guess I should remove the valve cover for inspection?

 

The compression came up quite a bit, this suggests the loss is
through the rings,or perhaps a collapsed piston.
To be sure I would do a cylinder leak down test.
Or a simpler method,I would....

Get some air pressure in the cylinder via the spark plug hole using a fitting
making sure #4 is in it's compression stroke and both the valves
are closed.(Caution...the pressure may push the piston down so I keep hands and body parts away from the moving parts...camshaft,chain and fan belt ect.)

Apply the air pressure and listen to where the air is going, crankcase, exhaust or intake manifold.
Good luck and keep us posted

 

Did you have a cat plug up prior to this? My 5.8 in my 96 lost compression on #5 due to a plugged cat. Ran fine till cat plugged off. Replaced cat and was low on power. Had 150-165 in other seven cylinders and 40 dry in five would go up to 100 or so with 50 weight in the cylinder.
Never did fix it. Had 250'000 kms on it traded it as is.

 

K, I'll check with air.


NovaNewfie, I assume when you say cat, you mean catalytic converter? It's not plugged but if it does, I'll just remove it completely.

Be back in a day or so with some feedback.

 

Yeah I meant the converter. Removing them is easy, getting one of these engines to run right with them removed isn 't. Not like the older ones.

 

Good luck Draft horse,there are some videos of cylinder leakdown tests
on you tube

 

While holding the crank shaft with a Johnson bar, I put air in the cylinder with piston all the way up; the air would give the piston a good jolt. I moved the piston up & down several times while pressurized with air; at one point, lots of air came out from the oil fill hole. I inspected with my Borescope and found something weird on the piston that I can't figure out what it is.



It's the best picture I could get. Does that look like a damaged piston or what the heck is it?

 

Idk what that is,will a magnet grab it or is it aluminum?
The "one point" where "lots of air" was coming through
the oil filer would of been when both valves were closed.

I think you found your leak

I would be removing the cylinder head at this point

Keep us posted
If you decide to open it up we want pics
Good luck

 

Is it possible to remove that piston from the bottom without removing the crankshaft? This is just my spare truck while I rebuild the trany in my F350 so I don't want to spend too much $ as I will sell it once I get my F350 back on the road.

Any idea if the piston will slide by the crank & block?

Thanks

 

No. The head has to be removed and the pan has to be removed to disconnect the rod.

You could do a cylinder leak down Test and listen for where the air is escaping, but you really have already determined that it is a piston/ring/cylinder problem when you did the wet compression test.

At this point, there's no way to know exactly the nature of the problem without pulling the head.

Sorry for the bad news and best of luck.

 

Maybe a used engine might be the way to go Draft Horse

 

Feb '13 i had bought a 2002 F250 with a 5.4. It lost compression on # 8, 500 miles after I bought it. I'd bought from a dealer and bought warranty. They put a used engine in it. I paid the core charge and brought the old one home. Never did get around to pulling the piston though. Pulled the oil pan and it didn 't look like I could pull the piston with out taking the crank out.