mustangdriver91

Hey everyone well I was wondering what the stock psi compression was on a 1991 5.0 mustang, so in a short while when i run a compression test ill know whats good and whats not. Also just saying i dont need to know it's 8:5:1 or whatever i mean like should it be around 170psi, or what?
Ok then i would like to know what range is still considered good and isnt bad enough to cause poor performance compared to stock, thanks sooo much guys.
Also im wondering because the 1991 mustang i bought seems to have no ***** compared to the last one i had, and it is bad comp or it needs a tune up bad because my 84 c20 with a stock 454 is faster then it and its a 16-17 sec 1/4 mile truck lol.

mustangdriver91

Oh and im hoping it's maybe a bad spark plug wire or two since when i pulled one off yesterday the little thing came out that contacts tip of spark plugs in boot, and it has been sitting.

mustangdriver91

WELL there was one cylinder that wouldnt go above 10psi, so im guessing the motor is junk since i cant tear into it to replace the picton or rings.
Anyways my freind says it might have a hole in the piston, but i know ill be stoping by the last owners house and getting some money back for it.

mustangdriver91

EDIT: so a guy i know said to put a teaspoon of motor oil into the bad cylinder and then let it run for awhile.
So all i need to know rite now is because the last owner let it sit for 2years and says that it probably dumped fuel into cylinder because spark plug wires are old that the oil should work and restore compression, so can someone tell me exactly how i should do this and how long untill re checking compression agin?

arnie93lx

Pull the cylinder head and check the head gasket and the associated valves with the cylinder. It could also be those giving low compression numbers. While you are in there you can check the piston, etc.

mustangdriver91

Well i tried the trick with filling the cylinder up with 15-40 heavy duty desiel motor oil and let it run for 2 minutes then got the same 10psi, then tried agin and same 10psi compression so i dont think it's the valves being stuck or bad gasket because it would have more then 10psi and it would most likely just leak out pressure.

mustangdriver91

But if the valves are messed up on that cylinder then it would cause this issue rite?

arnie93lx

If a valve seal or head gasket is leaking it can cause this. Also broken or damaged piston rings can cause this too, depends.

mustangdriver91

Wait but if it was a bad head gasket then chances are that it would have more then one cylinder lose compression, and they should have more then 10psi on them sill lol they would have kinda average compression then just leak out pretty fast if its bad. But i dont think the cylinder with 10psi lost and compression or at least know from when i looked so does that say anything.

dawson1112

Each cylinder is sealed separately by the head gasket, it is entirely possible to have a blown head gasket only on one cylinder.


Just pull the heads and get a visual , one of the valves could have a burn spot on it. Seen it in my old f150 started running like poop with no power, pulled the heads and one of the valves had a pencil lead sized hole on the edge where it meets the head.