What causes low compression in two cylinders in a v8?

1997 Ford Expedition 4.6L has low compression on #2 and #5 cylinders. Have put on computer and read misfire on #2 and #5. Checked and replaced everything according to haynes repair manual such as coil packs, spark plugs and wires, o2 sensors, even fuel injector. pretty much the whole nine yards any suggestion would be greatly appreciated.What causes low compression in two cylinders in a v8?
Remove all spark plugs, get a compression gage and run a compression check. Then get an oil can and squirt a few squirt of oil in the spark plug well. If the compression comes up then it is your rings, ir the compression does not come up then it the problem is in the upper end of the motor. I believe that this vehicle has hydraulic lifters and it is not uncommon for the lifters to bleed down causing compression problems.



If your head is compromised then some garages have the capability to test the coolant for carbon-carbon monoxide which may indicate a compromised head/head gasket.



If your engine oil %26amp; coolant is not contaminated and no white smoke then it is doubtfully that you have a compromised head/head gasket. Most likely a timing or valve train issue. I had an experience with Fords timing gears. A metal sprocket with plastic teeth. I had the sprocket teeth wheel separate from the metal and this was causing the timing to start slipping and loosing time.


Before you listen to everyone else on here, check the symptons before it's assuming a head gasket. Sure, a head gasket does cause low compression BUT IT USUALLY CAUSES IT IN TWO ADJACENT CYLINDERS (next to each other). That is where it blew usually. Anyway, to the symptons, the number one thing is white smoke out of the exhaust at start up, when driving or anything. Is it doing that? If not, thank God! Head gaskets suck and are costly if you don't do it yourself. But anyway, after that, check your oil. If your oil looks like chocolate milk (light brown milkish color is the best way to describe it), then coolant is getting with your oil which is usually caused by the head gasket and would lower the compression. Then, bubbles in radiator, constantly overheating, loss of coolant and compression. But if you said you checked that, then that's good.



Unfortunately, compression is made by the piston rings. But there are other possible causes which still suck no matter what.



It could also be a cracked head, cracked block, broken valve component - spring, cracked seat, bent valve, bad lifter.



It is usually caused by worn piston rings especially if the car was pushed extremely hard by the driver.



Typical symptoms of worn rings include low compression, oil burning (oil level gets lower and lower which causes BLUE smoke out the exhaust), spark plug fouling and elevated hydrocarbon emissions.



Good luck with it.



What causes low compression in two cylinders in a v8?
if you have low compression then the injectors, coil packs, plugs o2 and all that wont have any effect.



it comes down basically to..worn rings..or a bad head gasket.



seeing as the cylinders really arent close together ( where is part of the gasket went it would effect both cylinders) id bet its the rings.



you can do what is called a leakdown test...where itll put compressed air into the cylinder, and you basically listen to wear its leaking (crankcase, cylinder head, exhaust manifold)



its propably time for a rebuild!
Two adjacent cylinders low indicates a bad head gasket--but that doesn't sound like you--unless #2 and #5 refer to firing order? Otherwise the only reasons for low compression would be bad valves or valve adjustment (very unlikely with hydrualic valves) broken piston or bad rings or cylinder wear. The stuff you did would not do anything to the compression.What causes low compression in two cylinders in a v8?
Unfortunately, in most cases the reason for dropped compression in the cylinders is worn piston rings, these rings can be replaced, but it is costly unless you know an adequate shade-tree mechanic who will do it for a reasonable amount. Good luck.