I have been on this site for years. I was hoping someone can help me. I have low compression on one cylinder and get 4 mpg on my 05 neon. Does this usually mean I have a bad exhaust valve. Could the fuel is leaking into the oil?

 

Whatever caused the low compression problem probably resulted in excess oil or coolant being ejected into the exhaust which destroyed the O2 sensor and that killed your milage. You're gonna have to strip the head off and have a look, could be a broken valve, or a broken/melted piston, or just a blown head gasket.

 

Wouldn't I get a O2 code? I have a general misfire code. I have power the injector, I switched the injector between cylinders. There's no smoke coming at of the tail pipe, in the driveway. The oil level stays the same, I don't smell fuel. Thanks

 

First define "low compression". Are you calling it low because it's 150 PSI while the others are at 160 PSI or is low because it's 0 PSI? What is the code(s) you're getting?

 

The compression test in order 160,35,148,165. I found the spark plugs gaps had gotten to wide over time, I replaced them. Number 1 spark plug wire was half broken. I now get 20 mpg, but normal mpg is 30. I have been smelling rotten eggs, must be from the bad exhaust converter. The oil level never goes down. No smoke coming from the exhaust. I have a #2 missing code. Not much power going up hills.
There have used engines here but they drained the oil, they run about $120.00, no warranty. $225.00 with a 7 day warranty. They can't tell you if they had run. If the car was totaled on the side of the car and has 100,00 maybe its a good engine ? You have to remove it your self.

 

The low compression indicates a burnt valve. The cylinder head has to come off to be repaired or replaced. The rotten egg smell usually means the catalytic converter is loaded up and is restricting exhaust flow. This could explain your lack of power. The burnt valve probably contaminated the CC.

 

I added a little oil to number 2 piston and the compression went from 35 to 150 psi. So do I need a leak down test then?

 

No you need a motor. If the compression came up to 150 PSI after adding oil to the cylinder the rings are gone and either needs to be rebuilt, if possible, or replaced with a good used motor. You probably will also need a new cat. converter most likely. You probably would be money ahead just to junk it and get a different car.

 

Okay that tells you that you have a very worn ring, a cracked ring, or a scored cylinder wall. To bad, the head is easier and cheaper to repair.

 

The old spark plug on the low compression cylinder had no oil on it, doesn't that sound strange? It looked just like the other three plugs.

 

If the problem is worn rings, the plug may not have oil on it. Most of the time oil fouling plugs is caused by leaks through the valve stems.

 

No it's not strange because there are 2 compression rings at the top of the piston and the oil ring is at the bottom. So you can have cracked compression rings but still have a good oil ring that is keeping the cylinder clean of any extra oil.

 

Thanks I didn't know that. Still seem strange that a little oil doesn't make it to the top.

 

Oil would have to be sucked past the oil ring and compression rings, mostly as vapor, during the intake stroke. This will get burned on the next power stroke, assuming it doesn't all get pushed back into the crankcase during the compression stroke due to the poor ring seal. The power stroke will make a lot of smoke, but leave little oil on the plug.

On the other hand, if the valve stems are leaking, it would occur when the engine is not running (unless the leaks are really bad, in which case oil will get sucked in during the intake stroke again). They can leave a lot of oil around the plug, which show up the next time you start up as lots of smoke, and the plugs would be fouled.

 

It does, otherwise the piston ring would destroy the cylinder walls in no time at all. It is a miniscule amount and it gets burnt off so you never notice it.