Gas in Oil

[zmanss]

I have a 1991 Oldsmobile 98. When i got the car i knew there was gas in the oil, however a mechanic that had worked on it said he replaced 3 bad injectors that were constantly open, so i figured an oil change would be all i needed. I changed the oil; same problem. I replaced the injectors with new ones; same problem. I replaced the fuel pressure regulator; same problem.

The car is running on all six cylinders, and the spark plugs look good. However, i changed my oil last night and today i'm already losing oil pressure and have to stay on the throttle just to keep the car running. I'm also getting terrible gas mileage. My next step is to either run a compression check or replace the lower intake manifold (where the injectors are located). I have been advised that these sometimes crack allowing gas into the oil. Any suggestions/comments would be greatly appreciated.

Thanx,

zmanss

[ivymike1031]

That story about the intake manifold doesn't sound right to me. When does the oil go anywhere near the intake manifold?

I'd look at a bad head gasket or bad piston rings as the problem. Either one might show up when you do your compression test. I'm leaning towards a head gasket, since your car is running so poorly. You didn't say what type of engine you have. The running poorly definitely has something to do with the poor fuel economy. I'm not so sure that the "fuel in oil" problem is related, but it may well be.

I'm not sure how you can tell that there's gas in your oil. Have you had an oil analysis performed? Blackstone Labs in Ft Wayne, IN, does them for about $25.
http://www.blackstone-labs.com/

I don't usually pay any attention to those oil pressure gauges- it seems that they're wrong more often then they're right.

[zmanss]

The engine is a GM 3800 (3.8L). The camshaft is located in the sump near the crank, then transfers motion to the valves via push rods. These rods are visable when the intake manifolds are removed. You can see them at the top of the block (middle of the V). obviously these require oiling, the heads drain oil into this area. This surrounds the lower intake when it is in place. If the intake is cracked, there may be a route for fuel to escape which would put it directly into oil.

The car runs great, when it runs; the problem is keeping it running at idle after it has warmed up. I don't think it's the head gaskets b/c i don't have a coolant/oil mix and the car actually runs and has in this condition for quite a while. If the gaskets were bad they would have surely blown by now. I am also cautious about piston rings, b/c i have no problem starting the car. If i had no compression; would i not have trouble starting. I would also expect to be smoking heavily if there were bad piston rings and it doesn't smoke.

Thanks for the suggestions

zmanss

[ivymike1031]

Originally Posted by zmanss

The engine is a GM 3800 (3.8L). The camshaft is located in the sump near the crank, then transfers motion to the valves via push rods. These rods are visable when the intake manifolds are removed. You can see them at the top of the block (middle of the V). obviously these require oiling, the heads drain oil into this area. This surrounds the lower intake when it is in place. If the intake is cracked, there may be a route for fuel to escape which would put it directly into oil.

The car runs great, when it runs; the problem is keeping it running at idle after it has warmed up. I don't think it's the head gaskets b/c i don't have a coolant/oil mix and the car actually runs and has in this condition for quite a while. If the gaskets were bad they would have surely blown by now. I am also cautious about piston rings, b/c i have no problem starting the car. If i had no compression; would i not have trouble starting. I would also expect to be smoking heavily if there were bad piston rings and it doesn't smoke.

zmanss

okay, so it's a v-engine, with a cam-in-block configuration (the sump is aka the oil pan; I'm guessing the cam isn't way down there). The intake manifold closes up the top of the "valley". Then a cracked manifold could possibly leak into the valley and dilute the oil. How about the intake manifold gasket? It would allow the same leak path...

A note about pushrod lubrication - usually the pushrod only needs to be lubricated at the ends. This is typically accomplished by feeding oil through the lifter to the lower end of the pushrod, then up the pushrod to the top end.

A leaky head gasket won't necessarily allow coolant-to-oil transfer. It could be blown between two cylinders (if they are siamesed, which they almost certainly are), or between a cylinder and a pushrod hole (depending on layout), and no coolant would ever escape. You'd probably see a ridiculous amount of PCV flow, though...

A leaky intake manifold gasket (or cracked intake manifold) could definitely explain a rough idle - you'd be sucking excess air, and as a result you'd be running pretty lean. You've said it's fuel injected, so I assume it has O2 sensors. What are they saying about the mixture? Changing an intake manifold gasket isn't usually all that difficult, and it might be worth your while just to do it and see what you find.

How about the EGR system? A bad (plugged) EGR valve could also lean out your mixture at idle.

[jeffescortlx]

Originally Posted by zmanss

The car runs great, when it runs; the problem is keeping it running at idle after it has warmed up.

When a car is cold and first started, the ECU ignors the O2 sensor signal and runs in closed loop mode, that is a preprogrammed fuel map.
Once the car is up to operating temp the system goes into open loop and takes the info from the O2 sensor and makes any changes needed.

When a O2 sensor goes bad, it gives the signal of being lean when it's really not. So at first your car runs OK because its in closed loop mode. than when your car swithces to open loop the bad O2 signal is srewing with your A/F ratio. So your way rich, that why it can idle very well, and why there is unburnt gas getting into the motor.

O2 sensor's should be changed every 60k, when's the last time you changed yours?