1991 GMC Jimmy Engine runs rough then stalls when hot

BORGANISM

MEMBER
1991 GMC JIMMY
6 CYL
4WD
AUTOMATIC
150,000 MILES

Hi,

I have recently bought a GMC jimmy after driving it for 3 months with no problems it has now started to run rough and then stall after driving it for 15 minutes. If I try to accelerate when it is running rough it stalls on me. I have to nurse it through the revs until it is happy again. I cant see anything externally wrong with the carburetter, although the breather tube from the rocker cover was leaking a fluid which I belive was water into the top of the carby (suspect water since it leaves a milky residue like water and oil mixing). I have since disconnected it and pluged it, but it still runs bad. I am hoping there may be another solution apart from a cracked head or leaking main engine gasket. Anyone have any ideas?

My next step is to make sure the cylenders do not have oil / water / gunk in them but I think thats a long shot.

Thanks.

Paul

1 Reply

DAVE H

MECHANIC
13,384 POSTS

POSITIVE CRANKCASE VENTILATION (PCV)
The PCV system provides more effective elimination of crankcase vapors. Fresh air from the air filter housing is supplied to the crankcase, where it is mixed with blow-by gases and passed through a PCV valve into the intake manifold. This mixture is then passed into the combustion chamber and burned.
The PCV valve provides primary control in this system by metering the flow (according to manifold vacuum) of the blow-by vapors. When manifold vacuum is high (at idle), the PCV restricts the flow to maintain a smooth idle condition.
Under conditions in which abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), system is designed to allow excess gases to flow back through crankcase vent hose into air inlet.
Spring pressure holds PCV valve closed when engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, a condition which could result in hard starting.
During engine operation, manifold vacuum pulls the valve open against spring pressure, permitting crankcase fumes to enter the intake manifold. Should the engine backfire, the PCV valve closes to prevent ignition of fumes in crankcase.