5.7 Hemi Excessive Crankcase Pressure

TL;DR: Every running engine puts some combustion gas past the rings into the crankcase. The ventilation system's job is to pull that gas back out and burn it. On a 5.7 Hemi you get "excessive crankcase pressure" one of two ways: the engine is making more blow-by than it should, or the ventilation path that removes it is restricted. The symptoms look identical from the driver's seat - oil pushed past seals, a dipstick that will not stay seated, oil consumption, whistling or a rough idle - so the whole job is figuring out which one you have before you spend money. Test the ventilation path first, because it is cheap, then measure the engine.

What the Hemi's crankcase ventilation system is actually doing

The Gen III Hemi - the 5.7 that Chrysler has run in Ram trucks, Durangos, Grand Cherokees, Chargers and Challengers since the early 2000s - is a pushrod overhead-valve V8. Two valves per cylinder, hydraulic lifters riding the cam in the block, pushrods up to rocker arms under the valve covers. Many of them carry MDS, Chrysler's cylinder-deactivation system, which parks four cylinders under light load using special lifters.

None of that changes the basic physics of crankcase ventilation. Combustion pressure leaks past the piston rings into the crankcase on every power stroke. That gas is hot, it carries fuel and water vapour, and it is acidic. Left in the crankcase it pressurises the engine and contaminates the oil. The positive crankcase ventilation system uses intake vacuum to draw that vapour out of the crankcase, through an oil separator, and back into the intake to be burned - with a fresh-air path coming in from the clean side of the intake tract to replace what leaves.

The whole system depends on being able to flow freely. Restrict the outlet and pressure builds. Restrict the fresh-air inlet and you get vacuum where you wanted flow. Either way, the crankcase stops breathing properly, and the pressure has to leave somewhere. It leaves through whatever seal is weakest - usually the rear main, sometimes the valve cover gaskets, sometimes the oil pan rail or the dipstick tube.

Symptoms worth taking seriously:

Cause one: a restricted or failed ventilation path

Start here. It is the cheap end and it is a genuinely common failure, because everything in that path spends its life in hot, oily, acidic vapour.

Sludge and coking. The passages, the separator and the valve itself accumulate deposits over time - faster on an engine run on short trips, faster still on one that has gone long between oil changes. A partially blocked path does not throw a code. It just quietly raises crankcase pressure until a seal gives up.

A stuck valve. The PCV valve is a metering device with a spring-loaded plunger. Stuck closed, it stops the crankcase venting and pressure climbs. Stuck open, it becomes a large unmetered vacuum leak, giving you a rough idle and lean fuel trims. Both fail the same simple test: with the engine off, a serviceable valve rattles when you shake it.

Cracked or collapsed hoses. Ventilation hoses on an engine with twenty years on it get hard, then crack, usually at the ends where they are stretched over a fitting. A crack lets unmetered air in; a collapsed hose stops flow entirely. Check them cold and check them with the engine running, because some only open up at temperature.

A blocked fresh-air side. This is the one people forget. If the clean-air feed from the intake tract is plugged, the system cannot circulate. Follow the whole path, both directions, before you condemn anything.

Confirm the exact configuration, routing and component location for your specific model year against the factory service information - Chrysler revised the Hemi's ventilation hardware over its production run, and what is true for one year is not automatically true for the next. Do not buy on a picture; confirm the current part number against the manufacturer's catalogue for your engine and year. Seals and gaskets you disturb during the job come from gaskets and seals, and application-specific hardware sits in 5.7 Hemi parts.

Cause two: the engine is making too much blow-by

If the ventilation path is clear and pressure is still high, the engine itself is the source.

Worn or stuck rings. The rings seal the combustion chamber against the cylinder wall. When they wear, when the bore wears, or when the ring grooves fill with carbon and the rings stop moving freely, combustion gas gets past. High-mileage engines and engines that have been run hot or run on contaminated oil are the usual candidates. Replacement rings live in piston rings, but understand that ring replacement is an engine-out, machine-shop conversation, not a driveway repair - the bore has to be measured and, in most cases, honed.

Fuel dilution. Fuel washing down the cylinder wall strips the oil film the rings need, and diluted oil loses viscosity. Chronic misfires, excessive idling and short-trip use all feed this. Cure the cause first, change the oil, then re-measure - some engines recover a surprising amount of ring seal once the oil is clean again. Break-in and additive products are in oil, coolant and additives.

Cylinder or valvetrain damage. A holed piston, a cracked ring land, a dropped valve seat or a burnt valve all produce a sudden, large jump in blow-by rather than the slow climb wear gives you. These usually announce themselves with a misfire on one cylinder. Valvetrain components are in valves and valvetrain; if the measurements come back bad enough, a complete engine rebuild kit is the honest answer.

MDS lifter trouble. On MDS engines, a failed deactivation lifter can damage the cam lobe and, in the worst cases, send debris through the engine. That is not itself a crankcase pressure fault, but it belongs on the list of things to rule out on a high-mileage Hemi that has started behaving differently. Delete hardware, where it is legal for the vehicle's use, is in AFM/DFM/MDS delete kits.

How to diagnose it in the right order

1. Look and listen at idle. Oil cap off - a healthy engine burbles gently. A steady, forceful puff of vapour that will lift the cap is significant blow-by. This is a rough screen, not a measurement.
2. Check the ventilation path end to end. Valve rattles when shaken? Hoses soft, cracked, collapsed? Fresh-air side clear? Separator and passages free of sludge? Clean or replace what is restricted, then re-test - a surprising number of cases end here.
3. Measure crankcase pressure. A low-pressure gauge or manometer teed into the crankcase, read at idle and at a steady raised rpm, turns a guess into a number. Compare against the specification in the factory service information for your engine.
4. Run a compression test. Uniform low readings point to rings or bore wear; a single low cylinder points at a valve or a piston.
5. Run a leak-down test. This tells you where the pressure goes. Air heard at the oil fill or dipstick is past the rings. Air at the intake is an intake valve, at the exhaust an exhaust valve, in the coolant a head gasket.
6. Read the oil. Fuel smell, low viscosity, coolant contamination or metal all redirect the diagnosis.

Do steps 4 and 5 with the engine warm and the throttle held open, and follow the tester manufacturer's procedure. Gauges and testers are in engine stands and tools.

What actually fixes it

If the ventilation path was restricted, cleaning or replacing the restricted components fixes it, and the seals that were weeping usually stop once pressure normalises. Replace a seal that has been distorted by pressure; do not replace one that simply got wet.

If the engine is making genuine blow-by, no bolt-on solves it. A catch can reduces oil carry-over into the intake and is worth having on a hard-worked engine, but it does not reduce blow-by - it only catches what the blow-by carries. Treating high crankcase pressure by venting the crankcase to atmosphere hides the symptom, dumps oil vapour where it does not belong, and is not emissions-legal on a road vehicle.

Fix the cause. Where the measurements say the rings and bore are worn out, the repair is an engine rebuild or a replacement long block, and the earlier you accept that the less collateral damage you pay for.

Frequently Asked Questions

How much crankcase pressure is normal on a 5.7 Hemi? A healthy engine runs at or slightly below atmospheric pressure at idle, not above it. Rather than working from a number you found online, measure yours with a manometer and compare it against the specification in the factory service information for your model year.

Will a catch can fix excessive crankcase pressure? No. A catch can traps oil mist so it does not reach the intake and valves, which is worthwhile on its own merits, but it does nothing about the volume of gas coming past the rings. If the pressure is high because the engine is worn, it stays high.

Can a plugged PCV valve blow out a rear main seal? Yes. Positive crankcase pressure pushes on every seal in the engine, and the rear main is often the one that surrenders first. If you replace that seal without clearing the ventilation restriction, expect the new one to leak too.

Why is my dipstick popping out of the tube? That is real positive pressure in the crankcase, and it is worth stopping to diagnose rather than taping down. Check the ventilation path first, then measure crankcase pressure and run a leak-down test.

Does oil consumption always mean worn rings on a Hemi? No. Oil pulled into the intake through an overwhelmed separator, a leaking valve cover, worn valve stem seals or an external leak all consume oil without any ring wear. Find out where the oil is going before you assume the worst.

Does MDS cause crankcase pressure problems? Not directly. MDS lifter failure is a well-known Hemi wear item and can cause serious internal damage, but excessive crankcase pressure traces to ring and bore sealing or to the ventilation path. Rule out both before blaming the deactivation system.

Should I keep driving a Hemi with high crankcase pressure? Only far enough to get it diagnosed. Pressure forces oil past seals, oil loss leads to bearing damage, and oil carried into the intake fouls valves and sensors. It is a cheap problem early and an expensive one late.

Sources

Written from the Core Powertrain Parts technical team's shop experience on Gen III Hemi engines, cross-checked against Chrysler factory service information for crankcase ventilation configuration and test specifications, and against standard diagnostic procedure for compression, leak-down and crankcase pressure testing. Component configuration varies by model year - confirm routing, specifications and current part numbers against the factory service information and the manufacturer's catalogue for your exact engine and year before ordering or torquing anything.

Questions on a specific application? Email sales@coretransmissionparts.com. Free shipping on orders over $70.