5.9 Cummins 24V Excessive Crankcase Pressure
TL;DR: Every diesel makes some blowby, and the 5.9 Cummins 24V is no exception - the question is always how much, and whether it changed. High crankcase pressure shows up as vapour pouring from the breather, oil pushed past the valve cover gasket or the dipstick tube, and in bad cases a dipstick that will not stay seated. Before you assume worn rings, rule out the cheap causes: a restricted or collapsed vent path, an overfilled or fuel-diluted crankcase, and a turbocharger with a blocked oil drain. Only after those are eliminated does a compression or cylinder leakdown test tell you whether you are looking at a ring and bore problem or a broken part.
What "excessive" actually means on a 24V
The 5.9 Cummins 24V is an inline six, direct-injection, turbocharged diesel with a pushrod valvetrain and no spark plugs, coils or knock sensors to chase. Combustion pressure in a diesel is far higher than in a petrol engine, so a healthy 24V pushes a steady, visible stream of vapour out of the crankcase vent under load. New owners see that for the first time and assume the engine is finished. Usually it is not.
The useful definition of excessive is comparative, not absolute:
- It got noticeably worse over a short period.
- Oil is being pushed out of joints that were previously dry - the valve cover, the front or rear main seal, the dipstick tube, the fill cap.
- The dipstick lifts, whistles or blows out of its tube at idle or under throttle.
- Oil consumption climbed at the same time the vapour did.
- The vapour is heavy with liquid oil rather than mostly mist and water vapour.
Any one of those is worth investigating. All of them together usually means a mechanical fault rather than normal wear. Parts for the platform are grouped under Cummins 5.9 24V parts.
Measure it before you diagnose it
Guessing at blowby is how people end up buying a short block they did not need. Do this before anything else, and record the result so you have a baseline to compare against later.
The simple version. With the engine at operating temperature and idling, pull the crankcase vent hose free and hold a sheet of stiff card loosely over the open end. A healthy engine puffs; a badly worn one will push the card away and hold it there. Then load the engine - in gear against the brake, or better, on a road test - and watch how the volume changes. Blowby that is mild at idle and enormous under load points at rings and bore. Blowby that is heavy at idle and does not change much often points at something else entirely.
The better version. Fit a manometer or a low-range pressure gauge to the crankcase, either at the vent outlet or through the dipstick tube with the tube sealed. Measure at idle, at a fast idle, and under load. Compare the readings against the crankcase pressure specification and test procedure in the engine manufacturer's service literature for your build year - the acceptable figures and the measurement method are published there, and the number depends on how and where you measure it. Do not use a value quoted on a forum for a different engine family.
Then isolate the source. A cylinder leakdown test tells you where the pressure is coming from. Air escaping into the crankcase and out the vent is rings or bore. Air out of the intake is an intake valve. Air out of the exhaust is an exhaust valve. Bubbles in the coolant is a head gasket or a cracked casting. A compression test done to the factory procedure will back that up. Torque anything you remove and refit to the values and sequence in the service manual.
The cheap causes, in the order worth checking
A restricted crankcase ventilation path
This is the first thing to check and the one most often skipped. The crankcase has to breathe. If the vent hose is collapsed, kinked, packed with sludge, iced up in winter, or - on trucks fitted with a filtered ventilation setup - clogged with an element nobody has ever changed, pressure has nowhere to go but past the nearest seal. The symptom looks exactly like worn rings: oil at the valve cover, oil at the rear main, a dipstick that will not stay down.
Pull the vent hose off, look through it, and clear or replace it. If the pressure symptoms disappear with the vent open, you have found your problem and the engine is fine. This costs nothing to test and it is the single most common false alarm on these engines.
An overfilled or fuel-diluted crankcase
Too much oil in the pan means the crankshaft is whipping through it, aerating the oil and driving pressure and oil mist into the vent. Check the level on level ground, cold, with the correct dipstick for the truck - dipsticks get swapped between applications often enough that a wrong one gives a wrong reading.
Fuel dilution does the same thing and adds a second problem. On the VP44-era 24V trucks and the later common-rail 24V trucks alike, leaking injectors, a failing injection pump, or a failed lift pump that lets the injection system run short of supply can all put diesel into the sump. Diluted oil thins, foams and vents heavily, and it accelerates bearing and bore wear the whole time it is in there. If the oil smells of fuel or the level is rising, stop driving the truck and find the leak.
A turbocharger oil drain problem
The turbo drain line has to return oil to the pan by gravity. If it is kinked, sludged, or fitted with a damaged gasket, oil backs up in the centre housing and pushes past the turbo seals - into the intake, into the exhaust, or both.
The relationship runs in both directions, and that matters for diagnosis. High crankcase pressure will push oil past healthy turbo seals, so an oily charge pipe on a truck with a blowby problem is very often a symptom, not the cause. Fix the crankcase pressure first and re-inspect before condemning the turbocharger. Replacement units and drain hardware sit under turbochargers.
When it really is the short block
If the vent is clear, the oil level and condition are right, the turbo drain flows, and blowby still climbs sharply under load, you are into the engine.
| Finding | Most likely cause | What it usually means |
|---|---|---|
| Blowby rises hard under load, compression even across all cylinders | General ring and bore wear | High-mileage rebuild; not urgent if oil consumption is manageable |
| One cylinder low on compression and leaking to the crankcase | Broken or stuck rings, scored bore, damaged piston | Teardown; a single failed cylinder rarely stays isolated |
| Blowby plus coolant loss and bubbles in the overflow | Head gasket or cracked casting | Pressure-test the cooling system and inspect the block |
| Blowby plus metal in the oil filter | Bearing, piston or ring land failure | Stop driving it; cut the filter open and identify the metal |
| Blowby with fuel-thinned oil and one rough cylinder | Injector dumping fuel into a bore | Fix the injection fault before any rebuild, or you will do it twice |
Cylinder washing deserves a note of its own. An injector that leaks or atomises poorly puts raw fuel down the bore, strips the oil film off the cylinder wall and wears rings and liner far faster than mileage alone would. On a 24V with high blowby and one rough-running cylinder, test the injection side before you decide the short block is worn out - and confirm the current part number for any injector or pump against the manufacturer's catalogue for your exact build year, because the VP44 and common-rail trucks do not share those parts.
If the engine does come apart, plan the rebuild as a set rather than a series of individual purchases: rings and pistons under pistons and rings, the sealing package under gaskets and seals, and complete kits under engine rebuild kits. Bore measurement and honing decisions belong to your machine shop; buy piston and ring sizes after the block is measured, not before. Budget for the machine work as well as the parts - a straightforward diesel short-block rebuild is a substantial labour-hour job at any shop, and the machining bill is often the larger half of the invoice.
What to do this weekend
Check the vent path. Check the oil level and smell the oil. Look at the turbo drain. Take a blowby measurement and write it down with the date and mileage. That sequence costs an afternoon and no parts, and it settles the only question that matters - whether you are fixing a hose or planning a rebuild.
If the numbers say the engine is tired but it still runs and pulls, you are not obliged to act immediately. Monitor oil consumption, keep the service intervals short, and let the trend tell you when. What you should not do is keep driving a truck with metal in the filter or fuel in the sump.
Questions about a specific fitment or build year: sales@coretransmissionparts.com. Orders over $70 ship free.
Frequently Asked Questions
How much blowby is normal on a 5.9 Cummins 24V? Enough to see, especially under load - these engines have always vented visibly. There is no single universal figure worth quoting; measure crankcase pressure with a manometer using the procedure in the engine manufacturer's service literature for your build year and compare against the published limit. What matters most in practice is whether it has changed.
Can high crankcase pressure blow out a rear main seal? Yes. The rear main is often the first seal to give up because it has the largest diameter and the least mechanical support behind it. Replacing the seal without fixing the pressure just relocates the leak, so measure and correct the crankcase pressure first.
Why is my dipstick blowing out of the tube? That is crankcase pressure taking the path of least resistance. Check the vent hose for a blockage first, then the oil level and condition, then the turbo drain. If all three are clear, do a leakdown test - a broken ring or a cracked piston will do it.
Does an oily intake charge pipe mean the turbo is bad? Not on its own. Excessive crankcase pressure pushes oil past turbo seals that are perfectly serviceable, and a restricted turbo oil drain does the same thing. Sort out crankcase pressure and the drain line, then re-inspect before replacing the turbocharger.
Will thicker oil reduce blowby? It may mask it slightly and it will not fix anything. Use the viscosity grade the engine manufacturer specifies for your climate and duty cycle. Heavier oil in a worn engine raises indicated oil pressure and slows cold-start flow without restoring ring seal.
Is fuel in the oil related to crankcase pressure? Directly. Diluted oil foams and vents heavily, and it strips the film that keeps rings and bores alive. Find the source - injectors or the injection pump - fix it, and change the oil and filter before you run the engine any further.
Sources
Written from the engine manufacturer's published service and troubleshooting procedures for the 5.9-litre 24-valve ISB family (crankcase pressure measurement, compression and cylinder leakdown testing, crankcase ventilation inspection), general diesel diagnostic practice for blowby and oil-consumption faults, and Core Powertrain Parts technician experience with 1998.5-2007 24V trucks. All torque values, crankcase pressure limits and part numbers should be confirmed against the service manual and manufacturer catalogue for your specific build year before work begins.