Head gasket failure is the diagnosis every CJ340 owner fears when the temperature gauge climbs or white smoke appears. Sometimes it is right. Often it is not — a blocked radiator, a failed thermostat or a leaking hose can produce many of the same signs.
This guide explains the symptoms of a failed head gasket on the Peugeot XDP 4.90, how to prove it before spending money, what usually causes it, and what a lasting repair involves.
Symptoms that point to the head gasket
A head gasket seals three things: combustion pressure in the cylinders, coolant in the water jacket, and oil in the galleries that run between block and head. The symptoms depend on which of those seals has failed.
Combustion gas escaping into the cooling system is the most common failure on a diesel. The high compression pressure pushes gas into the coolant, so you see bubbles rising in the expansion tank or radiator neck with the engine running, coolant being pushed out of the overflow, and hoses that feel rock hard soon after a cold start. The engine then overheats under load because gas pockets stop the coolant circulating.
Coolant getting into the cylinders shows as persistent white exhaust smoke with a sweet smell, falling coolant level with no external leak, and sometimes a rough first start as a cylinder clears. Coolant mixing with oil shows as a creamy, mayonnaise-like residue on the dipstick or under the oil filler cap — though a little condensation there on a jeep that only does short trips is normal and not proof on its own.
- Bubbles in the coolant with the engine running
- Coolant pushed out of the overflow; hoses hard soon after start
- Overheating under load, normal at idle
- Persistent sweet-smelling white smoke
- Unexplained coolant loss
- Milky oil (after ruling out short-trip condensation)
Prove it before you strip it
Three tests separate a real gasket failure from a cooling fault. First, a combustion-gas (block) test: a tester drawn over the coolant with a chemical fluid that changes colour when exhaust gas is present. It is cheap and quick and is the single most useful test.
Second, a compression test on all four cylinders. Two adjacent cylinders reading low together is a classic sign of a gasket failed between them. Third, a cooling-system pressure test: pressurise the cold system and watch whether the pressure holds. A drop with no visible leak means coolant is going somewhere internal.
Before condemning the gasket, check the cheap causes of overheating: radiator flow, thermostat, fan belt, hoses and coolant level. Our overheating guide covers the full checklist. A gasket that is replaced without curing the original overheating will fail again.
Why XDP 4.90 head gaskets fail
The usual cause is overheating. One hard climb in low range with a partly blocked radiator, or a trip with a slow coolant leak, can warp the head or damage the gasket. That is why the cooling system is the first thing to fix, not an afterthought.
Age is the second cause. A gasket that has done thirty years of heat cycles and been through a period of standing with neglected coolant can simply give up. Corrosion from old coolant also attacks the sealing faces around the water passages.
The third cause is a poor previous repair: a head refitted without checking flatness, reused head bolts where the manual calls for new ones, or the wrong gasket thickness.
What a proper repair involves
Once the head is off, have it checked for flatness and cracks by a machine shop. Indirect-injection diesel heads can crack between the valve seats and the prechamber; a light skim is fine, but a cracked head needs repair or replacement.
On many Peugeot diesels of this family, the head gasket is supplied in several thicknesses, and the right one is chosen by measuring how far the pistons protrude above the block at top dead centre. Check your workshop manual for the XDP 4.90 procedure before ordering — a gasket that is too thin lets the pistons touch the head, and one that is too thick costs compression and makes cold starting harder.
Follow the manual for the head bolt tightening sequence and torque, and replace bolts if the manual specifies it. While the head is off, it is the cheapest time to reface the valves, check the injectors, fit a new thermostat and flush the cooling system.
Parts to order together
Order a complete gasket set rather than just the head gasket: the job disturbs the manifold, rocker cover and hose seals as well. Nonu Ventures lists a complete cylinder head gasket set with seals for the CJ340 and MM540. Add fresh radiator hoses, a thermostat and coolant.
Quote your engine number (DJxxxxxx) when ordering, and check the gasket thickness marking against the piston protrusion measurement before fitting.
Can you drive with a failed head gasket?
Not safely for long. Gas in the cooling system causes overheating, and overheating turns a gasket job into a warped or cracked head. Coolant in the cylinders can wash oil off the bores, and coolant in the oil destroys bearings. Sealant additives may buy a few days to get the jeep home, but they are not a repair and can block the radiator and heater core.
If a combustion-gas test is positive, park the jeep and plan the repair. It is a job a competent home mechanic can do on this simple engine, with the machine shop handling the head.
Frequently asked questions
How do I know if my CJ340 head gasket has failed?
Bubbles in the coolant, coolant pushed out of the overflow, overheating under load and unexplained coolant loss are the main signs. Confirm with a combustion-gas (block) test and a compression test before stripping the engine.
Is white smoke always the head gasket?
No. Grey-white smoke on a cold start that clears when warm is usually normal. Persistent, sweet-smelling white smoke with coolant loss points to the gasket.
What causes head gasket failure on the XDP 4.90?
Mostly overheating, followed by age and poor previous repairs. Fix the cooling system before or with the gasket.
Which gasket thickness do I need?
Measure piston protrusion at top dead centre and select the gasket per the workshop manual. Quote your engine number when ordering.
Keep yours on the road
The part this guide covers — Complete cylinder head gasket set — ships worldwide, OEM or reconditioned. Quote your chassis number (CJ3B-4 DK…) when enquiring so fitment can be checked.