Fig. 1: EGR valve with EGR cooler

Creeping coolant loss

leaky EGR coolers often go unnoticed

Pierburg | Motorservice
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Information on diagnostics

A leaking EGR cooler can be the reason for loss of coolant. How exactly can leaks occur in the EGR cooler? What are the most common causes of failure? You will find out why you should check the EGR cooler first before replacing cylinder head gaskets, cylinder heads or gaskets of wet cylinder liners in this article.


Since the introduction of emission standard Euro 4, the use of EGR systems with EGR coolers is becoming increasingly widespread. EGR coolers are not typical wear parts. But the EGR cooler can still fail over the life of the engine.


In EGR coolers, the engine coolant acts as the cooling medium. The coolers are made of stainless steel or aluminium. In unfavourable or unintended operating states (e.g. if running the engine with highly sulphurous fuel or biofuel), the formation of aggressive combustion products can increase. Over longer periods, this may lead to internal leakage, which is also associated with a gradual loss of coolant. In the search for the cause of this loss of coolant, cylinder head gaskets, cylinder heads or the seals of wet cylinder liners are frequently replaced – without success.

Fig. 1: EGR valve with EGR cooler


The most frequent cause of failure is a leak originating from the exhaust-gas side. One less common cause is pitting originating from the coolant side. Unsuitable cooling agents can result in corrosion or cavitation here.

As the exhaust gas back pressure is greater than the pressure in the cooling system during operation, the loss of coolant is not always immediately noticeable. When the engine is switched off, the coolant agent seeps into the engine’s exhaust tract or intake air system. If the EGR cooler is positioned higher than the intake and exhaust valves, this can cause coolant to accumulate in the combustion chamber of one or more cylinders. When the engine is started, “water claps” occur, with severe damage to pistons, cylinders or connecting rods.

Schematic diagram of cooled exhaust gas recirculation | Pierburg | Motorservice
Fig. 2: Schematic diagram of cooled exhaust gas recirculation

01 Air mass sensor
02 Charge air cooler
03 Charge-air temperature sensor
04 Regulating throttle
05 EGR valve
06 EGR coolers
07 Turbocharger (turbine)
08 Turbocharger (compressor)


To prevent expensive and time-consuming engine repairs when searching for coolant leaks, check for leakage from the EGR cooler before you start to dismantle the engine.

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