Nissan Patrol ZD30 Engine Problems.


This page represents my opinion on the many documented premature ZD30 engine failures experienced by Patrol owners in particular. The main issue of relevance is the melting/burning and cracking of pistons and in some cases, cylinder heads.


Nissanís ZD30 Patrol has suffered a dismal reputation due to these relatively common issues and I for one believe they havenít yet been completely resolved. After a number of early failures, Nissan did increase the oil capacity and lower it's viscosity. This seems to have reduced the number of engine failures, or at least given the engines a longer lifespan, so one would assume the problem was oil related. There have been many assumptions of the cause and this is just another one that could be considered.


These engines run relatively high boost levels and as clean diesel engines, they also use high levels of exhaust gas recirculation (EGR). This in turn reduces the amount of NOx released into the atmosphere and enables the vehicles to pass stringent emission standards.

One problem is that the high boost levels create high pressures in the crankcase which in turn pushes reasonable amounts of oil past the crankcase baffle and into the engines intake system. This oil finds its way into the intercooler and eventually into the inlet manifold, where it mixes with EGR gasses to form a thick black greasy substance that restricts the inlet manifold runners. In most cases the runners most affected are the rear ones, which are closest to the EGR valve itself.


Overboosting is fairly common with ZD30 Patrols. Early signs are blowing off intercooler hoses and in many cases the owners simply refit them and tighten them so they canít blow off again. This can result in the hoses splitting or worst, an intercooler rupture. Itís quite common to see an intercooler leaking oil as a result of overboosting and excessive amounts of oil in the intake system.


The ZD30 engine management system relies on input signals from various engine sensors. The Engine Control Unit (ECU) determines fuel delivery (timing and duration) depending on input signals from the Mass Air Flow Sensor (MAF), Throttle Position Sensor (TPS) and engine speed. The volume of air entering the engine determines how much fuel is injected, but the ECU doesnít monitor or have mixture control over each individual cylinder. The engine lacks an exhaust temperature sensor, so it canít know if a particular cylinder is being over fuelled or running rich.

As a result of this and the inlet manifold restrictions mentioned above, the front cylinders can run lean and the rear cylinders rich. Itís common knowledge that over fuelling a diesel or running it rich will develop excessive heat and the end result is a melted or cracked piston or cylinder head.


Along with this, MAF sensors are known to give faulty input voltages to the ECU if they become contaminated or insulated against the cool induction air passing through them. I believe this is a result of shutting down a hot engine, where some oily fumes can travel up to the MAF sensor and contaminate it with oil against the normal direction of air flow. A contaminated MAF sending lower voltage signals to the ECU will usually create a lean condition, but the power loss will increase the drivers throttle input which can increase the fuel load and increase high combustion temperatures even further.


The engine does have a boost sensor, but it plays no part in fuel mixture control. The ECU will reduce boost when it gets too high, but this system is very slow to react and boost spikes of up to 25psi are common.

One of the advantages of Variable Geometry Turbochargers, is that they can be controlled electrically using vacuum rather than pressure. The ECU controls a vacuum solenoid using pulse width modulation (PWM) and by rapidly switching it at different pulse widths, it controls how much vacuum gets to the turbochargerís actuator and in turn how much boost the turbo produces at any given rpm and load. This is very important because it enables the ECU to reduce boost at any time, even under load or when the throttle is partly pushed open.

The ECU can reduce boost to promote maximum EGR flow by reducing intake pressure while opening the EGR valve with the throttle pushed open under light load conditions. This forces more exhaust gas into the inlet manifold and combustion chambers, which in turn reduces peak combustion temperatures and NOx emissions. When this happens, EGTís rise because there is less cool oxygen enriched air entering the combustion chambers and more hot exhaust gas, further increasing exhaust temperatures.


Any combination of the conditions mentioned above can cause an engine failure due to excessive combustion temperatures, but there are relatively easy and cheap ways of monitoring and controlling these systems. Taking some precautionary measures should reduce the chance of an engine failure or at least prolong the engineís life. In most cases, modifying an engine or its controls can be risky to say the least, but in this case itís risky not to.


What can be done to reduce the chance of an engine failure?


Most importantly, fit an EGT and Boost gauge to monitor whatís happening inside the engine. They wonít stop it from melting a piston, but should warn the driver of any pending issues before they become terminal.


Anything that can be done to lower EGTís will be beneficial.


Fit an Oil/Air Separator or Catch Can to reduce the amount of crankcase blow-by oil that enters the intake system and contaminates the MAF sensor. These are common amongst racers and the Mann & Hummel Pro Vent or a CKD Separator are the units of choice.


Although illegal to do in most parts of the world, blocking the EGR valve does have benefits in stopping hot exhaust gasses from entering the combustion chambers and will lower EGTís. It also reduces the chance of the inlet manifold runners becoming restricted by the thick black grease when it mixes with oil from the crankcase and helps to keep the engine oil cleaner for a longer period.



Blocking the EGR valve will increase boost, so something has to be done to lower boost and also stop the ECU from reducing boost to promote EGR flow. The easiest and best way that I have found is to fit a Manual Boost Controller and take boost control away from the ECU. To do this I use a Multi Stage TDi Dawes Controller to set two maximum selectable boost levels and a needle valve to control and set the spool up rate. This mod also stops high boost spikes and the possibility of overboosting.


Other ways to decrease exhaust temps even further include fitting a free flowing exhaust system and pressurizing the fuel supply to the injection pump using an electric supply pump. These mods can help, but are optional.


I posted this page in an effort to help any ZD30 Patrol owners that have concerns regarding the reliability of his or her car since receiving many emails from concerned owners. I hope it helps to explain a theory that I, and quite a few others have on the Patrol Forum regarding the engine failures. There are a few other theories that have some merit and could also be contributing factors, but I feel that these are the main ones. There is much additional information on the Patrol forum and it is well worth the time to research.