Exhaust-gas recirculation puts exhaust back into the intake so the combustion charge is cooler and nitrogen oxides fall. Diesels need it because their exhaust still contains oxygen and a three-way catalyst cannot reduce NOx there. Petrol engines use it too, at part load, for NOx and for fuel consumption. The hardware fails in the same places on both: the valve sticks, the vacuum or the motor that moves it is lost, or the passages and the cooler fill with soot. The symptoms are not the same, because diesel exhaust carries far more soot and petrol EGR rates are often lower. This page is the fault-finding. How and why the system is fitted is on petrol and diesel emissions.
It does not describe how to remove or switch the system off. A valve that is stuck and a calibration that pretends the valve is still there are different jobs. The second is an emissions defeat, and the MOT inspection of missing or modified emissions equipment is set out on the emissions page.
Common diesel engine problems and common petrol engine problems both link here.
What is being asked of the valve
HELLA publishes these maximum rates and the exhaust temperatures at which it says the system is active. They are HELLA’s comparison, not a setpoint for every map.
| Diesel | Petrol | Direct-injection petrol | |
|---|---|---|---|
| EGR rate, maximum | 50% | 20% | Up to 50%, depending on homogeneous or stratified operation |
| Exhaust temperature while EGR is active | 450°C | 650°C | 450°C to 650°C |
| Why it is used | NOx, soot and noise | NOx and part-load fuel consumption | NOx and part-load fuel consumption |
HELLA separates internal EGR, which is exhaust left in the cylinder by valve overlap and by variable valve timing, from external EGR, which is a pipe and a valve between the exhaust and the intake. A cam-timing fault can therefore change the internal portion without the EGR valve having moved. External systems are pneumatic on older engines and electric on later ones. High-pressure and low-pressure loops, and why a diesel needs more of the gas than a stoichiometric petrol engine, are on the emissions page.
How it fails
HELLA calls the valve the largest source of faults in the system, because of the load it sees. Oil mist and soot reduce the opening until it closes up, so the recirculated quantity falls and the emissions change. Heat accelerates that. The same deposits can hold a valve open. The two positions are different engines:
- Stuck or blocked shut. Less EGR than the map asked for. NOx rises. On a diesel the intake is the cleaner of the two failures, and the driver may notice little except a fault code, a failed emissions test where NOx is measured, or more combustion noise. On a petrol engine the same loss can show as knock or a higher combustion temperature at the loads where EGR should have been present.
- Stuck open. Exhaust in the intake at idle, where the map wanted the valve shut. Rough idle, stalling as the throttle closes, black smoke on a diesel, and a flat response. This is the failure owner forums usually mean by “EGR problem”.
- Moves, but the passage does not flow. HELLA notes that an electric valve can pass an actuator test and store no fault while the opening is so restricted by deposits that the control unit is not getting the area it asked for. Commanding the motor is not a flow test. On a running engine the air-mass or manifold-pressure reading has to change when the valve is commanded, in the direction the procedure expects.
Pneumatic valves add another set of faults that are not the valve head. HELLA lists vacuum hoses that leak, are kinked or are swapped, a pressure transducer that does not pass vacuum, and a thermovalve that does not open when hot. If the vacuum never arrives, the valve never opens, and replacing the valve changes nothing. HELLA’s own vacuum figures, about 300 mbar on a petrol diaphragm valve and about 500 mbar on a diesel one, are approximate values for the valves in that article. The vehicle procedure and its limits replace them.
Electric valves fail in the motor, the position sensor and the wiring as well as in the seat. Position feedback that does not follow the command is an electrical or mechanical fault at the valve. Position feedback that follows the command while air mass does not move is a flow problem: passages, cooler, or a valve whose pintle is moving in a hole that soot has filled.
Diesel
Low-speed hesitation, a lump at idle, soot in the intake manifold and a filter that fills too quickly are the cluster. The AA lists an EGR or fuelling fault that makes excess soot as one of the reasons a particulate filter will not regenerate. Cleaning or replacing the valve and leaving the cooler and the manifold ports blocked puts the same soot back on the new valve. Carbon deposits and cleaning is the map of where that soot sits and which cleaning method reaches it.
The cooler is the part that gets mistaken for a head gasket. Exhaust gas passes through a core surrounded by coolant. Soot in the gas side restricts flow, which looks like a valve that will not pass enough gas, often with black smoke and low intake flow. A perforated core lets coolant into the gas path. The coolant level falls with no puddle, the exhaust can show white vapour that does not clear, and the cooling system can be pressurised. That is the same picture as head gasket failure. A combustion-gas test proves exhaust gas is in the coolant. It does not name the component. Isolate the cooler and pressure-test its coolant side before condemning the gasket, and do the gasket tests before condemning the cooler. Coolant loss and overheating as a cooling-system question are on engine overheating and coolant faults.
An air-mass meter coated in oil, a boost leak, or a variable-geometry turbo that sticks at low speed produces the same low-speed flat spot. HELLA’s diagnostic note is the right order: if the control unit is given the wrong air mass, it calculates the wrong EGR quantity. Compare commanded valve position, reported position, air mass and manifold pressure. A valve that is being told to open because the air reading is a lie should not be replaced first.
Petrol
A port-injected engine does not fill the valve with diesel soot, but oil mist and heat still seize pintles, and the vacuum plumbing on older systems still perishes. A valve stuck open upsets idle in the same way, because exhaust is diluting a charge the idle map did not ask to dilute. A valve stuck shut is quieter in the cabin and louder in the NOx result. Direct-injection engines, which HELLA allows a much higher rate on some operating modes, have more of the diesel-shaped complaint when the rate is high and the passages coke. Inlet-valve carbon on those engines is a parallel problem, not the EGR valve itself. It is covered on the carbon page.
A vacuum leak at the EGR pipe joint is also an unmetered-air leak. It can set mixture and misfire faults that look like ignition or injectors. Smoke-test the pipe with the valve commanded shut before splitting the fault into a coil and an injector.
A usable order of tests
- Fault memory and freeze frame. Note whether the code is “position”, “circuit”, “flow too low” or “flow too high”. Flow and position are different claims.
- Look at the pipes and connectors. HELLA’s first step for a pneumatic system is the vacuum lines and the electrical connections, before any substitution.
- Command the valve, where the controller allows it, at warm idle. Position should follow. Air mass or manifold pressure should move. If position follows and the air reading does not, inspect the passages and the cooler rather than buying another actuator.
- On a pneumatic valve, a hand vacuum pump tells you whether the diaphragm holds and whether the rod moves. HELLA warns that the test values in a general article are approximate. Use the vehicle figures once you have them.
- If coolant is disappearing or the header tank is pressurising, pressure-test the cooler and run the head-gasket tests. Do not treat one positive combustion-gas result as a gasket.
- After a repair, confirm the air-mass change on a road test at the load where EGR is used. Idle is not that load on every engine.
Sources
- HELLA Tech World, exhaust gas recirculation. Rates, pneumatic and electric valves, soot and oil mist closing the opening, vacuum leaks, and a contaminated valve that still passes an actuator test. HELLA states that its test values are approximate and that vehicle-specific figures are required.
- The AA, diesel particulate filters. Excess soot from EGR or the fuel system as a reason regeneration does not keep up.
- Petrol and diesel emissions on this wiki, for high-pressure and low-pressure EGR and the NOx chemistry.
- Engine Australia, service bulletin SB041. What a combustion-gas result and a leak-down test can and cannot localise. Used here for the overlap with a cooler that also joins gas to coolant.