Tip & How-To about Audi A4

Diesel EGR failure

An EGR valve usually fails prematurely due to a carbon build up within the inlet side of the engine. This means if you have a faulty EGR it is without doubt there will be an underlying cause.

You need to suss out what is causing the excess carbon build up in the first place. This, 9 times out of 10 will be the mass air flow sensor. It may still be within tolerance so that the EML (Orange light on dash with picture of an engine) is not emitted but be close to the end of it's working life. Live data is required to prove this but also beware of split hoses on the inlet side of the engine as they will also have a negative impact.

If the above faults are not rectified quickly the carbon will work it's way to the catalyst or DPF (Diesel Particulate Filter), these parts can be very expensive so act sooner rather than later.

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Check engine light. Code: Exhaust gas

Usually this is due to carbon build up. Fuel injector cleaners will not help. It involves removal of the EGR valve and inspection of the valve for carbon and the passages for carbon. You can also test the valve itself prior to removal if it is vaccuum operated. Unplug the vac hose, plug it and then install a piece of vac hose long enough to reach it without bending over the engine. While the engine is idleing apply vaccuum to the hose via a pump or the old fashioned way via sucking on it and note engine idle change. If none either the baffle is bad inside the valve or it is clogged. Hope this helps.
Also, some EGR valves have a vac control solenoid and those go bad too. If the idle does change via the procedure above then suspect the control vac solenoid.

Jul 15, 2010 | 2000 Honda Passport

1 Answer

egr valve when does it operate

The EGR (exhaust gas recirculation) valve

What is it? This is a device that reduces engine emissions of nitric oxides by allowing a measured amount of exhaust gas to re-enter the intake manifold and mix with the air prior to entering the cylinders. The EGR can be vacuum or electrically driven or a combination of both.

Where is it located? The EGR is most often associated with the intake manifold. The EGR will be sited at a point of contact with both the exhaust and intake manifolds. If the exhaust manifold is remote, for example on the other side of the engine to the intake manifold, there will often be an exhaust feed pipe leading from the exhaust manifold to the EGR sited on the intake manifold.

How does it work? Generally intake manifold vacuum acting on a diaphragm draws up on a valve to open a connection between an entry port from the exhaust gases and an exit port to the intake manifold. An open EGR port makes starting difficult so the valve operation is often impeded by an electrical over-ride until the engine has warmed up, as signaled to the ECU by the coolant temperature sender. In some cases, a differential pressure feedback exhaust (DPFE) sensor fed by pipes from the exhaust feed to the EGR informs the ECU when and by how much the EGR should be open.

Symptoms of faulty EGR

The EGR has two possible fault modes either it is a) open when it should be closed or b) closed when it should be open. (some EGR faults are in fact more likely to be DPFE related)
  • Poor idling: ‘hunts’ at idle and generally rough engine performance. This is due to the air ingress through the split diaphragm or the EGR valve being left open, either event compromises the intake manifold vacuum and in turn this interferes with the ECU’s ability to deliver accurate fuel to air ratio for a given engine speed.
  • Misfire and ‘pinking’: if the EGR is jammed closed or the exhaust feed pipe is blocked (e.g. gummed up with baked carbon) the engine ignition runs lean and very hot, so hot in fact that the fuel air mix can spontaneously ignite causing a misfire. The misfire if pronounced can, in turn, be detected by the engine knock sensors and this gives the appropriate on board diagnostic error codes along with the check engine light. Due to the diesel engines creating more soot in the exhaust a blocked EGR is a much more common complaint in diesels than petrol cars.
  • Surging: A sticking EGR valve can lead to an effect of rhythmic surging a bit like the MAF fault, especially in turbo charged engines. Normally on a warmed up engine the vacuum from the inlet manifold opens the EGR. If the EGR fails to open promptly the turbo can be subject to a slight boost. Extra fuel/air mix is pushed into the inlet manifold as a result, increasing pressure (decreasing the vacuum). Without vacuum in the inlet manifold the EGR closes again but if sticky does so only slowly. This may allow inlet gases to flow through the EGR into the exhaust manifold for a split second. There is a time lag in the sequence of these events leading the engine to surge in cyclical manner.

How to check? With the engine running it may be possible to force the EGR valve open by pressing on the diaphragm with one’s fingers. Failing that pulling off the vacuum line to the EGR, blocking the pipe on the inlet manifold and applying vacuum to the EGR to monitor function will determine if the diaphragm is split. Removal of the EGR assembly and examination of the ports and valve mechanism will reveal any carbon build up.

How to fix? If the diaphragm is split then the EGR needs to be replaced. Most EGR problems are linked to carbon soot build up and this can be cleaned using a cloth, brush and carburetor choke cleaner spray. If the exhaust feeder pipe is blocked this can be cleaned using a piece of frayed hand brake cable as an internal brush. This makeshift brush can be further enhanced by mounting it in a drill to sweep dirt from the interior of lengths of pipe. Some cars (Hondas are a good example) have an elaborate passage way system to provide exhaust gas to each inlet pipe runner. The only way to clean these out is by removing the blanking plugs (no easy task) and then using the makeshift rotary wire brush and carburetor choke spray. New blanking plugs have to be reinstated to make good the passage ways assembly.

Jul 14, 2010 | 1992 Ford Explorer

2 Answers

what does a PO401 means for a 1998 Mercury Sable

P0401 OBD-II Trouble Code - Insufficient EGR Flow

What does that mean?
EGR stands for Exhaust Gas Recirculation. It is part of the vehicle emmissions system, and is used to reduce combustion temperature and pressure to control Oxides of Nitrogen. There are generally three parts to the EGR system: the EGR valve, an actuator solenoid, and a differential pressure sensor EGR (DPFE). These things work together to deliver the correct amount of recirculation based on engine temperature, load, etc. The P0401 code means that OBD detected an insufficient amount of EGR.

You may notice drivability problems such as pinging (a.k.a. pre-ignition knock) when the engine is under load or the vehicle is at higher speeds. There may also be other symptoms.

A code P0401 most likely means one or more of the following has happened:
* The DPFE (differential pressure feedback EGR) sensor is faulty and needs to be replaced
* There is a blockage in the EGR (tube), most likely carbon buildup
* The EGR valve is faulty
* The EGR valve may not be opening due to a lack of vaccuum

Possible Solutions
In fixing this code, it is quite common for people to just replace the EGR valve only to have the OBD code return. The EGR valve is not always the culprit.
* Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's & DPFE voltage. There should be a noticable difference in RPM's with the EGR open
* Clean out the EGR valve and/or tubing to remove deposits
* Check the voltage at the DPFE, compare to specified values (refer to a repair manual for your specific model)
* Replace the DPFE sensor (with a good quality / OEM one)
* Replace the EGR valve

Hope this help to solve it; remember comment and rated this help. TY for using Fixya.

May 06, 2010 | 1998 Mercury Sable

4 Answers

I have a 1995 GMC Jimmy,rough idle, carbon smell, strong gas smell coming from the left driver side, check engine light is on. Has a hard time turning on...Any suggestions?

My 94 Jimmy had similar problems, until the dirty EGR valve was replaced. We tried a million things, first. But the improvement with the new EGR valve has HUGE.

Nov 17, 2009 | 1995 GMC Jimmy

2 Answers

Check Engine Light Just Came On..

Remove the EGR valve and clean it out. It's probably carboned up really bad constricting the exhaust flow. Also remove the EGR tubes and clean them out as well. I use brake cleaner and keep spraying the pieces out until it's pretty clean and re-assemble.

Feb 04, 2009 | 2000 Ford Explorer

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