A2 1.6 FSI Very rich mixture??

North West

New Member
I've recently bought a dead A2 1.6 FSI. I thought it had a blown head gasket but it turned out one piston had broken below the rings and so it had no compression.

I've rebuilt the engine and it now has excellent compression and starts instantly.

At idle it runs very rough but runs fine at high RPM and VCDS shows intermittent misfires.

The main problem appears to be a massively over rich mixture and I have a regular code for fuel pressure too high.

Does anyone know about the operation of the intake manifold control valve 036906336. This looks like its been removed and messed about with before. Can this cause excess fuel pressure? It looks like its an expensive part. Or could the isssue be a defective injector?

Thank you for any help.
 
The intake manifold flap vacuum actuator moves the flaps between the positions used in the different running modes of the engine. What can happen is that the intake cokes up and the flaps jam causing the plastic actuator arm to break. If the flaps are stuck you will get flat spots in power, irregular idling etc. but not a problem with fuel pressure.

The FSI engine ECU regulates the fuel rail pressure electronically. This is unlike more basic engines like the A2 1.4 petrol which uses a simply mechanical feedback from intake vacuum to control the pressure regulator.

The FSI has both an electronic pressure sensor and control valve for the high pressure fuel rail. If the pressure sensor or valve isn't working correctly. then the fuel rail pressure won't be correctly regulated and the engine won't run properly. If you believe that the pressure sensor reading is correct and is saying that there is excessive pressure then there could be something wrong with the regulation valve.

It could also be an injector which is blocked or failing to open properly. If this is happening then its more likely to be one injector that's failed (rather than all of them). Are you getting all the misfires on one cylinder or is it on all of them?

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Note, there is also a low pressure supply fuel metering valve and a mechanical bypass valve which manages the feed and return from the tank and the supply to the high pressure side.

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Given the recorded fault though I would suspect a problem with the high pressure side.

regards

Andrew
 
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The misfire varies between cylinders. The mixture is so rich it drinks fuel at idle. The flaps work fine, I've had the whole lot stripped and they are moving freely. VCDS says fuel pressure test is passed when checking blocks but consistently gives the fuel pressure too high error code?
 
Hi,

As a faulty injector is a possibility it seems to me a simple course of action would be to remove injectors and send away for testing and cleaning. Not a five minute job and a £100+ cost element I know, but it would remove a big factor and maybe worth doing from a maintenance viewpoint for improved performance and mpg. Ignore if this was done at the ideal time when the lower intake manifold was stripped and cleaned.

Andy
 
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The misfire varies between cylinders. The mixture is so rich it drinks fuel at idle. The flaps work fine, I've had the whole lot stripped and they are moving freely. VCDS says fuel pressure test is passed when checking blocks but consistently gives the fuel pressure too high error code?

What are the high pressure readings for idle and high revs?

If the pressure sensor is detected as faulty then the regulation valve is driven at fixed value which I would expect to give too high a delivery pressure for correct idling and too little pressure for high revs.
If the pressure regulator is detected as fault, its left closed. It works as a bleed off for extra pressure from the fuel rail so the pressure will be too high all the time. Note that there is a mechanical spring activated excessive pressure release in the valve which limits the upper pressure to about 120bar to prevent damage.

To me it sounds more like a regulation problem, as if an injector would not be delivering properly I would expect one cylinder to be different to the others and the pressure regulation trying to compensate for the actual fuel delivery out of the rail.

I would suspect the sensor and control valve first before injectors.
Note that it doesn't mean you don't also have a different problem with the injectors and still need to send them off for cleaning (or replace). The FSI doesn't run properly if the injector spray pattern is off so if the injectors are dirty, you will get misfires, but not a high pressure issue.

regards

Andrew
 
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