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2001 Audi A4 - Rough Idle (misfiring)

I have a 2001 Audi A4 1.8t (AWM engine, B5 platform). When I start the car, I get misfires on all cylinders (very rough idle, sometimes it stalls when the engine is cold) and the car appears to be running lean. Once the CEL comes on, it runs fine (albeit the slight loss of power do the ECU going into limp mode to correct the problem).

So far, I’ve done the following myself: new O2 sensors, new SAI pump, new EGR valve, new diverter valve, new N75 valve, new vacuum hoses, cleaned the throttle body, new MAF sensor, cleaned the air filter, new plugs, and new coils. When the problem persisted, I had my mechanic change the timing belt, check for vacuum leaks, change the valve cover gasket, replace the fuel pump and filter. None of this has solved the problem! This is driving me nuts.

At this point, there are only two possibilities that I can think of:

  1. Catalytic converter is clogged? I pulled out the pre-cat O2 sensor, started the car, but it still misfired. This seems to mean that there’s another problem (i.e., if the cat is clogged, it’s in addition to something else).

  2. Dirty fuel injectors? I’ve put two bottles of Techron through the car to no avail. From the outside, they look pretty dirty, so I’ll definitely try removing them, cleaning them, and replacing the gaskets. Hopefully this solves it.

Is there something I’m missing?

What DTC’s did you get from the ECU?

P0300, P0301, P0302, P0304, P1128, and P1136. The specific cylinders tend to vary on what triggers a code first, but 3 of 4 seem to trigger within 10 minutes of being cleared.

So, bank 1 lean and misfires on cyl 1,2,4.

Have you checked fuel pressure at the rail?

-From the resource-
P1136 - Audi - Long term fuel trim, idling, Bank 1 - system too lean
Cause: Fuel pressure/pump, injector(s), intake/ exhaust leak, AIR system, hose leak

P1128 - Audi - Long term fuel trim, entire speed/load range, Bank 1 - system too lean
Cause: Fuel pressure/pump, injector(s), intake/ exhaust leak, AIR system, hose leak

No, but given that I’ve replaced the pump and filter, wouldn’t that eliminate problems from the rail on back? What should the correct pressure be? I’ll try and pick up a gauge tomorrow to check it.

The DTC’s plus the work you’ve described suggest a fuel delivery problem.
I’m not familiar with the engine specifically, but I can give some generalities that may be useful. That said, I don’t know what the fuel pressure should read. The internets or a manual may be helpful there.
Clogged injectors will give you a weird number. Damaged or obstructed lines ahead of the filter would not be corrected by a pump/filter change and would give weird readings. I believe there is also a fuel regulator in line after the filter that could be obstructed or malfunctioning. The rail itself may be clogged. The injectors may be clogged or may not be working correctly.

Cool. Thanks for the tips. I think I’ll try all of the above. It looks like the regulator is supposed to run at 4 bar (58 psi).

Based on this, do you think it’s safe to rule out the catalytic converter then?

A clogged converter will generally cause an engine to run rich. The converter is easily checked with a vacuum gauge but the problem there is that for some odd reason many mechanics do not use them or know how to interpret them.

Any chance this could be caused by a loss of residual fuel pressure? The system should hold pressure while the engine is at rest. If pressure is lost (usually a faulty check valve in the fuel pump) the reason for the engine running rough is because it’s taking time to burp the air out of the fuel lines. This is a common problem with many cars: it’s not Audi specific.

If the engine is up and running fine once warmed up you might shut it off and wait a few seconds before attempting a restart. If it restarts without stumbling then there could be a check valve problem in the pump.
This can also be verified by connecting a fuel pressure gauge and keeping an eye on it as the car sits for a while.

Hope that helps some.