cjromney

First off, sorry for the long post. I just want you to know what has been replaced recently for better insight into possible problems.

I have a 1994 90 CS Quattro with the 2.8l 12v V6. I purchased it 3.5 years ago. The check engine light has always regularly turned on and off while driving. It ran very well other than a slight stumble at idle and some slight hesitation/pulsing when accelerating, but I wasn't too concerned about it. While having the safety/emissions done 2.5yrs ago, my mechanic(friend) offered to hook it up to his diagnostic computer to look things over. I don't remember the specific codes at the time but he said it was from an occasional misfire. I ended up replacing the spark plugs, plug wires, ignition module, and coils. None of which made a difference. So having run out of money, I gave up at the time.

Then the car started burning some oil and leaking very badly. Long story short, it was the valley pan (which was installed with bolts only finger tight) and the driver side head gasket leaking. I was too busy at the time and so the car was parked for about 2 yrs before I fixed those issues. I also replaced the valve seals while the heads were off, the water pump, timing belt, serpentine belt, and sent the injectors out to be serviced(fixed occasional misfire and slight hesitation/pulsing under acceleration).

Once all back together the car ran great, except that the CEL still regularly turned on and off while driving and for an inconsistent idle. It would jump around between 700-800 rpm. I tried the "paper clip" method to retrieve the codes (Later found out I was doing it incorrectly. Will explain) and was given 2342 - O2 sensor 1. I decided to go ahead and replace both, and with the Bosch OEM 3 wire style. There was no change. I tried getting the codes again but came up with the same single code. Started with checking for vacuum leaks. Found the Throttle body seal to the intake plenum to be the culprit. I replaced it and all the rubber vacuum lines.

Now the car idles better during warm up, but once warm it idles worse. It will even randomly stumble to about 250rpm then jump up to 1000-1250rpm and will often die when a load is applied the engine. Especially when working the power steering at idle. I did all sorts of research on 12v.org, this forum, and others. I tried testing and cleaning the ISV(9.4 Ohms, connecter has about 5.5v at idle, 6-6.5v when engine load applied/idle stumbles).

I have been playing with different things and have found the the engine idles better and does not stumble when I allow a vacuum leak. I also cleaned the MAF and it seamed to get a little worse. I then found that when I unplug the MAF the idle stumble goes away.

In playing with things I have discovered that my ECU requires that I jump the pins to get the codes between each code to move on to the next one (was confused because 12v.org states that some 92-93 models require this method, not my 94. Who knows how many codes there were that I didn't get that might have helped). Now the codes I get are 2341 - O2 Sensor Control 1, 2331 - O2 Sensor Control 2, and 4343 - EVAP Purge Valve.

I am positive that the 2341 and 2331 codes are not being caused by a vacuum leak(have checked 5 times). I tested the EVAP Purge valve and inspected the wiring(25.9 ohms, connecter has 13.4v when running). It clicks and doesn't seem to have anything wrong with it, but there must be if the error code is there(comes back after ECU reset).

My questions now are:

1. Does anyone know where I could get the EVAP Purge Valve (p/n stamped on valve - 078 135 517 - Dealer listed p/n 078 133 517F) other than the dealer? (preferably new)

2. Could the EVAP Purge Valve really be the problem with the idle?

3. Should I be looking at my MAF even though there are no error codes for it, or something different altogether?

I am just getting really tired of throwing money and guesses at my car. I would be very grateful for any assistance.

Thank you and sorry again for the long post.

Prospeeder

MAFs almost never set codes. if you unplug it, and all the symptoms pretty much go away, car might be alittle slower, than that is the maf. Its a pretty good way to check a maf.

hubturn

I've spent some time researching your problem. I remember a really good write up on the ECM and how it works but can't find it. During initial warm up you are in the "open loop" phase. The ECM is commanding certain functions to occur, such as a richer mixture, etc. After warm up you enter the "closed loop" phase. Clearly, you have problems when entering the closed loop phase. The oxygen sensors come into play when in closed loop, the ECU doesn't like the parameters of what they report,(assuming they are reporting correctly) and a code is triggered. Go back to 12v.org and look at "how it works." Scroll to ECM revisions, click on MMS300, (which you probably are) and at the bottom of that page click on Sensor and Actuators. This is a great diagram of the inputs and outputs of the ECM. http://www.12v.org/engine/index.php?section=hw&ver=3 That is for general knowledge. You are probably going to need a Bentleys manual and a Vag-Com to get to the root of the problem. The manual to help you test the inputs/outputs of the ECM and to, in detail, list what the block values should be for the components. The Vag-Com will be able to interrogate the ECM and display the block values. That said, it could still be something as simple as a coolant temperature sensor sending bad info to the ECM, or an air leak (intake or exhaust manifold maybe) allowing inaccurate air readings to go to the ECM. Could even be an intermittant short or broken wire from a good sensor that results in bad data to the ECM. By the way, when replacing the oxygen sensors, you didn't get any of the thread anti-seize in the O2 sensor slots, did you? Also, don't spray any silicone type of compounds near the wiring. The O2 sensor will draw that in (yeah, through the wiring or through the intake)and be ruined. That's a warning in the Bentleys manual. Best of luck.

Luxus Panzer

WHAT?

Unplug that MAF on a 12v and the engine will stall out right away.

cjromney

But you can restart the engine and compare how it runs to when it was plugged in.


Thanks for the help so far. I have decided to just try a new MAF. It will be here Tuesday. I'll let you know if it works. If not I'll be investing in the VAG-com tool.

hubturn

The Ross-Tech Vag-Com is a software program and connector that compares to the Volkswagen Audi Group (VAG) tool - VAG 1551. Go to their website. There is a learning curve in using this software and in understanding the block values. It is very easy for just doing basic scans for codes. The block values require a bit more thought and possession of the Bentley's manual for your car to understand what the block values should be. The block values may be that a component should send signal 01 when "on" and 02 when "off." That's a pretty simple explanation, but when the value should be on and it's not, you have your offending sensor pinpointed. There are probably some experts on this subject as well as some good posts. Search the forum. I'm still a bit of a rookie on block values.

cjromney

Yet another disappointment. Time to start saving for the Ross-Tech Vag-Com.

Prospeeder

well sorry man. Boy it really sounds like its got an air leak somewhere. Its possible. the EVAP purge is stuck open. If its stuck open, its going to run rather rich pulling in excess fuel vapors, and maxing out the o2 sensor signals and setting codes. Lets say, an evap line was broken. Now instead of pulling vapors, its pulling fresh air, a direct air leak no doubt. Just a thought. As a quick test, block the line from the evap purge.

cjromney

Yeah, I tried that. That was exactly what I was thinking as it is running rich.

upallnight

You could have a bad fuel pressure regulator. The diaphram could be torn and the vacuum line that operates the fuel pressure regulator could be sucking additional gas into the intake manifold. Pull the vacuum line from the fuel pressure regulator and see if it is wet with fuel, it should be dry. If it is wet that is your problem.