Golf 3 AEE Engine: Marelli - RPM Fluctuations

Hello everyone,

My Golf has been exhibiting the following strange behavior for a few weeks now:
1. The idle speed is extremely high, at approximately 2000 RPM (even when the engine is warm).
2. Idle speed fluctuations (sawing at approximately 100 RPM).
3. Most concerning: While driving at a constant speed with the accelerator pedal pressed, the engine suddenly loses power, as if the accelerator pedal has been released abruptly. When you slightly adjust the accelerator pedal (more or less), the engine resumes normal operation, but this happens very abruptly. When I disengaged the clutch during this phenomenon, I noticed that the engine was experiencing significant fluctuations in RPM, rapidly jumping back and forth between approximately 1400 and 2500 RPM, as if someone were rapidly moving the throttle lever back and forth.

- I installed a different, new throttle body, but the problem persisted.
- Could not detect any air leaks.

I have a used engine control unit (ECU) for replacement, but I was told that due to the immobilizer system, it won't work and needs to be reprogrammed. Can this be programmed with the full version of VCDS? A local workshop said no.

I recorded these states while driving with VCDS, and when I experienced the glitches, I added markers in the log file, specifically markers 2-5.

Can anyone give me advice on how to narrow down the problem?

Hello, che-sven.

Hello, I have the following addition to further narrow down the scope:

The following error did not occur after deleting [something], but it is now persistent.
00516 - Idle speed switch (F60)
27 - 10: Implausible signal - sporadic.

This suggests a problem with the throttle body unit. Since I already replaced the throttle body, I suspect there might be a wiring issue. Does anyone know what "27 - 10" refers to?
Is a short circuit detectable from the message?

Hello,

Quote:

implausible signal - sporadic

is 27 - 10.

Have you already clicked on the label file within the measurement data blocks and checked whether the status of the idle switch is displayed in a measurement data block (MWB1 field 4, see label file)?
You could start by checking if everything is working correctly, and then inspect and repair the wiring and switches.

The idle throttle angle at the end of the log file also doesn't match the percentage readings from the throttle position sensor.

Pot failure, twisted wires, wiring issues...

Based on my initial assessment, I would recommend taking a closer look at the throttle position sensor and its wiring.

Best regards, Rainer.

Hello Rainer, thanks for the Rü-Me!

I probably didn't understand how to check the idle speed sensor status. I'll take another look in VCDS...
Otherwise, I'll be looking at the wiring tonight. Hopefully, I'll find something this time, as I haven't had any luck so far.
Can we rule out a defect in the MSG module at this point? I have a replacement module available, but I've now discovered that it needs to be reprogrammed (due to WFS). Is it possible to do this with the licensed VCDS version?

Best regards, Sven.

Hello,

You can customize the WFS and replace the engine control unit.
http://wiki.ross-tech.com/wiki/index.php/Diagnostic_Procedures#Common_Procedures

It will be either WFS1 or WFS2.

Best regards, Rainer.

Thanks again for the helpful tips. I've also reviewed the label file, and it seems understandable so far. I'll let you know when I've checked further.
Best regards, Sven.

-hello-
Well, the solution, contrary to our initial assumption, turned out to be a faulty ignition coil module!
It must have had a bad connection, and the moisture that got in probably caused it to malfunction intermittently. Ultimately, it just stopped working completely.
Best regards,

Hello everyone,

'I've also been experiencing intermittent idling problems with my 1996 Golf 3 MKB AEE for some time. Occasionally, the idle speed starts to fluctuate, ranging from 1500 to 500 RPM, until the engine stalls. After driving for a while, it usually returns to normal. It happens most often in traffic jams, i.e., after a long period of idling. Sometimes it also occurs after a longer period of coasting (downhill driving), or lately, more frequently just before the engine reaches its operating temperature, i.e., after a few minutes of driving. There are no issues during cold starts or at higher RPMs; everything seems fine there.'

Previous measures:
- New spark plugs and distributor cap.
=> I noticed that one spark plug was much whiter than the other three. However, after replacing the spark plugs (approximately 200,000 kilometers), the condition of all the spark plugs remained uniformly good.
- The entire intake system was checked for leaks.
=> apparently no vacuum leak.
- Ignition module (ignition coil) replaced (new part).
=> no effect
- The throttle body has been removed and thoroughly cleaned several times.
=> Every time, there were no problems for a few days, and then it would start again (I have no idea if that was a coincidence or not).
- Replaced the throttle body with a used one.
=> no effect

I've now recorded a log and noticed the following:

- Firstly, it's noticeable that the throttle angle at idle is outside the range specified in the Labelfile (0-7 degrees according to the Labelfile, 10-12 degrees in reality). Is this an error in the Labelfile, or does it indicate a problem? - Secondly, the intake air temperature and coolant temperature readings are swapped. I believe this is an error in the Labelfile.
- Thirdly: Whenever the engine speed drops, the air-fuel mixture becomes extremely lean (Lambda 0), and the 'engine load' (vacuum in the intake manifold) is higher than usual at idle.
- Generally, it seems like the engine is running very lean.

I have attached the log file. An example of an outlier can be seen in line 338. From line 764, I disconnected the oxygen sensor. After that, the engine at least doesn't stall, but the intermittent rough idle persists. The idle. You can't see it as clearly in the log file anymore (line 884), but you can definitely feel it.

Dear readers, I'm at my wit's end. Does anyone have any ideas, perhaps inspired by the log file??? 'Speculative parts swapping' is really no longer worthwhile for this vehicle (260,000 km, 5 months remaining TÜV).

Best regards and thank you in advance.

Thomas.

Is the hose/pipe connecting the intake manifold to the bypass valve completely sealed?

Hello Ruedi,

Yes, absolutely. I disassembled the pipe to be able to examine it from all sides. I also cleaned everything. It doesn't seem to be a leak.

'I removed the intake manifold sensor three days ago and cleaned it with brake cleaner. It worked fine for two days, but yesterday evening, while the engine was still slightly warm, it started idling erratically again, and now the engine is running rough at idle.'

I'm going to go crazy!

Regards,
Thomas.

I replaced the oxygen sensor (new part) and the catalytic converter (used part) a few days ago. The TÜV (German technical inspection authority) accepted it like that . Since then, there have been no more idle fluctuations. Because it only has a pre-catalytic converter sensor, the problem was probably due to the oxygen sensor.

VG
Thomas.

Thank you for the feedback .