Audi 100 (C4) TDI: Engine Misfire Troubleshooting

Hello everyone,
'Recently, my tractor has been exhibiting a new issue: engine misfires. This misfiring occurs sporadically, both with a cold and a warm engine, and at all engine speeds, at a constant speed, and during acceleration.'
As I mentioned, the jerking occurs sporadically. Sometimes I drive 200 km without noticing anything, while other times the problem occurs every few kilometers. Sometimes it's a very violent jerk, other times it's just a brief hesitation. Could this be due to a faulty throttle position sensor? If so, how do I check it? I'm having the error memory read out today; hopefully, something will be found there.

Regards,
Mathias.

Hi Mathias,

I'm never sure if a particular model has an NHG (National Hydraulic Gear) transmission... if it does, that could potentially be the cause of the problem.

However, the main relay for the power supply to the EDC (usually labeled No. 109 on the housing) can also occasionally cause such problems.
If the error memory contains all sorts of random information, the relay should always be the first thing to suspect.

ulf wrote:

...with the 2.5s, I'm never sure if it has an NHG. If it does, it could potentially be the cause. ...

...if it's a 5-cylinder engine, then it's not significantly different from the 4-cylinder VE TDI.
So, it should have an NHG (Neutral Heat Gauge). By the way, I heard that in this engine, the NHG is supposedly responsible for crankshaft knocking and timing belt failures!!! Apparently, it's caused by the jolts/jerks!

Er hat einen Nadelhubgeber, sitzt am 4. Cylinder.
According to Etzold's wiring diagram, the relay is labeled J249 and is located in the auxiliary relay holder "I" inside the water tank. This is the same location where the fuse box was in previous models, the Audi 100 C2 and C3. However, I haven't personally verified this.
I've also experienced misfires before. Once, it helped to replace the worn-out carbon brushes of the alternator. But that might have been a coincidence. The second time, both air bubbles in the fuel supply and a misadjusted injection timing could have been the cause. I fixed both at the same time, and the misfire disappeared. However, it tended to misfire more often on warm afternoons after work, and it's not warm weather now.
Hopefully, this brings more clarity than confusion.
Christian.

First of all, thank you for your suggestions. Unfortunately, I won't be able to have the error codes read until the end of the week. Perhaps one of the experts can give me a tip about what it might be.

The problem is solved; it was actually the injector control valve. I had the other injectors checked and replaced at the same time.

Mathias wrote:

The problem is solved; it was actually the needle lift solenoid.

Hi Mathias,

Nice that you're posting the results.

Unfortunately, that's not the standard procedure here (yet), otherwise we would already know a lot more about whether and how certain "interesting" problems have been solved.

The problem is solved; it was actually the needle lift solenoid. I had the other injectors checked at the same time.

Hey Mathias,
I'd also like to know how much the repair cost. I think I need to start looking into the injectors on my Volvo with the 2.5L AEL engine as well.

Best regards from Bielefeld.

Peter.

'The standard nozzles cost €77 each, while those with a needle lift injector cost around €150. I have the invoice in the office. Please note that the prices do not include installation.'

If you have some experience with DIY projects, you can easily install the NHG yourself.

As far as I can recall from my faulty knock sensor, it couldn't be classified as defective in the error memory. The best way to test it is still to tap the injector with the knock sensor while keeping the engine at approximately 3000 RPM. If the engine starts to stutter, then the knock sensor is faulty.

best regards

Hannes.

If you have some experience with DIY projects, you can install the NHG yourself without any problems.


Sure, you can do that, but I wasn't feeling like it with this awful weather. My workshop took an hour to replace all 5 injectors, so it's hardly worth it to do it yourself. Hannes, I already answered you on the Meckisforum, but here's the full explanation: The injector control valve appeared multiple times in the error memory, which is why it was replaced. The workshop has a lot of experience with diesel engines and said that if the injector control valve shows up in the error memory for the AAT (common rail diesel engine), then it's definitely the problem (unless there's a broken wire, but that's usually obvious).

@Mathias:

"The NHG (knock sensor) wasn't showing any error codes in my system, but as soon as I disconnected the cable, the check engine light started to blink. But it was still defective... it seems like the whole thing with the error codes is a bit strange; it often feels like a game of chance."

The hour at the friendly VAG dealer costs 50 euros, and with that, I know what I can do better with the money... I also quickly replaced the nozzles at home. I actually only need the VAG dealer to read out error codes (which they do for free for us)...