Intermittent limp mode mystery

Car: Jetta Mk4 99.5 TDI ALH. Mods: mikroTuning chip (Alligator?) and 0.205 nozzles. 330000 km (200000 miles).

Symptoms: Last week, while "cleaning my intake" on a highway on-ramp (with a moderate up-grade), I was at about 3000 RPM in 4th and still accelerating firmly, ready to shift. Suddenly, no more power. Car went limp, barely accelerating. According to my ScanGauge unit, boost was 5-6 PSI at the most and it showed. Acceleration felt worse than a stock TDI. No CEL.

Rebooted the car, and it was fine (though I was afraid to really punch it). Now the limp mode has come back a few times during the week, even without hard acceleration (as I said, I was easy on the go-pedal): sometimes, while cruising steady, if I wanted to simply adjust my speed, even with a slight press on the accelerator I felt a lack of power. This went on until I turned off the car. But it never occured *during* an acceleration (maybe at the begining so I can't notice the change).

So I met a local guy with good knowledge in TDIs to analyze this.
- We test drove the car, hard, and it boosted fine. Of course.
- No codes showed up in VagCom.
- Actuator moves allright.
- Turbo vanes too (but we didn't open the turbo)
- Intake manifold was dirty, but not clogged. Anyways it was on my to-do list.
- He tested a vacuum (N75? I think) and no bad results.

We noticed the EGR has spat some oil on top of the valve cover, dunno if it could be related. But it was fresh/wet oil, and no sign or prior contamination on the otherwise clean cover. Though there seemed to be an older dirty spray pattern on the cowl area behind the EGR, but it's not fresh.

I'm now looking at this thread for more ideas: Diagnosing and Fixing Limp mode for A4 1.9TDI ... but we've already ruled out many causes.

I don't think it could be the MAF, since it's an intermittent trouble. MAFs are either fine or totally borked, and performance simply degrade over time, without remission, right? Anyhow I'll test it just in case.

According to the ScanGauge, I have a spike at 20-21 PSI when accelerating, which (as I now know) is too much for the *gasp* stock turbo. But it has been this way since I've had the chip, and never went in limp mode before. Sure thing I'll keep a eye on the boost from now on.

Any further input is appreciated.

Thanks all!

edit: Air filter is somewhat dirty and due for change, but still have a lot of breathing area. The snow screen is *gone*, so it isn't clogged.

Strange that you have no codes. Limp mode would trigger a code. that makes me think you have a MAF problem.

For MAF problem you can Graph the MAF on Vag-com, on a hard pull with the pedal on the floor.

Here's a working but not "necessarily known good" pattern.



Without over analizing it seems the above graph "over-reports" at low rpm and "under-reports" at high rpm. It seems airflow would more closely follow rpm's behaviour, this is why I said, "not necessarily known good". The car ran fine though.

For limp mode....Graph requested map and acutal map on vag-com, during a hard pull, preferably as the problem happens.

After the initial spike, "Acutal" should follow "Requested" similar to this one.




Read through all the links I posted in grizzly's "how to" thread, to familiarize yourself with common problems.

And dont forget about Drivebiwire's "how to" on fixing the vnt.

http://forums.tdiclub.com/showthread.php?p=1340914

Can scanguage pull up acutal map and requested map for comparison?

.

*scratches head*

Car runs fine. No limp mode for a week. I'm less and less afraid to push it, and the car pulls strong (broke my MPG record this week by the way, 46MPG average for the whole tank). That's why I didn't update... I thought there would be some development.

If the trouble comes back, I wont turn off the car and head for my local mechanic for logging purpose.

Oh well, thanks for the graphs examples and hints to what to log. In a way I hope I won't have to use this info, but I'll be glad to have them if I need it!

I don't think the SG can fish the MAP requested data... Well it could, but I can't find the coding. I remember it's been requested in the OBD forums here, but I don't think anything came out of it. I will search for it.

Problem did come back 3-4 times the last week (often while cruising, never during an acceleration), but couldn't get to a VAGCOM in a practical manner so still in the dark for the logs.

I tried to unplug the MAF. Car runs worse than when it goes fine, but still better than when it creeps into "limp" mode.

But, the way I understand how a MAF works (or doesn't ), either it's fine, it degrades performance over time, or is totally fubar. Can it "work fine" intermittently?

I have been having the same problem, after a good hard pull I go limp. err, I mean I go into limp mode after I punch it... well that just sounds dirty no matter what.
I will be taking off my intake to clean it as I know its gummed up as well as replacing as many vacuum lines as I can find.

Did you ever find a fix for your problem?

Check your VNT actuator for full 1", range of movement- couple ways to do this:
1st way: disconnect the vacuum line going to the n75 from the actuator. Attach a mityvac to this line and pump up to 18-20HG while watching the VNT actuator arm. Arm should move smoothly until you hit 18HG, - should have moved abt 1 inch.
2nd way: Undo the 2 nuts holding the actuator onto the turbo, and move the actuator arm by moving the whole actuator assembly. Again, arm should move smoothly, abt 1"

If it sticks, or doesn't move the full 1", you have a sticky VNT that needs cleaning.
I recommend you try the chemical clean method before going to the mechanical clean method.