Jetta Limp Mode - No Codes

Has anyone got experience with limp mode and no codes? My car will limp from a start and clearing codes while driving unlocks normal boost and fueling operation, it lasts about 30 seconds and limps again, but still no codes.

Background:
- Recent full engine rebuild, all internals new
- Recent stage 2 tune from Malone
- Recent turbo replacements and upgrades

I had one day of normal operation after the turbo upgrade and feeling confident I changed the injectors. The next time driven it limped and it has been limping continuously even though I reverted the injectors.

Tune has MAF delete but MAF is new, with MAF tune also limps
Occurs on 2 Bar and 3 Bar map sensors, output looks normal either way

All sensors checked seem normal. The only code set is the fan since I have it bypassed.

Any help would be appreciated as I have a tune and turbo investment that I cannot use since it only limps and will not drive normally.

C'mon tdi guys, any ideas? I hesitate to call this limp mode as the logging really only indicates poor fueling. Just took my son out for a drive and had him clear codes while driving - it fairly gives a kick in the pants when the codes are cleared - briefly. I loaded a stage 5 tune from Malone at the time. I have a Darkside gtb1749v turbo and when the codes are cleared it boosts like you wouldn't believe. But lasts about 30 s and goes back to a limp. Still no codes and no clue. I am seriously down to ECU as a thing to change because I can find no values remotely out of spec. This thing is killing me!!!!!!

As the ECU is mainly monitoring emissions-related system failures there are several boost system failure modes that won't trigger a CEL.

Best bet: track down the excellent "diagnosing limp mode" in Canadian Grizzlies signature and follow it step-by-step.

First step will be to graph MAF actual-vs-requested during a 3rd gear pull and see what's actually happening to your boost.

Thanks for the link - what a great thread for diagnosis. Canadian Grizzly has it going on! Unfortunately, since my car limps out of the gate, there is not much I can do regarding logging. Logs appear like you would expect limp mode to look like > under-fueling, low boost etc. I can clear the codes and the car takes off like a rocket. I have replaced or confirmed good all parts that would normally be suspect and am down to ECU/Software. There is no code set, no CEL, and no indication of a problem except the poor performance. In terms of troubleshooting for instance: I have ran Malone tunes with and without MAF and no difference.

Can you clarify what you mean by these two seemingly opposing statements tho?

texadian said:

There is no code set, no CEL, and no indication of a problem

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texadian said:

I can clear the codes and the car takes off like a rocket.

Click to expand...

Are you using a VW-specific scanner like VCDS (Vag-com) ?

Try zip tie mod.

I am using VCDS, the only fault posted in the engine group is:
005746 - Coolant Fan Control 1
P1672 - 000 - Open or Short to Ground
Freeze Frame:
Fault Status: 01100000
Fault Priority: 0
Fault Frequency: 1
Reset counter: 255
Mileage: 294010 km
Time Indication: 0
Date: 2000.00.00
Time: 09:03:57

I have been struggling with this issue for quite some time. Initially, after looking at the logs, Malone guys felt like it was the turbo and since I had installed a cheap chinese GT1646 clone I tended to agree. I changed to a Darkside GT1749VB (no big change in performance). When comparing logs to other BRM logs I can see that boost specified and boost actual are very close but fuel quantity is about half at the same rpm and load. Seems like limp mode. On a whim, though I had no codes but the fan present, I cleared the codes while driving and everything lit up, briefly...20+ pounds boost, super acceleration and some smoke out the tail pipe finally. This lasts about 20-30 seconds and then back to what seems like limp mode but no codes (except the fan) are posted.

I have considered that the fan bypass may be causing the limp condition but all research says this should be a non critical code. In addition, I had this bypass in place several years ago when the car was operating normally with an expected style of limp mode upon underboost etc. ie: a code was set and the limp mode would clear when the car is shut off and was permanently fixed when the mechanical root cause was addressed.

This is a real puzzle for me and I really appreciate your kind replies.

Not the ECU. Installed a used from Malone and same scenario. Going to rig the feedback on the fan circuit next, then I guess I'm looking at the wiring harness and maybe the injector loom.

texadian said:

Has anyone got experience with limp mode and no codes? My car will limp from a start and clearing codes while driving unlocks normal boost and fueling operation, it lasts about 30 seconds and limps again, but still no codes.
-snip-

Any help would be appreciated as I have a tune and turbo investment that I cannot use since it only limps and will not drive normally.

Click to expand...

I had a mystery limp mode issue once that I finally traced to a leak in the vacuum reservoir that is part of the valve cover. It did not throw codes, but would go into limp mode. The vacuum reservoir is comprised of a plastic cover glued over a depression in the main valve cover plastic casting. The glue in mine failed, resulting in a crack that leaked air into the vacuum reservoir. The super easy fix was silicone rubber applied to the joint all around. Problem solved for my case.

Problem resolved: Malone guys had seen where an inferior connection at the injector harness can cause high error rate. Sure enough, I cleaned with 'De-Oxit' and reconnected - no more issue.

I think that is one diabolical piece of VW code! Limp but don't tell me why - nice.