Another limp mode with no codes

Dave128

Well-known member
Joined
Oct 25, 2012
Location
PA
TDI
96 passat B4V
So I have been going thru the "troubleshoot low power" notes. I am checking things off the list but was hoping someone could narrow down my issue(s). It drives well to 2000rpm, then goes into limp mode. When the rpms come back down below 2000rpm, it's fine (doesn't stay in limp mode.) I have floored it above 2000 and it seems to surge a little bit. The other thing is that when I roll up to a stop sign, the rpm drop to 550, sometimes it even stalls. If I touch the go pedal, it will come back up to the 900rpm that it is usually at. Does this sound like a fuel issue? Any idea/suggestions are greatly appreciated.
 

Abacus

That helpful B4 guy
Joined
Nov 10, 2007
Location
Relocated from Maine to Dewey, AZ
TDI
Only the B4V left
Two things:

1) Just a guess, but you probably have the black plastic case ECU with the BK designation. While not OBD-2, it doesn’t ‘latch’ the error codes and will reset them when the condition corrects.

2) The RPM for the BK was 860 and not 903, that came with the GQ ECU update. I’m thinking you have a boost leak in the piping, or the hose inside the ECU that’s causing the limp mode and the car adjusts things to compensate. As to the RPM drop, it may be fuel related or air. A Vag-Com log would tell a lot.
 

Dave128

Well-known member
Joined
Oct 25, 2012
Location
PA
TDI
96 passat B4V
A bit more background. I recently had a lower mileage engine and trans swapped by my local garage. The previous engine was stock and ran fine with this setup till I blew a heater core and cooked itself.

Abacus -Thanks for the info and a big reminder. item 1 - I do have a GQ ECU with a slight tune. It's a RC3 setup for 110hp injectors, 5th injector delete and something about the EGR duty cycle being such that it doesn't open often? Sorry don't exactly remember what he said.
item 2 - My Vagcom says that MAF (specified)240-340 but the reading is 750mg/str. MAF (actual) 240-340 but reading 442.2. Not sure how to interpret it. Would the tune have made the MAF specified that high? What Groups should I log?

Vince - Thanks for the info. I will check the ECU hose. I think I replaced it a while back(10 years ago...that went fast!) but it doesn't hurt to check it out.
 

Abacus

That helpful B4 guy
Joined
Nov 10, 2007
Location
Relocated from Maine to Dewey, AZ
TDI
Only the B4V left
If you have the GQ already then I have no idea why it'd return other than perhaps a boost hose. I have seen something similar when a compression clamp snapped but was still in place.

It's common for tuners (or ones for which I have experience) to set the MAF for a constant max flow for more fueling. One of mine is set to 1,000 mg/str constantly and the another is set to 850.




As to datalogging, I will let some of the tuners chime in, but the Vag-Com is VERY useful for figuring out what's going on.






 

garciapiano

Veteran Member
Joined
Feb 12, 2018
Location
Los Angeles
TDI
1997 Jetta TDI (1Z)
I had a similar issue recently. Double check that the vacuum hoses are good and new and not leaking or cracked - that includes the ECU.

On my car it turned out to be a bad N75, unable to push enough boost pressure to the wastegate due to a failing internal diaphragm.

The turbo will overboost without the N75 working properly. The N75 is what allows the turbo to bleed off boost. It uses the turbo's boost pressure itself to actuate the wastegate.

Thus, to protect the turbo, the car goes into limp mode if it can't actuate the wastegate (or see that the boost pressure has leveled off).

The maddening part of this is that the car will not always throw codes in this case, making the diagnosis difficult. In my case, I got a boost deviation code... but only sometimes. Not always. I tried many different things but a new N75 is what fixed it.
 

Dave128

Well-known member
Joined
Oct 25, 2012
Location
PA
TDI
96 passat B4V
Latest update. I needed to walk away from the car to keep my sanity. Decided to take it to a recommended TDI mechanic from the list. They looked at it and decided that they did not want to work on it. They did think that the problem is with the turbo or wastegate. I was able to check them both out and they seem fine. Turbo is free and has no play. Wastegate works with a bike pump. The ECU has a new hose so I don't think that is a problem. I have a N75 on order and should be here soon. Hopefully, that is the problem.
In the mean time, I started to wonder if my no boost has something to do with the replacement engine not having an EGR. Would that cause a no boost condition? The car also stalls often when coming to a stop sign. Any thoughts?
 

volksguy

Veteran Member
Joined
May 29, 2003
What is The ECU?
Do you have one of the Famous?
Hans Auto Parts ECU'S That will not show Trouble Codes.
 

Dave128

Well-known member
Joined
Oct 25, 2012
Location
PA
TDI
96 passat B4V
It's a GQ ECU with a slight tune (see above). It has always shown codes with the previous engine.
 

jdulle

Veteran Member
Joined
Feb 4, 2017
Location
Ithaca, NY
TDI
96 B4, 97 B4
How new is your fuel filter?
Also, I have found that older vacuum lines can build up crud inside that blocks them and sends the car into limp mode. Might want to check that they all flow easily.
 

Dave128

Well-known member
Joined
Oct 25, 2012
Location
PA
TDI
96 passat B4V
jdulle, thanks for the response. the fuel filter is new. I am going to replace it anyway. The vacuum lines have all been replaced and seem fine. I just replaced the N75 and still have the problem. I have a spare MAF sensor that I'm going to swap on. I guess I am going to go thru the limp mode/low power check list again and see if I missed something. this is really becoming frustrating!!
 

Dave128

Well-known member
Joined
Oct 25, 2012
Location
PA
TDI
96 passat B4V
Volksguy, I have the original BK that I be swapping back in. It just seems strange that everything was working fine before the engine swap.
 
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