P1556 Limp Mode Diagnosis Problems

[Rick Blalock]

Hey guys, I was hoping someone could help me out with a limp mode problem I have been having. Heres the basic run down: It starts up running fine and once I get goin up a hill on the highway or try to pass someone it shuts off the turbo going into limp mode. Here is what I have done so far:

- Opened up the snow screen and cleaned it out, it was caked on.
- Replaced the air filter.
- Connected my hand vacuum pump to the line going down to the actuator, i can hear it engage at about 5in of vacuum and then I can hear the arm go back down gradually as i release the pressure. I did this back and forth many many times to make sure it is free.
- I tested as many of the vacuum lines as I could get pulled off. Had to replace a couple of them and a T connector that broke when taking the lines off.
- I erased the code off the computer and it eventually came back after getting it into limp mode on the highway... cycled the key and all power was back.
- I measured the electrical resistance on the terminals on both the N75 and the EGR solenoid on the firewall next to it... Both read 16ohms as they should according to ALLDATA.

I was wondering how I can go about testing the N75 without VAGcom or anyone else around to help out... I read this thread but am unsure if the actuator arm is supposed to move with just turning the ignition to ON:
http://forums.tdiclub.com/showthread.php?t=179589&highlight=p1556

Seeing that the engine would be off and that the actuator needs vacuum to engage, I would think that wouldnt work...

Thank you guys very much for all of your help! Hopefully we can figure this out..

 

[puter]

Sounds like a classic maf problem to me.

Try running with the maf unplugged and see if the problem goes away, if it does then clean the connections on the maf with some deoxit and unplug/plug it back in a few times. If this fixes it then your connection was dirty, if not then replace the maf.

 

[Rick Blalock]

Sounds like a classic maf problem to me.

Try running with the maf unplugged and see if the problem goes away, if it does then clean the connections on the maf with some deoxit and unplug/plug it back in a few times. If this fixes it then your connection was dirty, if not then replace the maf.

Since the problem is not there on start up, would it be ok to unplug the MAF while the engine is running after it goes into the limp mode or must the engine and ignition be off when unplugging it?

I went ahead anyways and cleaned the connections with some quick drying automotive electrical connection cleaner that i already had from O'Reilly's... It says it dries without residues so i figured it cant make anything worse.

I also went ahead and replaced all the vacuum lines with about 9 feet of line from Auto Zone. Just to eliminate that possibility...

 

[puter]

Since the problem is not there on start up, would it be ok to unplug the MAF while the engine is running after it goes into the limp mode or must the engine and ignition be off when unplugging it?

I went ahead anyways and cleaned the connections with some quick drying automotive electrical connection cleaner that i already had from O'Reilly's... It says it dries without residues so i figured it cant make anything worse.

I also went ahead and replaced all the vacuum lines with about 9 feet of line from Auto Zone. Just to eliminate that possibility...

I never recommend unplugging any electrical component while the engine is running. What you want to do is unplug it and see if you can get the problem to happen.

Best way is to find a steep hill and try to go up it at a low-ish RPM. This is pretty sure to send you into limp mode if your MAF is on its way out.

 

[Matthew_S]

This problem is usually caused by a vacuum leak, sticky vanes/actuator or a bad N75. Connect a vacuum gauge to the hose that goes to the vacuum ball on the side of the engine. Vacuum should hold after shutting the engine off. If you don't have a vacuum gauge just pull the hose off the vacuum ball a minute or so after shutting off the engine and it should hiss if there is still vacuum. You could also have someone shut off the engine while you are watching the anti-shudder valve. If there is adequate vacuum it should fully close for a couple of seconds.

If the vacuum system checks out okay it will be difficult to diagnose without VAG-COM or a lab scope unless you start throwing parts at it.

 

[Rick Blalock]

This problem is usually caused by a vacuum leak, sticky vanes/actuator or a bad N75. Connect a vacuum gauge to the hose that goes to the vacuum ball on the side of the engine. Vacuum should hold after shutting the engine off. If you don't have a vacuum gauge just pull the hose off the vacuum ball a minute or so after shutting off the engine and it should hiss if there is still vacuum. You could also have someone shut off the engine while you are watching the anti-shudder valve. If there is adequate vacuum it should fully close for a couple of seconds.

If the vacuum system checks out okay it will be difficult to diagnose without VAG-COM or a lab scope unless you start throwing parts at it.

Before I replaced all the lines I did actually rig up a T connector in the line that goes down to the actuator. From this I was able to connect my hand pump (which i think is the generic form of a mity vac) which has a gauge on it. When I started the engine up the gauge when up to around 15in of vacuum or so and when i shut it off it gradually dropped to like 1-2 then went down very slowly from there. Even before then, when the engine was shut off, and I were to disconnect one of the vac lines I would hear that little "whooosh."

I guess my concern with running with out the MAF plugged in was that it wasnt safe, but apparently its ok. I will try doing this today

 

[Rick Blalock]

Well I just got back from drivin around with the MAF unplugged... Holy crap my engine has SO much more power! I was unable to get it into limp mode even when pushin it over all the hills that normally triggered it. I'll try cleanin the connections one more time. I already cleaned them but hadnt driven the car since before I unplugged it.

Also, oddly enough my daytime running lights were working as well... maybe its just a coincodence, but they hadnt worked in months... it'll be interesting to see if they continue to work with the MAF unplugged!

 

[puter]

If it's not oxidation on the connector then you can get a new MAF for ~100 from tdiparts.com or worldimpex.com

They sometimes run sells on them (I got mine for ~90 thanks to a sell) so check both sites.

Edit: I also recommend using deoxit rather than automotive cleaner, the problem with the maf is caused by oxidation, not contaminants, and I'm not sure how focused on oxidation a generic automotive cleaner is, deoxit is designed for oxidation and oxidation along. Also be sure to plug it in and unplug it a few times after you spray it (both the maf and the wire harness) to help break up the oxidation. If your problem goes away, then just do that every few months or when the problem persists and you should be good.

You can also look into the 'cage mod'. Be warned, some members say the silicone grease breaks down the wire harness, I personally don't buy that silicone breaks down polymers and so I did the mod. Note: This mod does _not_ fix the problem, it is designed to prevent it.

 

[Matthew_S]

If the MAF is so bad that it is having a significant effect on the power cleaning it probably won't help. It is certianly worth a try because sometimes you get lucky. Also, be sure that you replace the air filter since that is probably what killed the MAF in the first place.

 

[puter]

If the MAF is so bad that it is having a significant effect on the power cleaning it probably won't help. It is certianly worth a try because sometimes you get lucky. Also, be sure that you replace the air filter since that is probably what killed the MAF in the first place.

There's been a lot of evidence lately about oxidation being a problem IMO. I've seen several come through that were going into limp mode due to overboost and cleaning the oxidation off fixed it. You definitely know a lot more about these cars than me tho, so I wouldn't be surprised if I were wrong

I didn't even think about the fact that the filter probably killed the maf if it's the sensor that is bad, good to know.

 

[Rick Blalock]

If the MAF is so bad that it is having a significant effect on the power cleaning it probably won't help. It is certianly worth a try because sometimes you get lucky. Also, be sure that you replace the air filter since that is probably what killed the MAF in the first place.

Just ordered a new MAF from Advance Auto Parts. It was $111 but I won't have to wait around for it.. it'll be here tomorrow. And it comes with a lifetime warranty

The car really performs waaaayyy better with the MAF unplugged!! I still cannot believe how much more power it has, I can run my AC now without being bogged down significantly. I did replace the air filter and cleaned the snow screen yesterday. The air filter wasn't too bad but was probably due, and the snow screen was completely caked over with bugs and dusts...

I'll post the results of the new MAF tomorrow

 

[Matthew_S]

There's been a lot of evidence lately about oxidation being a problem IMO. I've seen several come through that were going into limp mode due to overboost and cleaning the oxidation off fixed it. You definitely know a lot more about these cars than me tho, so I wouldn't be surprised if I were wrong

I didn't even think about the fact that the filter probably killed the maf if it's the sensor that is bad, good to know.

I've never seen MAF problems on these cars caused by oxidation on the connector. I know many have reported having it happen though. Personally I think that the MAF problems are greatly exaggerated. There are so many posts here where someone threw a MAF at the problem with no effect. I'd bet half the MAF sensors that are replaced are perfectly good. The last 3 or four MAF sensors I replaced were 8-9 years old and had well over 150k miles on them. My MAF ('02 ALH w/90k) is original and survived at least 10k with the K&N that was in the car when I bought it.

In my experience (w/VW and Nissan mostly) the number one killer of hot film MAF sensors is the air filter.

Friends don't let friends let Monkey Lube change their air filter...

 

[puter]

I dunno, When I first got my car the maf went out the next day (the dealership tried to claim they didn't know...They also told me they had checked it before I bought it...I had asked them specifically about it). I replaced the maf and no more limp mode. month or two ago my car went into limp mode, cleaned the contacts and limp mode went away. I have a suspicion the old maf I replaced just had dirty contacts....so I have kept it in case this one goes bad.

Obviously one car is not a good example. But I do agree that I have seen a lot of people throw a maf it it without fixing the problem, usually I tell them not to and they do it anyway. I always recommend driving with the maf unplugged and see if it goes away.

imo the maf is a common cause of limp mode and power problems, and is also the easiest thing to rule out since all you have to do is unplug it and see what happens. That's why I always point them in that direction first, it can save a lot of money and time to at least rule out the maf, much easier than replacing or checking vac lines etc.

 

[Rick Blalock]

imo the maf is a common cause of limp mode and power problems, and is also the easiest thing to rule out since all you have to do is unplug it and see what happens. That's why I always point them in that direction first, it can save a lot of money and time to at least rule out the maf, much easier than replacing or checking vac lines etc.

Dang, I wish I would have found this more prominent in all my searches for P1556, limp mode, power problems, etc etc. Most of the threads say check vac lines, VNT, etc waaay before checking the MAF. I can't even remember how many hours I spent tryin to pull off old vac lines and connecting my mity vac to check for leaks... and all the frustration that went with it!!!

Well hopefully people will find this thread in the future should they have the same problem...

 

[Canadian_Grizzly]

Rick glad you found the fix... I put together the writeup based on the causes that were most predominant on the forum.
I will add this of things to check when trying to diagnose limp mode... as well as link this thread as an example.
Cheers,
Byron

 

[puter]

Dang, I wish I would have found this more prominent in all my searches for P1556, limp mode, power problems, etc etc. Most of the threads say check vac lines, VNT, etc waaay before checking the MAF. I can't even remember how many hours I spent tryin to pull off old vac lines and connecting my mity vac to check for leaks... and all the frustration that went with it!!!

Well hopefully people will find this thread in the future should they have the same problem...

Matthew_S is probably right about the most common cause, he has a lot more experience than me.

I just figure that the MAF is way easier to check than vac lines etc. and it is a fairly common cause.

*shrug*

 

[Rick Blalock]

Just put the new MAF on and cleared the codes... OMG even more power!! I can't believe it, its never felt this peppy, its like drivin a sports car hahaha!

Thank you guys all for the help, if you were local I'd definitely send ya'll a six pack!

Rick

 

[puter]

Not a problem, I'm glad it was an easy fix. Woulda sucked if you had to really tear into it.

I like Fat Tire

 

[epp]

I am also getting a P1556 code with loss of power. I unplugged my MAF but don't see any real difference in how the car is driving. I also noticed that the engine is not shutting down as smoothly as it should. I started checking vacuum lines and noticed a line off from the tee connected to the N18 to the N239. I replaced this line and also the line from the N239 to the antishudder valve. I am still getting the same code with loss of power and I don't see the antishudder valve move when I shut off the engine. I can move the antishudder valve manually or with my mityvac. I can also move the VNT actuator with my mityvac from the hose on the N75 (Ithink my vnt vanes and actuatur or ok). I'm suspecting that I may have more of a vacuum problem than a MAF problem. The closest size vacuum line that I can get locally is 5/32. Would it be alright to use this size for all the lines until I can order the correct sizes from mcmaster?

 

[Matthew_S]

You definitely have a vacuum leak and it may not necessarily be the hoses. Vacuum leaks can be caused by a sticky N18 or N75, a cracked vacuum pipe to the booster, leaking vacuum pump among other things. Use your mityvac to pull vacuum on the system then go around with a pair of needle nose pliers pinching off different sections of the vacuum system until it holds vacuum. By doing this you will be able to narrow down the source of the leak.

 

[epp]

Thanks for the great suggestion. Is there any place in particular that would be best to attach the mityvac?

 

[Matthew_S]

Pull the hose off the vacuum ball and attach it there. Make sure the vacuum ball holds vacuum too.

 

[epp]

Thanks. I'll give that a try this weekend.

 

[SS930]

MAF or not?

I was driving down the road last night and all the sudden my 99.5 Jetta seemed to have no power. I replaced the MAF with another I had sitting around (which I thought it was okay, but I'm not 100% sure of it) and this didn't help. I disconnected the MAF to drive to work today to see if that would help, but it still has not power. Does this mean the MAF is BAD or GOOD?

The MAF I took out of the car has about 150k miles on it. The other (which I thought was possibly good) looks very new, but I have no idea about it. The car has 317,250 miles on it.

I don't have a VAG/computer to read codes.

Any thoughts?

 

[puter]

You will need to have the codes read.

See my sig for a link on troubleshooting low power/limp

Matt:: I'm pretty sure that vac wont hold at the line from the res. If I remember correctly we tested it this way and it did not hold (slow drop) but I did not have a leak either.

 

[SS930]

You will need to have the codes read.

See my sig for a link on troubleshooting low power/limp

Matt:: I'm pretty sure that vac wont hold at the line from the res. If I remember correctly we tested it this way and it did not hold (slow drop) but I did not have a leak either.

Thanks. The problem is not resetting after I turn the key off, so I'm assuming this is not an over boost situation. Also, when I noticed the car acting funny and starting to lose power, I was just driving down the highway without really laying my foot into it... So again I would question it over boosting.

Since it's not resetting with the ignition, I'm assuming it's a bad MAF issue. Based on this quote in your link, it sounds like mine is bad...

"Bad maf
One of the easiest to check...but you will get a CEL. All you have to do is disconnect the MAF and give it a test drive. If power is same or improved, replace."

 

[puter]

Bad maf usually causes limp mode.

if your problem is not being resolved by restarting the car then i would be suspicious of the vac lines or turbo more so than a maf.

 

[SS930]

I sprayed some brake cleaner on the MAF and the car runs much better now. I don't know how long this will keep it running well since it has about 168,000 on it... although I think I put the MB MAF in it last time around, so maybe their stuff last longer than the VW MAF.

 

[puter]

They have actual maf cleaning fluid, I have no idea what break cleaning fluid would do to the maf element.

I also recommend cleaning the contacts with some de-oxit.

 

[SS930]

I think MAF cleaning fluid = non-chlorinated brake cleaner. Either way, it made a big difference.