Low boost on stock A4

[youloze]

Hey all, I'm having a low boost issue on a 2002 A4, completely stock. I've already performed multiple boost leak tests and have found not a single leak (surprised). I also found that my lines from the breather tube were all filled with crap and actually clogged completely so I replaced everything including the breather pipe and ran another boost leak test and still no leaks. Pretty sure the pancake valve is good since I can take the oil cap off and oil dipstick out without issue. I haven't actually properly tested the diverter valve yet, I did take the line off that connects under the plenum to the solenoid and sucked on it and put my tongue on it. It held vacuum. I plan to actually take the DV off this weekend and properly test it though.

The most boost I will see in any gear and any load is 6'ish and the car is very sluggish. I'll usually hover around 5 though and even lower at times in higher gears. I've pulled codes and there is not a single code stored. The only thing I can think of is the DV is bad. Any other thoughts as to what could be wrong? I've read that limp mode will put you at about 5psi but I dont have any codes either. I also have seen in my research that there is a hose you can disconnect from the N75 valve that will bypass the controller and run strictly off of wastegate pressure. Haven't tried that yet either. Can someone direct me as to which line I remove and do I have to cap the line or the port at all?

I've owned a B6 previously and I dont remember it being nearly this sluggish although to be fair it would actually hit full boost.

Thanks!

[FrankenA4]

I'm fairly certain if you just unplug (don't remove hoses) the N75 valve then it should pull straight off wastegate pressure. Otherwise, it's more than likely a vacuum leak of some sorts. Even though you did a boost leak test doesn't mean it's not there. Test your DV as you have mentioned, test the N75 valve, and do whatever you can to improve on your boost leak test to attempt to find something.

Something I do is hook up my scanner and pull Short Term Fuel Trim values. Idle the car, and lightly spray brake cleaner over all vacuum lines, any differentiation in value will pinpoint your trouble spot. Sometimes vacuum leaks aren't large enough to be obvious to us, just to the car. Troubleshooting is the most difficult part.

Good Luck!

[youloze]

Thanks FrankenA4. I thought I've read that if you just unplug the N75 that you will get no boost at all? Maybe it was bad info. I'll try it out and see what happens. I'll properly test the DV this weekend as well. Good thinking with the Short Term Fule Trim and brake cleaner. I'll give that a shot as well. Although I would think the leak would have to be significant for me to be missing 3-6psi. But who knows lol.

Thanks!

[youloze]

Well I'm bummed. I took the diverter valve off and tested it and it holds pressure perfectly. I guess I'll try unplugging the N75 tomorrow to see if anything changes.

[CardFan]

My MAF sensor went bad recently, put me into limp mode with no codes. If you have vag-com (or access to one) you can check to see if the readings are in the proper range or not.

[youloze]

My MAF sensor went bad recently, put me into limp mode with no codes. If you have vag-com (or access to one) you can check to see if the readings are in the proper range or not.

Thank you. Do you know what the proper range is by chance? I don't have vag-com but I do have means to log another way.

[CardFan]

Some info here and here.

[youloze]

Thank you! I'll check those out.

[youloze]

So I thought that I could log MAF readings with the Torque app but I guess not. At any rate I cleaned the MAF last night with MAF cleaner and still only seeing 5-6psi :-(

[texasboy21]

So I thought that I could log MAF readings with the Torque app but I guess not. At any rate I cleaned the MAF last night with MAF cleaner and still only seeing 5-6psi :-(

5psi is limp mode. Did you scan the ECU using an Audi specific tool, or a basic scanner? Scan and ideally log the car with Vag Com.

If this is absolutely out of the question, disconnect the battery for a few minutes. This will reset the ECU and delete any codes. If your boost returns, you will need to find the fault once you hit limp again.

[youloze]

I can hit as high as 7 psi though so I'm not sure that it is actually in limp mode. I scan using the Torque app and the Carista app, both of which are very good and have always picked up the correct codes when I have had a check engine light. Unfortunately I do not have a VagCom and do not have access to one so that is not an option :-( I'll try disconnecting to battery to reset the ECU and report back.

Thanks

[texasboy21]

I can hit as high as 7 psi though so I'm not sure that it is actually in limp mode. I scan using the Torque app and the Carista app, both of which are very good and have always picked up the correct codes when I have had a check engine light. Unfortunately I do not have a VagCom and do not have access to one so that is not an option :-( I'll try disconnecting to battery to reset the ECU and report back.

Thanks

Might want to try the boost leak test again. How high did you pressurize the system? You will want to take the reading from the car, not the compressor.

[youloze]

I've done a test twice. I pressurized to 15psi (using compressor gauge). I dont have an actual boost gauge, using an app to pull the boost numbers. I am going to pick up a cheap autometer gauge today so I can make sure the numbers I'm seeing are actual. I'm pretty sure they are accurate though because the car is extremely slow.

[SaabtoAudi]

I've done a test twice. I pressurized to 15psi (using compressor gauge). I dont have an actual boost gauge, using an app to pull the boost numbers. I am going to pick up a cheap autometer gauge today so I can make sure the numbers I'm seeing are actual. I'm pretty sure they are accurate though because the car is extremely slow.

It is a stock 1.8t trying to push a heavy car, did you expect it to be fast?

[youloze]

Thanks for the sarcasm. I've owned a B6 A4 in the past and it was not this slow. I know they are not "fast" but something is wrong and I'm trying to figure it out. Thanks for the help << that was sarcasm too.

[diagnosticator]

Replace the crankcase pressure regulator valve. (Pancake valve,) Besides getting plugged with sludge, the diaphragm inside gets stiff over time from heat and aging. Even if you clean it out, it won't function properly with a stiff heat aged diaphragm. I don't necessarily think the CPRV is the cause of the low boost, but degradation of all the various system parts adds up to overall sub-optimal operation.

The ECU uses the MAF meter output signal, and the output from the Manifold Absolute Pressure sensor and the boost pressure sensor in the intercooler to measure boost pressure. Check the MAP and Boost Pressure sensors for clogging with sludge.

[youloze]

Thank you diagnosticator. I noticed when I was cleaning out the PRV that the date code on it was '02 so it's definitely original. I'll buy a new one and swap them out. I will also check the noted sensors for grime and clogging.

Thanks!

[jakeR88]

I hope you can find out the problem, I'm having the same issue.

[youloze]

I hope you can find out the problem, I'm having the same issue.

Ugh that sucks dude. I'll report if I figure it out if you promise the same :-D. BTW can you list the things you've done so far to try to fix it?

[jakeR88]

FML!!! Changed most if not all my vacume hoes, new dv sprayed the crap out of the engine bay with carb cleaner to see if the car would stall (it didn't), cleaned mass air and added a boost gauge to check psi and I'm still at 5 psi!!!

[youloze]

FML!!! Changed most if not all my vacume hoes, new dv sprayed the crap out of the engine bay with carb cleaner to see if the car would stall (it didn't), cleaned mass air and added a boost gauge to check psi and I'm still at 5 psi!!!

LOL sounds like we could be twins. Are you hitting close to 5 psi at all times or can you reach 6-7 psi at times? I range from 4-6 psi with the occasional 7 psi. I'm going to hook up a boost gauge some time this week to make sure of my actual boost but I believe the numbers I'm seeing based on how weak the car feels.

[jakeR88]

I'm hitting 4/5

[youloze]

Yuck, you're worse off than me. I'm going to borrow my friends test boost gauge and hook it up in the next couple of days to get my actual boost numbers and report back.

[Northern_B6]

Quote from this thread: http://www.audizine.com/forum/showth...derboost-Cause

I've never heard of removing the o2 sensor to see how much boost changes, but it does make sense. If the cat is plugged you are creating a lot of excessive back-pressure that will restrict the amount of boost you can run. Opening up a small hole will at least allow SOME air to flow.

They need to check the pressure levels pre and post cat to be 100% certain. Or just take it off and look. But even if it doesn't solve your problem, a clogged cat is a clogged cat...get rid of it.

Might be of some help.

[youloze]

Hmm, I never even thought of a clogged cat. It's definitely something to consider, although with only 120k on my car I would think that may be premature BUT a lot would depend on how the car was driven prior to me owning it. Lots of stop and go, in city driving would clog it faster. Maybe I'll take the o2 out and see if I gain anything.

Thanks!

[jakeR88]

My car does get hot real fast under the hood much sooner then a car should and a friend said that he thinks it's clogged, I ordered a test pipe to see if it works.

[youloze]

My car does get hot real fast under the hood much sooner then a car should and a friend said that he thinks it's clogged, I ordered a test pipe to see if it works.

Good luck, hope it works out. The trouble with replacing it with a test pipe is you are going to gain power out of it because of the lack of cat so the results may be skewed. So make sure to log boost before and after and post your results!

[youloze]

Quick update on my situation. I borrowed my friends extra boost gauge that we use for testing and I hooked it up last night. Before that I also cleared the OBD2 using my Torque app and disconnected the battery for good measure. I'm happy to say I am now spiking at about 11-11.5 psi and generally holding 9-10 psi throughout. The car also now feels like it should.

So in short for JakeR88 and anyone else in the same boat even though you may not have any codes try resetting the ECU and disconnect the battery to see if it helps any. Actually worked for me.

I'm also going to replace the pancake valve since its original.