Slow to build boost

[Matata]

Hi all,

Need some help diagnosing a boost issue with my 02 A4. The car has a full stg 3 apr kit (gt28 turbo, apr tune, carbonio, apr r1 dv, apr front mount..) with 177K on the original motor and around 40K on the stg 3 kit.

The issue is the car is very slow to make boost. At 3000 rpm I am making around 5-7lbs of boost, and full boost (25lb) does hit until around 4500rpm and above. The other issue is boost used to come on really hard and strong, and now it feels very linear. The car used to start pulling hard at 3K rpm, so I know its making boost much slower than before. Once I hit max boost (25-26lbs) it holds perfectly, so I don't think it is a boost leak. Another issue I have noticed is the vacuum pressure fluctuates around 15-16 at idle with the ac off, and maxes out at 19-20 while decelerating with my foot off the gas. Not sure what it should be at.

Possible culprits:

Bad N75 valve
Bad turbo wastegate
Dying motor

No codes, no burning oil, or smoking. Getting tired for a car that has no low end power.

Help!?

[Fast4esT]

First guess is spark plugs, coil packs or a vacuum leak. Is there any hesitation while it builds boost?
I find with my unitronic 630 tune sometimes I can't get over 15 psi while other times I hit 25 easy, not sure why but assume the computer doesn't like what the software does.
Bad n75 won't let you hit peak boost
Not sure what the wastegate will do if failing but still wouldn't expect full boost at all.
Computer is fusing it back based on a sensor somewhere, check for codes?

[Fast4esT]

Your vacuum levels sound right

[Matata]

Thanks for responding.

Basically if the car is at 3K rpm, and I step on it, it just slowly climbs and wont hit peak boost until it get around 4300-4500. It just feels really flat. How would spark plugs or coil packs have an effect on it?

Ok so I can rule out a bad N75 valve. If I have a vac leak, wouldn't I be getting low vac pressure at idle? No codes.

[diagnosticator]

Did the slow boost show up suddenly or gradually? Perform a compression test to gauge the mechanical condition of the engine.

[EBG 18T]

It sounds like a good time to run a compression test on the motor. I am assuming the motor is all original and stock with 177k miles now?

[lilman05]

It was more gradual but seems to have gotten much worse. I will runs compression test and report back. I feel like it's time for a rebuild...!

[Matata]

Sorry above post with with my other old account. Anyway, quick update. I haven't run a compression test but I drove the car quite a bit today, and it pulls hard once boost is at 25 lbs, and holds perfectly. It doesn't feel like a dying motor (low compression) since once it makes the boost, it goes! After doing lots of searching, a few others had similar symptoms with bad O2 sensors. I need to rescan the car, but does anyone think this could be the case?

[Fast4esT]

What's your mpg doing? O2 sensor is cheap from gen 1 TT at autozone

[diagnosticator]

Sorry above post with with my other old account. Anyway, quick update. I haven't run a compression test but I drove the car quite a bit today, and it pulls hard once boost is at 25 lbs, and holds perfectly. It doesn't feel like a dying motor (low compression) since once it makes the boost, it goes! After doing lots of searching, a few others had similar symptoms with bad O2 sensors. I need to rescan the car, but does anyone think this could be the case?

The static and dynamic compression pressure developed must be above the minimum and is the most important factor for good low RPM torque and turbo spool up performance. At higher boost levels, the boost pressure easily compensates for low compression. Therefore, a compression test is important to determine if low compression is provoking the low RPM torque being weak, and the associated slow turbo spool up. Or, it will show the compression is not the issue and the problem is due to something else.

[old guy]

This comment from your first post leads me to believe you have a Vacuum/Boost related issue somewhere rather than a compression problem " Another issue I have noticed is the vacuum pressure fluctuates around 15-16 at idle with the ac off, and maxes out at 19-20 while decelerating with my foot off the gas." At idle with the A/C off you should be closer to 20 in-Hg. Decelerating with full engine braking should produce close to 24 in-Hg.

Try logging your actual boost against the requested boost. This will tell you if the ECU is holding back boost for something out of wack or if ECU is calling for full boost but the engine just can't get there because of a pressure loss somewhere.

[diagnosticator]

This comment from your first post leads me to believe you have a Vacuum/Boost related issue somewhere rather than a compression problem " Another issue I have noticed is the vacuum pressure fluctuates around 15-16 at idle with the ac off, and maxes out at 19-20 while decelerating with my foot off the gas." At idle with the A/C off you should be closer to 20 in-Hg. Decelerating with full engine braking should produce close to 24 in-Hg.

Try logging your actual boost against the requested boost. This will tell you if the ECU is holding back boost for something out of wack or if ECU is calling for full boost but the engine just can't get there because of a pressure loss somewhere.

If the compression is low, then the vacuum pressure in the intake manifold will be effected as well, resulting from the same piston ring and or valve seat leakage that is causing the low compression.

[quattro guy]

Reviving this thread. I've been having almost identical issues as the OP with boost build up. My current set up is e85 motoza tune running a f21l mixed flow. When i first installed the turbo the car ran great. I wouldn't see boost until i hit 3,200-3,400 rpm, but when i did it would jump from like 2psi where it held steady until i reached the stated rpm all the way to like 23-24 (spike) then come down to a steady 21psi and hold there until fuel cut (rev limiter) within a matter of 1 second. The boost would come so strong that when the road was slightly wet and i would be in 2nd gear, that the traction control would start blinking because i was loosing traction. I was a happy camper no doubt.installed the turbo on 6/14/2016 with 185,805m. Now i have 200,150 and the problem has gotten bad enough to where I'm not happy driving the car anymore because it frustrates me. I do hit full boost at or a bit over 4k but literally takes 3-4 full second and about 1rpm to reach there. Here's the odd thing, on some days it drive perfect. Like i hit full boost at like 2,800-2,900k. I leave it in the same gear and rev up to 3.5k and it goes from -21 to 21psi within 1 second. It hits so hard that my clutch slips. But that's one every blue moon. When its one of those days i feel happy and have a smile.99.9% of the time my Boost creeps. I wanted to see if the OP has found a solution. Thanks everyone.

[quattro guy]

[quattro guy]

This video shows what im talking about. My Boost issue is worst now. Forgot to mention those where @ WOT

[Human Garbage]

Check the wastegate arm to see if it is loose. You can stick your fingers in there (when its cold of course). If there is any fore/aft play, that could be your problem.

My K03 started to do that and it was because the wastegate flapper wasn't sealing and bleeding off boost. It took about as long as your video to hit full boost. I couldn't be bothered to take the turbo off or try and adjust it in situ, so I threw a spring on the wastegate flapper arm to remove the play and help hold it shut. Problem solved... For now. New turbo is my long term solution.

[quattro guy]

Well check this out. Compared to now that builds boost fast. Ill upload a video shortly

[quattro guy]

1st pull was in 2nd gear. From not touching the throttle to WOT in every pull.

2nd and 3rd pull done in 3rd gear.

[Human Garbage]

Yeah check that wastegate. That ain't right at all. It should literally snap up to requested boost. It sounds like its hissing as boost builds too. Mine did that because of the partially open flapper.

[quattro guy]

I looged requested vs actual and im way off. I will defenetly check wastegate. The hissing sound has always been there sice the beggining. Idk why my turbo sounds like if i had a BAT or something its that loud when spooling. I dont maind it but whatbi do mind is the boost build. Ill chime back in and update. I will try and get it done this weekend. Havent had time these days been remodeling my room.