Slight surge under hard acceleration

[KMinAF]

Under hard acceleration with 7lbs boost (stock turbo, K&N, 3" pipe, 2kmiles on engine) I am experiencing a slight surge at rpms above 4k.

There are no air leaks in the intake tracts and it is not a missfire. I am thinking fuel delivery maybe a faulty/dirty injector.

Any insight or ideas would be appreciated.

 

[hellraiser456]

compressor surge is almost always caused by a faulty BOV. this happened becuase the BOV is unable to release the boost pressure when the throttle plate closes. this causes the pressure to come back to the turbo instead and harshly disrupts the compressor wheel speed thus causing the noise. closing the throttle will be the only time you will experience compressor surge(when your intake goes into vacuum).

What kind of bov do you have? are you using the stock ct-26?

i doubt a fuel problem would cause compressor surge. and if it was you would notice it in the drivablity of the engine before you noticed it in the turbo.

I would insure that you have no post throttle body intake leaks. i would also make sure that your BOV has a good vacuum source. These are usually the causes of BOV problems. Also i would make sure that your BOV is working properly as well.

 

[SMP142]

are you talking about compressor surge? or are you saying your boost slowly drops as your rpms increase?

 

[KMinAF]

The symptoms occur under hard acceleration (3/4 to wot) and at 7PSI which is where the MBCV is set. Boost pressure is steady and begins to drop off after engine load decreases.

I am thinking weak or faulty injector. Is there a way to check them short of removing them and having them serviced?

Commpressor surge wouldn't apply here because that only happens when the throttle is suddenly lifted i.e. to change gears causing a reverse pressure against the turbo. The symptoms as mentioned only happen under steady acceleration.

 

[SMP142]

are you still running the stock downpipe and stock exhaust? if you are then it sounds pretty normal to me. basically what is happeneing is that you are getting close to equal backpressure on your turbo. if there is nowhere for the exhaust to go, it will just build backpressure. that it why it only does it after the engine load decreases. during a load the pressure is alot more on the manifold side. or, it could be the diaphram on your wastegate is going bad and not holding your wastegate at the right spot. since you have a boost controller, you might be able to try adjusting the arm on the diaphram. if you can tighten it up some and see if it helps any.

 

[titus]

i would bet:faulty bov!

 

[KMinAF]

Titus, Why do you think faulty BOV?

SMP142, the only stock piece on the exhaust is the elbow. Is there a way to verify that the wastegate is operating at the correct pressure?
PS Thanks for your service in the AF it is greatly appreciated

 

[SMP142]

first i would just see if you can move the diaphram with your hand via moving th rod or arm back and forth. it should have good tension. your running a boost controller on a stock turbo right? well the stock boost pressure should be in the 7psi range with the stock dp. so with a dp and exhaust you should be in the 9-10 range. mine ran at 10 with the new dp and a test pipe. i am not sure what you feel comfortable with, but you could alway take out the boost controller and see what it runs at. or open it all the way. but if your running stock hg and other things i wouldnt recommend it. i hit fuel cut with a stock hg once and i got kinda scared it was blown lol. but it held!

bov could be a factor, what kind is it?

 

[KMinAF]

BOV is stock and when I checked it by trying to blow through it is was tight with no appearant leaks. The turbo is the stock unit with 125k miles on it. No smoke and no oil leaks. Head gasket is Cometic with factory bolts torqued to 75lbs.

What RPM did you achieve 9-10 PSI? Now I am concerned because with the 3" pipe I have never gotten that high except when my previous MBCV went bad and statrted to spike and cause the system to hit fuel cut, (a feeling that always is un-nerving). I am now using a Dr Jonez MBCV and the boost comes on nice and smooth.

 

[SMP142]

i was hitting 9-10 early. 3k maybe. its been awhile so i dont remember exactly. have you ever ran without the mbcv hooked up on that exhaust? so you hit 4k and your sitting at 7psi. then does it slowly bleed off to nothing or does it just drop down to like 5 or so?

 

[KMinAF]

Slowly drops down to 4 - 5 psi. After rebuilding the engine I did run it for some time without an MBCV and it would max out at 5psi

 

[SMP142]

maxing out at 5 seems kinda low. i would check out the diaphram. adjusting the linkage on the diaphram might help. you could try "shimming" the wastegate also if that doesnt change things. just be careful if you do that. it just takes a little bit to tighten things up.