My 20b once reached full boost at 4k....now I cant get full boost (17psi) until 7k'ish in 4th gear. 3rd gear only to about 12 psi. It does boost quickly to 10 psi at 4k still though
I have pressure tested for a boost leak until I'm blue in the face....I have small leaks from the exhaust manifold but nothing that seems major....put in new gaskets (the expensive ones) just to help.
I'm looking for some ideas.Tthe wastegate seems to be working right. I was wondering if the turbo (PT74) needs bearings. I had the bearings replaced by Precision about 2 yrs ago after I twisted my eshaft but one would think they should be OK.
Any ideas?????

 

Check for binding of the turbo by spinning the compressor wheel.

Have you done anything recently with your boost controller and which one do you have? This is the type of thing I see when the duty cycle is turned all the way down.

What wastegate do you have?

 

turbo seems to spin fine.
I have a Greddy Profec-b and a HKS WG. I took the WG apart and the shaft was not sticking...everything looked good. I'm wondering if it is a boost controller problem. I think I am going to put a stronger spring in to see what happens.

Yes, we turned the boost controller up and it is at the highest boost setting and gain. I was trying to run 17 psi on a 8 lb spring. Is that what you mean by turning the duty cycle all the way down?

What do you think? Thanks for the help

 

Duty cycle on the B/C is the gain setting. Is this the old style Greddy with the 3 *****? Which solenoid do you have, large black plastic box or the small square 3 port? If you have the really old profec with the dip switches you may need to change the configuration.

Have you tried switching it over to the other boost setting? High or low depending on the one you are having the issue with currently.

17psi on an 8psi spring is not a big deal, I run 17 on a 10psi with a 60mm WG and have no problems, can easily go up to 20+.

Pressure/leak test the vacuum lines going to the solenoid and to the WG.

 

I have the small black 3 port one.
I rally haven't messed with the low boost setting since I got several different boost leaks fixed. Fixing the boost leaks (mainly cracks on the exhaust manifold) haven't made much difference.
I hear different experiences on springs vs. max boost....some say 10 psi over the spring and others say 2 times the spring...either way, I would be close....and I am running the boost setting at the max. The gain was at max but I turned it down but no better results.
I think I am going with a bigger spring and see if the response is better. I'll double check the lines on the WG system.

 

Cracks in the manifold are exhaust leaks, not boost leaks. Depending on how massive a pre-turbo exhaust leak is it will effect spool up.

A higher rated spring is not going to fix the actual issue. You said that the car used to reach 17psi @ 4K, which means that your current spring was capable of achieving the desired boost level. My guess without seeing your car is that you have an issue with the boost controller or the lines to and from the solenoid.

 

What boost controller would you recommend if I changed it.
Yes, I've had exhaust leaks at the manifold that affected building boost before.
I've had leaks on those lines before. I've used steel braided rubber hose that has melted. I've since wrapped them in insulation. What do you use for lines? What do you think about silicone hose?

 

I use SS braided. The lines are routed too close to the manifold/downpipe if you melted through SS lines. Silicone will melt even quicker. In extremely tight setups we have run solid metal lines to the wastegate.

Since you have had supply line issues before then I would say that the BC probably is not the issue.

 

Like banzai said check your lines.. how is the diaphragm on the wastegate? i had this problem before on my old setup and it was my HKS wg not staying completely shut.

 

I looked at the diaphragm and it looked OK. I tested it and it seemed to move fine. I have tested the lines and they seemed OK. They are wrapped in insulation so I might retest.

 

I pressured up the hose to the top (spring side) of the WG and I could hear some air leaking. I pulled the hose and it had no leaks...I either have a leaking diaphragm or cap. I haven't had a chance to pull it yet. I wouldn't think you could hear a leak on the diaphragm though. Thoughts? I might be able to work on it tomorrow?

 

hitting 7k in 4th gear in a boosted 20b? you're goin really really fast lol

 

Yeah..I don't like doing that on the street but I had to check it out. We have a long bridge that is 3 lanes wide on each side and not much traffic. In fact, late at night there is no traffic. I wouldn't go this fast if there was traffic around...(other than a motorcycle)

 

Just for archive purposes.....The issue was not enough spring for the WG. I increased the pre-load on the WG with the set screw and boom I have full boost again.