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Good evening all, just wanted to relay my recent experience with a wastegate issue.

I was driving home last night. I was on the highway and cruising between 65 and 70. I noticed that at 65 mph, even on the downward inclines, I was running a minimum of 5 psi of boost. Thats way to much for a downhill. Accelerating from 65 up to 70 mph had the gauge pegged at 10 psi. maintaining 70 mph was giving me about 7-8 psi on the gauge.

I've always had very strong boost from my GM3. It's never, ever gone over 13 psi, and that was a result of really getting on the pedal, uphill on the interstate with a full load. I usually can top out at 10 psi in 4th gear under any driving condition. Checking the wastegate, it was always pretty hard to move by hand, which I assumed was just because of the spring pot holding it closed. I usually cruise at about 4 psi at 70 mph, so last night I knew something funky was going on.

This morning I removed the spring pot thing from the turbo, and noticed that the wastegate arm was very difficult to move to the open position, and it was sticky no matter what position it was in. So I sprayed the wastegate shaft down with penetrating oil, which freed it up nicely. I placed a few drops of engine oil around where the arm shaft enters the turbo body, which made it nice and smooth. After reinstalling the pot, it is now possible to move the arm by hand with a little but of force.

The truck sure does run better now that the wastegate is free and working like it should. What I did this morning didn't really affect my power output much, but I will say that the truck feels "lighter" on acceleration. I'm guessing thats because the engine doesn't have to deal with the massive backpressure from the high boost and a badly flowing turbo.

As far as boost numbers, I top out about 6-6.5 psi, and the boost starts to drop off at about 2200 RPM. I now cruise at 2 psi at 50mph, which is much better than before.

So the moral of the story is, if you're experiencing an overboost condition, check your wastegate and make sure it's opening freely.
 

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Possible that cleaning debris out of the arm's bushing by solvents and lubricants will help. Another possibility we run into during rebuild is the bushing has too much wear that the wastegate arm run through requiring a new bushing to be machined into the housing. The loose shaft will bind up, seen plenty of turbos with this problem, and of course a lot of the GM1-4's with the issue, the GM5 is especially bad about it and why it was updated to the GM8.
 
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