Need help… no boost

[Slow998]

Hey guys, I’m beating my head off a wall trying to figure out a low boost issue. Here’s what’s going on…. I took my sled to the lake a week ago and it ran fine. I’m running it with a TD header tune, up to 315hp. At that time it was running 19.4 pounds of boost. The lake was slushy and I would spin and grab and ended up blowing a belt on a WOT pull around 133mph, somewhere around 8800 rpm. The belt disintegrated into 1000 pieces. Put a new belt on and kept playing for a few passes longer. Next time out, noticed that I could only see 17.8-17.9 pounds of boost, but played with clutching to see up to 9200 rpm and would run 138 mph gps. So, I brought it home and checked for boost leaks… which is where I’m stuck now. My boost leak tester will come to just shy of 18 pounds and you can hear the air flowing out the exhaust. When I remove the air pressure, it holds 15+ pounds for a minute or longer. At first, I figured I had an exhaust valve not seating, so I continued to pull the valve cover off and check valve clearances, which are all perfectly in spec. I pulled the header off, and the first few times could hear the air coming through cylinder one. Had a helping hand roll the clutch over and the air would change to cylinder 2, then 3, but always only at 17.9 pounds. Compression is 150 pounds across all three holes…. What am I missing? If it was a valve problem it would leak all the time and wouldn’t have 150 pounds of compression across all 3 cylinders…. Same with piston/ring problem… cam timing is all on the mark with all
marks parallel with the top of the head and dots on the cams pointed up.

 

[0degC]

Hey guys, I’m beating my head off a wall trying to figure out a low boost issue. Here’s what’s going on…. I took my sled to the lake a week ago and it ran fine. I’m running it with a TD header tune, up to 315hp. At that time it was running 19.4 pounds of boost. The lake was slushy and I would spin and grab and ended up blowing a belt on a WOT pull around 133mph, somewhere around 8800 rpm. The belt disintegrated into 1000 pieces. Put a new belt on and kept playing for a few passes longer. Next time out, noticed that I could only see 17.8-17.9 pounds of boost, but played with clutching to see up to 9200 rpm and would run 138 mph gps. So, I brought it home and checked for boost leaks… which is where I’m stuck now. My boost leak tester will come to just shy of 18 pounds and you can hear the air flowing out the exhaust. When I remove the air pressure, it holds 15+ pounds for a minute or longer. At first, I figured I had an exhaust valve not seating, so I continued to pull the valve cover off and check valve clearances, which are all perfectly in spec. I pulled the header off, and the first few times could hear the air coming through cylinder one. Had a helping hand roll the clutch over and the air would change to cylinder 2, then 3, but always only at 17.9 pounds. Compression is 150 pounds across all three holes…. What am I missing? If it was a valve problem it would leak all the time and wouldn’t have 150 pounds of compression across all 3 cylinders…. Same with piston/ring problem… cam timing is all on the mark with all
marks parallel with the top of the head and dots on the cams pointed up.

Seems like your sled is working pretty good? In the boost leak test you should be able to see it hold more than 18psi if get engine in correct spot but always a bit leaking it seems at valves, never all closed. Are you running the stock turbo waist-gate? With the header and free flowing muffler you may not see much more that 18lbs of boost with stock waist gate for any length of time even if running the higher boost tunes.

 

[0degC]

It is definitely a concern that your sled ran 19.4 psi before your belt explosion and now under 18psi.
My Hurricane Header tune sled also runs over 19 psi so over 19 is not unheard of with a header and stock wastegate.

View attachment 175893

Yes some will run higher for sure depends on strength of your stock spring. And not sure if different headers make a difference but my only experience is with TD header.

 

[Slow998]

The faster pass is obviously on better ice conditions, but boost is low at 17.8 pounds. The second log is the pass just before I blew the belt. This motor held around 22 pounds, if I remember correctly, last time I checked it

 

[Slow998]

Also, not sure why the throttle position is so choppy on the logs sometime?

 

[sideshowBob]

Looks to me like you have a TPS issue as it should not give an erratic readout once its held wide open...100% should be 100% so as not to send confusing info to the ECU.

 

[Slow998]

Looks to me like you have a TPS issue as it should not give an erratic readout once its held wide open...100% should be 100% so as not to send confusing info to the ECU.

Agreed, but the rpm doesn't fluctuate while riding it and the boost is always steady on the gauge. Makes me wonder if its a communication issue or actually a TPS issue

 

[earthling]

You see a boost spike after every throttle glitch and a small drop in speed after the big one at 143. This seems like its a real issue with the TPS and is likely keeping you from building boost.

 

[Slow998]

You see a boost spike after every throttle glitch and a small drop in speed after the big one at 143. This seems like its a real issue with the TPS and is likely keeping you from building boost.

On the log with low boost, the throttle reading is steady? Right now it shows boost is 17.9-18.1psi, which is exactly where it starts leaking on my boost leak tester.

 

[earthling]

Oh.. you are right.. hmm.. The TPS thing seems to be real at least that there is a correlation between what the sled thinks is happening and what is happening to boost. Can you switch the scale on the left so it is boost instead of mph?

 

[NYTurbo]

Does it idle nice and smooth ?

 

[Slow998]

Does it idle nice and smooth ?

It starts well and idles smooth.... on the log, it doesn't fluctuate more than 70 rpm at idle.

 

[NYTurbo]

Usually when you spit a valve shim it idles a little rough. Blowing a belt at WOT without a valve shim retainer kit in place can throw a shim as rpms sky rocket and float the valves.

 

[Turboflash]

Compression test?

 

[Slow998]

Compression test?

150psi across all 3 cylinders. Cam timing is lined up.