Loss of power on the top end

[srichard]

I posted a question in the Accessories forum, but my question has evolved and i think is now better suited for this forum:



I noticed in the past couple of days that the truck was down on power towards the top of the RPM range.

I had been driving fairly conservatively until last week when i installed the Scotty 5 and wanted to put it through its paces to check for EGT reductions.



What I did notice is this:

1- I can't boost past 38psi. It used to boost well over 40psi before (Quad race box). Also, in 2nd gear, it won't go over 32psi, the boost just builds too slow.



2- the pedal, which used to "feel light" on the 100 and 130hp settings on the box doesn't have that light feel anymore - but maybe that's in my head?



3- the truck feels no different than before (other than the throttle pedal feel) under part throttle, mid range acceleration burts. It's only on a WOT, using all the RPMs that I really notice a big change.



The way I see it, I either have a boost leak, a fuel delivery problem, or maybe the Quad box has a problem.

I know that the latter is very unlikely since there has been almost no reports of actual box failures. So before I start buggin the friendly tech support guys at Quad, I'd like to get some of your opinions, ideas.



Any troubleshooting tips would be greatly appreciated.

thanks in advance.

 

[BigGunZ]

The fact that you lost boost would lead me to first check for leaks.

 

[JCleary]

Quoted from srichard's accessories forum thread:



I am starting to wonder if myabe my fuel pump of injectors could be having problems. I mean, the truck feels like it's running out of steam on the top end, but the low to mid range seems ok.

I put on the box when the truck had about 4000 miles, it's now 2000 miles later, and I wonder if my fuel delivery is just not keeping up.

It's frustrating that I am not sure how to go about trouble shooting it. I'm pretty experienced mechanic when it comes to gas engines, but I am still learning about diesels...



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You may have a problem with the rail pressure relief valve. Check for any seeping from the copper washer on the rail relief valve fitting. There's a procedure outlined in a Cummins manual to check the valve, but I wouldn't feel comfortable doing it.



I've heard you can pressurize the system by using a 4" pvc pipe with a cap and air fitting on it. Hook that with a silicone coupler on the turbo and use REGULATED 20psi shop air to pressurize the system. Use some windex and spray all the hose connections.



You're only supposed to do this for a minute or less, as the turbo is spinning while you're checking for leaks.

 

[srichard]

hmmm, interesting idea about the leak testing. I was wondering about that as the truck is not exactly generating squat for boost while just sitting there running.



With the quad box off, it seems to run just like stock, and boosts up to 32psi fine.

 

[JCleary]

Stock you're running lower fuel pressure. So if the Quad is turned up, I know it commands 27k pressure in the rail (what Quad will admit) or more. I have no way to verify one way or the other.



Anyway, your valve could be opening at that pressure, lowering your power output.