boost, no boost

[DF5152]

see sig for truck info as fars as milage just hit about 14k. the truck only gets used for long trips or when i need to move something. so here is my problem recently went away on vacation, no trailer on back. drove about 800 miles south no problem. on the way home about 200 miles into the trip i noticed my boost would drop off to zero, cruizing at 75mph. it usually sits at 10psi. back off the gas and hit it again and bosst would come back up for a little while then disapear. truck seems to run fine the whole time. now what really got me... slowing for a toll at ezpass pulling through about 15-20mph i hit the gas and the truck lurches forward I look down boost is at 20+ and we are off to the races. my foot wasnt to the floor. It did it a few more time during the trip no major problems. now checking the waste gate should i be able to move it by hand cause i cant seem to. boost doesnt seem to come on till about 2000 rpm plus or minus a few max boost is still about 22 or 23 on an autometer 60 psi guage. any ideas???

 

[JCleary]

Perhaps you have a leak in your air line for the boost gauge?

 

[DF5152]

I checked that couldnt find one though about a faulty guage but the toll booth thing got me a little more concerned.

 

[Tomeygun]

when the boost drops, do you get a surge of black smoke?



In other words, is the fuel still there when the boost drops, or does the boost drop BECAUSE of the reduced fuel?

 

[nickleinonen]

my foot doesn't need to be on the floor to make over 20psi boost. . i can get that at between 1/4 and 1/2 throttle... with no load real load, there will be low boost... but going 75mph, there should be boost... what was the egt reading when the boost was at 0?



you can move the WG by hand if you are superman... the spring in them is pretty strong [needs to keep closed against 20+ psi with a larger than 1 sq inch area, so that spring tension might be close to 50lbs]

 

[DF5152]

egt about 600-650. what rpm do you start seeing boost?



didnt notice any black smoke but then i wasnt looking.

 

[rbattelle]

Mine is like Nick's... I can make boost at pretty much any RPM and throttle position under load. As soon as the engine loads up, the boost starts going up.



When I first read your post I thought "leak in the turbo-to-manifold plumbing", assuming the gauge is good. But the sudden jump to 20psi is weird. Could it be a leak at the exhaust manifold venting turbine energy out to atmosphere? A tiny hairline crack might have sealed when the manifold heated up and expanded during your slow-speed at the toll so when you hit the gas on the other side you had full drive pressure at the turbine and away you went!



I'm just throwing out things to check. A hairline crack in the exhaust plumbing could be seen as a dark black soot mark. Leaks in the air system would be more difficult to find... check the boots.



-Ryan

 

[DF5152]

checked all the boost connections no luck. could my lift pump be going bad and not supplying enough fuel to make boost?

 

[nickleinonen]

what rpm do you start seeing boost?

i start seeing boost from about 1200 rpm, but you really feel the engine start to fuel at 1500rpm and boost will get to 20psi in an instant there.



i also know my engine will make 10psi boost no load sitting on the governor... [found that out when checking oil pressure]

 

[betterthanstock]

DF5152 I think you need to exercise your wastegate regularly by stepping on it until it opens, at least once a week. Otherwise carbon builds up and can cause it to get sticky. Don't try it by hand, it will fix itself, just drive it like Cummins designed it to be driven!