Low Power, Low Boost, Black Smoke

jok · July 15, 2015, 04:22:34 PM

My 1990 Prevost with 8V92 has gradually been loosing Turbo Boost. We just returned from a trip from Michigan to Georgia and back through NC, VA and WV. From a stop it takes forever to get up to speed, with plenty of black smoke and 4-5psi of turbo boost, even on flat roads. Rarely I can get 9psi, depending on speed and grade. Mountains/hills are very slow unless I hit the hill at 70mph. If nothing gets in my way I can cruise up the hill at close to 70. If I have to slow down or hit the hill at a slow speed I just have to stay in the slow lane with the heavy trucks. Once it gets up to 70 mph I have good power. I just installed a new radiator (easier job than I imagined) and overheating is not an issue. I have no DDEC codes. I pulled the exhaust and intake tube at the turbo and there is no slop or drag. I don't seem to have an exhaust leak since the engine compartment is staying clean.
I have seen in the archives it could be "Turbo Boost Sensor", or "Blower Bypass". Not sure where they are.
Any ideas?
Thanks,
John

stevet903 · July 15, 2015, 04:46:38 PM

You should be seeing boost in the low 20's loaded at full throttle. The boost sensor has a hose on it that could be cracked or disconnected.

Here's a picture:

http://miscpartsmanuals2.tpub.com/TM-9-2320-360-20-2/TM-9-2320-360-20-20306.htm

I would recheck for an exhaust leak. The manifold gasket between the head and manifold will blow out and you can see a soot mark where it is leaking. I have also had the exhaust pipe crack at the v-band clamp close to the turbo. Exhaust leaks before the turbo = low boost.

robertglines1 · July 15, 2015, 05:13:04 PM

look under manifold for exhaust leak. 2 times to me on 8-V92

Jon · July 16, 2015, 05:33:43 AM

8V92 exhaust manifolds usually crack at the ears, they have exhaust gasket failures and sometimes the pipes from the manifolds to the turbo crack and eventually fail. Look for soot. That tells you where your turbo boost has gone.

opus · July 16, 2015, 10:24:45 AM

Air filter?

Jon · July 16, 2015, 02:19:24 PM

I would put the filter last on my list of things to look for just from past experience which included everything on my list at one time or another.

Darkspeed · July 16, 2015, 02:27:53 PM

Sounds like a failure of the waste gate (exhaust side) or relief valve (air side).

luvrbus · July 16, 2015, 03:41:20 PM

Quote from: Darkspeed on July 16, 2015, 02:27:53 PM
Sounds like a failure of the waste gate (exhaust side) or relief valve (air side).

They don't have a waste gate or a relief valve on a 8V92 he should be flashing a code if the turbo sensor is bad I would bet on a exhaust leak or the air filter

jok · September 18, 2015, 05:09:02 PM

Thanks for the replies. I finally had time to pull the exhaust manifolds. I found that I did indeed have exhaust leaks at the gaskets on both sides. Additionally I found that an end was broken off at the transmission end of the driver side manifold. It was still in place but leaking. I bought new gaskets from NAPA, which did not hold up more than a few miles. I then bought some from Detroit Diesel and they work great. I also replaced the air filter. The boost didn't get up to the 20's as some have mentioned (made it up to 17 psi from 7-9 before). But the power much better.