akghound

I need help and advise. I just re-rebuilt my Cummins, with less then 6500 miles on a complete rebuilt I managed to over heat the exhaust and melt a piston (#4). Running an 11 plate. As a result of this I tore it down again and honed out the cylinders, replaced #4 piston and the rings on all the pistons. Then trouble strikes with a fouled injector which I replaced, on the test run the exhaust temps were still out of control but I kept them under 1300*. About 30 miles after the startup I lunched the turbo. The shaft broke between the turbine and blower. I could not find any debree or anything involved inside the turbo to cause it to jam. the blower fins are not damaged but the exhaust fines are toast from spinning after the shaft broke. I sort of expected trouble with this turbo after the heat it had been through and happened to have another one on hand. I installed it yesterday and realized that the waste gate was hanging open so today I adjusted that and took a test drive. About 5 miles into the drive I broke the new one in the same way, the shaft let go. I have not taken it off the truck yet, so I don't know what it looks like inside. It never went above 30psi boost, 1200* egt. or 2700 engine rpms. Do any of you have any ideas on what is up?
Thanks .... Ken Gardner.

infidel

Maybe just plain ol' bad luck?

wilderness

What is your timing set at. Are you running wvo through this truck too?

akghound

Timing is set at factory setting, can't remember what that is. I haven't run any Veg oil since the rebuild.
I just plan HATE "Bad Luck"

A couple more questions;
What is the max boost this engine should require with the #11 plate, stock intake and 3.5 straight pipe exhaust?

Can excessive boost actually raise exhaust temps?

When the first one failed I had hit 34psi for about 2-3 minutes. What would the combination of high boost pressure and elevated exhaust temps do?
Thanks ...Ken

cumminsdriver635

These turbo's are pretty tough. Mine got worn so I replaced it, but you shouldnt break one like that so easily. I would pull the oil tube off the top of the turbo, and put it in a bottle. Start the truck just long enough for oil pressure to start coming up, and shut it down. Make sure it is pumping oil into the turbo. I dont see a turbo lasting even a few minutes if it wasnt getting oil, but it wouldnt hurt to be sure. 35 psi is not too much at all on these turbo's. I have seen them run 50 and 60 psi for a while, though I wouldnt reccomend that at all. You do need to figure out why your EGT's are running high if they are. With just a plate in mine, I was hard pressed to hit 1300* Check the governor arm in the pump, and be sure it is hitting the plate in the correct spot

Eric

v8440

Ditto on checking the turbo oil supply. Could be a clogged supply hose or something plugged up with machining dust in the block.

akghound

Great, you guys are getting me thinking. The oil supply is working OK, plenty of pressure, infact due to the stock oil sender failing I have a temperary oil gauge on the supply side of the turbo. After the turbo fails engine oil is pumped into the intake side of things, tons of smoke!
Is it possable to "Over Boost"? It seems to me that the more Boost Pressure there is the more the Exhaust would be restricted, due to the increased load on the turbo, meaning greater exhaust pressures.
If this is possable, is there a point where the exhaust manifold pressure could be greater then the intake manifold pressure?
If so then the exhause gasses would be forced into the intake manifold during the moments that the valves overlap, open at the same time. Now if this is the case, hot exhaust and new boost air would be pushed into the cylinder causing the cylinder to over heat, thus higher and higher exhaust gas temps.
Is this possable?
What is the max REQUIRED boost psi with a #11 plate, would it be more then what a stock setup would require?
Is it wise to run higher then the required boost pressures?
Ken Gardner

v8440

Having the exhaust manifold at higher pressure than the intake is standard operating procedure under any real boost. This is accounted for in the cam profile. We're not suggesting the engine has low oil pressure overall, what some of us are talking about is whether or not the oil supply hose to the turbo itself could be blocked.

akghound

The oil supply to the turbo is fine, lots of flow and pressure.
While talking to another Dodge owner it was brought up that suddenly letting off the throttle at high boost and RPMS can snap the shaft. How likely is this? In both cases I did suddenly let off the throttle.
Ken

infidel

This is entirely possible, I've seen it happen several times. It's called barking the turbo, a search here will tell you more.
Pressurized air in the intake system has nowhere to go but backwards when you let off the throttle rapidly, worse at high boost.

My understanding is it won't cause the turbo to rotate in reverse but will cause it to come to an instant complete stop. Besides snapping the shaft I've seen turbo barking cause the lid to blow off the stock airbox bending the latch clips in the process and blow a BHAF right off the end of the intake tube.

The way to bark the turbo the worst is by tapping the brakes with the cruise control on while at high boost. It seems to let off the throttle faster than if you you just ease up with your foot.