Rebuilt Turbo with Low Boost - 300SD
 

Hi All,

I just rebuilt the turbo in my 1985 300SD and I'm seeing low boost now. It peaks at about 8 psi. (I'm sniffing the boost line at the ALDA with a boost/vacuum gauge.) It spools up right where it should; just a hair under 2,500RPM's but it also sounds like a police siren instead of the jet engine that it used to be. The shop that got the parts for me also checked the balance and declared it good. The rebuild kit included new thrust washers/seals, bearings, and new nuts & bolts. I did not touch the wastegate.

Per Click & Clack, they say if you hear a police chase under your hood, its the sound of a turbo headed for the recycling bin. "The next sound you'll hear is the whooshing of money leaving your wallet..."

My only suspicion at this time is the nut that holds the impeller on the shaft - the guys at the shop said just get it nice and tight, maybe 25 ft-lbs. I got it to about 22 and decided that the little nut had seen quite enough thank-you. I suspect I may have over tightened it which does not allow the turbo enough play to spin up properly - am I off in the weeds? What else could it be?

Thanks!

Travis

If you can hear the turbo at all then there is a problem. You should not be able to hear the turbo over the engine noise. Usually the seals on the U-tube from the air filter are not sealing.

The torque only holds the nut on the shaft, it does not alter clearance.

8psi is 100% normal if you didn't touch the wastegate, its what these cars came from the factory set at.

I'd start by checking the u-tube seals then for air leaks between the compressor outlet and intake manifold. If you don't find anything then take the turbo back apart and start again.

My suspicion is that the turbo is not spinning freely in the bearings. Therefore the speed of the turbo is reduced at any given exhaust volume.

Remove the U-tube and attempt to spin the impeller with your fingers. It won't "spin freely" but it should rotate with just the slightest finger pressure..........almost wants to continue to spin when you remove your finger...........but won't quite do that.

If the capability to rotate is any less than the above..........you've got the bearings too tight for some reason.

With any new bearings and seals there will be a short breaking in period, but you shouldn't hear any noise. If the rotating assembly was balanced right they should have had a mark on the turbine wheel, compressor wheel, and nut that should be lined up. Also how you tighten/loosen the nut makes a difference. On both the turbine wheel and the nut you should not use an ordinary wrench or ratchet as you may bend the shaft. You need a T-bar wrench so that all you are applying is torque and little or no side to side force.

Edit: Also check to make sure you haven't bent a compressor blade. All blades when viewed from the side should come up to an even plane. A bent blade will make a whirring noise too

How do you loosen bearings, Brian? I am picturing a sealed housing with bearings inside of it that should either spin or not straight from the factory. Is that not the kind of bearing in a turbo?

Agreed........they're not adjustable. It might be possible that the bearing is pinched in some fashion and is applying undue constraint to the shaft. I never had one apart, personally, so I'm at a bit of a disadvantage.

Took it for a spin (out to a junkyard with some old Mercedes carcasses BTW) today. It peaks at 9psi of boost which, according to ForcedInduction, is perfectly normal. I would also agree with the other comment that there is a break-in period. It seems to have quieted down a bit, but I will still replace the U-tube seals as one of them is in really bad shape.

I forgot to do a 0-60 time on it today - I'll try that soon. If I remember correctly, that should be in the neighborhood of 16 seconds, no?

Thanks again guys!

Travis

1985 300SD on SVO since 2006

If I really mash the throttle on my straight piped 300SD I can see as high as 11-12 psi of boost....isn't it set at 12 from the factory....but as the waste gate spring weakens over time it falls to 9-10ish?

I am wondering what assembly instructions did you use? (In fact was it a KKK turbo or a Garrett turbo?)
On the larger Garrett (Air Research) turbos that used to go on the Detroit Diesel engines (T10A40 turbo) the process was to heat the compressor wheel, line it up with the marks torque it to specs; after cooled back to room temperature it was torque again to specs. After which you used a dial indicator to check the axial and end play of the shaft. Before running the turbo you are supposed to squirt clean oil into the oil inlet to pre-lube it.
Garrett turbos have 2 full floating bearings, the KKK I do not know about.

No, 11-13 is the stock pressure but they are set low from the factory.

Assembly instructions? :-) Made it up as I went. I really didn't have time to get anal about it - put it together and go. I used a normal socket wrench on the nose of the turbo, torqued it to 22 ft lbs and put it together. I did, however, fill the turbo with oil before I started it.

I'll probably just leave it for now, but if I do anything, I'm considering shimming the ALDA as the adjusting screw is at its stop. I may also try to adjust the wastegate to get that few extra psi out of it.

Thanks again all!

Travis