Hello,

My 1987 300SDL is really slow (0-60 in 60 seconds) and measures zero psi boost when driving with a pressure gauge attached to the intake manifold and run back to the driver’s seat. But the engine otherwise fires up eagerly when cold and runs smoothly.

When I remove the ALDA, I get really black smoke under load, but the power loss does not improve. I'm suspecting that the turbo is not spooling up. When I removed the black intake hose on the turbo I can see that the turbo is spinning at idle. So it looks like the turbo is not completely stopped up. Also, the turbo has no axial or radial play that I can tell.

Next, I measured exhaust back pressure via the M10x1.0 threaded port on the front of the exhaust manifold and found zero psi at idle and 2 bar at 4000 rpm which the FSM says is normal. So I'm guessing that my catalytic converter is not plugged up. Somebody please correct me if I’m wrong on this.

I removed the waste gate rod and found the waste gate lever was moving freely, and the waste gate could be actuated with 14 to 18psi. So the waste gate externals appear to be normal.

I have also changed fuel filters, bypassed fuel tank with a small fuel tank, and disconnected the air cleaner. I could feel smoothness increase after I changed the fuel filters, but none of these things improved the low power problem.

The flex tube on the exhaust manifold is leaking exhaust and is ahead of the turbo. Is this exhaust leak enough to keep the turbo from spooling? I'm guessing not, because I drove with the M10x1.0 threaded exhaust port open and noticed no improvement, but I would appreciate any comments.

I need to repair the leaking exhaust manifold which requires turbo removal, which would give me the opportunity to rebuild the turbo. However, before I tear down the turbo, I would like to know if anybody can see if I've missed anything in my troubleshooting, and/or could somebody give me some confirmation that I should condemn the turbo?

Also, I'm a bit short on turbo theory. Can a turbo fail to spool up even if it spins freely at idle? Could the turbo be partially seized? Maybe with carbon?

Any advice would be much appreciated.

Bypass the vacuum line

__________________

1986 300SDL 440,xxx

If the ALDA is deleted, the overboost switch has NO EFFECT. Your symptoms all point to a turbo that isn't working. You could have a broken shaft (would let the compressor spin freely) or you could have a waste gate that's stuck open. The latter will essentially let pressure bleed around the turbine instead of through it. Manifold leaks will definitely affect performance, but shouldn't kill the ability to build any boost.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

On second inspection, I feel a tiny amount of play when I try to move the shaft from side to side. Also, I tried spinning the wheel. While I can rotate it smoothly with my finger, I cannot get the wheel to "freewheel" after I stop spinning it. It seems like the wheel is moving smoothly by hand, but will not continue spinning after I let go. In contrast, I tried to spin the wheel on a known good turbo on one of my om617, and that turbo wheel easily continues to "freewheel" so much after I spin it, that it will "bite" my finger if I put my finger back into the turbo. The problem 300SDL turbo will NOT freewheel, but I cannot describe it as "binding" either since it, too, rotates smoothly by hand.

I have read that carbon can build up and impede rotation of the wheel, and my symptoms sound similar. Does anyone have experience to recognize this as confirming that "not freewheeling" will cause a turbo not to spool up enough to even produce any boost?

If the bearings are in spec and oil is present on the bearing faces, it won't freewheel. They're bronze bearings bathed in oil, not ball bearings. A small amount of radial play is acceptable, try pushing and pulling the shaft in and out, it shouldn't move.

As suggested before, check that your wastegate is closed. If you have a garrett turbo, it's easy enough to check, it has an external pod and linkage that goes to the wastegate. If you have the ARV on the compressor housing, its possible that it's stuck open too which would bypass boost pressure back into the intake side of the turbo.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I only found some radial play, but no axial play. See my original post. I already described how i tested the waste gate. I also have the ARV disconnected from the vacuum hose. Maybe ARV is stuck open. Do you know how to test that condition?

Diseasel, also why am I seeing freewheeling on my om617 known good turbo when you said that freewheeling is not possible? How do you explain that?