1994 S350 slow acceleration diagnosis input appreciated

[Actros617]

Well recently i've acquired a 1994 S350 for a hardy laugh (~$200) and for the price it came with some perplexing problem. Now I'm well aware of the 3.5L OM603 been the infamous rod benders but I'm beginning to doubt that its plagued problems is in this case.

Initial Problem
Very slow acceleration almost like driving a 240D on its last legs 0-60 is 40-45 seconds,

So what do we have:
*A W140 350K miles chassis with a probable (undocumented) engine replacement some time within 2-10 years ago.

*Previous owner bought it in 2015 and drove it till 2017 then it sat in a shed till 2019 after it begin to developed poor acceleration, unable to maintain speeds higher than 70 mph (floored) and previous owner loss interest

*Hack job straight piped the exhaust with 3 replaced pipes welded to stock (Resonator, catalytic converter? and muffler delete)

*Possible rust/bio growth in tank as there are few sediment in the new primary filter.

*A clean valve train and what appears to be a recently replaced new timing chain and chain guides, (when i replaced the valve cover gasket)


What i've done so far:

*When thru all the vacuum rubber lines in the engine bay and replaced them.

*When thru all the hard plastic vacuum lines and intake manifold port clean them out as needed (near the intake manifold ports for the ADLA were clogged!)

*Replaced the vacuum hose on the turbo transducer (will be converted soon to pressure setup soon)

*Replaced, fuel filters both primary and secondary, air filter, engine oil (delvac 15w40 for now)

Diagnosis performed so far

*Vacuum leak check as followed:
A. Turbo wastegate vacuum pod, no leaks, actuates with vacuum
B. Egr pod slow leak 5mmhg per second (disabled and plugged),
C. Vacuum transducer not holding pressure but will go up to 10 mmhg and leak out
D. Brake booster line to vacuum pump check valve no leak

*Turbo boost checked as followed:
A. Initially 0 psi and was very loud exhaust suspected wastegate was not closing
B. Sorted out various vacuum leaks and replacing the rubber lines in transducer got boost but very low 6-9 psi will reach 10 but at 3500-4000 rpm (Nominal boost 12-14 psi @ 2500 rpm), acceleration much improved can get up, maintain and exceed 60 mph but still slow acceleration 0-60 25-30 seconds
C. Check for boost leak from turbo to intake manifold none found
D. Attached a hand held vacuum pump to turbo wastegate vacuum actuator, was able to shut wastegate completely but still low boost 6-9 psi till 3500 rpm then goes up to 10 psi, acceleration still slow
E. Turbocharger inspection, very little shaft play forward and backward, almost no play in up and down, compressor wheel has some strange looking rust spots on one of the blades, other wise some fine scoring on the compressor turbine, inside the compressor housing has small non penetrating hole about the size of an eraser tip doesn't seem to be leaking (no air leak when under boost)

*MAF
Will trigger turbo limp mode when plugged in (no boost) due to EGR disabled, when unplugged has boost 6-9 psi but acceleration slow

*Compression test
A. Unlikely an issue as it starts up almost immediately afterglow plug cycled and after sitting overnight (70*F ambient)...however awaiting on diesel compressor tester to arrive
B. No smoke or oil consumption (after driving 300+ miles after purchasing from previous owner)

Impressions
So far all fingers points to the turbocharger delivering boost thus slow acceleration however i can't seem to wrap my mind on what about the turbo that may cause it to have performance issues and not sure on what other test I'm missing out here, I hope this isn't too much information, I did all I know to try to diagnose this problem but im still stuck, any advice or input will be appreciated.

[Diseasel300]

The turbo is only half the battle. Fuel enrichment is the other. If you have extra boost but no extra fuel to use with it, you gain no power. Since you're making 12-15PSI of boost, the turbo works. You need to figure out why you're getting no fuel. Try connecting the boost line from the intake manifold directly to the ALDA and see if you get an improvement. If you do, you likely have a problem in the lines, switchover valve, boost sensor, or elsewhere. The 3.4L is not slow, it should be like a gas car off the line. In city traffic, I'd even call it quick compared to my 300SDL.

[Maxbumpo]

When you disabled the EGR, on that car you need to do some other gerry-rigging to make sure limp-home mode / no boost does not happen. MB made it harder to defeat the EGR, it is not a simple block-off.

Low boost sounds like turbo /waste gate problem, maybe more work required there. Straight pipe means a blocked exhaust is not an issue, right?

After that, typical check list. Fuel filters and screen in tank, air filter, ALDA actually working, maybe re-seal ALDA or adjust / shim.

[ccrelan]

Don't these cars have biodegradable wiring to the wastegate actuator that goes bad and the wastegate then defaults to no boost??

[Actros617]

Originally Posted by Diseasel300

The turbo is only half the battle. Fuel enrichment is the other. If you have extra boost but no extra fuel to use with it, you gain no power. Since you're making 12-15PSI of boost, the turbo works. You need to figure out why you're getting no fuel. Try connecting the boost line from the intake manifold directly to the ALDA and see if you get an improvement. If you do, you likely have a problem in the lines, switchover valve, boost sensor, or elsewhere. The 3.4L is not slow, it should be like a gas car off the line. In city traffic, I'd even call it quick compared to my 300SDL.

It only boost up to 6-9 psi and when it gets up to 3500 rpm is when it barely gets to 10 psi, I'll see if can direct connect the manifold to the ADLA and report back. I also had a 300SDL and those things are a rocket once the turbo is spooled and boosted.

Originally Posted by Maxbumpo

When you disabled the EGR, on that car you need to do some other gerry-rigging to make sure limp-home mode / no boost does not happen. MB made it harder to defeat the EGR, it is not a simple block-off.

Low boost sounds like turbo /waste gate problem, maybe more work required there. Straight pipe means a blocked exhaust is not an issue, right?

After that, typical check list. Fuel filters and screen in tank, air filter, ALDA actually working, maybe re-seal ALDA or adjust / shim.

Correct, from what i've read MB had made it harder for one to delete the EGR a complete delete of the transducer and conversion to pressure operated wastegate is required. The exhaust is wide open no resonator or muffler and perhaps no catalytic converter ( theres 3 straight pipes welded on to the existing stock exhaust). How does one goes about resealing or testing the ADLA, and whats does the shim do?

Originally Posted by ccrelan

Don't these cars have biodegradable wiring to the wastegate actuator that goes bad and the wastegate then defaults to no boost??

The wastegate is not directly operated electronically, but also by vacuum via transducer, Air flow sensor, egr valve and a few 3 way vacuum switch over valves.