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Old 01-03-2004, 01:29 AM
Registered User
Join Date: Feb 2001
Location: Midwest, USA
Posts: 171
1986 300SDL loss of power

I have a friend with a 1986 300SDL. I am looking for advice for her. Her car starting losing power and driving really slowly up hills and was eventually only able to go about 40 on the highway.

She took it to a shop that told her it had about 6 months to live. She told me that they said it had bad compression. She said it burns no oil.

I went out to start it up at below freezing temps. It started right up after sitting for a long time. I don't think an aging engine would do that? One of mine certainly does not! But, it runs great.

When reading her diagnosis from the shop, it mentioned that the turbo was not kicking in. They felt it was stuck and got it to read a slight reading, but suggested a new turbo was in order.


I think it odd that they said her engine was worn out, but at the same time suggested it was just the turbo on paper.

My question is, perhaps just a turbo boost pressure switch? I also suggested the ALDA. I didn't see any checks of either of these things.

They are not wanting to lose this car. I don't feel that a diesel car that starts right up in 10 degree weather and that burns no oil is a worn out engine.

Any ideas or suggestions I will pass on to them.

If anyone has a test of the turbo boost pressure switch, I'd appreciate it and any other information, I'll print out and pass to them.

I'm hoping they will start posting here.

They are now no longer able to drive the car at all. It has no power.

Thanks for any ideas.
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Old 01-03-2004, 08:36 AM
Registered User
Join Date: Apr 2001
Location: Near Omaha Nebraska
Posts: 19
Check and clean the boost pressure sense tube from the manifold to the pump. Also check to make sure the over boost relay/valve is not stuck or blocked. Check the pressure switch that runs the relay.

My 87 300SDL had enough carbon after 220K miles that there was no boost signal getting to the IP.. The valve was gummed up. I cleaned it out with a shot of Kroil, and after it was working, I later replaced it with a new one to prevent troubles later on.

1985 300D - 190K Miles - 26 MPG on first trip home - getting rust fixed and repainted.
1987 MB 300 SDL - 238K miles - Sold
2003 DC Ram 2500 QuadCab 6 speed, 4x4 Cummins - 82K miles - 20+ MPG
1999 VW A4 Jetta TDI 5 speed - 83K miles - 50+ MPG is easy!
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Old 01-03-2004, 09:41 AM
engatwork's Avatar
Join Date: May 2000
Location: Soperton, Ga. USA
Posts: 12,232
Replace the fuel filters too.
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Old 01-03-2004, 10:24 AM
Registered Diesel Burner
Join Date: Aug 2001
Location: Northern Virginia
Posts: 2,911
Right - the fuel filters are the first thing to just change. They're probably due and can easily create the symptoms.

Also, a seized turbo could do some of the same things. This is also easy to check with little in the way of disassembly. All you have to do is pull the boost sense hose on the overhead intake manifold housing and verify that you have some boost when revving the engine up to 2500-3000 rpm or so.

You can either feel the air rushing out or use a small length of line to a boost gauge - or put the hose in your mouth and let it puff out your cheeks.

Be aware though that this area of the intake manifold can clog with soot from the combined EGR and crankcase oily vapors. If there is no boost present at the boost sense connection, the next step is to take out a few screws and remove the "crossover" pipe from the turbo to the intake manifold housing. With this done you should easily feel the turbo generate its airflow.

Those are some very basic checks.

1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles
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Old 01-03-2004, 10:45 AM
Registered User
Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Probably a combination of clogged fuel filters and a bad turbo.

Clogged filters will greatly reduce power -- to the point the car won't climb hills or accelerate much. Easy fix, but I suggest getting the engine hot first and filling the spin-on with clean fuel -- no hand pump on this application.

If the small pre-filter is black, watch the new one, you may need to get some algicide from a marine supply place to clear the tank.

Pull the intake hose between the air filter housing and the turbo (ENGINE OFF!) and reach in and gently spin the turbo -- must rotate freely with minimal free play. If it binds at all, or sticks intermittatnly, or moves axially, it's toast. May be fine, no fuel will result in no turbo action to speak of!

Do the filters and drive it -- that may fix it completely.

You get to look like a genious and she saves tons of money!

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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