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Old 04-09-2000, 09:48 AM
Posts: n/a
Over the winter my turbo would not function until the cooling thermostat warmed things up a bit, then it would operate fine. Took it to my independent and he was stumped by it. So I figured summer's almost here.... I'll just keep driving it. Yesterday however the turbo stopped working altogether. Can still get the car up to speed, but it's a little like trying to get a train up to speed. Any ideas on what the problem might be before I race up to the dealership. I do believe the turbo is fine but something is preventing it from activating. 336k miles and still going!

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Old 04-09-2000, 01:43 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I doubt that you have a turbo problem.

There are a couple possibilities. The most common is that the turbo boost signal isn't getting to the aneroid on the top rear of the injection pump. Without that signal the pump will not give the extra fuel necessary to use the extra air coming into the motor under boost. This is easy to test with a vacuum/pressure gauge. Tee into the line that carries this signal further to the blue vacuum amplifier on left fender. This line comes from the hollow screw that attaches the line to the aneroid. Its usually easiest right at the hollow bolt. We use a section of the hard plastic door lock vacuum line to allow us to drive the car and monitor the boost gauge inside the car (be safe here please - use a second person to watch the gauge). You should have around 10psi boost at around 2500rpms full throttle. The waste gate will prevent the boost going higher.

If you don't have this boost check the boost at where it comes from the intake (this also could be done first but the answer lies in finding the restriction to the signal and verifying the true boost). The common restrictions are: the overboost protection switch-over valve and the bolt where it comes from the manifold.

If you decide that you actually don't have any boost you must verify the position of the pop-off valve. The late (85) 617 and early 603 motors used pop-off valves in the intake to prevent the turbo from loosing velocity when the throttle is let up. The concept here is that when the engine speed slows drastically the turbo speed has to also due to the pressure building behind the intake valves. This pressure slows the turbo drastically. So upon vacuum signal the pop-off valve opens venting this pressure back to the turbo intake allowing the turbo to pump in a circle instead of into the motor. As soon as the throttle is depressed the vacuum signal goes away and the engine goes back to power with the turbo at close to full speed (solves turbo lag problems).

If this stays open the turbo does nothing. I would check the vacuum to the pop-off valve or just disconnect it for testing. For vacuum to be the problem would require vacuum to be present all the time.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 04-09-2000).]
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Old 04-09-2000, 02:10 PM
Posts: n/a
Wow! Thanks Steve. Awesome information. I'll get w/my mechanic tomorrow. In the meantime - I just dug out the repair order from my attempt to get this problem fixed (2/22/2000) when the turbo was only inoperative when the engine was cold. Does this shed any more light on the problem? "Test operation of Turbo Charger. Test pressure switch at intake. Check operation of overload protection valve. Replace valve assembly". Steve, they then put on a new overload protection valve($63.64) ..... but the problem did not go away.
Thanks in advance.

[This message has been edited by Bigrusty (edited 04-09-2000).]
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Old 04-10-2000, 12:09 AM
Aaron's Avatar
Join Date: Aug 1999
Location: Cincinnati, Ohio
Posts: 1,937

Don't forget to check those vacuum lines and fittings as well. With age, they get brittle and split. That could be the cause of your problem. Good luck!

Aaron Greenberg
MB technician
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'77 450SL
'80 300SD
'85 380SE
'89 420SEL
'93 300E 2.8
'74 Jensen Interceptor Mk.III
'81 DeLorean DMC12
'85 BMW 745i Turbo
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Old 04-10-2000, 04:15 PM
Posts: n/a
Thanks guys. I got my neighbor (a forklift mechanic) over here yesterday afternoon. We went by your recommendations and even though we did not have a vacum tester.....we found that one of the plastic vacum lines was severely restricted by an oily gooey substance..... I think it was the hose from the boverboost protection switchover valve. We put it back together and she runs like a scalded dog now...... and viola....she now runs awesome even before the thermostat opens!!!!!... best we can figure... the goo completely restricted the line until the engine compartment reached operating temperature... and then the goo softened enough for a little vacum to get through. Thanks for the advice. You guys are awesome.
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Old 04-10-2000, 09:57 PM
Registered User
Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
This is the anaroid supply line. I have seen many stop up. Good find.

Donnie Drummonds
1981 280GE SWB
1987 16V

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