Under full throttle accelleration the wastegate on the turbo is opening and closing (fluttering) so I am not getting full boost. It's not the overboost switch kicking in because I bypassed it and I am still having the same problem. I suspect the vacuum pump might be the culprit. I do not have a vacuum gauge but I noticed that when I take the hose off the vaccuum pump and put my finger over it (the vacuum hose thatfeeds to the turbo boost and EGR circuit) that the vacuum is good at idle but drops to almost nothing when I rev the engine. The manual says that vacuum should be about 700 mbar at idle. It doesn't say what should happen to the vaccum at higher rpms. Is the vacuum supposed to decrease at high rpm or is there a problem with the vacuum pump?

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Walter
1992 300D 175K

AFAIK the 92 2.5 doesn't have an overboost switch. Are you sure you didn't disconnect the MAP sensor? That will definitely disable the wastegate actuator.

Sixto
93 300SD 3.0

Perhaps overboost switch is the wrong term but, the part I disconnected is fed with manifoold pressure by a small line that goes from the intake manifold to the ALDA and this electronic component. I was under the impression that this component sensed excess manifold boost pressure and then sent a signal which would cut vacuum to the wastgate actuator. No vaccuum to the wastegate actuator results in the wastegate open which cuts the boost to the intake manifold. Nevertheless, I bypassed this unit by blocking off the line that goes to it. After blocking it off the the waste gate is still fluttering between open and closed under full throttle. As I said in my original post I suspect the vacuum pump. Although it produces good vacuum at idle, it's vaccum suction at high rpms drops to almost nothing. I don't think it should do that (although I don't know how the vacuum output of the pump is controlled). It would seem to reason that the vacuum output of the pump should remain constant or increase from low to high rpms. Afterall, the wastegate needs vacuum applied to it to remained closed. It for some reason it needs to open at higher rpms there are other components in the system that that cut the vacuum to the wastegate actuator.

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Walter
1992 300D 175K

The vacuum from the vacuum pump should definitely remain constant (or nearly so) at all rpm's.

If you've hooked up a vacuum gauge directly to the vacuum pump and have confirmed that it won't provide vacuum at higher rpm's, then it's definitely the culprit.

But, don't condemn it without a test right at the output from the pump.

What about your brakes? Without vacuum, you'd be in a world of hurt when you hit the brakes out on the highway. They would feel very hard to push. If the brakes are fine, then you cannot condemn the pump.

Brian,
The Brakes are fine. The vacuum pump has two lines coming from it, one large going to the brakes and a small one that goes to the systems that go to the EGR, transmission, air controller and turbo wastegate. It is this small line is where I am sensing poor vacuum. If I disconnect it near the pump and hold my finger over it at idle it pulls pretty well. When I rev the engine to high rpm there is hardly any suction. I also thought the brakes would be affected but they are fine. Shouldn't the vacuum be the same to both lines coming off the pump?

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Walter
1992 300D 175K

That's what I would figure. Have you tested the smaller line with a vacuum gauge to confirm your suspicions?

If confirmed, you've found your problem.

Remember, a properly operating brake booster will retain enough vacuum to make 1 or 2 stops. I would isolate the pump from all users (brakes, etc) and check the vacuum from the pump at different engine soeeds. If you now have good vacuum then determine which system has a leak. I know for a fact (been there done that) that if the wastgate controller, egr controller or the one for that other thing in the intake system goes bad, you loose vacuum to all three. I am referring to my 91 300D so if some of the above doesn't apply to your car then ignore it.

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84 500 SEL (307,xxx miles)

Agreed. It's far more likely that a leak is present than the vacuum pump has failed only on the small output line.

There might be an internal check valve to isolate the brake line from the small fitting. In the 140 Diesels engine shut off vacuum is sourced from the brake booster line.

Can you swtich ACC settings at high RPMs? There might be a reservoir for this circuit, though.

Are transmission shifts particularly stiff at high RPMs?

Does the vacuum pump have a flat front cover or are there 4 screw heads protruding from the front cover?

Sixto
93 300SD 3.0

Shifts are harsher at high rpms which ties in with the low vacuum from the small end of the pump at high rpms which is causing the my turbo boost problem as well.

I want to check for a leak in the brake booster circuit before I replace the vacuum pump. However, If I had a leak in the line going to the brake booster, wouldn't I tend to have trouble starting the engine because a vacuum leak would tend to not pull the fuel shut off open after the car sits?

Do these check valves in the vacuum pump somehow issolate the large and small vacuum outputs in the pump from one another? I have the old style pump with the flat cover, no screws on the outside.

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Walter
1992 300D 175K

3) Turbo wastegate vacuum pod. The wastegate has a vacuum failsafe that is normally open with zero vacuum. Bypass is to connect the wastegate vacuum
(which is the electrical/vacuum pod closest to the passenger side headlight, low in the inner front fender), and hook it directly to the main vacuum system. My wastegate vacuum pod leaks, and I need to replace it. Bypassing is only temporary, as that particular failsafe is not good to ignore for too long. The key to preserving the computer happiness is not disconnecting the electrical harness to the vacuum pod.

What happens when you do this?

__________________
1999 E300DT (131,800) 154,000 Black on Black SOLD

2006 CLK 500 coupe Capri Blue on Grey (zoom,zoom)
47,000mi

04 VW TDI Passat 80,000mi
(Techno)

How to eliminate oil dependency through market-driven approaches.
“We could cut oil use in half by 2025, and by 2040, oil use could be zero,”
The Sound of Diesel Speed
Ode to MB

Previuosly I had hooked the small vacuum line directly to the turbo wastegate actuator and I still have the same problem which means the problem is at the vacuum pump or a leak from the vacuum pump to the booster hose for the brakes.

__________________
Walter
1992 300D 175K

I'm not sure if your car routes vacuum to the shut-off actuator through the key switch or an electric switch. The key switch vents the actuator in the run position so residual vacuum isn't a problem. Likewise the electric switch only holds for about 5 seconds then vents the actuator.

Sixto
93 300SD 3.0

Did you disconnect the the brake booster first and plug the vac hose?

http://www.carsoft.ru/avtorepair/Diesel602_603/70007.pdf

As of February 1992, a modified vacuum transducer has entered production. The new part can be identified by a black ring, however, the part number remains unchanged. In case of customer complaints of engine surging at approximately 50-60 MPH, smoking rough idle after start, or poor power output on vehicles with earlier production dates, the tests in this bulletin should be performed; see Testing.
Pressure control flap test (engine 602 only)
1. Connect vacuum tester to vacuum actuator (arrow, Figure 1).
2. Apply vacuum slowly until lever (a) contacts stop (b).
3. Vacuum reading should be 385 +/- 5 mbar. If necessary, adjust by loosing screws (c) and sliding vacuum actuator (100) in the slots. Release vacuum and repeat test if needed.

Vacuum pump test
1. Connect vacuum tester (019) to accessory connector on vacuum pump (arrow, Figure 2).
2. Disconnect main vacuum hose (B) from brake booster (A) and plug.
3. With engine idling, reading should be 700 mbar after 30 seconds. See barometric (altitude) correction table in this bulletin.


Barometric (altitude) correction (see chart)

Readings must be adjusted according to the prevailing barometric pressure to compensate for differences in altitude (deviation from sea level). When performing the vacuum pump test, reduce the reading obtained according to the altitude dependent value in the chart.

Vacuum system and consumer test

1. Using a Y-connector, connect vacuum tester into consumer connector on vacuum pump.

2. With engine idling, check that reading is at least 600 +/- 30 mbar.

3. If vacuum is below 570 mbar, check the following for leakage: line to vacuum control valve (65) (transparent), line to consumers (grey), vacuum transducers (Y31/1 and Y31/5), as well as the black/white vacuum supply line. To check, disconnect and plug lines at T-connector or double T-connector (arrow, Figures 7, 8, 9, and 10) one by one.
4. If vacuum is greater than 650 mbar, check for possible leakage in the vacuum line system.


Vacuum transducer test (Y31/1, Y31/2, Y31/3, Y31/4, and Y31/5)

1. Using a Y-connector, connect vacuum tester to consumer connector on vacuum pump.

2. With engine idling, disconnect connector VAC from transducer Y31/1 and plug line. If vacuum reading is below 650 mbar, replace vacuum transducer Y31/1. Repeat procedure for vacuum transducer Y31/3 (124.128 model year 1990-91) and Y31/4 (140.134)

3. On engine 602 only, with engine idling, disconnect connecting line VAC from transducer Y31/2 (model year 1990/91) or Y31/5 (starting model year 1992) at the Y-connector black/white line and plug. If vacuum reading is below 650 mbar, replace vacuum transducer Y31/2 or Y31/5.

Model years 1990/91
Engine 602.96 Model 124

Y31/1 EGR valve vacuum transducer
Y31/2 Pressure control flap vacuum transducer
Y31/3 Wastegate vacuum transducer

Model years 1990/91
Engine 603.970 Model 126

Y31/1 EGR valve vacuum transducer

Starting model year 1992
Engine 602.96 Model 124

Y27 EGR switchover valve
Y27/4 Boost pressure control switchover valve
Y27/5 Pressure control flap switchover valve
Y31/1 EGR valve vacuum transducer
Y31/5 Boost pressure control/pressure control flap vacuum
transducer (not visible, located under air cleaner housing.

__________________
1999 E300DT (131,800) 154,000 Black on Black SOLD

2006 CLK 500 coupe Capri Blue on Grey (zoom,zoom)
47,000mi

04 VW TDI Passat 80,000mi
(Techno)

How to eliminate oil dependency through market-driven approaches.
“We could cut oil use in half by 2025, and by 2040, oil use could be zero,”
The Sound of Diesel Speed
Ode to MB