Hi,
I am a decent machanic but a diesel dummy as in my log-in name.
I have a 1991 300d 2.5 turbo that is having intermintant boost issues that seem to be and electronic based problem. I installed a boost gauge so I can track whats going on and half, some, most, or none of the time my car will boost and when it's not, I can get it to attain full boost just my activating the Accl. lever of the cruise control. Once it starts to boost it runs great and may go 1 min./day/week working correctly before the boost drops out again.
Whatever electrical device the Cruise Control circut is controling must be getting or not getting a signel from whatever is supposed to control it. What controls the boost system, is it the CPU or something else? What can I look at ?
thanks in advance!
kevin

You might read wbda####'s post on 2.5T boost problems on this thread:

617 in 124 lOOK

__________________
'91 300D 2.5 Turbo 330K
'00 VW TDI Golf, 190K
'67 BMW R50/2
'73 Norton Commando Interstate

1.check the mechanical "Doggie Bones" and cable connections between the :
accelerator pedal and the injection pump.[Lengths and ease of operation(lubrication)]
2.check the mechanical connectios between the cruise control actuator (motor)
and the I.P., the transmission, the accelerator linkage.[Lengths and ease of operation (lubrication)]

There is no direct electrical connection between the cruise system and the EDS
(Electronic Diesel System)[engine computer].

What you describe is that the cruise system is somehow affecting the boost...
Only possible through the "Fuel Rack Position Sensor" getting stronger information from the mechanical connection between the cruise and the I.P ; than the information from the accelerator pedal mechanicals.

The boost is controlled by the EDS through two Vacuum/Electric Transducers.

Look at the Vacuum/Electric mess just in front of the coolant surge/overflow tank(in front of the battery) on the passenger wheelwell.

If compress ignite's suggestion doesn't work....

Try looking for a code with a home made pulse counter.

93 300D 2.5L Turbodiesel Power Loss

Do a couple of searches for "2.5 powerloss" also search using GSXR as poster for "turbo"

Also check for posts by TimFreeh

__________________
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

Thanks for the replys!
Here's more info I sent to one of you!

Hi and thanks for the thoughts.

I have worked through many things on the car as best as I can fumble through them. This car has 3 vacuum transducers, one boost control, one EGR, and one flapper valve gizzmo. This entire set-up was only on 90-91 300d's and was completely changed on the 92 model. The boost control(waste gate) transducer is in the best shape of the 3 loosing the least amount of pressure of the 3, loosing only 20 (whatever the units are?) the other 2 loose about about 40 units each of a system total of close to 600.
The change is instant when you hit the CC Accl. lever, doing let say 45 mph with sluggish pull and showing maybe 1/3 of full boost with pedel on the floor. Hit the Accl. lever and in less than 1 sec. full boost and the car takes off great. That's why I think it's some elec. thing because it happens much faster than the Cruise can even react mechaniclly.
Thanks for any more thoughts and help.

kevin

DD,

When you push the cruise control lever for Accelerate (Electrical)I.E.
INSTANTANEOUSLY the cruise control MOTOR (Electric) moves the
MECHANICAL linkage[which connects with the "Rack Position Sensor"(
senses the frequency of fuel distribution of the Injector Pump)which
inputs Electrically to the EDS...which CONTROLS the wastegate functions.

Look under the hood ,when you move the accelerator the connection to
the I.P. should move the exact amount AS IF THE CRUISE LINKAGE WERE
PROVIDING THE MOVEMENT.Any difference needs to be adjusted.

The linkage performs other tasks simultaneously,most notably increasing
fuel distribution.

YOU most likely have a LINKAGE problem...Cable,"Doggie Bones",adjustment.

The wastegate on these models begins bleed off of the precious BOOST
long before the set maximum P.S.I.
Even if the system is functioning perfectly.

Thanks for taking the time to respond !
I will look over linkage adjustments as I don't know what the Rack Posision Sensor or the I.P. is (please tell me more), but as I mentioned I feel it reacts much faster then the CC could even take effect IMO because when I am trying to activate the cruise control it takes several seconds to come in.
I installed a PSI boost gauge last night so I can reference the numbers to all.
40 mph pedal to the floor I have 6 psi of boost(in the no power mode) and as soon as I touch the ACCl. lever, boost spikes near 15 psi settleing back to 12 psi for 5 sec., then drops back to and maintains 6 psi and I've never lifted of the floor.
The 5 sec. of boost before dropping back to 6 psi is a constant if I repeat the test everytime, except during the few times the systems seems to work properly.
Most of the time, the very first time I floor it from a stop the boost with spike to 16-17 psi then fall back and maintain 10-12 psi as long as I keep accellerating (when boost is working anyway), but some, half or all of the time when I lift off the gas and then hit it again it will max out at 6 psi which really offer's no extra accel.
I seem to be seeing something new too, at a steady state of accellerating the boost will climb and drop in equal multi sec. cycles between 7-11 psi.
Is the waste Gate normally Open or Closed ? Most US systems I've worked on(non-diesel) are normally closed and a boost setpoint opens it.

Here is some other info I'm sharing with compress ignite, thanks again for helping me out !

I mention the CruiseControl only because it will affect it almost every time, but like this mornings drive in, the system worked almost the entire time getting what I would consider full boost(around 12 psi) 90% of the time. Off a stop light with full throttle it will spike near 15 psi and quickly drop to 12 psi and hold that through all the gears until I lift off the throttle. Most of the time something is limiting boost to 6-7 psi under all the same conditions and I change nothing. Actually the whole time I've owned this car it Never boosted over the 6-7 psi (without hitting the Cruise lever) until I got it back from a MB shop who said it was my timing chain (3 deg. retard.) He had left off a major vacuum hose at the first Y- connector to the transducers which I found and plugged back in. For some reason the car boosted correctly for the first time I've owned it and over most on the next week it ran great! Then over the next few weeks it started to do more and more of the limited boost thing, maybe the EDS is learning something bad.

Why I keep thinking there has to be some CC elec. connection is that I don't see (or understand) how with the pedal to the floor that the CC could add anything to something already maxed out. The linkages Can Not move any farther, can they? Doesn't the throttle cable move the fuel rack and the Fuel Rack Position Sensor? Also other FI Turbo cars I've worked seemed to also have the same or similar 5 sec. over/extra boost when the cruise is activated, like I feel in this car too, most likely controlled by the EDS. Can you tell me why boost would drop off from 12 psi to 7 psi after 5 sec. of full boost after activating the cruise control and I never lift of the gas. It is some switch/solenoid/EDS dropping out or coming in IMO.

What is the ALDA and can the pressure switch affect any of this ?

Thanks for helping this DieselDummy!

I will try to spend some time on likage adjustments today and see if I can find a problem in that area.

What else ?

Thanks and Happy Hoildays everyone!

Back again.

If this were a linkage problem like some of you suggest, than that machanical action would produce the same result every time, would it not???
I can floor the gas pedel 20 times and get different results each time, from having 5-6 psi of boost to 12 plus psi of boost. I would think if it were a mechanical linkage problem it would react the same every time you douplicate throttle movement.
Also the boost drops off to 6 psi after 5 sec. of full boost when using the CC lever.
thoughts

OK, now I'm getting No boost ! Hitting the CC lever does nothing either like before.

On a 91 300d 2.5 turbo, is the waste gate normally closed or open ? I'm assuming it's normally open and the vacuum transducer provides vacuun to pull it closed as computer dictates. All the transducers run off vacuum not boost, correct ? I'm also assuming most of the older MB turbo's work the other with boost pressure going to the waste gate and it opens in over boost cond.?

I change nothing and will go from limited boost to no boost to full boost. The last two days have been no boost.

OK, well I re-booted the computer by disconecting the battery when I left work yesterday to see if it changed anything because I have had two days of a No boost slug fest.
results, Full boost and proper running for the most part for the remainder of the day, then this morning it started out good then dropped back into limited boost . The computer must be getting bad info from one or more of it's sensors and is removing/retarding boost to protect the engine.

Is there a way to use a Multi Meter to read codes) pulse singles ?

Check my other post

__________________
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

PMARI,

Thank You!
The link in your first post provides the pathway to solution.

If you want the codes and another trouble shooting procedure, give me a PM and your emaill address. I have a copy of Mitchell's disks that has a decent write up.

Was it a code 5??

__________________
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