Boost - or lack thereof
 

Hey all
I have been wanting to check the boost pressure on my W140 OM603.971, I don't think it's as high as it should be, seems she has some troubles on steep hills sometimes. So today I finally found the boost gauge and I used a T to integrate this into the line coming from the intake manifold to the air pressure sensor. For awhile I had 2psi under hard acceleration, but after a while I lost all reading on the gauge. I don't believe it's leaking anywhere, it's all plastic pipe except for the rubber T and a short piece connecting to the gauge.

So...someone who's been down this road give me some advice. I've traced all my vacuum routing and it looks to be correct except for perhaps the key shutoff switchover valve where according to the diagram my top / bottom hoses are crossed.

Someone give me some clues where to start on this

Thanks
Steve A

start by blowing into your boost pressure line from the manifold side. - eliminate the pump / alda / vacuum hoses etc..


if that checks out ok, inspect your intake manifold gasket for air leaks (putting a fine powder around the outlets might provide some info)

This does not sound like a small leak.... ps


what type of boost controller are you operating with currently?

I don't believe there's a leak at the intake, the engine behavior hasn't changed since I got the car and less than a year ago the manifold was off the car to change the fuel shutoff valve.

Already blew through the line, there's no blockage anywhere. I don't believe it's leaking, the fittings are too tight and I can blow into the end of the fitting and get the gauge to lift 1psi.

The car is stock, so there's no special boost controller other than the overboost solenoid from MB.

I read elsewhere that the solenoids can get clogged so I think it prolbably wouldn't be a bad idea to clean it.

Thanks
Steve A

Doesn't the 603.971 have the dreaded vacuum-actuated wastegate, which will be open unless sensors tell it that it's okay to close and send vacuum its way?

this is what we're getting at:

Boost actuator swap

Yes, I see what you mean now. I had read some other posts about EGR removal and it mentioned the wastegate situation but I wasn't aware that there was actually any real computer control on this engine.

I'll have to do some investigation about this.

Is the computer interface non-standard such that only MB readers can pull codes? Or is there a way to reset errors? I suspect the disconned EGR vac hose could have caused a problem, it was disconned when I got the car, the hose was cracked.

I might need to look in to switching to this wastegate.

As a side, shouldn't the car have an engine light that tells you if there's a fault in the system? I have no problem dash lights and from what I remember there's no specific check engine light on here.

As much as I would prefer to leave the OE system alone it might just be easier to fix with a different wastegate controller.

Thanks
Steve A

Well I may have found the problem, or at least one of them.

I noticed that the part numbers for the overboost control valve and the egr controller valve were the same so I put a vacuum gauge on the port leading to the wastegate and it failed to hold. When I put vacuum to the same port on the EGR control valve I have good vacuum.

Since I don't have maintenance manual nor do I have diagnostic manual for the rather complex vacuum system on these I can't say this is a problem but I find it unusual that I would get different results for the same valve on the same port.

When I put vacuum to the wastegate I get a good reading, as in the diaphram doesn't appear to leak. If it did leak I would expect to have boost of some kind, as it is perhaps the control valve is applying vacuum all the time?

Any comments on this?

Thanks guys

Steve A

You might be misunderstanding the function of the valve.

as Babymog suggested... the wastegate is always open until the required vacuum is applied.

you have a pm about the FSM (manual)

I think you mean overboost solenoid... it does not control flow until high boost pressures occur... it should not hold vacuum.

Correct, I went back and tested all of the solenoids and they function the same, so that's a waste of time. I did however, find a rotted bit of hose at the EGR, replaced that anyway.

A new development however, after buttoning up everything and going for a ride I had boost in the range of 10psi (as good as I could get in the few short blocks of travel in my neighborhood) but when the engine passed 80c I lost all boost again, so this suggests there's something else at fault.

Thanks for the commentary, and yes, I have good vac at the waste gate actuator.

Steve A

that was quick.

nice job. Maybe a temp sensor to the comp?

did you get my message about the FSM?

I got your message about the FSM, thanks for the tip!

Could be a temp sensor, not sure yet. I have to do some digging in the FSM to see what controls what and which gadgets send info to what else etc.

I would like to have a permanently installed boost gauge in the car but locating it is the biggest issue and since I have a list of things that need to be fixed, a permanent install will have to wait. If I can get this resolved I'll leave it alone for a while, I have some transmission work that needs to be done (slow reverse engage) and an annoying vibration when the car gets over 80c, which is frequent now that summer has arrived. Oh and the AC system needs to be repaired (God help me on that one!) but I don't think the evap is bad...or at least I'm not thinking it is right now.

Thanks again

Steve A