Time bomb? overboost protection switchover valve - PeachParts Mercedes-Benz Forum

erubin · #1 09-08-2003, 03:37 AM

I've got all the creature comforts and mechanicals taken care of with my family's 82 300D, 180K miles. The car runs great and the 20 year old interior is holding up with the kids and general abuse.

The overboost protection switchover valve was bypassed on purpose by original owner's mechanic.

Should I fix this or is this a turbo-engine safety devise that never saved a 300D?

If you say it's risky to bypass it I will plunge into the repair. If it serves no practical purpose i'll ignore it. Anything in between puts it on the back burner (could take up to a year to getting around to fixing it). I guess these systems get bypassed because the valve goes bad or maybe it's just a hose problem.

If anyone has an engine disaster experience due to a bypassed system i'll be really concerned.

Thanks

The Warden · #2 09-08-2003, 04:06 AM

I would get it fixed, simply 'cause you're risking overboosting the engine with it disabled (I think so, at least; I'm not sure exactly how it's plumbed; either disabling it removes any form of limitation of boost or dumps all bost, making the turbo innefective). If I'm right and disabling it allows the turbo to pressurize the intake without any form of release, you're potentially risking blowing a head gasket or damaging a piston if you work the engine very much. IDI engines have a very high compression ratio (21:1 versus 17:1 on a direct-injected diesel), and won't stand much more than 10 to 12 psi of turbo boost without risking damage. I'm assuming that the overboost protection valve is doing the same job that a wastegate (which is usually an integral part of the turbo) would do. I can't remember for sure if there's also a wastegate on these turbos, or if the overboost protection valve performs the wastegate's task.

Just my $.02...good luck!

P.E.Haiges · #3 09-08-2003, 04:25 AM

Erubin,

Hook up an intake manifold pressure gage and see how much boost you are getting. If you are running over 12PSI, either fix it or watch the gage and back off on the accelerator when you get more than 12 PSI.

P E H

R Leo · #4 09-08-2003, 01:27 PM

From the (misleading at best) OM617.952 FSM:

The boost air pipe (ed: actually the manifold) is provided with a pressure switch (ed: boost pressure sensor). At a boost pressure above 1.1 ±0.15 bar gauge pressure, the pressure switch will close and will connect minus (negative) to changeover valve (overboost valve), which is connected to plus (positive) via terminal 15. The changeover valve interrupts the connection boost air pipe (what we call the overboost line) to ALDA aneroid capsule on injection pump. The aneroid capsule is connected to atmosphere and the injection fuel quantity is thereby reduced. If the pressure drops to below 1.1 ±0.15 bar gauge pressure, the pressure switch will again interrupt the minus (negative) connection and will connect the ALDA aneroid capsule again to boost air pipe.

Translated to 'real-world' English...
The overboost valve is controlled by the boost pressure sensor. This sensor is located on the intake mainifold and limits the amount of pressure available to the ALDA if manifold pressure exceeds 1.1 Bar. If the wastegate fails to limit boost, and the manifold prsssure exceeds 1.1 Bar, the overboost valve is switched on by the boost pressure sensor. Switching on the overboost valve blocks the flow of pressurized air to the ALDA. This moves the fuel rack so that it limits the amount of fuel delivered to the injectors and prevents the potential overloading of the engine.

#5 09-08-2003, 04:37 PM

I doubt there will be many engine disaster stories out there that have been related to an engine overboost problem. That being said I would bet my next pay raise a lot of former good running diesels have failed early in life because of an overboost history. Just one of those things most good techs will not look at once they see a blown out engine.

erubin · #6 09-08-2003, 08:16 PM

P.E.Haiges,

I guess you're implying that if the pressure is within the acceptable range, under .8Bar (12psi) then the Boost Pressure Control Valve (wastegate?) is doing the job of overboost protection. The secondary level of protection provided by the Overboost Protection Switchover Valve (called the Changeover Valve Boost air pipe in RLeo's diagram) does not come into play if the primary level of protection (wastegate) is working correctly. But if the pressure is over 12psi doesn't that mean the Boost Pressure Control Valve (wastegate) needs to be fixed? If the first level of protection failed why would I only want to fix the second level protection? Is it common for the primary protection system to fail and it's the second level that is doing all the protecting?

Do the boost pressure control valves (wastegate) fail alot and makes the backup system (changeover valve overload protection) very critical?

R Leo,

Thanks for the description and diagram. Based on the diagram I will need to not only test the valve and piping but the sensor as well. If the valve leaks and I replace it who's to say that the sensor is not defective as well.

Finally, why have two levels of protection if you can't tell if one of them has failed from a driveability standpoint. If one system fails but you keep driving it puts all the responsibility of protecting on one unique system and from that point forward you loose the benefits of two protecting systems. The average owner is not going to test the two sysyems regularly to verify they are both functional. The secondary system with its piping, sensor and valve probably go bad all the time but how does a user know it if the primary system is fine (and vise-a-versa).

Maybe I'm not understanding something here.

#7 09-09-2003, 04:20 PM

When you put your right foot to the floor on a properly running MB turbo diesel it will be running with the wastegate open from about 2000 rpm or a little less all the way out to the limit (~5100 rpm). The wastegate holds the boost pressure below 1 bar. Eventually some wastegates WILL fail - most fail by not completely closing or by opening too soon tnus holding boost well below the ~0.9 bar limit. A few will stick shut and that will allow boost to build up to 2 bar (or more) and it only takes minutes at that boost to melt pistons or do other serious (and VERY expensive) damage - and there is usually NO warning. Just climbing a mile long hill flat out can do it! The overboost protection circuit guards agains that and it should be quite noticable when it kicks in. The engine isn't just limited in power as when the injection pump governor hold the engine to 5100 rpm - it drops instantly about 30% in power when the pressure signal to the ALDA is shut off and all pressure controlled fuel enrichment is turned off!

Running with the overboost disabled is fine as a test, but to do it for any length of time is NOT prudent although MOST turbos do not have the wastegate fail closed. If your turbo should fail that way with the overboost protection disabled, you could turn you engine into a doorstop/anchor VERY quickly.

Marshall

kashbaugh · #8 09-10-2003, 01:27 AM

I have often wondered why MB didn't put some type of boost meter or overboost light or SOMETHING on the dash. Once I get all of my gremlins under control I have thought I might try to invent something. Any thoughts? Or has someone else already done it? Cheaply?

kma

erubin · #9 09-10-2003, 04:22 AM

Thank you. i guess i'll just fix it. In writing/thinking about the problem and then getting your descriptions and advise I'm that much more prepared.

My plan for fixing the bypassed 300D overboost switchover valve:
-vacuum test changeover valve (I tend to suspect all vacuum and pressure activated gizmos on my MB cars)
- check/test the system piping
- test the pressure switch

Missing anything?

Oh yeah, time permitting i'll do a boost pressure measurement per Marshall with a rigged up hose/gage contraption.

R Leo · #10 09-10-2003, 06:05 AM

QUOTE]
I have often wondered why MB didn't put some type of boost meter or overboost light or SOMETHING on the dash.
kma [/QUOTE]

Essentially, the overboost valve is doing this exact thing except, instead of flashing a warning light at you, and hoping you throttle back, it's slightly more Teutonic and actually prevents you from toasting your piston crowns.

I made a simple boost gauge out of a $5.00 air pressure gauge (see pic — the real gauge has numbers 0-30) from W.W. Graingers (http://www.grainger.com/Grainger/productdetail.jsp?xi=xi&ItemId=1611669055), a brass barbed fitting and 3-4 feet of appropriately sized rubber tubing.

For testing purposes, I pulled both of the lines off of the overboost valve, stuck them into an extra rubber vacuum "t" that I had and used a plastic barb fitting to attach the rubber line to the pressure gauge. I snaked the tubing and gauge back out through the left rear corner of the hood and in through the open window of the car and just sat the gauge on the dashboard.

Then, I drove Marlene around town and made some trips on the highway to see what kind of boost was being generated. At full chat (climbing a hill, 3500 rpm, full throttle) she makes just under 10psi of boost. Attaching the test gauge to Lilly on similar runs produced almost identical results.

Since the power at full throttle, high boost conditions isn't a real problem in these cars, I left the wategates untouched. That all said, I'll probably do a boost test on an annual basis or if a performance issue arises.

R Leo · #11 09-10-2003, 06:12 AM

Thinking further about this, and considering all the crap that accumulates in the intake tract of these engines, it might not be a bad idea to remove the manifold pressure sensor regular basis to clean the sensor port and insure that the opening in the manifold is clean as well.

kashbaugh · #12 09-11-2003, 01:40 AM

R. Leo,

I wasn't thinking of an indicator of some kind in_stead_ of the switchover valve(?), but in addition to it. That way, if all the safety devices failed (I know, that would _never happen!) you would still get notified and could throttle back. Just me wanting a belt AND suspenders, and still wanting to hold my britches up by hand.

I like your guage! I'm gonna have to make me one of them. Especially as I try to determine if I have any boost at all right now!

kma

mr. Goodwrench · #13 09-26-2003, 07:32 PM

Volvo Turbodiesels had a spring loaded dump valve on the front of the intake manifold. The dump valve had a plunger which dumped excess pressure and made an electrical contact and illuminated a overboost warning lamp in the instrument cluster. This was in addition to a wastegate on the turbo and a manifold pressure activated smoke limiter (ALDA?) on the injection pump. No switchover valves were involved. The mounting for the dump valve had about a 1-1/4" flange. The air dumped was routed back to the intake side of the turbo. Pretty ingenious.

DslBnz · #14 09-27-2003, 12:32 AM

I have an SDL which is limited, as Marshall said, by fuel enrichment properties.

But the question is, why so high in RPM's? It limits itself practically at 5500! I thought the set redline was 5150 +- 50, but I can bring it up to 5450, before it cuts the fuel out. Hmmm...,

I read on MBUSA that the '97 E300 had a redline of 5450 rpm, so it seems to be set like that car.

Fredmburgess · #15 10-22-2003, 06:16 PM

Ok, so I know you'll all throw stuff at me, but I've got my '84 300D turned up to max just under 13psi boost at full load, max rpm. Have been flogging the poor thing up and down I-81 the past two months at autobahn speeds on diesel and/or WVO and nothing has gone "Bang!" or melted ... yet! I'll let you all know if (when?) something bad happens, but on the trip home tomorrow it'll turn over 396,000 miles and seems to be running better every trip! A little more boost ain't necessarily bad...and it certainly does make this sucker go! Besides, isn't the upper limit of the spec for boost just over 15 psi?