Uh oh...

I was fiddling with things today (replacing fuel hoses, etc) and decided to do some cleaning in the engine compartment. While doing so, I found something that surprised me. The valve on the firewall that senses boost pressure (the line coming from the intake manifold and the line coming from the ALDA attach to it) has been bypassed. The two lines are now connected directly to each other, although I was surprised to find the ALDA line going into the banjo fitting that (what looks like) a vacuum line also goes to. I don't know how long it's been like that, but it's certainly from before I got the car.

First, how do I ensure that the valve's actually in working order? Second, what exactly can bypassing this line do? Last time I found a turbo boost line altered on a vehicle, it allowed the engine to overboost and overfuel, and irreparable (at least without completely rebuilding the engine) damage had already been done, so you can see why I'm seriously concerned about this... BTW, the line is partially cracked at the ALDA side, although that may have just happened. The car (at least I think ) drives perfectly...

Any thoughts would be appreciated. Thanks in advance!

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999

You can test the valve by applying 12V to it and checking that it doesn't let air through. If it doesn't let air through without +12V then it's clogged. I cleaned one on an 81 300SD with brake parts cleaner and it worked fine afterwards. I didn't have the car much longer after the cleaning so I don't know how long it lasted.

I don't think you'll damage anything if your wastegate is set properly. How old is that turbo anyway?

You can hook up a boost gauge or an idiot light that comes on when there's a ground signal from the overboost sensor on the intake manifold. If you drive hard and the never see 10psi or the light come on, then nothing would cut the boost signal to the ALDA.

I don't think there's any risk of ruining your engine with the overboost solenoid bypassed unless you run flat out all the time.

Clarification - the overboost solenoid gets a constant +12V and ground when the pressure sensor on the intake manifold senses too much pressure... whatever that threshold is. By suggesting that you test with +12V I mean test it on a bench.

Sixto
95 S420
87 300SDL

Sixto,

Thanks on the testing. I'll give that a try tomorrow.

The turbo, AFAIK, is original. I do have to admit that Italian tune-ups aren't uncommon for me but I don't typically cruise any faster than 3000 RPM or so.

I have no idea if anyone's monkeyed with the wastegate. I haven't touched it. If that valve works perfectly, then whoever did this may also have messed with the wastegate...if the valve doesn't work perfectly, someone probably bypassed it instead of replacing it...

Here's to hoping... *fingers crossed*

Also, just thought about this...is there a way to test the sensor on the intake manifold that works the boost switch? I guess, since I'm going through everything else, I should also cover this...

The sensor opend the valve at 14.5psi. I don't know of a way to test it. At $56, it's not an item you should replace right away unless you KNOW it's bad.

http://catalog.eautopartscatalog.com/mercedesshop/wizard.jsp?partner=mercedesshop&year=1982&make=MB&model=300-TDT-001&category=B&part=Boost+Pressure+Sensor

It's my belief that ANY car/truck with a turbocharger should have a boost gauge of some kind. You should get one asap and find out exactly how much boost you have. If you keep an eye on it every now and then to make sure it does not go above 13psi, you should be fine.

The turbos in these cars will go over 15 psi (I got scared when testing my boost controller and hit 16, so I let off). I know when warmed up, mine does it quite easily when cruising at highway speeds and hitting the throttle relatively hard (not getting to kickdown though).

Fortunately now my boost controller keeps the boost at 11 and under. But Id worry about it getting too high, since I know how easily mine overboosted. Even if heat wasnt an issue, Id bet the alda has some sort of a limit and you could probably blow out the diaphragms in that. Just a guess though

JMH

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Chances are the valve is clogged or blown out -- it should hold pressure indefinitely when off and vent out the top cap when energized.

The boost signal line probably goes both to the ALDA and the vac amplifier for the tranny control -- shift is modulated on both throttle position and amount of boost.

Here's what I would do: put the line to the ALDA back on the switchover valve, the attach some hose to the other side and blow into it. If the valve vents out the top, it's shot and you need to replace it. Pull the line to the ALDA off it the valve holds, and make sure you can freely blow through it -- if not, replace it.

Next, find the pressure sensor on the intake manifold and pull the wire off. Should be the only electrical connection on there, but I'm not exactly sure where it is on the 617. Pull the wire. With the ignition on, the switchover valve should be closed (hold pressure), with it grounded, it must vent out the top. If the valve works, but stays open (vents) with the switch connected, the switch is bad, replace.

Check the banjo bolt -- you should be able to blow down the line to the switchover valve freely. If not, remove the banjo bolt (rear of the manifold) and clean it out.

Check all the connectors and plastic lines on the boost signal system. All of them usually have clamps, else they tend to blow off. Replace any cracked plastic lines or hard, mushy, or torn rubber bits.

I have a sneaky feeling you may get more power when you are done.

I would not drive for a long time with the overboost protection disabled. Wastegate failures on these turbos is usually a bad spring, limiting boost to minimal pressure, but I'd sure hate to find out that mine was the exception....! RUnning without one when you need it is a good way to melt the pistons.


Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Believe me, I know about that one...that's what happened to my old Scout. Someone had bypassed the wastegate, allowing the engine (198ci Nissan turbo diesel) to overboost and overfuel. By the time I found out and repaired it, the truck had already cracked a piston, although it took almost a year to manifest itself...

So, the switch in the intake manifold completes the ground circuit? Also, when you say "hold pressure", I presume you mean that the air's allowed to go through the lower hose fitting and the upper fitting, and there's a third fitting on top that serves as a "blow-off"?

Just wanted to double-check...thanks!

I got the valve pulled, and sure enough, the bottom nipple is all clogged up.

Will I damage anything by immersing the whole thing in lacquer thinner and letting it sit awhile to soften up however much gunk is in there?

Also, is there supposed to be a plastic cap on top of the top nipple (I presume, the one that allows the boost to vent)? It fits rather loosely, so I'd presume that air could still get out...

Thanks!

The valve parts are plastic and/or rubber. Laquer thinner will probable cause some trouble.

You need a new switchover valve. Not too expensive, I think. The plastic cap is factory, it keeps crud out.

You may be able to ream the hole out, but don't count on it. Try some brake parts cleaner, and blow backwards through it. If it clears, you're good to go, at least for a while.

Switch grounds the switchover valve, its supplied with voltage all the time.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Figures...after not getting a reply immediately, I tried soaking it anyways. I do'nt know why, but i kept refreshing the window, and your reply didn't show up 'till just now...but I've been soaking the thing and still no joy, so you're probably right that I need to just replace it. It's about $30 from Fastlane.

I guess I'll let it soak overnight and see if anything happens, but I won't expect anything...

Has anyone ever heard of a wastegate failing and staying closed? Everthing I have ever seen/heard regarding wastegate failure is they fail open and no/not enough boost is produced. As long as the wastegate hasn't been adjusted by an overzealous tinkerer you won't have any problems. $30 on a new valve is a waste IMHO. $30 will buy you a nice 0-15psi boost gauge from www.egauges.com or www.dieselpage.com Then you can monitor your boost yourself which is also a great diagnostic tool. MB overengineered these cars, designing every possible failsafe so the "luxury car owner" didn't have to think about what really happens under the hood. RT

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When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K

I've thought about putting a boost gauge, along with a pyro, in the car. However, I have yet to figure out where to mount the gauges where it'll look halfway decent.

Also, will I notice anything, besides a power loss due to some lost boost pressure, by driving the car with the hose disconnected?

I ask 'cause I want to get tbe car back on the road (between the fuel stuff and this, the poor thing's been sitting a week), and if I don't get the replacement valve, I definitely want a gauge (again, where to mount?) and would like to be able to drive the car before that...

Thanks!

BTW, unless DIS has 'em cheaper, I'll probably get any gauges straight from Isspro... Also, DIS is a good group of guys; that's where I got the injector pump for my truck two years ago...

I have my boost gauge mounted in my passenger inside vent. You could also use the other center vent. Since MB designed these with such poor airflow from the start, you shoulden't miss them much. It could be possible to mount them in the vents and have the ducting still blow air around the gauges.

On the steering neck before the instrument cluster is anothe popular spot. With alot of work, you might also be able to mount it above the climate control buttons.

As for the bost line, get a 2-3" length of rubber tubing/duct tape and connect them directly. It's likely that if nothing has happened yet due to being bypassed, nothing will happen before you get the new overboost protection valve.

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Back in the days of tuning turbo gassers I knew to keep it under 1250F or some such number. How hot is too hot for an MB Diesel?

The folks at www.westach.com make some interesting derivatives of aviation gauges for automotive use. If you have a 240Dt you can get a 4 face EGT gauge http://www.frostalarm.com/cat/p15.html

Sixto
95 S420
87 300SDL

The "magic" numbers are 1250° pre-turbo and 1000° post-turbo.

I putzed around on DIS's site and found that they have 0-15 psi gauges...very I might get two of these mounting cups and put them on the A-pillar...again, haven't decided for sure yet. I don't want to lose my two center vents and I don't think there's space for a 2" gauge up by the ACC controls...