Free Horsepower for 1987 300D (and 300SDL?) - PeachParts Mercedes-Benz Forum

gsxr · #1 03-27-2002, 05:14 PM

This is a copy of a message I posted to the MBZ.org diesel email list:

===========
Hi all,

While doing some other stuff to my second (running) 300D, I finally tried disabling the "cruise surge fix". This was an add-on to reduce cruise control surging on hills, or some such malarky. It was due to the operation of the EGR and/or air recirc valves, I believe. If these have been disabled, it's my opinion that the cruise retrofit is unnecessary. For the record, on my first 300D, I NEVER had any surging - and that car never had the retrofit installed (still just 2 s/o valves - this car also has the EGR & recirc disabled.)

How do you tell if you have this? Look at the plumbing between the intake, ALDA, and tranny valve. Normally there are only 2 switchover valves. The cruise "fix" adds a third, with some restrictors and a filter. I disconnected the third with it's plumbing. It should go from intake, to overboost s/o valve (#1), out to ALDA, and "Teed" from there to the tranny switchover valve (#2, which softens cold shifts).

I did this temporarily and from the seat of the pants, it feels like there is significantly more power from ~2000rpm up - basically where the turbo is working. I believe the restrictions limited, or slowed, the signal to the ALDA and therefore reduced fuel enrichment. Marshall mentioned this a while back. Oddly, the first time I stomped on it and the boost came up, I got a MASSIVE black cloud out the rear for about 3 seconds. And this car NEVER smokes like that. It did not do this on subsequent throttle mashings, and has not since then.

Hmmmm. Anyway, I think I might reconnect it to try to verify the improvement, then permanently disconnect the "retrofit", and remove the valve and assorted plumbing. I'll try to take some pictures too.

Note: The retrofit installation is explained in detail in the OM602/603 engine manual. Look in there for diagrams and more info. But I recommend anyone with this "fix" in place at least *try* disabling it, you might be pleasantly surprised!

[:-)

goldenbear · #2 03-27-2002, 06:55 PM

Dave,

Did Dr. Booth chime in on your post? I'll check mine tonight.

gsxr · #3 03-27-2002, 08:44 PM

Marshall's only comment was something like "Doug Q. had long claimed the cruise surge fix robbed power."

After studying the vacuum diagrams some more, I understand how it works, and why it robs power. What it does is normally send the boost pressure signal to the ALDA through a restriction orifice, to make the ALDA see a more constant pressure, even if the actual pressure is changing frequently. This keeps the fuel supply relatively stable even if the boost pressure is varying, therefore reducing power "surge" at cruising speeds. The 3rd switchover valve is wired to the "WOT" switch on the throttle linkage, AFAICT. So in theory, when you floor it the 3rd s/o valve should energize, and bypass the "restricted" circuit to the ALDA. If it all works as designed, you lose no power at full throttle, but you DO lose power everywhere except 100% throttle. Yuck!

And like I said, I think this "fix" is ONLY to "band-aid" the air recirculation valve in the first place - which was only there to "provide more favorable conditions to the trap oxidizer", according to the OM603 service manual. And since the trap is removed due to the factory recall, there is NO point is leaving the air recirc connected - it just bleeds off boost at certain RPM's. Disconnecting it *should* cure the cruise surge, making that whole 3rd s/o valve & restrictor unnecessary. Both of my cars have the EGR & recirc disabled (solenoids & all plumbing physically removed), and have NO "surge" problems whatsoever. (And they're pretty quick now, too!)

gsxr · #4 03-28-2002, 12:42 AM

Here's an update, copied from my post to the email list:

===========

I wanted to verify the "seat of the pants" power increase befor removing the unnecessary stuff in the engine compartment. I plugged the hoses back in the way they were (reconnected the cruise fix) and went for a drive. Yep, it was slower. Then I went to disconnect it and found the manifold hose popped off. Huh? I'll spare you the diagnostic details, this is what I discovered:

1- My "cruise fix" was plumbed incorrectly. I studied the manual and it makes sense. It puts a restrictor in line to the ALDA, but bypasses it (returns to normal) at WOT per the WOT switch on the throttle linkage. However, mine - had it worked - would cut the boost signal to the ALDA at WOT! The way it was plumbed, the "T" went to the wrong spot on the overboost valve, so if the overboost sensor ever kicked in it wouldn't work (cut boost)! Unbelievable.

2- The WOT switch is either faulty or there is a bad connection. Engaging it does not energize the 3rd s/o valve. The valve itself is electrically good (clicks audibly when fed 12VDC). I need to further test the WOT switch.

3- The 3rd S/O valve leaks internally. In its normal position it should be open from 1-2 and have 3 closed. It would seep some pressure into 3 when it shouldn't have. Since it's removed now it's obviously not a problem. I think the oil in the lines (blowby from the intake) ruined the tiny little rubber seal on the plunger internally. (I tore one apart once, there's very little to them inside!)

4- This car is stinking fast with the ALDA disconnected! My other 300D would drop from 13 seconds 0-60 (with ALDA) to more than 20 seconds with the ALDA disconnected. This car ran a sub-15 second time with the hose at the manifold pulled off! =:-O From memory, this car (with cruise fix) would do low 13's 0-60.

5- With the cruise fix permanently removed, good hoses & clamps installed, it now runs consistently 11.0-11.3 seconds 0-60 with a stopwatch (factory spec is 10.9 seconds for 0-62mph). I have a G-Tech Pro but didn't have time to dig it up and use it. I'll do that when I get the blue beast on the road again, for some more comparison. I'm pretty happy with it right now tho.

6- The tranny now flares when the car is cold, until the 50C switch above the T-stat disengages the "cold shift" s/o valve. I turned the modulator at the tranny in 1 turn CW, but now it's a tad harsh when warm. I think I need to fiddle with the knob on the blue flying saucer? (Justin? Any tips?)

7- This car has a new 50C temp switch. It is closed when cold and opens above 50C. I checked the one on my blue car and it is always open (always thinks it's hot). So my blue car has never had the "cold shift softener" circuit operating! Hmmm. Either I'll fix the other car too, or remove all the shift softening stuff if I feel it doesn't make that much of a difference when cold.

Sooooo, it appears I spoke to soon. The cruise surge "fix", when installed and operating correctly, should not reduce full throttle power. It will reduce part throttle power by dampening (delaying) the fuel enrichment to the ALDA. I still think it's worth disconnecting but your results may not be as drastic as mine, given all the stuff that was wrong on my car! Of course, I took a couple of photos. I'll upload them shortly and post the URL.

Best regards,

Dave M.
Sacramento, CA
1987 300D - 229kmi (still waiting for brake pads)
1987 300D - 237kmi (cruise fix just removed)
1984 300D - 208kmi (about to get new air cleaner, sunroof motor, & door lock vacuum element)

gsxr · #5 03-28-2002, 01:04 AM

OK, sportsfans, pictures before & after of the plumbing are posted at the URL below. Notice how much less clutter there is with it removed!

http://www.meimann.com/images/mercedes/OM603_intake/

Regards,

jcyuhn · #6 03-28-2002, 08:28 AM

Try unplugging the electrical connector to the 50C "tranny softener" switch and see how you like it. This will stop the flaring, though it may firm up the shifts more than you like. Remember, MB trans shift more firmly when cold. I found that I needed to disconnect mine after changing to synthetic ATF (Mobil 1). No flaring, and the syn ATF shifts very nicely on the coldest mornings. Problem solved.

gsxr · #7 03-28-2002, 10:00 AM

Jcyuhn,

True, unplugging it will fix the flaring. However, Mercedes designed it to work without flaring. I'd like to be able to figure out how to adjust it, rather than disconnect it... just so I know how it works! There's a knob on the blue vac amplifier, I think I'll play with that.

goldenbear · #8 03-29-2002, 04:03 PM

Dave,

Would you mind detailing the location of the air recirc valves or add a pic? I've plugged the vac line going to the EGR, but am not clear on the arv.

gsxr · #9 03-29-2002, 08:22 PM

Chris,

The air recirculation valve is the only item on the turbocharger with a vacuum hose attached to it. It is in a slightly different place depending on if you have a Garrett or KKK turbo. On the KKK, it's right next to the air intake, where the rubber "elbow" attaches. I'll have to look at the Garrett and/or take a picture. Make sure you plug the hoses tightly if removed. The air recirc is on all 87 300D's, but may only be on the 87 300SDL, not the 86... the 1986 model didn't have the trap oxidizer.

HTH,

gsxr · #10 03-31-2002, 07:19 PM

Here's the latest on the power/acceleration tests on my car. I'm also posting this as a new topic, to see if I can get tips on adjusting the wastegate...

================

Hi all,

I tested my boost pressure yesterday, and also timed the car with my G-Tech Pro. The pressure is consistently 12.0-12.5 psi max at full load. I then removed the rubber wastegate hose and plugged it, and went for a drive. The boost would go easily over 15 psi, near ~17 psi or so. I was too chicken to do a redline run to see what the max would be. I also checked my overboost sensor, it engages at 17.5 psi. It did not cut in on the short (1-mile) test drive. Spec per the book is 0.85-0.95 bar, or "13.75-14.25 psi". Is it just me or does that math not add up? Shouldn't that be more like "12.25-13.75 psi", if 1.0 bar is ~14.5 psi? Hmmm.

I then disconnected the ALDA and reconnected the wastegate hose and drove the car. Same 12.0-12.5 psi max boost, even with NO fuel enrichment! That kind of contradicts Marshall's theory, unless I'm missing something? I plugged the MityVac onto the canister to check the operation. The control rod doesn't budge until 15 psi, which is reaaaally weird... That should make the operational boost 15, not 12, shouldn't it? If not then I guess there is no way to set the boost with the engine off. Bummer! Any adjustment would require a test drive to verify.

So I'd like to increase the boost to ~14-15 psi instead of ~12. AFAICT, I need to shorten the threaded rod from the wastegate can to the valve. It looks like a total bear to access. Anyone BTDT? Any tips?

Oh, yeah - acceleration. Per stopwatch I was getting 11.0-11.5 for 0-60 times. The G-Tech Pro gave me ~10.75 times. With "powerbraking" against the torque converter, it dropped to 10.0. It seems the G-Tech disagrees with my speedometer by 3-4mph and gives optimistic numbers. I do have different wheels/tires, however the circumference is within 1% of stock and I tend to believe the speedo over the G-Tech. I wish I had something more accurate to test with! Oh well...

Regards,

Dave M.
Sacramento, CA
1987 300D - 229kmi (brake pads on the way, so I'm told)
1987 300D - 237kmi (running pretty darn well with a cracked head)
1984 300D - 208kmi (needs a new turbo - anyone have one for sale cheap?)

surfblau · #11 07-08-2002, 05:44 PM

I have just plugged the recirc and EGR vac supply lines at the vacuum pump in order to get a hold on the source of my very hard shifts when cold.

As I plug up more of the vacuum leaks, my shifts seem to be getting harder.

Did adjusting the saucer help you, or are your shifts still prety firm?

psfred · #12 07-08-2002, 07:28 PM

Surfblau:

You need to verify that the 50C switch is closed below 50 C, and that you have good vac to the entire tranny cicuit. I replaced nearly all the rubber bits and still had a problem until I discovered the leaking EGR and recirc tansducers.

By the way, I did have cruise control surge problems with the recirc sort of working -- this may be the problem. I had runaway going up hills -- would end up 5 mph or so faster than set, then have the car dive back down in speed suddenly. Went away when I plugged the air recirc control line.

My brother doesn't have any trouble with his 87 300DSDL, with a brand new turbo (just got the trap oxidizer replace this winter!).

I'm planning to replace the actuator for the air recir to see if I can boost milage a bit (I got 36 with it working, 30 without it). I'll probably disconnect it again if I get cruise control surge back.
Peter

surfblau · #13 07-09-2002, 12:10 AM

I have heard mention of it on this board, but I cannot find any mention of it in the 126 manual with the vacuum diagram from the injection system - section 500.

Can you describe it, or where it is in the vacuum lines?

Any other ideas?

alec

psfred · #14 07-09-2002, 09:11 PM

Both vacuum actuators are up by the air filter. Should be obvious which one works what. The vac supply is the two branches on the passerger side of the 5-way "T" at the front of the engine.

From the actuators (really switchover valves, sort of) there is a vac line to the EGR and recirc valve. On our turbos, it is in front, very obvious. Dont' know were it is on the others.

On the W126 you can get to the vacuum actuators without removing the air cleaner. On the W124 it has to come out.

Peter

surfblau · #15 07-10-2002, 12:30 PM

It sounds like the 50C switch is the same as recirc transducer. If that is the case, like you said, it is pretty obvious which circuit is the EGR and which is the recirc.

I may be looking at replacing the recirc transducer.

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