'85 300D very slow revving - PeachParts Mercedes-Benz Forum

ironandsteel · #1 10-25-2018, 05:25 PM

I'm trying to get the '85 300D turbo running decently. It had been sitting for several years before I got it.

After changing crankcase oil and trans filter and fluid, air filter , fuel filters, primer, it would start right up, but the engine had very little power, and it would sputter sometimes.

I drained the tank and removed the strainer- it was a terrible mess. I cleaned and replaced. While apart, I blew compressed air back through the steel fuel line and the return line.

I did the diesel purge thing- 1 can lasted about 23 minutes.

At this point, it is definitely better, but it is still pretty slow to rev when you goose the throttle- it seems like it takes a a second before it gets up to high rpms. I also have a 240D and I think it may rev a bit quicker.

My next step is to adjust the valves. I've done this and I made custom 14mm offset wrenches for this purpose.

So at this point, it is close to driveable, but it doesn't acellerate like I think it should, especially being a turbo.

What else can I try? I'm pretty sure I'm not getting air incursion. I put on yet another primary filter and hoses and the primer pump feel right.

Maybe I need to open up the turbo and make sure it is good shape. Kent Bergsma discusses this.

Maybe I should change the large fuel filter again since I did the diesel purge and maybe it clogged up. Or maybe I should do a 2nd can of diesel purge. By the way, I'm running off-road red diesel fuel while doing my testing, since I have a big tank of it for tractors.

I do have at least some sort of vacuum leak- it takes a few seconds before it shuts off. There are several more vacuum lines and gizmos in there as compared to my 240. I guess some of it is related to the EGR and other pollution devices.

tyl604 · #2 10-25-2018, 05:52 PM

May need a new switchover valve or maybe the plastic lines to it are leaking or attached to the wrong nipples.

Here is mine and this is the new style; actually upside down from what came stock on my 1981 300SD. I could not get over 30mph when the old one got clogged up and this solved the problem. The top nipple is open to the air so do not plug it up.

ykobayashi · #3 10-26-2018, 07:51 AM

Hey,

I just went through this on my 85. My car is a low mile car but very neglected by the po. I solved my low power issues over the last couple of months.

Here is what I did in chronological order. It seemed that it wasn’t one thing that was messed up. There were lots of little issues robbing power at different rpm and load ranges tha needed to be fixed.

I checked and adjusted valves like you. This cleaned up the idle and got some power back. I adjusted the accelerator linkages and set the idle. No big improvement there. I adjusted the rack damper bolt and smoothed the engine out but that didn’t do much. The bolt was pretty much bottomed out by then.

I opened the turbo on the intake side by just taking off the rubber boot. Checked to see that the turbo spun freely (with the car off

). It was a bit sooted up and draggy so I sprayed it with carb clean and it loosened. I got some power back.

Checked the boost pressure at the ALDA under load. This will check the turbo banjo, switchover valve and lines all in one step. This is a common issue that will rob power. It was ok.

Changed an air filter. Bypassed the trap Catalyst which helped a lot. My cat was apparently restricting exhaust flow. But the car wasn’t 100% right at this point. Yes it made power but it wasn’t tractable power for around town driving. It was good wide open now.

The car now made power wide open but off the line it stuttered and just revved slowly.

I changed the fuel strainer and filter. And I changed to a cigar hose. The po used a regular fuel return hose. This helped the tip in power.

I turned my ALDA screw out 1/4 turn CCW. This helped. I started with 1/2 CCW but the exhaust seemed to laden with fuel and was nauseating. I gave up some power but it smells better now.

Then I decided to cut the locknut on my rack damper bolt down 0.060” and adjust it in a bit. This helped the shaking at tip in. Before this the car would shake as I pulled up and down my driveway. It struggled at low throttle position maneuvers. I mean I couldn’t floor it to make power to climb my sloped driveway to my garage door. Once I got the bolt screwed in enough it could heft its mass smoothly with ease and control. Before this point the pedal had to settings, up and floored.

With all those little things the car is now making good smooth power. I cannot say it was any one thing that fixed the power. Many problems contributed to the power loss so I had to undo them one at a time.

I’ve discussed the car a lot in my Got a 1985 300d thread. It was severely neglected. Basically the MO of the PO was if something broke, just keep driving. So eventually nothing worked and the car got parked and left for dead. Thankfully it had a good chassis, engine and trans. So I’ve spent two months fixing little things and gradually getting this thing drivable.

Put it all together and the car runs beautiful now. I’m still scratching my head thinking that it wasn’t just one thing. It was like four things that just weren’t right.

Hope that helps.

tyl604 · #4 10-26-2018, 09:35 AM

Yko - great post.

Diseasel300 · #5 10-26-2018, 09:51 AM

Don't underestimate the turbo as being part of your problem. A partially stuck open wastegate or a coked up turbine shaft will rob you of power.

I went through the latter in my SDL (which is a different turbo design, but still similar in construction). The rear oil seal ring had excessive side clearance allowing excessive oil leakage/consumption. The side effect was coke and carbon buildup everywhere on the turbine side, including in the ring gap itself. The turbine shaft was free to turn but stiff feeling.

I installed the rebuilt donor turbo on the car last night and the difference driving it is profound. Same model turbo, same turbine, same compressor trim but with a fresh rebuild and everything in tolerance. The car drives so much smoother and the boost comes on significantly sooner and more gradually instead of just suddenly kicking in.

I bring up my experience here because my SDL has always been kinda sluggish, especially off the line. Once boost is built, it was fine, but the top end was always weak, seemed to die off abruptly >4K RPM. I did everything suggested above, had good boost, clean path to the ALDA, even removed the ALDA at one point (which made it worse!). I knew the turbo was in poor shape, I just didn't realize how poor until it was addressed.

Make sure your wastegate is fully closed and the EGR is not stuck open (block it off with a plate if you want). If the turbo can't spool then it can't make boost. If it can't make boost (or if the boost can't reach the ALDA) then you get no fuel enrichment. You're effectively then running an N/A engine with an intake restriction and cut-back fuel.

ironandsteel · #6 10-26-2018, 11:54 AM

ykobayashi - thanks so much for the detailed post!

I will pursue the things you mention.

One thing I don't under stand is this: "Then I decided to cut the locknut on my rack damper bolt down 0.060” and adjust it in a bit. " - can you or somebody give me a bit more info on what you are referring to?

I think that it would be ok to totally bypass the switchover valve between the manifold and the ALDA, right? I've seen where some folks have done this.

Also- there is another device that hooks to the alda- it is a blue plastic item with 2 vacuum lines that is attached to the driver side wheel well. I wonder what this is for, and if it is needed.

I'm fond of the idea of simplifying the vacuum lines as much as is reasonable. And I think I'll do the ERG delete.

Diseasel300 · #7 10-26-2018, 01:26 PM

Quote:

Originally Posted by ironandsteel

I think that it would be ok to totally bypass the switchover valve between the manifold and the ALDA, right? I've seen where some folks have done this.

Also- there is another device that hooks to the alda- it is a blue plastic item with 2 vacuum lines that is attached to the driver side wheel well. I wonder what this is for, and if it is needed.

I'm fond of the idea of simplifying the vacuum lines as much as is reasonable. And I think I'll do the ERG delete.

I have the switchover valve bypassed on my car and I'm not losing sleep over it. The wastegate operates well before the switchover valve should ever operate and the fuel system is pretty well maxed out with the boost fully built anyway.

The blue thing on the wheel well is the vacuum amplifier for the transmission modulator. If it's working, it's worth leaving. It improves shift quality over the VCV directly operating the modulator. You should have 4 hoses on it by the way. Vacuum, Boost, Modulator, VCV.

tyl604 · #8 10-26-2018, 02:04 PM

I do not see any logical reason to bypass the switchover valve especially since it was put there as a safety device by the MB engineers to prevent overboost.

Bengoshi2000 · #9 10-26-2018, 02:24 PM

When I got my '91 300D it had been sitting for the better part of a year. Acceleration was sluggish, it was nailing and had a rough idle when cold. I ran two tanks of B99 biodiesel through it (about 1000 miles of driving), then changed all the fuel filters. Acceleration was much improved, nailing was eliminated and the idle smoothed out.

Just an idea if you have a source of B99 available.

jsp300D · #10 10-26-2018, 05:20 PM

checked any vacuum leaks? especially around the turbo seal.

turbo also checked on play?

g-wizz · #11 10-26-2018, 08:49 PM

when i first bought my car the turbo didnt even spin freely cause there was so much build up. try to clear that out? just rev it repeatedly.

ykobayashi · #12 10-27-2018, 10:54 PM

Quote:

Originally Posted by ironandsteel

One thing I don't under stand is this: "Then I decided to cut the locknut on my rack damper bolt down 0.060” and adjust it in a bit. " - can you or somebody give me a bit more info on what you are referring to?
.

Okay, look at a rack damper bolt over here:

http://www.peachparts.com/shopforum/diesel-discussion/183139-so-what-does-rack-damper-bolt-617-actually-do.html

See the lock nut around the middle? It’s is long. I made it shorter with a file so I can screw the bolt into the IP a little more. Mine was turned in so far it was bottoming out. Not much more but maybe 0.030” or so. It made a big difference in smooth delivery of power at low throttle.

As I said I did a lot of little things and all of them helped in different ways to make a great running engine.

I deleted my EGR. My egr and saucer had leaks so I just ripped it all out. I followed rollguys advice and just ran vacuum to the VCV and then to the trans. Vacuum to the locks, climate control and shutdown. It is really simple. Plumbed like my 82 300sd.

I was too lazy to set vacuum at my VCV and modulator so I just used some orifices the PO had plumbed into the system (he had them everywhere

) to restrict VCV bleed off till the car shifted appropriately - firm but not jarring.

Good luck. I’m really happy with the way my 85 CA turbo runs. It has this nice tall gearing and drives differently than my 82 300sd.

Diseasel - thanks for the tip on the turbos. I will have to keep an eye on my SD as it’s endplay is detectable. This is the kind of thing that can carry on for years and rob performance before it gets fixed.