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I’m struggling to find a starting point. But a bit of background. Bought a 91 350SDL, great car, sloppy paint on the rear. Interior is pretty spotless, no rust. The issues might be related, or they might not, and that is why I’m here. You get in the car, glow, start. Everything is great so far. Oil pressure is pinned to 3, rpm might be a bit high, I would give you a number but the tachometer is burned out (blown resistor on the actual gauge). So the issues arise once the car is up to operating temperature. Putting it in gear drops the RPM noticeably. You go about a drive, shifts smooth unless you accelerate with pedal 3/4 way down, then there’s jerking before the gear change (this is not an issue at cruising speeds). Let’s say you want to put the car in neutral or park after a drive. Once it’s P or N, RPM gradually surges over 5 seconds (over 3k for sure), once again, I would give you a number, but the tacho is out, keep up. The quickest remedy is to turn A\C on, any load on the engine returns the idle to a lower manageable RPM. This whole time however the oil pressure gauge has been goin up and down. At idle, (at operating temp) oil gauge is is at 1.5 and it naturally goes up with RPM. At cruising speeds it’s usually at a high 2 to 3 (I’m assuming rpm is about 2500/3000 here).
There are some power grounding issues around the vehicle I’ve been plugging here and there. Aux fan had a faulty ground. I wired to the relay so it’s constantly on when the A/C compressor is on. Cabin lights don’t function, I’d blame the shorted out light at the bottom of the driver side door, melted planting, charring and all.
The car doesn’t generate any noticeable smoke, not even during quick accelerations, a bit of white smoke at start up (2 bad glow plugs, ordered)
Any help would be greatly appreciated, if guys have any unrelated tips, feel free to share.
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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Welcome Aboard, man!

I speak general W126, but the diesel version I am not much help... I am sorry.

Others with more practical knowledge will chime in for sure!

MBL
 

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350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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If the idle is not steady in neutral and the throttle linkage is not moving, the most likely cause is the injection pump governor. First thing I'd do is change both fuel filters and treat the fuel with a heavy dose of a quality additive. I prefer FPPF Total Power, personally, YMMV. Make sure no air is getting into the fuel system. If there are no apparent leaks, the only way for air to get in is between the tank and the fuel transfer pump. Everything after the pump is under pressure.

As far as the shifting goes, the engine doesn't know when the trans is going to shift, so I'm not sure how that could be anything other than a trans issue, or maybe a kick down cable adjustment. Unless the engine is consistently running out of fuel at that power level. Can you get it to jerk at the same RPM but under light throttle? If it's only heavy throttle then, again, the fuel filters/fuel delivery are suspect. Otherwise, similar advise; change the trans filter, drain the torque converter and refill with new fluid.
 

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1983 380-SEL, '67 220D (sold), '65 200D (sold), '75 L-206D (sold)
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Double check the engine ground is attached correctly to the body. Not sure on your model. Usually, it is a braided cable below the power steering pump and connects to the fender where there would be a ignition coil for a gas engine. Sometimes someone works around that area or changes a power steering pump and does not reattach the ground correctly. Strange effects bad grounds can happen.
 

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Discussion Starter #5
If the idle is not steady in neutral and the throttle linkage is not moving, the most likely cause is the injection pump governor. First thing I'd do is change both fuel filters and treat the fuel with a heavy dose of a quality additive. I prefer FPPF Total Power, personally, YMMV. Make sure no air is getting into the fuel system. If there are no apparent leaks, the only way for air to get in is between the tank and the fuel transfer pump. Everything after the pump is under pressure.

As far as the shifting goes, the engine doesn't know when the trans is going to shift, so I'm not sure how that could be anything other than a trans issue, or maybe a kick down cable adjustment. Unless the engine is consistently running out of fuel at that power level. Can you get it to jerk at the same RPM but under light throttle? If it's only heavy throttle then, again, the fuel filters/fuel delivery are suspect. Otherwise, similar advise; change the trans filter, drain the torque converter and refill with new fluid.
Got a huge care package coming in. Oil change, fuel filters, transmission filter. I suspected a torque converter, but you’ve convinced me. So the jerky rpm is difficult to explain. I can slowly accelerate to 100mph no problem but if I’m going 80mph and go half throttle it jerks back and forth. It occurs at the top end of the rpm range.
 

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350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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I can slowly accelerate to 100mph no problem but if I’m going 80mph and go half throttle it jerks back and forth.
How much throttle to slowly get to 100 mph?

Have you looked at the fuel? It should be clear, not cloudy, and sort of straw color. Where are you located? Are we talking great white north? fuel jelling is a distinct possibility. Are you still running the tank of fuel that was in it when you got it?

Change the filters and run a double dose of the additive of your choice. See if things improve.
 

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Discussion Starter #7
How much throttle to slowly get to 100 mph?

Have you looked at the fuel? It should be clear, not cloudy, and sort of straw color. Where are you located? Are we talking great white north? fuel jelling is a distinct possibility. Are you still running the tank of fuel that was in it when you got it?

Change the filters and run a double dose of the additive of your choice. See if things improve.
It’s whenever I accelerate rapidly at high rpms. If I ease it to 100mph it eventually gets there. I wish I could give you rpm numbers. Might wire up a stand-alone Tacho just to get some readings. Any cheap recommendations?
I’ve done a couple of rounds of additives. I’m looking at liqui molly diesel purge in a couple days when I do all the maintenance.
Not sure if I mentioned this, but I do have oil seeping out of the top of the injection pump, at the barometric pressure thing. Could have been due to mismatched vacuum lines which I since redone after acquiring a diagram. That same diagram shows a vacuum component that I seem to be missing. It was “vacuum multiplier” or something of the sorts. It’s located driver side over the wheel well. My buddy’s 87 300sdl has it.
 

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UPDATE:
So I’ve been doing some work past few days. Here’s where we are. In an effort to replace my glow plugs, I noticed an oil leak at the intake manifold. So I took manifold and diesel lines out (which were the biggest pain ever). Replaced gaskets on intake manifold. Deleted EGR while at it. The EGR was pushing oil back into the intake. Found it better to just block it off. Cleaned out most the oil from the intake manifold, there’s plenty of carbon/oil buildup on the valves. I’ll try to upload a picture. After that and the oil change I turned the car on without the turbo linked up to the intake just to see if the idle surged. Sure enough, once the car got up to operating them and I moved shifter from P to D and back to D the idle creeped up. Plot twist, I turned the ignition off and the car was still on roaring with me running to the engine compartment to push the wonderful stop lever. (Forgot to plug ignition vacuum back up after installing manifold). So idle surging might be a vacuum issue, it’s not a throttle linkage issue, I lubed them nice. Wasn’t oil coming from turbo either. Could be IP, I’m saving that for last.
Solved my grounding issue as well. Engine ground seemed fine, so I moved to the battery ground which seemed to be bolted up to a painted surface. A wire brush and 30 min looking for right sized wrench later and ground is good! Still need to go back and remove all the separate grounds on some systems to see if that issue is fixed.
There was also a line at the waste gate that was broken, I replaced that. Noticed it when working on EGR. Might have fixed jerking at night rpm, we will wait and see, couldn’t take the car out as it was still on the lift waiting for fuel filters and drive shaft adjustment (wasn’t put on correctly after universal bearing install. Has a shake at low speeds).
2618280
2618281
 

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350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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Unlike with a gas engine, a vacuum leak won't affect the idle. Air is unrestricted in a Diesel (other than the obvious issues, it should run just fine even with the intake manifold completely removed) and engine speed is controlled by the fuel rate. If no oil is getting in from external sources (crankcase vent, etc) then it has to be the injection pump. The fact that it shuts off with the key or the shutoff lever also indicates it's the injection pump causing the elevated idle.

Your second photo appears to show some contamination in the injector inlet. I hope that is not the case.

Still, step #1 is always to ensure the injection pump has a good, uninterrupted supply of clean fuel.
 

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The 350sd/sdl uses a electronic control system called EDS (Electronic Diesel System) specific to these and the w124 diesels. On the rear of the injection pump there is a 2 pin plug which controls the injection pump. Upon unplugging this, your idle will default to around 650 RPM without increase for AC. Sometimes the control module fails, other times other sensors, as the whole system runs off a resistance loop. It also is shutdown if the OVP relay is bad/fuse blown. The EDS controls the tach, as it compares the AC RPM to engine RPM to disengage the clutch if belt is slipping. Since your tach isn't working, you may want to check power to EDS computer and try unplugging the connector on the IP (top of rear of pump, vertical connector.
 

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Discussion Starter #11
The 350sd/sdl uses a electronic control system called EDS (Electronic Diesel System) specific to these and the w124 diesels. On the rear of the injection pump there is a 2 pin plug which controls the injection pump. Upon unplugging this, your idle will default to around 650 RPM without increase for AC. Sometimes the control module fails, other times other sensors, as the whole system runs off a resistance loop. It also is shutdown if the OVP relay is bad/fuse blown. The EDS controls the tach, as it compares the AC RPM to engine RPM to disengage the clutch if belt is slipping. Since your tach isn't working, you may want to check power to EDS computer and try unplugging the connector on the IP (top of rear of pump, vertical connector.
My OVP is working fine, the tach is a burnt resistor on the tach itself. I’m reading voltage at the 2 pin at the back of the cluster for rpm. I believe the previous owner had messed with the IP. I’ll try some remedies I’ve been reading about tomorrow. But the idle creep and failure to return to idle might be a different problem than the different isles under different loads. I’ve been in 2 SDLs other than mine where RPM doesn’t vary under loads.
 

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Discussion Starter #12
Unlike with a gas engine, a vacuum leak won't affect the idle. Air is unrestricted in a Diesel (other than the obvious issues, it should run just fine even with the intake manifold completely removed) and engine speed is controlled by the fuel rate. If no oil is getting in from external sources (crankcase vent, etc) then it has to be the injection pump. The fact that it shuts off with the key or the shutoff lever also indicates it's the injection pump causing the elevated idle.

Your second photo appears to show some contamination in the injector inlet. I hope that is not the case.

Still, step #1 is always to ensure the injection pump has a good, uninterrupted supply of clean fuel.
My train of thought was since Alda controls fuel enrichment during high boost that it was somehow faulty getting intake manifold pressure therefor enriching fuel at idle somehow making idle to creep up but then I saw a post (pic attached) that made sense and describes my problem.
2618505
 

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SOLVED!
We have done it... mostly. I’ll itemize this and list the fixes.
  1. Surging/Stuck high idle: well, after months of just messing with the idle adjustments, it turns out that alone wasn’t enough to fix it. The trick was to bring up the base idle on back of the IP then lower the idle adjustment that’s connected to the throttle linkage. I turned CCW about two full turns while sliding the idle adjustment back about an inch. If I had the Tacho I would give numbers but, car isles fine, and returns to idle perfectly now.
  2. Jerking at high rpm: well this one was due to a loose hose that is connected to the waste gate, replacement has car driving smooth through all RPM ranges.
  3. Rough start up: most obvious, 3 bad glow plugs, replaced, been starting up with minimal smoke in 30 degree weather.
  4. Shaking at rapid acceleration: driveshaft was installed incorrectly, adjusted, still mild shake, way better than before. Noticed a wobly universal bearing I think it’s called. The rear one is brand new, front, not so much. Rear flex disk seems new, front on has minor crack between two bolts (culprit?)
  5. Grounding issues: battery negative had a bad ground: took bolt out, sanded, went at it with wire brush, put bolt back into body. Although there’s a solid connection, I still have a bad ground to the aux fan. Radio speakers go out at loud volume, cabin lights don’t work. (Bad fuses? They look okay) this will require more work.
So I never replaced fuel filters when I changed oil. I really wanted to check things off the list, god to say fueling system is good. Minor diesel leak at lines, so will change filters in the next month while I change fuel lines.

I wanted to thank you guys for your help, you suggestions helped directly or made points that lead directly to fixes.
Time to daily my 91 SDL :)
 
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