['87 300D] Misfire at 3k RPM under load
 

Hey guys, I've seen that this seems to be the place to find answers to diesel Mercedes problems so I'm posting here trying to help a neighbor out.

He has a 1987 Mercedes 300D that has a strange misfire right around 3000 RPM and up. It seems to run perfectly fine until you hit the magic 3000 RPM when you'll suddenly start experiencing it. It also can be revved right up through the 3k range with no problems whatsoever when in neutral with no load. Now, he has thrown a lot of money at the car in an attempt to fix the miss such as fuel filters all around, replacing leaking injector seats, and rebuilding primer pump. He has also tried running some diesel fuel additives to clean the injectors and the fuel as the car sat for nearly a year (though the miss appeared before it was parked).

Here is the weird part... I was pretty sure it was a fuel issue until another neighbor of ours came out touting that he was sure it was completely electrical and pulled the computer connections and sprayed them down with an electrical cleaning solution. This caused a number of things that didn't work to start working and a number of things that did work to stop working (like the blower fan and tachometer). The owner and I were surprised at what an impact that made and we noticed during this that there was a small red relay looking device to the (if you are at the front of the vehicle) right of the computer connections that are in the far left back side of the engine compartment that when pressed a certain way would actually race and/or stutter the engine speed.

One time after jiggling this relay the miss was completely gone. For a short while, anyway. It eventually returned but it definitely made me think twice about my original conclusion and made me search out an answer all over the internet which I haven't been able to find. So I am hoping someone here can provide some insight or direction on where to head with this mind boggling issue.

Thanks in advance! And if you need more info on the car or what has been done just ask and I'll get you the info you need.

The red thing is the OVP relay, it powers the tach and electronic idle control (hence what happened when you poked it). I'm not sure how the electronic idle control would influence anything at 3000rpm though.

I suspect air in the fuel lines getting in a higher rpm. On top of the electrical issue, replace that relay to fix the idle and tach.

If this were an earlier engine .. up to 1985.. I would suggest those symptoms might mean valve spring (S ) was weak....

Pretty hard for a diesel to "miss" right? Seems like all it could be is fuel since there's really no electric involved in firing the cylinder.

As mentioned above, the engine is completely mechanical other than idle control.

It is possible that a valve spring has broken, I haven't seen it but theoretically possible (coaxial spring, so one can break and you have a weak spring remaining).

Fuel supply is the most common, is there smoke when this happens? If there is white smoke, it's getting fuel but missing on one cylinder (or more). If black smoke, it's starved for air (check air intake, filter, airflow meter, etc. for blockage). If neither, possibly a blocked Catalyst (if it has one, might still have the trap-ox which is a football sized can on the exhaust next to the battery compartment) or failed turbo.

Smoke is a big help here, let us know.

When he said that I assumed it was not a valve spring... but usually that would be a prime suspect.....

quick test for plugged cat: undo the EGR tube to open the exhaust manifold, yes it will be noisy but you'll quickly find out if the miss is restricted exhaust related

quick test for restricted air: take out the air filter

test for air entering fuel system: use a large plastic pop bottle full of diesel, disconnect fuel lines so suction from fuel pump (side of IP) is drawing straight from bottle, and disconnect return line and stuff it in the bottle too. (clamp and plug off where these lines used to connect to prevent tank from spilling)
After you run it a while to work out air from doing this operation, see if bubbles continually come from the return line. Go for a drive with load, with bottle secured under hood.

test for too-soft return check valve in the IP, by pinching off the return line so it is nearly shut. Go for a short test drive.

Wow, thanks for the fast responses! I'll be sure to relay some of this info to the owner.

As far as smoke goes I'm pretty sure when this happens there is no smoke.

And sorry for describing it as a miss, it's the first diesel I've ever tried to trouble shoot and I didn't even think about that, haha.

welcome to the forum. now you have to get a mercedes of your own you know!

most likely candidate is fuel lines/filters. air is the issue under load, if the tank screen is plugged, or partially plugged, at 3K the engine is pulling a lot of fuel, and ANY line deficiencies would result in air sucking into the engine. you can pressurize the fuel system with the car off, by putting a shop vac into blow position, and putting the hose into the fuel tank. then follow the lines, and look for wetness.

How do you feel the miss? Is the miss intune with the engine RPM (this would be very fast and almost impossible to feel I think). Or is more of a cyclic feeling of the car losing power and then getting it back?

good point, it could also be a bad motor mount, and you are FEELING the engine vibration under load. these motor mounts are hydraulic inside, and the owners (her?) car may have a rupture in one or both...

I was thinking more specifically over boosting. 3k is where the turbo should be around full pressure in 3rd gear and up. I disagree with everyone else here I think this is either over boost or injection pump/injector related.

yup. 3 K turbo is cranking for sure. overboost, hmm
lets see what the details are.

No smoke, hmm. Might even be a locked fan clutch.

Thanks for the info on what that is! We'll look into cleaning it up or just replacing it outright.

Right around the 3k mark the car cyclicly loses power and gets it back. I could try and get a video of it if you think it would help.

Yea, don't think it is a motor mount because the car is smooth as butter any other time. And I've owned a car that I've put solid motor mounts in so I think I would be able to differentiate motor vibrations from the issue that is occurring.

How would we check for over boosting? Like I said, the car runs fine until you hit the 3k mark. You can feel the turbo kick in and put you back in the seat under 3k RPM.