I've researched the forum and found a number of posts related to this issue; however, no confirmed solution or fix. Essentially the engine starts up and runs perfect; however, at about 55 - 65mph at a steady speed the engine begins "bucking" or hesitating and then surging. If I accelerate the problem stops immediately .... it only occurs at light throttle pressure at constant speed and not all the time it only happens intermittantly. Since it has occurred over multiple tanks of fuel I don't suspect poor fuel is the cause. The vehicle has 193k on the odometer and I've got relatively new injectors and all new fuel lines and delivery seals .... so there is no air in the fuel system and I seriously doubt that air in the fuel is the problem. I also recently replaced the K40 relay so that can't be it. Additionally, there are no trouble codes stored in the computer so I don't think its the flaps in the intake system since on the '97 models there is an electronic feedback loop to the computer to indicate if they are open or closed and improper operation throws a code. Since the problem starts and stops rather immediately, like a light switch turning on and off, I suspect some electrical control issue. Any ideas as to where I should check/investigate next?

David Roseman
Long Beach, Ca

Any thoughts? Anyone? I was doing a little research on-line and one possiblity that arose was the "pressure sensor" or MAP which is located on the drivers side of the engine bay and connected to the intake manifold via a small vacuum hose. Has anyone had trouble with the MAP? What problems did it create? What problems were resolved by replacing it?

Thanks for any insight you can provide.

David Roseman
Long Beach, Ca

David,

I'll respond but don't really have much more to offer you. The only time I have experienced your symptoms is with an air leak in the fuel lines - hesitation at speed that disappeared with acceleration. I know you have covered this - but you may consider if there are any o-rings you may have overlooked, it only takes one to cause problems. Having said that, I have a '95 and yours is going to have the additional sensors, etc. I lack the familiarity of your model and can't really offer anything in that area.

Doesn't really sound like filters - but if you haven't changed them for some time, it would be one more thing to check - as well as the orings there too.

Good luck.

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Does sound like air - new seals and lines not withstanding.. Did you replace the o-ring behind the shut off valve and the o-ring at the pre-filter? Things electrical - like the manifold air pressure sensor would likely kick a code if they were acting up, that's why I'm inclined to think it might still be an air / fuel issue. Sorry if this doesn't help!

The manifold pressure sensor disconnected will throw a P0400 code. Problems with the resonance flaps will throw a P0200 code.

I would first look at the EGR valve and confirm operation. It sounds to me like the intake diverter valve may be asking for exhaust gas (and possibly not getting it), and when you hit the throttle, it opens back up to the main air filter box. Just a guess though.

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Thanks for the response. I've changed the o-ring behind the shut-off valve as well as the one at the prefilter. I've had problem with air in the fuel lines before so I know what that feels like (more of a miss or stumble) and I'm almost 100% certain it isn't air leaks becasue this seems more like an immediate shutoff and then surge. I drove the car all over today (all under freeway speeds) and zero problems with excellent pickup and go. I also reved the engine a number of times at idle and could see no air bubbles in any of the new fuel lines. I'll check out the EGR operation .... maybe there is something with the control system. However, if the EGR works fine at lower engine speeds and lower exhaust pressure shouldn't it work even better/smoother at higher RPM's when there is more exhaust pressure and vacuum available? I'm I missing something? What control system works worse at higher RPM's? Thanks for your help.

How new are your fuel filters?

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resonance flaps

1996 E300 Diesel Surging. Please Help!

E300 resonance flaps, vacuum trasducers, and lower control arm bushings

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1996 E300 110,000
1985 300TD 212,000
1980 300D 238,000

The fuel filters were replaced at the same time as the fuel lines and o-rings. I took the intake crossover pipe off and cleaned it along with the flap. Both were dirty but not all that bad. The flap seemed to work fine using a hand vacuum pump. I did notice that the vacuum lines controlling the flap by the EGR valve and the vacuum line to the flap on the crossover pipe were cracked so I replaced both. I also swapped out the pressure sensor with another one of my cars that uses the same part (the problem didn't migrate to the second car). The car seems to be operating fine now ..... I'll test it on a longer freeway run over the weekend. Thanks for the suggestions.

I took the car on two long trips this past week. One over 800 miles and the other over 500 miles. The car performed perfectly with no problems. So I guess it was the flaps ... just not sure if it was the cleaning of the flap in the crossover pipe or the replacement of the cracked vacuum lines that did it. Thanks to everyone that provided suggestions/help.

David Roseman
Long Beach, Ca

Thanks for posting a resolution.

Resonance flaps or vacuum lines. That will give someone researching in the future a place to start.

__________________
1998 E300 +310,000K
1996 E300 +460,000K

My guess would be the cracked vacum line; I had this issue with my 96 and 97.

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Keith Schuster
2006 E350 98K miles
2013 Ford Explorer 15K miles

Ok, for all those 1997-99 E300dMB owners experiencing hesitation and suspect o-rings, air in fuel lines, k-40 relays, egr sensors as source of the problem but are frustrated to find out that it isn't, try cleaning your gas tank. I did all the above only to find out that my gas tank was so dirty that muck has covered much of the fuel filter located inside the gas tank. I replaced again the fuel filter in the engine and had the tank removed and cleaned. It is now running so smooth as though it was brand new. It is frustrating that MB bulletins do not indicate this as a potential problem.

I have the exact symptoms as the OP, and I also have a 97, but with only 92,000 miles on it. I'll check the vac lines this weekend. Love threads where OPs describe resolution.

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Another potential cause for this bucking / surging while cruising at steady speed low load is the transmission. I have seen where a high mileage never serviced 722.6 was getting starved for fluid because the filter was so gunked up. Replacing the transmission fluid and filter cured the problem.

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1998 E300 turbodiesel

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