450se Motor Bucking

[brettj]

Hi All,

I've done lots of work to my 75 450SE D-Jet. Last summer the cam shafts, timing chain etc were all replaced. The motor runs so much better now! The only problems I'm still having is the engine still has a slight rough idle, I feel it is using too much gas, average mpg is 13.25 and worst of all the engine will buck periodically. For example I drove the car to Boston which is about 90 miles from my home. The car ran great at speeds of 70 - 80mph but suddenly after about 35 miles the engine started to lose power then quickly gain power repeatedly. This is what I mean about bucking. When the engine does this you can not pick up speed as giving the engine gas does absolutely nothing. Once you slow down to about 55 to 60 mph the probelm goes away. I've noticed the engine doing this in city driving as well at speeds of 45mph. The strange thing is that the car doesn't die but simply bucks a while then returns to normal. This issue only happens after extended use of the car. What could be the cause? I thought clogged fuel injectors but all of my injectors are new. To be on the safe side I used some fuel injector cleaner but to no avail. My thoughs have been to the throttle switch adjustment, the CPU or even the transmission but I'm at a loss on this one. Any ideas?


Thanks,

Brett

[Tony66_au]

First check for me would be the TPS, Mine was a bit grubby and after a clean and check a lot of small gremlins disappeared.

[djenka018]

A clue or a pointer may be in engine "bucking" at certain RPM.

Not sure of the breadth of work you've done, I remember D-Jet guys at BW 107 forum talking about clogged fuel filter, tank strainer or bad pump all leading to reduced fuel pressure, causing engine to starve at certain rpm's.

[brettj]

Hi,

Thanks for the replies. Tony, sorry to sound stupid but what part are you referring to, TPS? Djenka, you may be on to something there. I installed a new fuel pump a couple of years ago but never have done anything with the fuel tank. Maybe I do have a clog somewhere in the fuel system. I haven't changed the fuel  filter since I replaced the pump.  I'll have to check the filter  out too.

Brett

[jbrasile]

brettj,

The fuel filter/tank strainer  suggestion is right on, in addition I would also check if the fuel tank's breathing system is not clogged, being a US car you should a fuel vapor ventilation set up, if the system is plugged, vacuum will develop inside the tank and eventually the pressure will prevent the fuel pump from sucking enough gas to keep the FI pressurized correctly so the engine will begin to starve for fuel. Since it takes a while for vacuum to build  the problem only occurs after a certain period of time.

A simple test: with the tank about half full, drive the car until the symptom  begins to show itself, stop and open the gas cap, if you hear a loud suction noise that might be it. Now drive the car without  the cap and see if the problem goes away, if so then you've nailed it.

Tks,

Joe

[djenka018]

In addition to all previous, coil is known to induce problems of similar kind too.

Make sure you use D-Jet coil as a (trial) replacement, I think they're different from K-Jet or plane jane coils

[s class]

How's the points and condenser?  How's the points gap?

My D-jet 350 developed a problem whereby it would be running fine, then all of sudden it would drop to 4 cylinders, and then after while go back to 8.  The problem turned out to be a loose connector at the cable connecting to the injection trigger points on the side of the distributor. 

[motec6.9]

Hi brettj TPS means throttle position switch it is cobected to the throttlebody. But I would check tank breather first . Geoff

[brettj]

Thanks for all of the replies! Lots of great suggestions here! I have new points, condenser and in fact rebuilt the distributor this summer. I think the point gap is correct. I also have a new old stock TPS which I think is set properly. I also installed a brand new coil from Mercedes and that is working fine. To be sure I'll recheck all of those areas.

The fuel tank is a part that I have not worked on at all since I put the car back on the road 5 years ago. What I've noticed is  when I open the gas cap to fill up the tank that a big swish of air will come out of the tank. Seems like a lot of pressure has built up inside the tank and comes out the filler when I open the cap. Is that normal or is it an indication of a clog in the tank strainer?

Brett

[motec6.9]

Hi there is your answer then as the pressure builds up it stops the fuel flow. The fuel strainer as you called it allows air pressure the escape from the tank if this line becomes blocked . You have a pressure buildup in your tank.

[djenka018]

If driving with tank cap removed (and tank at near empty) causes no problems... there's a solution.

Few good years back I "fixed" a friend's W124 by drilling a 1mm hole in fuel cap.
It was mechanics idea and it worked. I vents the slow process of buildup of fumes pressure.

@motec
Fuel strainer I thought of is always submerged in fuel and is kinda like a first filter before the pump. Similar to the one in your dishwasher.
You're probably talking of that stuff at the top of the tank that connects to charcoal canister and intake manifold on smogged cars

[jbrasile]

Brett,

Perhaps I was confused in my assessment, I was thinking vacuum buildup and not pressure.... I bet your tank vent system is really plugged, as suggested here, with the tank about half full (this will avoid spillage) drive the car without the gas cap, I bet you won't see any issues whatsoever. The vent system has nothing to with the strainer as djenka018 pointed out, the strainer is the first filter the fuel goes through prior to leaving the tank, it is a good idea to replace it as well if that hasn't been done. Over the years the screen mesh will get clogged and sometimes will even come apart sending all kinds of debris into your very expensive D-Jet fuel pump...

You should check the condition of all tubing and also the charcoal canister in the engine compartment, it is supposed to absorb fuel vapors when the engine is not running and once the car starts there is a vacuum connection to the intake manifold that sucks the fumes into the engine. If the canister is saturated and/or the lines are plugged, there will be vapor buildup all the way back to the tank.

Let us know how it goes.

Tks

Joe

[brettj]

Hi All,

I been doing some research on the fuel evaporation system in my 450SE. From the parts involved in this evaporation process I've replaced the fuel pump, fuel damper, all of the hoses related to the fuel  pump and the fuel tank outlet pipe. Is the outlet pipe what everyone keep referring to as the strainer? I replaced all of those parts about 9,000 miles ago. The fuel filter was also replaced at that time. How often should the fuel filter be replaced? In the maintenance manual it states every 30,000 miles but that seems a bit long to me.

This afternoon we had a  snowstorm so I really couldn't work on the car but I started her up and at idle I get the usual rough idle. Thinking that the rough idle has to do with the vapor lock up in the fuel system, I removed the gas cap. A little bit of pressure was released when I did so. I let the car idle without the cap and the engine seemed to smooth out and run stronger. I put the cap back on and the motor seemed to roughen up a bit. With that said, I feel that perhaps the suction valve which removes the fuel vapors from the charcoal cansister while the engine is running is not removing the vapors causing the rough  idle and even the bucking. The only other thing I can think of is that the fuel filter is clogged and perhaps the stainer in the fuel tank outlet pipe is once again clogged.

Thanks for the info and I will try the driving without the gas cap test and look into the suction valve and charcoal canister operational issues.

Brett

[wacotech]

You can check the TPS by just disconnecting it, the car will still drive alright, if the bucking stops (normally at part throttle cruise amount) then you know what it is.

It is possible to clean up the tracks on the TPS, and then carefully nudge the contact arms over 0.5mm onto new track (you might need a magnified glass to check the arms are tracking good.) (a standard x10 jewelers glass is a use full tool to have). I did this repair because i had part throttle bucking and now it is perfect. A can of contact cleaner/lubricant is also use full for these jobs.

[brettj]

Thanks, I'll take a look at the TPS. The TPS in my car is a new old stock part. It should be working but we did have trouble setting it this spring after the cam job etc.