Bucking Motor

[brettj]

I'm finally getting some of the motor done on my 75 450SE. Today, I replaced the brake booster hose, manifold exhaust gas hose, the valve covers and gaskets and the spark plugs. I used the non-resistor type of plug which did seem to help make the motor run smoother. The strange thing that happend after this work was completed is that all of a sudden whenever  the car started up a hill the motor would lose power and start bucking. At one point I pulled over as I had my foot to the floor and the car wasn't going anywhere and then back fired.  When I pulled over and put the car in park the motor stalled. It kept repeating this behavior all the way home which is roughly 10 miles except the stalling stopped but you could feel the loss of power and then all of sudden the motor would return to normal. So this problem just kept going on and off.

I know that I need new fuel injectors,  replacements are on the way and will be installed next week but could it be only the injectors causing this problem? I was thinking that my throttle switch is incorrectly set.

Any ideas here?

Brett

[oscar]

You can discount the throttle switch.  IMO that wouldn't cause this behaviour under any circumstance.  To be sure, whilst this bucking is occuring, pull over, pop the hood and pull the plug off the TPS socket.  I would expect the bucking to continue.  The car can run normally without the TPS connected.  It might loose some responsiveness off the mark but not really noticeable if you're not racing around town. 

As for the bucking problem itself, can't think of anything at the moment apart from wanting to ask something simple like, have you got fuel in the tank??

[brettj]

Yes, I had a full tank! Perhaps the fuel pump isn't working. Would that cause bucking problems? I installed a new or should say rebuilt fuel pump in October. The new pump has made a wining noise ever since I put it in. I was told that some pumps are simply noisy and it is not an indication of pump failure at this point in time. I haven't been comfortable with that pump since it was installed.

[oscar]

Perhaps the fuel pump isn't working. Would that cause bucking problems?

It could and getting the fuel pressure tested would be a sure way to tell if the pump is delivering so to speak, but a fuel delivery problem was my line of thought when I asked about the amount of fuel in the tank.  Especially when you mentioned going up hill brought about the problem, I thought maybe a very low tank might be leaning things out which we know now isn't the case. 

But it's all too suspicious that other things were worked on immediately prior to the problem.  So ruling out fuel delivery for the moment, perhaps air and spark should be concentrated on instead. 

I'll throw a few more ideas about for you to look at

Regarding air - check brake booster hose connections and one way valve for vacuum leaks.  I'm not familiar with the exhaust gas hose and not sure what affect a blockage or leak there would have but give it a once over.

Spark - go over all your connections again, sequencing and check dizzy cap is securely fastened.  It might be too soon to read the plugs but pull one out of both banks just in case they can point the finger at something.

Lastly, d-jet stuff - during gasket and hose changes and removing the valve covers, there's a possibility something's been knocked loose.  OMG a eureka moment - vacuum hose to the MPS!!!  Check both ends, that's what I'm putting money on

[brettj]

I checked everything you suggested and nothing seems out of place. When I started the car today it turned over and immediately started bucking. I just let the car idle and all of a sudden the bucking stopped and the motor ran smooth. I backed the car out of the garage and let it idle more. Periodically it would start bucking and then again there would be a power surge and the motor returns to normal. I disconnected the hose to the MPS and the motor started bucking and go haywire. I put the hose back into the MPS and the motor returned to a smooth normal idle. I don't think this is an MPS issue.

I think this must be the result of one or more bad fuel injectors and that piece of crap rebuilt fuel pump I purchased from German Star. I'm going to put my old fuel pump back in and see if it makes a difference. The new fuel injectors will go in next week as well. This must be a fuel delivery problem.

I'm still finding small oil leaks under the car even after changing the valve covers. The leaks appear under the right side of the motor block. One oil leak is near the rear of the motor and the other closer to the motors midpoint both leaks are at the side of the motor at both points.  Are there any other oil hoses in that area?

[oscar]

Sorry brett.  I really thought the MPS hose would have been loose causing havoc to fuel mixtures

I suppose if the old fuel pump was working but was replaced as a preventative maintenance measure it'd be worth trying it out again.  If that cures the bucking hopefully there'd be some warranty to chase up with the replacement. 

What about the spark plugs, what did you buy?  Part number and what size gap if changed?

[s class]

You can check spark plugs, but its unlikely to cause such intermittent problems. 

[brettj]

The spark plugs I used are NGK BP5ES non-resistor type. They were suggested by Ton Hansen from the Mercedes Classics Center in Irvine California. I changed the fuel pump because I thought it may have been causing the warm start stalling issue. I guess I shouldn't have bothered. I'm going to put in the injectors and the old pump this Thursday. Hopefully, I'll have some good news to report!