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At My Wits End and on my Last Nerve

2K views 20 replies 12 participants last post by  JoeB 
#1 ·
Good Morning to all, Have a 73 450 SLC 81K original Miles, and have pretty much exhausted all of my resources on getting it run well.
When Cold starts up and will accelerate and shift like a new car.
Once up to operating temperature, starts to cut out when accelerating at 2500 RPM up through 3000 RPM ?
Have changed plugs, wires rotor cap new fuel filter new voltage regulator and alternator changed over to Electronic Points system Petronix.
And changed all of the Injector seals and new fuel hoses throughout the manifold.
Sooooo
No local Mercedes Mechanic in my area.
Anyone RX a Good mechanic in either San Diego or Yuma Az area ??
 
#14 ·
I like the way you use callouts to clarify.
Great job.
 
#4 · (Edited)
Check for any obvious vacuum leaks...spray WD-40 along all the areas with vacuum while it is idling and see if there is a difference.

Also, as the engine warms up, plug wires, leads, can become more vulnerable...check if you have spark when the engine is warm.

Check the color/condition of your sparkplugs - take pictures and post here.

Also, make a short video of how the car runs after it is hot and post it, we can help you better this way.
 
#5 ·
On the top of the engine computer is a knob. with the car hooked up to a smog machine, you can adjust this knob for ideal mixture. Its located in the right kick panel at the passengers feet. Given the fact that it runs great cold, I think its running too lean when warm. I've seen some run better with the air cleaner sensor in the snorkle unplugged.
 
#8 ·
No, I meant ideal mixture. I had a 72 280SEL 4.5 that was running poorly. On my car the computer was behind the passenger headlights. A rubber cover was removed and the screw (knob) was exposed at its top. We hooked it up to a smog machine to monitor the air/fuel mixture. Then slowly turned the knob a quarter turn at a time. If I recall, it made a difference at all engine speeds, just not idle. I presume the fuel filter is not restricted? and correct fuel pressure has been verified?
 
#9 ·
At Wits End

I pulled the EEG valve and looked for something obvioiusly broken but could not see anything. Is there a way to test, and repair, or replace ?
I have a Caveat, this hesitation and stumble only occurs under load. If in the driveway in Park, I can rev the engine all day up to Redline not a stumble hot or cold.
It only occurs under load in gear accelerating and again only at operating temp starting at 2500 K ?
I do appreciate all of your feedback, but with the plethora of ideas and recommendations, I would feel much better taking it to someone who actually knows what the hell he or she is doing, and has the proper tools.
 
#11 ·
I pulled the EEG valve and looked for something obvioiusly broken but could not see anything. Is there a way to test, and repair, or replace ?
I have a Caveat, this hesitation and stumble only occurs under load. If in the driveway in Park, I can rev the engine all day up to Redline not a stumble hot or cold.
It only occurs under load in gear accelerating and again only at operating temp starting at 2500 K ?
I do appreciate all of your feedback, but with the plethora of ideas and recommendations, I would feel much better taking it to someone who actually knows what the hell he or she is doing, and has the proper tools.
...fuel filters will do that when theyre clogged. i had one that ran fine stopped and in park, could rev the engine without issue but as soon as you dropped it into drive it would quit. i changed the fuel filter and the problem was gone. sometimes it really is the simple things.
 
#10 ·
I agree! This system was also used on early Porsches, Volvo's and VW's. Maybe someone local has experience with D Jet on those cars? Another thing to check is the TPS (throttle position sensor) on the side of the throttle body. A black plastic cover over it can be removed and the copper contacts can be cleaned with a pencil eraser. They can gum up over the years and cause a momentary bog at a specific RPM because continuity is lost and the injectors dont fire for a split second.
 
#13 ·
Fonzi is correct. The knob on the ECU only adjusts IDLE mixture. The idle mixture circuit is only active if the throttle plate is 100% closed (and the linkages to the TPS are properly adjusted.)

Mixture for all other conditions is controlled by a separate ECU circuit with adjustment at MPS.

The AAV is a place to start. When cold, the car should idle at quite high rpm - about 1100-1200rpm. As car warms up the AAV starts to close and when fully warm, should be at about 600-700rpm.

There are various other things that could cause poor running when warm. Best to first check all sensors using tests in shop manual. Set timing and make sure it advances to something like 25-30BTDC when engine is revved to 3000rpm. This is test of the centrifugal advance within the distributor.
 
#17 ·
I had these symptoms too - I was tuning a cold engine with a poor starting point.

Find the rheostat on your ECU (on the firewall forward of the passenger footwell). Set that rheostat to its center point; 11 clicks from either extreme (just a starting point). This function is only used for idle mixture. It has absolutely no bearing on your mixture above idle.

Timing First: Set your timing on a warm engine - most of us have found that the standard spec works better than the US spec (US spec is optimized for emissions). Since your problem is on a warm engine at cruising speed - start with TDC at 750 RPM. See if that lets you rev to 3000 RPM and what your timing is there (25-30 BTDC).

If your idle is rough, you can play with the ECU rheostat to attempt to smooth that out. As you do this, you will indirectly change your idle speed. When you get it smoothed out, adjust your idle speed at the big idle set screw.

More to follow if these steps do not help your problems . . .

Do you have the D-Jet test procedures?
 

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#21 ·
Set fire to it...

Thats a solution.
 
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