"Flat spots" in acceleration curve
 

My M103 (W124) idles fine and runs smoothly for the most part. But it has what I describe as flat spots in its acceleration curve. Under load (not necessarily even WOT, which I don't do much of), there are subtle losses of acceleration that give it a feeling of very slight momentary loss of power. I'm not talking about anything dramatic, it's very subtle, but it can be detected behind the wheel. No CEL, no codes.

Regular maintenance has always been meticulously performed since new. Recent servicing included fuel and air filters, correct copper non-resistor plugs, new OVP, fluid replacements always. Three times a year it gets a bottle of Redline injector cleaner spread out over 2-3 tank fulls. Fuel pump is original.

Hauled out the multimeter and tested all the electrical components in the CIS-E system. Pressure was not tested, nor was duty cycle. Otherwise, all of the FSM diagnostic tests were conducted. All but two tests were well within specs. The two problem tests are:

1) Throttle valve switch (S29/2): Measuring at the KE controller (N3) coupling, depressing throttle should take resistance from <1 Ohm to infinity. Instead, it reads 38.7K Ohms with throttle depressed. The same test at the connector (S29/2x) is supposed to give the same result, which it does, but once again at only 38.7K Ohms rather than infinity. That's the first problem with S29/2 (its idle speed reading). The companion test for full load speed also gives anomalous results. Instead of infinity going to < 1 Ohm upon accelerator depress, my result is infinity at all times. This is true both at the CIS-E coupling and at the S29/2x connector. The wiring tests fine at 0.2 Ohms. That tells me that the S29/2 switch is bad. Right?

2) Air flow sensor position indicator (B2): Measured at the B2 coupling, I get 2.4K ohms, against specs of 3.6K - 4.4K ohms. Slowly deflecting the air flow plate is supposed to cause resistance to rise continuously up to half deflection, and then to drop off again. My result is infinity at all times during deflection. Here, too, the wiring in the circuit back to the CIS controller tests fine at 0.1 ohms. I interpret this as a bad B2 sensor. Is that right?

My question: are these two components (S29/2 and B2) likely to be the cause of my acceleration "flat spots", or is it something more mechanical, like the camshaft, or even a compression problem? Thanks for wading through a long description.

I had this on my first Mercedes, a '91 TE/M103 at about 70,000miles (always maintained meticulously by me). I tried "everything" including a complete new fuel distributor, new ECU, etc., etc. to cure the problem. I also tried many fuel injector cleaners including the common dealer favorites.

The cure came when I ran a single bottle of RedLine Complete Fuel System Cleaner SI-1(I think it's called), cures the bad spray pattern etc. in many of my CIS cars and cured the part-throttle hesitation within the first 1/2 tank-full.

For $10, find a bottle and pour it in the tank, best money you'll ever spend IMO.

Your car will become addicted though, will respond well to the 1oz/10gallon maintenance dose for a while 'till it's cleaned up.

Have the same problem with my '90 300SE. Heck, for $10 I'll give it a shot. I've already ran Techron and Gumout through there with minor improvements, but nothing dramatic. WOT does not seem to be affected......

It does look like you were thorough in your tests, and the erroneous readings might be the real issue, or at least AN issue. I have always suspected that most fuel additives give a mostly placebo effect, but I'll try it anyway.

I'm also gonna try the tests you did, and see what I get.

Jeff, she already gets a maintenance dose of Redline SI-1 about 3 times a year.

Correction on earlier test result: I posted the wrong data for my airflow sensor (B2) result. Sorry for the confusion. I initially tested the coupling by error and got the results I posted. In fact, the correct proc is to test the sensor pins themselves. The correct result there was 4.2K ohms against specs of 3.6K - 4.4K ohms, so that's OK. However, upon throttle depress, the resistance climbs OK but then does not come back down as I continue to depress up to WOT. I don't know what to make of that. Similarly, the idle speed test of the throttle valve switch (S29/2) is supposed to go to infinity upon throttle depress, and my test result there was 38.7K ohms. To me, that's a failed test. Do others agree, or should 38,700 ohms be considered high enough to be infinity, in which case the test is satisfied?

I run the redline in every tank on my M103.....about 4oz per fillup....and its been running perfectly since I got it hooked on the stuff. It used to hesitate and idle rough, now its smooth and happy. Bought in cases of 12 the SI-1 isn't that expensive on a tank by tank basis. Less than a dollar.

any new word on this??

I'll add some new info....I went cheapo and tried midgrade in my 300E, and well well well, it runs perfectly fine/the same on it. I got 22mpg going 74mph for 250 miles with the A/C on full blast and the car full of camping equipment last week. I won't use regular, but since Midgrade works, that's what I'll be using....engine runs/sounds/feels/performs exactly the same as on premium....in fact, I think it idles a bit better and starts a little easier. I still use the redline in it to keep it clean though.

I had a similar problem with my M103 in my 300CE. It would hesitate at times, especially when cold. I too could feel a noticeable flat spot in the acceleration curve. I had recently done the head gasket and top end. All six injectors and seals were replaced as well as the fuel accumulator. Turns out that it was a defective fuel pressure regulator. The hesitation is gone and it starts without any problems now. By the way, I too have used mid and high grade fuels and have found that there is almost no difference in engine performance between the two.

Thanks for the comments. The fuel pressure reg is original, so I'll put that on the to-do list. I think the first step will be pressure testing to see what's what in that area.

Hmm....I have a spare fuel pressure regulator from the junk yard....Maybe I will try it out on my car to see what happens.

You got 22 with the ac on? Hmm. I must be doing something wrong. I have a m103 3L in my 190E and I got 17mpg with the ac on. I usually get around 21 with out a/c... I would love to get 22 with a/c on...