Normally, under heavy acceleration, my engine rpm increases fairly quickly in 1st gear. When it hits 3500rpm it feels like it gains about 20 horsepower and it really takes off. I assume this increase in power is due to the valve timing advancing.

Lately I have noticed, especially when the engine has just reached operating temp, that it hits 3500 rpm and it just sits there for a few seconds before the extra power kicks in and then it continues speeding up. It never runs rough, but just kind of pauses for a few seconds. I read that there's a computer that controls the cam advance mechanism but does not really go into detail. Has anyone had experience with related issues?

When I read the CEL codes with the cold engine it says EGR inoperative. With the engine hot, no codes.

 

are you sure the extra power isn;t your transmission just kicking down?

 

It's not a gear change. The RPMs don't jump up and the transmission stays in the same gear. I just feel a boost in power at 3500 RPM. Sometimes the boost is delayed and that's the issue. It could be a temporary lack of fuel or ignition timing, too. Valve timing seems the most likely to me since that would be about when it should advance, I would think.

 

I think I know exactly what you are talking about. I would experience this on my old S320 with 380K miles on the clock. Sometimes the engine would run very well and never miss a beat. Most times it would run so-so but then like a switch it would instantly take off and run great. This was always when the engine was running up through the RPM's and more distinct as load increased. This got progressively worse as the mileage increased on this car. I never really looked into it and just wrote it off because of the high mileage. I had this car for over 200k miles and it was already there to some degree when I bought it. I never had a fault stored for the cam advancer though there is one available. HFM will look for a manifold pressure change when activated and set a fault if the logic chain is not met. I am not sure if the advancer was the issue but the power change was so fast and drastic it certainly crossed my mind too. Maybe it was sticking and not coming in until later? I was also thinking it could have also been fuel related as the engine seemed to feel lean when not running 100%.

I was going to hook up the HHT and browse over some values while driving but never got around to it. I was also planning to connect a fuel pressure guage. The car is gone now. I kind of miss it.

 

Yes, that sounds like my car. What is the HHT? I thought if this is a simple issue I would correct it but it really doesn't happen that often, or at least I don't push it hard very often due to the age and worrying about the transmission, but maybe I should just leave it alone. My experience with European cars is that if you fix one thing it seems to trigger a domino effect and something else completely unrelated breaks. Thanks for the input!

BTW, how did you get 380K on yours? How much work did you have to do to it? Probably not the original transmission, right?

 

The HHT was the old Benz diagnostic tool before the SDS came out. It stands for Hand Held Tester.

Reverse failed in the original trans and that's why I finally got rid of it. I still got $2K for it and in 2 days on craigslist! The car was essentially bullit proof with minimal repairs through the years. I tore the engine down at 350K miles to replace the headgasket, timing chain, rails and sprockets. The cylinders still had the crosshatch and the chain was not really stretched at all! It didn't even need the chain items but I repaced them anyway since I was there. Great car and I would own another if I came across a low mileage example cheap. The oil was changed every 3k miles religiously with Castrol conventional oil and the engine looked like it had only 30k miles on it inside. Yes, it had 3 headgaskets through it's life but that's to be expected. It had been as far south as Key West and as far west as Vancouver.

 

Interesting, I did not know Mercedes Benz had a variable valve engine in the 90's.

 

The M104 had it starting in '93, or maybe slightly earlier on the coupes. That was cutting edge technology then, I would think.

 

You know, it does feel like it runs lean sometimes, and then it gets fuel and takes off. Would it hurt to run some fuel injector cleaner through it? I also had a thought that maybe the Mass air flow sensor or something else reacts slowly. Sometimes it seems as though it's "thinking" about how much fuel it needs for a second after I make an abrubt change to the throttle, then it reacts and runs well again. I think the car was driven very gently in the past; maybe the computer needs to be "reset" so it can learn my driving style. Can this be done?

That's encouraging that your transmission kept its reverse for 350k. I would settle for 200k. I only put about 7k/year on it.

 

i always thought it was normal for the car to take a few seconds before it speeds up. i thought maybe it was teh oxygen sensor that was lagging

 

You can reset the adaptation data but you will need equipment to do it. If you drive more agressive for a while it should change the adaptation data on it's own. I buy the cheap $1.50 fuel inj cleaner at Advanced auto parts by the dozen and put it in at every fill up.

Keep in mind the old S320 was mostly highway miles and rarely saw reverse compared to city driving. I was always on top of the trans servicing too.

The M111, M119 and M120 had the variable intake timing as well.

 

I don't think it's valve timing now. The fuel injector cleaner doesn't seem to be helping. It does feel lean, and kind of surges a bit under WOT, like it's trying to get more fuel. What else could cause it to run lean? You mentioned fuel pressure. What could make this low? Would an O2 sensor going bad make it starve sometimes? I wouldn't think it would be the fuel filter since it sometimes runs very strong.

 

OK, it's definitely running lean. If I remove the vacuum line to the fuel pressure regulator (max pressure) it runs better, especially at not-quite-WOT, but still not perfect. Could it be the fuel filter? Has anyone ever noticed an improvement after replacing their fuel filter? I never have on other cars so I guess I never gave much thought to replacing this one. I don't know how old it is; could be original. The OBD says code 1, which is no faults.

Is it a hard job to replace it?

 

my 1990 M104 runs the same way. Except mine runs far worse than yours- it has idling issues, but it's sporadic. One day the car will run perfectly, other times I have to give it gas for the first few minutes to keep it from dying.

It hesitates bigtime when accellerating from as top. It actually feels like one cylinder isn't firing or something... kind of a rythmic shake that increases in direct proportion to the rpm's. And then when I hit about 2k - 2.5k rpm's, my neck snaps back and it's as smooth as silk. Just exactly like you said- as if I'd flipped a switch.

I've replaced every d*mn part imagineable in this car. Injectors, wires, plugs, cap, rotor, 02 sensor, head gasket, valve job, cleaned the idle control valve, MAS Control unit (which houses the fuel pump relay in my model), fuel filter, crank position sensor, EHA valve (not sure if that's the right name), and more (I just can't remember),

Lots of this stuff was replaced for other purposes, obvsiously. But this problem has persisted with different degrees of severity for my whole 3 years of ownership. I've pretty much decided to live with it. I dont' want to keep putting money into this car!!

 

Some thoughts:

Have you checked the knock sensor, which if not functioning properly will alter ignition timing?

Fuel pressure regulator should deliver fuel at 5.4 bar / 78PSI.

Replace the fuel filter and make sure the return line from the fuel pressure regulator to the tank is clear.

What type of air filter are you using, stock or K&N style?

 

Stock air filter. I could drive around the block without it and see if it improves. Wouldn't a knock sensor set a code in the computer?

 

Knock sensor works on a vibration principle...when it "feels" detonation, it alters the timing and fuel delivery. It doesn't set a code as it reacts to the moment.

You might want to bypass the sensor to see if it is problematic.

Reason I asked about the filter is that if you were using an oiled K&N type, it could contaminate the MAF in your HFM system.

The MAF may also be causing your symptoms.

 

Thanks for the info. I though about the MAF, too. That will probably be the last thing I try due to the expense. Is there any way to inspect it? How do you
bypass the knock sensor?

 

I'm assuming a lot of things, but let's get back to basics.

Did you check for any vacuum hose leaks?

Did you replace the OVP?

OVP more important then you think, inexpensive and worth replacing, because it may be semi functioning and can give your symptoms.
The OVP and fuel pump relays do a lot more then you think. The OVP if not fully functioning can affect the pulse width to the injectors and cause what you are describing.
Try tapping it with a screw driver while the engine is running and see if you can notice any minor RPM change.

 

I did not replace the OVP but I did tap on it and it made no difference. I also replaced the fuel filter and hoses. They were so bad that gas started leaking out as soon as I touched one. This also made no difference.

I sometimes get a code 5- EGR Inoperative- but the egr valve does not stick when I suck on the tube, and I can hear it opening and closing. It does make it idle a bit rough if I suck it open while it's running.

Here's what the car does: During the first 10 minutes or so of driving, if I let it rev past 3500 rpm in 2nd gear, sometimes the engine will suddenly lose power (I'm not sure if it dies completely or just powers down considerably) and right after that it gives a hard CLUNK into 3rd (the clunk is because it has to take all of the slack out of the driveline due to the engine backing off right before the shift- flex discs are good) then it continues on as normal. So far it only does this once before it is completely warm and then after that if I let it rev past 3500 it just feels lean and sluggish and surges a bit like it's trying to get gas or the timing is being messed with. It's much less noticeable with the AC off.

Here's what's strange: Halfway home yesterday (30 minutes one way), after the 3rd gear clunk had occured once, it suddenly started running absolutely perfect. No more surging and plenty of power- just like it was brand new! I had to stop for 5 minutes and when I started it and drove off, it was back to the same surging and lethargy.

Does anyone have any more ideas?

1. O2 sensor going bad would set a code or make it run rich, I would think. I get 20-22 mpg in town so it's not running rich.

2. Knock sensors- it looks like there are 2 of them on the lefthand side of the block- is there anyway to test these?

3. Mass air flow sensor- could it be going bad and then suddenly work perfect?

4. Starting and idling is perfect so I don't suspect a vacuum leak.

Bicylindrico- did you ever replace the MAF or O2 sensors to see if that solved your problem? It sounds a lot like the same problem.

Any ideas would be a great help.

 

When driving home, I had to stop suddenly so I shifted manually from "D" to
"3" to lessen the wear on the brakes. When I resumed in "D" the engine was running perfectly. Now I notice that if it starts to gets sluggish, all I have to do is occasionally shift into "3" for a few seconds and back to "D" and it snaps out of it. What could shifing to "3" possibly have to do with how the engine runs? It always shifts through all 4 gears. If shifts very firmly so there's no mistake in knowing what gear it's in. Now I know more, but am even more confused.

I used to shift down to "3" fairly often in traffic until I started reading about all the transmission problems, then I just left the shifter alone. Now that I downshift to aid the braking, the engine is running better.

 

What is OVP?


Jim Coulter
'99 C280
My first MB

 

Over Voltage Protection relay, a common issue on these cars that causes intermittent voltage dips and losses to the engine control systems.