Your car should have an 02 sensor - at this stage someone could have removed it. It seems to be running in open loop mode and/or the chain is off a tooth.

 

I checked the timing and the throttle. And no it does not have an O2 sensor and neither is there a slot for it anywhere near the exhaust manifold. It a 1988 model not a 90. This problem is a recent problem. Also the MAF has no effect on the car. Even when unplugged the car idles the same.

Any other things i might have overlooked? No cat converter in this car and no 02 sensor. Also is there an OBD port on this car i can connect a scanner to? Cant seem to find one anywhere.

 

what MAF??

You'll need to lean it out @ the CO adjustment, look under the passanger foot panel/carpeting for an open connector, that's where the O2 would plug in.

Are you sure it's not actually on the limiter? When the fuel pump relays give out, sometimes they do this, ie artificially low limit

Where are you located?

Jonathan

 

Well i do think its a fuel problem. The fuel cuts off at 4K rpm meaning that under load its not getting enough volume though pressure is fine. Today i changed the fuel filter and car would not start. So after panicking a bit i find that there is no current going to fuel pump so the relay is suspect. I opened it and am getting it repaired (yes i am getting it repaired as a replacement is no where to be found

After i get the relay and hopefully the car starts, i will target the fuel pressure regulator up front under the airbox to see if there is a leak in the diaphram.

I dont see anywhere where an O2 sensor would fit. The car had no CAT so maybe its a pre-emission foreign market car. MAF sensor i am guessing was probably retrofitted on this car as i dont see a purpose for it. Where exactly is this 02 plug in under passenger side of car? Underneath carpetting or behind some interior bits inside. Car is RHD if that helps but bonnet release is on passenger side on left

Will keep you all updated as i hate it when people have similar problems yet never reply when they find a cure :thumbsup:

 

Fuel pump relay is very common failure item , can't you find a good one on eBay ?

 

O2 sensor plug on the US cars is under the pass side floor mat, behind a plastic access panel that covers the front of the foot well. It requires the removal of a 10mm plastic nut to gain access.

No surprised it's the FPR. If you are in a Pickle you can always run a jumper b/w the two middle pins where the relay sits, that will bring 12V directly to your pumps.

Where are you located?

jonathan

 

I am in Pakistan hence the unavailability of such regular items

Well i got the relay repaired and started the car up with a new fuel filter. The car idles good but again won't rev past 4K.

Also, without the air cleaner element in place the car will rev to 4.5K and than it sits there with no shuddering. With air cleaner element in place it wont rev past 4K and its shudders when it reaches that limit. So its actually better without the air cleaner (at least 500 rpms more limit and no body shaking). I also checked the fuel regulator at the engine near the throttle body and its clean with no leakage.

Still stumped. Car has low fuel in it. Going to top her off and see what happens. Maybe the low fuel level in the tank is straining the fuel pump. who knows

Thanks for all the replies and help

 

I have the same problem,

Mine is a 300 SEL 1990 and it wont rev past 4.5K. i have auto trans. and it only does this on the first gear. when it shifts to second @ 4.5K rpm and i keep the pedal to the metal it will go to 6.5K rpm on 2nd and 3rd gear (ran out of rood so couldn't test the 4th )

im not sure what the problem is yet but i hope to figure it out soon.

 

Guy's reality check required here Mercedes Benz, in their kind wisdom fitted a little ditty called the "fuel safety shutoff" this little electronic chap will not let you rev the engine beyond 4000 rpm when in Park or Neutral OK, so you do not have a problem.:thumbsup:

 

Thanks for the info

BUT i am not revving a standing car in nuetral. Its a manual transmission and in gear while DRIVING the car does not accelerate past 4.5K rpms...its limited by either a fuel cutoff or a problem with the airflow

So far all the fuel related stuff has been checked. Ordered a MAF..lets see if that helps. Else will need to start replacing fuel pump as maybe its no delivering enough volume under load.

No matter how much i am pressing the gas pedal the car will not go above 4.5K rpm...actually it feels just like a limiter...car reaches it..lunges a bit but refuses to go above 4.5K...like an anchor has been dropped at the back. runs perfect at low rpms though

stupid car:crybaby2:

 

EUREKA!

After reading this link :
http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/1475901-w126-6-cylinder-260se-fpr-petrol.html

It seems maybe the FPR on the car belongs to a different engine classification with a limit of 4.5K rpms! The FPR was CHANGED previously be a mechanic and this problem started right after it was changed. I just repaired the current one when it did not send voltage to the pump.

Is this true guys? Do FPR's have rev limits stamped on them??? I will see the part number of the one on the car and see what the original 300 sel engine part number is for the FPR

Remember, the car feels exactly like its on the rev limit. the fuel pump is working, new fuel filter, the regulator is clean with no leakage and FPR was changed before and this problem arose after its changing. I hope i am on to something here.

Quick confirmation on this theory required please.

Kudos to all those who have helped out so far :bowdown:

 

There are different revving FPR's so its a good theory. I dont know if there is one that has such a low limit though? Lets hope so cos that would be a nice easy fix wouldnt it.

 

I've never seen a FPR w/ a limit under 5500 RPM...and that was for a v8, which will give you a much higher rev limit in a 6.

Jumper out the fuel pump relay and go for a drive.

Pins 7 & 8 in the relay receptacle.

Jonathan