Miss firing badly when engine cold better when at working temp but still there..

[Steve Delves]

Been looking through the threads and can't find one with these symptoms...
Engine miss fires when cold, it's ok at idle but when you ask for power it's weak and miss fires a lot.
The miss firing reduces as the car warms up until when at working temp just above 80 degrees then there is just the occasional miss fire. - every few mins.
On expressway the car is fine. But driving in traffic when you have to accelerate from lights etc the engine will miss fire.
When at the lights the revs will remain steady then after about 20 seconds they will rise a little then settle back down.
Codes read by mechanics and they said "Engine Miss fire" keeps appearing but they can't track it down.
They have checked the rotors, caps etc and changed the cables to the spark plugs as well - not for new but off a friends E500 which worked fine. They also changed the spark plugs and checked the injectors...



Cheers

Steve

 

[gsxr]

Steve, this sounds a lot like secondary ignition problems. Have the insulators been replaced, or removed to inspect for liquid on the back side? If not, the next step is to remove them and inspect. How old are caps/rotors, and what brand are they? People have reported defective new caps or rotors out of the box, so swapping in different items would be another test. The issue isn't likely to be fuel or spark plugs.

Live data from the EZL will show which specific cylinders are misfiring, this will help pinpoint the problem, especially if the affected cylinders are from the same distributor cap. Adding ventilation slots in the caps is also recommended if you haven't done that yet.

A very long shot is a failing crank sensor (at the rear of the engine), these can cause some bizarre problems despite no fault codes indicating a bad sensor.

 

[Steve Delves]

Thanks Dave. The mechanics had done all you have recommended so we are going down the cranks sensor route now....
It was bad this morning did a 3km drive to the shops it was almost undrivable - dithering, cutting, no power when trying to accelerate. The car stood at the shops for 1 hour then was much much better driving home - faltered once.

 

[maw1124]

How did they check the injectors? Fuel pressure meter, spray patterns? And how old is the fuel pump?

When I had similar issues on another Mercedes it was the fuel pump. The car would coast just fine but as soon as I got into the throttle “dithering, cutting and no power” is a very accurate description. In 5 minutes my guy hooked up a fuel gauge to the rail, started the car, told me to hit the gas, and that was it: “Fuel pump; not enough pressure under load.” I was out of there in an hour with a new fuel pump and filter. What was funny there is, the car didn’t throw a CEL for lean condition.

Plus I see no mention of the pump and only the most vague mention of injectors.

maw

 

[Steve Delves]

How did they check the injectors? Fuel pressure meter, spray patterns? And how old is the fuel pump?

When I had similar issues on another Mercedes it was the fuel pump. The car would coast just fine but as soon as I got into the throttle “dithering, cutting and no power” is a very accurate description. In 5 minutes my guy hooked up a fuel gauge to the rail, started the car, told me to hit the gas, and that was it: “Fuel pump; not enough pressure under load.” I was out of there in an hour with a new fuel pump and filter. What was funny there is, the car didn’t throw a CEL for lean condition.

Plus I see no mention of the pump and only the most vague mention of injectors.

maw

Thanks for this. I’ll get them to check when it goes in… should be in the next week.

Steve

 

[gsxr]

Thanks for this. I’ll get them to check when it goes in… should be in the next week.

Steve

FWIW... usually, if the fuel pumps are at fault, it causes a smooth loss of power. No misfiring, just feels like the engine is stuck at idle while your foot is on the flor. And the power may come back to normal randomly. I've had this happen on a couple different cars. To pinpoint conclusively you attach a fuel pressure gauge that can be viewed while driving, and drive until the power loss occurs. If the pressure has dropped dramatically, the pumps are at fault. If the power loss occurs with normal fuel pressure, it's something else.

If there is misfiring occurring, it's not the fuel pumps.