W124 300E No good duty cycle reading - PeachParts Mercedes-Benz Forum

AndreiFS · #1 03-25-2006, 12:01 AM

'90 300E 110,000 miles

Hi guys, I just installed a new coil, rotor, distributor cap and wires. After some initial misfire which seems to have cleared up after some driving, the car seems fine. When I try to read duty cycle, however, I don't get a reading. With the engine off but key on I get a steady 51.1% which indicates oxygen sensor failure, correct?
When I switch the car on, the initial reading fluctuates about 50% but then after about 4 mins. jumps to about 85 then 95 then 103.0, where it remains steady. Unplugging the oxygen sensor doesn't seem to make a difference. Thick smoke also pours from the back of the car but it's hard to tell what colour it is under fluorescent lighting. It looks light gray and gets thicker when I accelerate.

Any ideas? Is the O2 sensor a likely culprit?

A264172 · #2 03-25-2006, 12:26 AM

with the o2 sensor unplugged... you should get a static (non-fluxuating) 50%

AndreiFS · #3 03-25-2006, 12:37 AM

You're right. I don't think I gave the engine enough time to adjust when I unplugged the O2 sensor.

AndreiFS · #4 03-25-2006, 09:53 PM

Okay, it misfires again. This happens when accelerating abruptly from idle. I unplugged the EHA and there was no noticable difference in performance. The duty cycle read 84.4 steady. After plugging the EHA back in the duty cycle steadily increased until topping at 103.0. With the car off but key "on" the duty cycle read 31.2 steady with the engine cold. What is going on?

PS. The car only misfires at full operating temperature.

A264172 · #5 03-26-2006, 10:59 AM

Quote:

Originally Posted by AndreiFS

... With the car off but key "on" the duty cycle read 31.2 steady with the engine cold. What is going on?

PS. The car only misfires at full operating temperature.

That means the leads are reversed (should read 70%) I had the same problem when I first tested my duty% the positive lead in the #2 pin and the negitive lead in the #3 pin will give the correct reading... (something about mercedes useing reverse duty cycle) So your real percetage is heading toward 0 (richening)
Does the car misfire with the o2 sensor unplugged? (fully warmed up) If not try unplugging the coolant temp sensor.

AndreiFS · #6 03-26-2006, 04:58 PM

I see. Thanks a lot.
The car misfires with the O2 sensor off as well as with the EHA off. The smoke also remains the same. There seems to be no difference. I will now try unplugging the coolant sensor. Is the coolant temp sensor also known as a coolant temperature switch? Since there is no difference with the EHA off, could this be pointing towards a mechanical fault such as the distrbutor or fuel pressure regulator?

dkveuro · #7 03-26-2006, 07:38 PM

Which pin is giving you your % reading ?

This sounds like an intake air leak, as it does so when leaner and warm.

Check the hose/s to and from the idle valve...they split.

Are you unplugging the O2 under the passengers side carpet ?

Are you sure you put the distributor cap on square and did not loose the center carbon contact.?

The engines on these will hunt at idle if the % is not set as close to 50% as possible.

The rear 'two pin' sensor on the intake side of the head is the CTS....warm should read 380 ohms from either pin to ground.
Cold is 2000 ohms or more to ground.

Is the missfire regular or random?
Which cylinder if any ?

Enquiring minds want to know.

.

AndreiFS · #8 03-27-2006, 01:40 AM

I'm getting the reading with the positive lead in pin 3 and the negative in pin 2. Turns out the battery on my multimeter was dying, I changed it and my readings are normal again.

I have checked for leaks once and not found any but I will try again.

I did unplug the O2 from the passenger side carpet, yes.

The distributor cap was indeed on square.

It hunts at idle very slightly. I would say less than 50 RPM +/-

I will try testing the CTS with you advice.

The misfire is irregular and only occurs when accelerating very rapidly from idle. By irregular I mean it is not evenly rythmic but does happen everytime I floor it in idle. If I do a sequence of floor, let go, floor, let go it backfires from the intake after about the third time. I don't know which cylinder it is. After 2500RPM I can't get it to backfire regardless of what I do.

Thanks a lot for your help, I will post the results of the CTS test.

Thanks, Andrei

dkveuro · #9 03-27-2006, 11:11 AM

Information now points to fuel pressure or lean .
If frequency not set to 50% +/- 5 it will hunt at idle.

The missfire you describe is at low speed and this means that as pressure to the injectors increases, missfire goes away.

Could possibly be a bad injector to.
These motors are tough and I have yet to find one with a valve guide problem or pistons rings worn out.

I would be hunting for an injector or fuel pressure problem, although the fuel pressure seems okay if you are able to run rpm's all the way to 5+ k with good power.

There is faint possibility there's a burnt valve seat.
Try this.....turn the mixture up about 1/4 turn and drive ....if miss has gone, this indicates a bad injectors or very lean.

These motors do eat cams though......have had several in last year with no lobes on various cylinders.... but that usually make for a rocker tick.

.

.

AndreiFS · #10 03-27-2006, 02:20 PM

When I turn up the mixture enough the misfire does go away. But then I get too much smoke from the back.

dkveuro · #11 03-27-2006, 05:19 PM

Okay...first....let's pull the valve cover...

.

AndreiFS · #12 03-27-2006, 11:07 PM

Sorry, I was wrong, the misfire doesn't go away no matter how rich I set the engine. I should also mention that the oil smells like gas.

AndreiFS · #13 03-27-2006, 11:09 PM

I've got a complete Air Flow meter assembly on the way (includes EHA, pressure regulator and distributor). I will try swapping that I guess. This is annoying because the car runs and starts fine except for this misfire. It doesn't even smoke from the back anymore.

dkveuro · #14 03-28-2006, 12:05 AM

Second then.....pull the spark plugs......you said nothing about them.

Perhaps it's time for a compression test.

When you check them, do so with working end down...I had a ceramic center crack on one plug that took forever to find...with plug pointing up, as you would normally do to see the gap , it looked fine....but when turned over, as it would be in the engine, the ceramic center dropped down over the center electrode and shut off the spark to the air/fuel.

.

AndreiFS · #15 03-28-2006, 03:59 PM

I did a compression test a few weeks ago and found that each cylinder jumps to 9 bar on the first stroke then 10 then 12. The deviation between cylinders was negligible. The sparkplug gap should be 0.032 correct?