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Old 09-16-2015, 03:42 PM
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Join Date: Sep 2015
Location: Kohler, WI
Posts: 2
'92 Mercedes 300CE Running Rich, Suspect TPS Sensor, need help confirming

I purchased this car with 48,000 miles in August and while in basically new condition it has some issues to work through. I'm pretty sure it was left standing for long time without running since the tires have a manufacture date of December 2000. The car originally was from Quebec.

Symptom: fuel smell at airflow meter flapper, 7.5 - 9V duty cycle (rich), intake misfire and stuttering acceleration below 2500 RPM.

- So far the tune up basics have been done: plugs, cap and rotor, fuel filter.
- All of the rubber Vac lines look remarkably in like new condition for the age.
- OVP is OK.
- Replaced Fuel Pressure Reg due to 6 . 6.5 bar reading. Now it's at 5 / 5.5 bar reading.
- Replaced Throttle Body Gasket due to leak.

I'm leaning toward a problem in the throttle position sensor, but I'm not sure if I'm diagnosing this correctly. Write ups are kinda all over the place on the TPS. Here's what I'm seeing.

- Deflecting the air flow sensor plate results in a standard 10% change, which rules out the Airflow Poteniameter.

- Measuring voltage at the x11 port at roughly 7.5 - 8.5V. When the throttle is engaged, it runs all the way up to 11.4v regardless of throttle position; no sequential voltage increase.

- Running the engine and measuring the duty cycle in volts, I'm seeing 7.5 - 8.5V at idle, 4.5 - 5.5 V at 2500 rpm. That's off I think.

- The curious measurement from the X11 is at a steady 1500 RPM which is where I'm getting a hesitation and popping in the exhaust. I'm getting a reading in the 6 - 7 v range, HOWEVER as the cycle progresses it drops out to 0v and then picks up at 6.5V. I don't see this at the 2500 RPM or any other range.

Measuring Ohms at the three pin connector for the TPS using K ohms I have one side that registers resistance based on throttle position. The other side doesn't register. Switch to M Ohms, both sides register resistance based on throttle position. With the throttle wide open or fully closed, I'm not getting an infinity ohn reading.

So how should I interpret this? I don't want to rip the fuel distributor off again without diagnosing this correctly.

Thanks - appreciate any help you can offer.
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Old 09-16-2015, 06:38 PM
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Join Date: Feb 2011
Location: Modesto CA
Posts: 2,532
Quote:
Originally Posted by Josecuervo View Post
I purchased this car with 48,000 miles in August and while in basically new condition it has some issues to work through. I'm pretty sure it was left standing for long time without running since the tires have a manufacture date of December 2000. The car originally was from Quebec.

Symptom: fuel smell at airflow meter flapper, 7.5 - 9V duty cycle (rich), intake misfire and stuttering acceleration below 2500 RPM.

- So far the tune up basics have been done: plugs, cap and rotor, fuel filter.
- All of the rubber Vac lines look remarkably in like new condition for the age.
- OVP is OK.
- Replaced Fuel Pressure Reg due to 6 . 6.5 bar reading. Now it's at 5 / 5.5 bar reading. CIS-E nominal system pressure is 6.3 bar!
- Replaced Throttle Body Gasket due to leak.

I'm leaning toward a problem in the throttle position sensor, but I'm not sure if I'm diagnosing this correctly. Write ups are kinda all over the place on the TPS. Here's what I'm seeing.

- Deflecting the air flow sensor plate results in a standard 10% change, which rules out the Airflow Poteniameter.

- Measuring voltage at the x11 port at roughly 7.5 - 8.5V. When the throttle is engaged, it runs all the way up to 11.4v regardless of throttle position; no sequential voltage increase.

- Running the engine and measuring the duty cycle in volts, I'm seeing 7.5 - 8.5V at idle, 4.5 - 5.5 V at 2500 rpm. That's off I think.

Duty cycle measurements are to be made in %s, or in degrees. 100% = 90 degrees. A dwell meter is convenient.

- The curious measurement from the X11 is at a steady 1500 RPM which is where I'm getting a hesitation and popping in the exhaust. I'm getting a reading in the 6 - 7 v range, HOWEVER as the cycle progresses it drops out to 0v and then picks up at 6.5V. I don't see this at the 2500 RPM or any other range.

The TPS is a switch, not a continuous sensor. Closed throttle sends one signal to the ECU, WOT sends the other, nothing in between.


So how should I interpret this? I don't want to rip the fuel distributor off again without diagnosing this correctly.

Thanks - appreciate any help you can offer.
See notes above.
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  #3  
Old 09-16-2015, 07:56 PM
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Join Date: Sep 2015
Location: Kohler, WI
Posts: 2
The multimeter I'm using is a Radio Shack unit that is true rms. However the duty cycle function isn't cycling, so that's why I'm using voltage. I can convert that to a 6.8 - 7.4 V as the 45 - 50% ideal range.

The stand differential pressure I've seen referenced for the bosch system is 5 bar in the upper chamber and 5.5 bar in the lower chamber. It was reading 6 over 6.5. From what I understand that won't have a huge difference on the operation of the vehicle. It just verifies that there are no restrictions in the fuel system

So what do I make of the momentary no signal at 1500 RPM? It seems like the CIS is getting an implausible signal and cycling.
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Old 09-16-2015, 08:33 PM
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Join Date: Feb 2011
Location: Modesto CA
Posts: 2,532
Quote:
Originally Posted by Josecuervo View Post
The multimeter I'm using is a Radio Shack unit that is true rms. However the duty cycle function isn't cycling, so that's why I'm using voltage. I can convert that to a 6.8 - 7.4 V as the 45 - 50% ideal range.

The stand differential pressure I've seen referenced for the bosch system is 5 bar in the upper chamber and 5.5 bar in the lower chamber. It was reading 6 over 6.5. From what I understand that won't have a huge difference on the operation of the vehicle. It just verifies that there are no restrictions in the fuel system

So what do I make of the momentary no signal at 1500 RPM? It seems like the CIS is getting an implausible signal and cycling.
1986 & later MB CIS-E systems operate at higher pressures than "stand".
The readings of 6 bar (upper), and 6.5 bar (lower) were correct. Low pressures result in leanness.

Re: implausible signal
Have you verified O2 sensor voltage?

Before going any further with the fuel system, it may be helpful to run compression pressure, and leakdown tests.
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