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300SEL running rich

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adidallas's picture
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I have a 1988 300SEL that is running with a rich fuel mix. It fouled out a new set of plugs within 300 miles. Ran great for those 300 miles though.

At the point the plugs foul, I get a little shimmy in the steering wheel on acceleration, fuel economy drops drastically, and she smokes on hard acceleration (gray/black...soot really).

Also, at start (hot or cold), the engine dies immediately after ignition. The second ignition always takes but is a bit rough for less than a second before dropping to normal idle.

The Bosch fuel injection manual suggests several possible causes:

Hot/Cold Start Failure:
a. cold-start valve or thermo-time switch faulty
b. fuel pump not running
c. air-flow sensor plate rest position incorrect
d. fuel pressure incorrect
e. coolant temperature sensor or wiring faulty
f. insufficient residual fuel pressure
g. fuel leak(s)
h. lambda control faulty
i. injectors faulty or clogged

Poor Fuel Mileage:
a. idle speed, ignition timing, and idle mixture out of adjustment
b. cold-start valve leaking
c. fuel pressure incorrect

I've replaced the cold-start valve and lambda (O2) sensor. Though I understand the O2 sensor is somewhat delicate and easily damaged...will likely replace it again soon.

The car sat mostly still for ten years, racking up 4000 miles from '97 to '07. I've dealt with quite a few dry rot issues (motor mounts, etc.).

I'm curious if there are any known common faults in this model with reference to any of the above.

On a related subject, I've looked high and low for a secondary market manual (Haynes, etc.) for the 300SEL with no luck. I ended up with a Haynes British manual for the E series, 200 thru 320, but have not been able to confirm that this manual covers the same 103 engine. Is anyone familiar with this?

All the best,

Chris

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marlinspike's picture
Hampton, VA
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Re: 300SEL running rich

Why get a secondary market manual? The Benz service manuals are far better.

As far as the problem, as you see there are many potential causes.

Is it rich throughout the rpm/throttle range or is it only rich at certain times? I presume you have checked for vacuum leaks (the first step in fixing a CIS or CIS-E injection is taking care of any and all vacuum leaks)?

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1985 380SE 5.6

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adidallas's picture
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Re: 300SEL running rich

marlinspike;151798 wrote:
Why get a secondary market manual? The Benz service manuals are far better.

The Benz service manual has been discontinued according to the dealer that I work with.

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marlinspike's picture
Hampton, VA
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Re: 300SEL running rich

While I wouldn't recommend buying pirated software if MB had the decency to still make the manuals (there once was a time when Mercedes would on purpose make the cars easy to work on and would advertise in the dealerships that they will sell you their service manuals), pirated versions are plentiful on the internet.

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1985 380SE 5.6

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alabbasi's picture
Carrollton, TX
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Joined: May 7th, 2004

Posts:
399

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1988 560SEC
Re: 300SEL running rich

If you need to take it to a fellow in Dallas that knows these cars. Give David Poole of European Performance a call on 469-688-0422. He drives a W126 and should be able to sort you out.

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Al
www.w108.org
check out the W114, W115 /8 enthusiast website  http://www.stroke8.org
 

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adidallas's picture
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Re: 300SEL running rich

Update.

On the recommendation of the dealer's parts dept., I have been adding Xylol into new tanks of gas. Also, I tried to adjust the air-flow sensor plate (not recommended).

The results: no smoking. good power and acceleration. fuel economy has increased significantly.

I put it through inspection again and the test results were worse than the first time.

All I succeeded in doing, while adjusting the air-flow plate, was choking the car out and delaying the ignition turnover. Now it takes two cranks to turn over the ignition every time, and even that's not certain.

Chris

adidallas;151533 wrote:
I have a 1988 300SEL that is running with a rich fuel mix. It fouled out a new set of plugs within 300 miles. Ran great for those 300 miles though.

At the point the plugs foul, I get a little shimmy in the steering wheel on acceleration, fuel economy drops drastically, and she smokes on hard acceleration (gray/black...soot really).

Also, at start (hot or cold), the engine dies immediately after ignition. The second ignition always takes but is a bit rough for less than a second before dropping to normal idle.

The Bosch fuel injection manual suggests several possible causes:

Hot/Cold Start Failure:
a. cold-start valve or thermo-time switch faulty
b. fuel pump not running
c. air-flow sensor plate rest position incorrect
d. fuel pressure incorrect
e. coolant temperature sensor or wiring faulty
f. insufficient residual fuel pressure
g. fuel leak(s)
h. lambda control faulty
i. injectors faulty or clogged

Poor Fuel Mileage:
a. idle speed, ignition timing, and idle mixture out of adjustment
b. cold-start valve leaking
c. fuel pressure incorrect

I've replaced the cold-start valve and lambda (O2) sensor. Though I understand the O2 sensor is somewhat delicate and easily damaged...will likely replace it again soon.

The car sat mostly still for ten years, racking up 4000 miles from '97 to '07. I've dealt with quite a few dry rot issues (motor mounts, etc.).

I'm curious if there are any known common faults in this model with reference to any of the above.

On a related subject, I've looked high and low for a secondary market manual (Haynes, etc.) for the 300SEL with no luck. I ended up with a Haynes British manual for the E series, 200 thru 320, but have not been able to confirm that this manual covers the same 103 engine. Is anyone familiar with this?

All the best,

Chris

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adidallas's picture
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Re: 300SEL running rich

Update.

I finally took the car to someone who knows what they're doing under a Mercedes hood.

Turns out two cylinders where not firing and the fuel distributor is leaking internally.

The following items were replaced:
-spark plugs
-distributor cap
-ignition rotor
-injector seals
-fuel filter
-ECU computer
-CIS-E coolant temp sensor
-O2 sensor
-motor mounts

My understanding is that this particular model barely passed emission tests when first introduced to the States and converted. Continuing to pass emission standards as it ages is an interesting task.

The car runs fantastic now. Starting is a bit rough though this is expected as the internal leak in the fuel distributor was deemed not worth the expense-to-benefit to fix.

All the best,

Chris

adidallas;152982 wrote:
Update.

On the recommendation of the dealer's parts dept., I have been adding Xylol into new tanks of gas. Also, I tried to adjust the air-flow sensor plate (not recommended).

The results: no smoking. good power and acceleration. fuel economy has increased significantly.

I put it through inspection again and the test results were worse than the first time.

All I succeeded in doing, while adjusting the air-flow plate, was choking the car out and delaying the ignition turnover. Now it takes two cranks to turn over the ignition every time, and even that's not certain.

Chris

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redlaser's picture
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Re: 300SEL running rich

On a related subject, I've looked high and low for a secondary market manual (Haynes, etc.) for the 300SEL with no luck. I ended up with a Haynes British manual for the E series, 200 thru 320, but have not been able to confirm that this manual covers the same 103 engine. Is anyone familiar with this?

All the best,

Chris

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Re: 300SEL running rich

Haynes book no 3253 covers the W124 series which is suitable for a W126 owner in many aspects..Haynes does not have a W126 book...The 5253 book covers W124 from 1985 to 1993 petrol and diesel..Haynes are not as good as a Benz manual but covers what a DIY amateur like me needs to know!!

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