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

 

While I'm new to Mercedes specific land.....


Well, you can rule out b, f, and g. Those would all result in a lean out condition, not rich.


The O2 sensor can be damaged by a couple of different things. Most two common: Leaded gas and never seize on the threads. Other than that, they should last a long, long time.

Assuming that the O2 is doing its job, the computer is either not getting the message, or whatever message it is sending out ot the ignition/fuel system(s) isn't being acted upon.

E should be cheap to replace, just do it. H you did already.

That leaves a shorter list:

Hot/Cold Start Failure:
a. thermo-time switch faulty

c. air-flow sensor plate rest position incorrect
d. fuel pressure incorrect

i. injectors faulty or clogged

Poor Fuel Mileage:
a. idle speed, ignition timing, and idle mixture out of adjustment

c. fuel pressure incorrect


I would start with the ignition system (cap, rotor, etc) and work my way towards the fuel system.

 

I tend to agree. I'm thinking fuel pressure should not be a problem with the rich mix.

I took the car in for inspection and the CO was 4.20. The limit is 1.20.

I attempted to adjust the CO mix under the air filter and succeeded in choking out motor with less than a half twist. No ignition till I opened it back up. My inclination is that the air intake is at the bottom of its range pointing to one, or more, of the sensors.

Thanks,

Chris

 

Actually, increasing fuel pressure will increase the amount of fuel through the injector. Drag racers do it when they have a good tune up, but the weather condtions require a little tweak.

I would save the injector issues for last, as they hardly ever fail. Having the fuel pressure checked (regulator failure) is fairly easy and shoudn't cost much if the ignition checks out OK.

 

Actually, b f and g cannot be ruled out because of the interesting ways these cars compensate. I had a fuel check valve problem which resulted in dropping fuel pressure when the car sat or was idle for a long time. The car ended up richening the mixture and applying throttle to compensate for the low fuel pressure at idle, and I would sometimes smoke at idle.

Also, have you checked the fuel injectors and seats, the seats are probably cracked and causing a vaccuum leak which disturbs the fuel air management.

 

It depends on where you are, if the car has been running fuel with ethanol (like in CA), many parts of the fuel system, including injectors, may be damages.

 

Ummm, B. is the fuel pump isn't running....

If the car runs, the fuel pump runs...

F. is a residual pressure issue that would only be a problem upon start up. If the fuel pressure at shut off either leaks out (see G.) or drains back past the regulator, then that would be an issue. Either way, those wouldn't result in a "pig rich" condition when driving.


G. is a fuel leak. If it leaks a little under "normal" use, its going to leak more if the fuel pumps try to compensate.


The poster said he's driving the car and it sounds like its running rich all the time.


I'm still betting on ignition.

 

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.

I would like to find a shop with the equipment to read fault codes. The dealership that I typically go to acts like they no longer have the proper equipment (or I've surmised this after they did not pull the codes the last time I took it in).

Is it unreasonable to think one, or a few, shops might still have test equipment for an 88 vintage?

Chris

 

You can take the car to AutoZone and get it done for free......

 

The problem is that now you will need an osilloscope to set the air/fuel mixture correctly, and a locking pin. Always a bad idea to touch that.

 

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

 

Hey Pal:
I've got a 86' 420sel that has a crank/no start/no spark condition that I'm working at! I see your update to this posting but let me understand something! Are you saying that after you reversed your actions for the CO mixture you were able to obtain ignition?
RSVP,
TBJCC001

 

If I understand your question, the answer is yes.

As it turned out, that had little to nothing to do with the problems I was experiencing.

Chris