I just took my 1991 300E in to have some problems resolved that I haven't been able to fix myself.

The car has 65K on the clock and has been treated with TLC since day one.

Symptom one was a rough idle. The problem was a collapsed motor mount (even though I couldn't tell it).

Symptom two was hard starting in hot weather and occasionally a backfire in the intake manifold. The problem was a leaking fuel injector.

Symptom three was poor gas mileage and spark plugs indicating running rich. The problem was dirty fuel injectors.

BTW, the poorly operating injectors made the O2 sensor test look bad.

I'm having the O2 sensor, the motor mounts and the injectors replaced.

It's going to cost me a lot of money but compared to a new car, it's cheap. Besides, I'll know that it was done right and they gave me a new Benz to drive while they worked on my car.

Just changed the sparkplugs yesterday(Bosch H9DC0, $3.30 apiece). Bosch O2 sensor to follow($80).

For geographical & political reasons, Mercedes-Benz parts are far cheaper around here :p

~Nautilus

__________________
1990 260E Sportline (that's 300E 2.6 for our American friends) -> sold
2001 E320 4Matic Elegance -> my Dad's daily drive
2005 Seat Leon FR 1.8T

Sparkplug change solved the problem just partly - it gives now 13 mpg instead of 10(speaking strictly city traffic), still not the promised 16 mpg.

Some people suggested the coolant temperature sensor can influence the ECU as well. (!?)

Still not diagnosed yet

~Nautilus

__________________
1990 260E Sportline (that's 300E 2.6 for our American friends) -> sold
2001 E320 4Matic Elegance -> my Dad's daily drive
2005 Seat Leon FR 1.8T

Finally happy!

Fuel consumption in city traffic had dropped to 14.5 liters/100km(that's roughly the promised 16 mpg), and 13.2 liters/100km combo(that's 17.7mpg; city + crowded highway)

Not diagnosed O2 sensor, yet done the sparkplug change I've written above, plus I've tightened the clamps to seal perfectly the vacuum tube from air filter to ICV. Not bothered yet either with the distributor cap(it's awkward; how does it fit!?)

~Thanks you all

Nautilus

__________________
1990 260E Sportline (that's 300E 2.6 for our American friends) -> sold
2001 E320 4Matic Elegance -> my Dad's daily drive
2005 Seat Leon FR 1.8T

Just for the record, I generally get 10 Litres/100km (24mpg) in mixed driving. As much as 15% better for free-flowing highway.

Thats with a 3 litre 5-speed.

I do not drive in the city or heavy traffic enough to have numbers for that.

__________________
1986 300E 5-Speed 240k mi.

An idea buzzing through my head of late:

Should I clean the fuel injectors by now? I can bet they were never cleaned, in 60K miles, and gasoline has pretty much dirt in it around here... actually there is "dirt with some gasoline in it"

1. The thorough procedure is to drive the car to a shop, and they will connect the fuel lines to a device which pumps a cleaning agent under pressure throughout.

On the other side, the complex and delicate construction of the KE-Jetronic tells me to be very careful with guys dealing with pressure devices around...

2. There are also "injector cleaner solvents" to be added to the fuel on a tank refill, which should - theoretically - flow with the fuel, and clean the injection system underway. I wasn't warned as to their effects. Can they do any harm(to catalysts, O2 sensors etc)?

If any of you have tried one of the above quoted methods, please tell which effects did they have(either good or bad).

~Thank you,

Nautilus

__________________
1990 260E Sportline (that's 300E 2.6 for our American friends) -> sold
2001 E320 4Matic Elegance -> my Dad's daily drive
2005 Seat Leon FR 1.8T

Just to follow up on my post above. I took my '91 300E to a dealer and essentially told them to make it run like new. They did better than that. It runs every bit as good as it did when it was new and they adjusted the vacuum modulator on the transmission to fix the hard shifting problem I've had since the car was new.

Now for the results. New motor mounts completely eliminated any hint of vibration. When stopped with the transmission in gear, you cannot feel a thing. In fact the first time my wife attempted to start the car she didn't know that it had started and tried again.

The hard starting, backfiring and poor fuel mileage were the result of leaking and dirty injectors. I told them not to fool around but to just replace them. The results are amazing. It's a new car.

I had a new O2 sensor and fuel filter installed but I don't think these components were really the cause of problems.

This dealership had a shop supervisor who seemed to know about CIS-E FI and guided the Tech in trouble shooting.

The car is perfect. It cost me $2,000 including all the other 60K maintenance items except an oil change. That's pretty stiff but on the other hand it's peanuts compared to the cost of a new car.

In fact, the dealer did such a great job on the car, I decided not to buy a new CDI from them but to keep on driving the '91 300E. I saved about $48K or so.

Even with only 65K on my car, both the motor mounts and fuel injectors were bad.

That makes the problem even more fuzzy, since I get slightly under 10 litres/100km(24-25mpg, about the official EPA rating) on free highway, yet 15-16 litres/100km(about 14-15mpg) and even worse, in city traffic....

I've tried standard 95-octane lead-free($0.79/liter) and 98-octane Euro 3($0.84/liter), now switched to OMV 100-octane lead-free Super($1.04/liter).

~Nautilus

__________________
1990 260E Sportline (that's 300E 2.6 for our American friends) -> sold
2001 E320 4Matic Elegance -> my Dad's daily drive
2005 Seat Leon FR 1.8T