93 300E with idle surge and poor gas mileage

[420sel man]

We have a great 1993 300E (3.2 liter) with 120K that uses almost no oil between oil changes at 3-4000 miles.
Gas mileage has dropped from just under 30mpg to just over 20mpg recently.
The other day the check engine light came on. About the same time the idle changed from a normally smooth 600-700rpm to a pulsing or surging 600rpm to 1100or 1200rpm (only after warm-up)
I checked the spark plugs, they were even but sooty black. I then retrieved the following trouble codes:
1. #2 oxygen sensor inoperative (replaced 7 months ago)
2. #4 air injection inoperative
3. #10 voltage at air mass sensor too high or too low
4. #26 up-shift delay switchover valve open or short (replaced 7 months ago)
5. checked for vacuum leaks, none found (yet)
6. read lots of archived threads on rough idle and other

I cleared codes but #10 is back.
Seems like the coolant temperature sensor might be the source of the problem. If it is sending the wrong signal saying the engine is still cold, thus the rich fuel mixture, bad gas mileage and rough idle.

Anyone have a value @ temperature chart for the coolant temperature sensor for this model?
Anyone have any other suggestions?
I really don’t want to start throwing parts at this problem. Thanks for your input!!!

[suginami]

Man, we have the same car, and I have never achieved fuel economy of just under 30 mpg. Even driving to Las Vegas with the cruise control pegged at 65 mph, I still got just 25-ish. I'm averaging about 21-22 mpg since I've owned the car, so I think your fuel economy is in the normal range.

A fluctuating idle is generally caused by either a vacum leak, bad mass air meter, or bad throttle actuator.

[420sel man]

Paul, could you please explain how I could better ascertain if the problem is the air mass meter or the throttle actuator. As I stated earlier no vacuum leaks found (yet).

BTW I have noticed that your vehicle seems similar. Ours really has gotten very good gas mileage (we keep a mileage log), we don’t push it to hard…

Anyone else like to comment?????

Would still like to know the test values @ temperatures for that coolant sensor....

[suginami]

Retreiving the codes in the DM could point you to the right direction.

The Mass Air Meter is so quick to replace that I think most shops would swap in a known good one to see if it solves the problem.

[gsl56]

I also have the same car. I also had the same problem you describe. I just replaced the MAF after much help from this site. I did a little testing on my own and found when I disconnect the MAF I would get the same symptoms. After I plugged it back in I tapped it a few times and the idle would surge. After a couple more taps it would return to normal. Mine also would get close to 30 mpg on the highway and around 21 everyday driving. When I had this problem the milage around town dropped to 18/19. Now after replacing the MAF it seems to run a little better and the idle is normal. Hope this helps.

[dtf]

I have the same engine in my '94 wagon with 233,miles.

1. My air flow meter caused the same symptoms last month. Was about $400 job at the dealer.

2. Even in the summer months I've never come close to 30 mpg. The best I get is 24 mpg. Share your secret (especially at $2.30 a gallon in CT!) !

[suginami]

Originally Posted by dtf

I have the same engine in my '94 wagon with 233,miles.

1. My air flow meter caused the same symptoms last month. Was about $400 job at the dealer.

2. Even in the summer months I've never come close to 30 mpg. The best I get is 24 mpg. Share your secret (especially at $2.30 a gallon in CT!) !

If you know you're MAF is bad, it is one of the simplets DIY jobs. All it takes is a phillips head screwdriver and about 15 minutes, incuding the time it takes you to get your screwdriver from the toolbox. The problem is that electrical parts are not returnable.

Again, not that I question you, but I'd like to state again that your fuel economy is incredible. I keep every receipt. And I mean every single one. I have my fuel economy recorded on an elaborate excel spreadsheet, and over many, many years, my average is stabilized at about 21.5 mpg, with mixed highway / city driving. Like I said before, my all time record was a run to Las Vegas. I literally filled up at a gas station before the freeway onramp, and upon arriving in Las Vegas, exited the freeway and practically coasted into the first gas station. My mpg was 25.9.

[gsl56]

Before I had the MAF problem I ran the car up to the White Mountains. I filled the car before I left with 93 octane fuel. I used the cruise most of the way at about 68 to 70 mph. When I refilled the car I my average was about 28.5. This was up and down the hills. My wife runs this every day to work and back (about 4 miles each way) plus the other run around driving she does. We refill the tank when she reaches 300 miles. It usually takes about 14 to 15 gallons to refill. Compared to a Chevy Astro van with a 4.3L 3.73 rear gears this car gets fantastic fuel mileage. I can understand a wagon getting a little less as is heavier than my 4 door.

With regards to replacing the part Suginami is right. I drove the car to the dealer removed the part and replaced it in their lot. It also saved me the core charge and an extra trip.

[420sel man]

Thanks guys!!!

I am going to assume that MAF stands for Mass Air Flow Meter……

If I had a parts car or a spare MAF laying around I would have already tried a swap….
Since I don’t, I am reluctant to lay down $300 to $400 to find out if this is really the problem.

BTW, on another recent thread there was some discussion on use of carb cleaner vs denatured alcohol as cleaning agents for a MAF. One could buy lots of cleaner for less than a new MAF…..Anyone else have some experience here?

Comments appreciated!

[420sel man]

I took the car into the shop today and after they listened to the surging idle they put a new air flow meter on. NO CHANGE in the surging idle!!!! They suggested replacing the wire harness……

Anyone have experience with deteriorated wire insulation inside the outer insulating sheath?
I heard this was the Germans' idea of making everything biodegradable…..

I've never replaced a wire harness in any other vehicle I've owned...

What sort of price should I expect to pay for a harness? Sources?

How much time should it take to install?

I have already repaired the insulation (with tape) on wires leading to the cam sensor while replacing the fan bearing bracket....

[pberku]

I have a '95 E300D, and had the identical symptoms that you describe. It tuned out to be the infamous wiring harness problem that afflicts our model cars.

In my particular case, I discovered that about 4" of the wiring harness behind the Battery, the one that amongst others, leads to the over-voltage relay had completely disintegrated. Most of the wires within that harness had lost all of their insulation, and were either touching each other or grounding to the chassis.

You may want to look at the wiring harness in that area yourself to determine its state. I am referring to the portion of the wiring harness that's behind the Battery and that can be accessed by removing that black cover on the firewall. You don't need to remove the battery to see it. Once you remove that cover you will be able to see the harness. I'll bet that its totally disintegrated.

I fixed mine myself by cutting the damaged 4" section, splicing and soldering new wires, and using heat shrinkable tubing and electrical tape to complete the repair.

Being a klutz, it took me about three hrs. to complete. Its now been about 4 Months, and the problem has not returned.

Phil

[suginami]

Originally Posted by 420sel man

I took the car into the shop today and after they listened to the surging idle they put a new air flow meter on. NO CHANGE in the surging idle!!!! They suggested replacing the wire harness……

Anyone have experience with deteriorated wire insulation inside the outer insulating sheath?
I heard this was the Germans' idea of making everything biodegradable…..

I've never replaced a wire harness in any other vehicle I've owned...

What sort of price should I expect to pay for a harness? Sources?

How much time should it take to install?

I have already repaired the insulation (with tape) on wires leading to the cam sensor while replacing the fan bearing bracket....

The wiring harnesses are bad on all engines from 1993-1995. They all need to be replaced.

The sad thing is this ----- I don't think changing your wiring harness will solve your problem, but you still need to replace it. It causes all sorts of problems, like misfiring.

The idle surge problem is surely your throttle actuator or a vacum leak.

Have your mechanic take another look.

[420sel man]

The wiring harnesses are bad on all engines from 1993-1995. They all need to be replaced.

The sad thing is this ----- I don't think changing your wiring harness will solve your problem, but you still need to replace it. It causes all sorts of problems, like misfiring.

The idle surge problem is surely your throttle actuator or a vacum leak.

Have your mechanic take another look.

Could someone elaborate what is it about these 1993-1995 wiring harnesses that causes them to fail. Knowing the cause of a problem is the first step to finding a solution that will last.

Also, has anyone had a similar problem (idle surge) that was in fact corrected by replacing the throttle actuator?
Are there other symptoms that are usually associated with a bad throttle actuator?
Seems like the car would not run very well at all (at other speeds) if the throttle actuator was bad. The car seems to do fine out on the highway.

I have a '95 E300D, and had the identical symptoms that you describe. It tuned out to be the infamous wiring harness problem that afflicts our model cars.

In my particular case, I discovered that about 4" of the wiring harness behind the Battery, the one that amongst others, leads to the over-voltage relay had completely disintegrated. Most of the wires within that harness had lost all of their insulation, and were either touching each other or grounding to the chassis.

You may want to look at the wiring harness in that area yourself to determine its state. I am referring to the portion of the wiring harness that's behind the Battery and that can be accessed by removing that black cover on the firewall. You don't need to remove the battery to see it. Once you remove that cover you will be able to see the harness. I'll bet that its totally disintegrated.

I fixed mine myself by cutting the damaged 4" section, splicing and soldering new wires, and using heat shrinkable tubing and electrical tape to complete the repair.

Being a klutz, it took me about three hrs. to complete. Its now been about 4 Months, and the problem has not returned.

Phil

Phil, sounds like you found the source of your problem and effected a good fix. Have you looked at other sections of your harness? Any idea why just a 4" section would deteriorate... or have you just not noticed any other deterioration elsewhere?

Would also like to hear from anyone else who has repaired a harness, sucessfully or unsucessfully.

[pberku]

I had wiring problem in two other locations. 1) The coolant sensor that activates the Auxiliary Fans. The insulation disintegrated and the 2 wires leading to that sensor were shorting. I fixed that. 2) Also the two wires coming from the AC speed sensor. Although these did not short, there was a leakage to ground from one of them, and since they carry a digital signal rather than simply voltage, they caused my AC compressor to not work. I ran two new wires and that fixed that problem.

The biggest mess was that 4" section that I was referring to. I suspect the problem there, was caused by a bad design that allows rain water from the windshield to accumulate behind the Battery housing leaving that section of wiring harness wet.

I am hoping to be able to effect repairs to the harness if / when new problems crop-up. Replacing the dumb thing is extremely expensive in both parts and labour. I am sure however that one day I will have to replace it all, By then however, I would have possibly sold this car.


Phil

[suginami]

Originally Posted by 420sel man

Could someone elaborate what is it about these 1993-1995 wiring harnesses that causes them to fail. Knowing the cause of a problem is the first step to finding a solution that will last.

Also, has anyone had a similar problem (idle surge) that was in fact corrected by replacing the throttle actuator?
Are there other symptoms that are usually associated with a bad throttle actuator?
Seems like the car would not run very well at all (at other speeds) if the throttle actuator was bad. The car seems to do fine out on the highway.
.

I struggle with the search function, but if you try a search, there is tons and tons of information regarding the wiring harness problem in the archives.

The most accepted version of the wiring harness is that MB got caught up in the environmental movement, and was using insulation on the wiring harness that would be easily recycleable. The problem is that the insulation bio-degraded in the car - due to heat.

Again, not to beat a dead horse, but the throttle actuator problem comes up again and again in the archives. There are two reasons that cause actuators to fail: they also have a wiring harness that has failing insulation. They also have potentiometers that wear out over time. Neither of these can be fixed or repaired.

I guess the consolation regarding the engine wiring harness and the throttle actuator is the insulation degrading problem has been fixed.