Ideas as to huge MPG drop since overheating? - PeachParts Mercedes-Benz Forum

ps2cho · #1 09-29-2010, 07:26 PM

It all started after the overheating issues I had...I ran the car at ~110C a few times while trying to figure out what was going on. I used the heater to keep temperatures down, but it never hit the red line.

Water pump and radiator later it is resolved and now purs at no higher than 95C in 115F heat here in Phoenix.
Could the overheating have done anything else?

Car idles smooth, no misfires....I have dropped from ~20mpg (~300mi to tank) to 12-13mpg (210mi to tank)

I've done a full tune up in the last 5k miles when I bought the car. All filters and fluids, rotor, cap, plugs wires, ovp...Duty cycle is ~45-50%.

Car drives fine although power seems slightly down, but nothing else out of the ordinary.

Any ideas how I can track down whats happening?
For my 87 260E

pawoSD · #2 09-29-2010, 11:39 PM

Failed engine/CIS temp sensors?

ps2cho · #3 09-30-2010, 02:19 AM

Quote:

Originally Posted by pawoSD

Failed engine/CIS temp sensors?

I replaced the 2 prong sensor 6 months ago after it cracked. A failed CIS temp sensor exhibits poor cold starts, but once warm no difference. I have no starting issues.

ps2cho · #4 10-01-2010, 01:21 AM

Just probed O2 sensor...Reads 0.8v. Definitely running rich according to O2 sensor.

Since the duty cycle reads 50-55%....what could be causing this rich condition? Injectors leaking?

Ferdman · #5 10-01-2010, 09:40 AM

Consider replacing the coil.

ps2cho · #6 10-01-2010, 10:58 AM

Quote:

Originally Posted by Ferdman

Consider replacing the coil.

How would the coil be causing overfueling?

Chas H · #7 10-01-2010, 11:17 AM

Do a compression test.

Rahulio1989300E · #8 10-01-2010, 11:35 AM

Quote:

Originally Posted by ps2cho

How would the coil be causing overfueling?

I think what he meant here was that since the oxygen sensor is in the exhaust system, which is after the cats it will only tell you if the exhaust gas is rich with fuel. The coil could have been damaged internally from the high heat and is no longer producing a powerful and 'complete' spark to burn all of the fuel/air mix in the cylinders before it is extracted out by the opening of the exhaust valve. A weak coil would leave an engine producing less power and give the impression to the oxygen sensor that the engine is running quite rich.

Other than that...

ps2cho · #9 10-01-2010, 03:22 PM

Is there a way to test the coil's output?

Merkey · #10 10-01-2010, 04:54 PM

Whilst its possible that its the coil, in my experience, they either work or they don't. If the coil was failing then it would not start well, and misfire at higher revs.
We did a compression test a while back, admittedly before the overheat, but I see no rerason for anything to go bad.. its not like you fried it...

My guess would be something in the FI domain.
You already checked the temp sender, so that was a start.
Do a fuel pressure test, see if something is odd... you know what it was before, see if it changed.
Maybe the EHA went bad?
Leaking cold start injector?
I am also concerned about vac leaks... remember you had that idle hose fail, even tho it was replaced when you got the car.

Another issue is use... are you are doing more short trips with the A/C on full... could be a factor?

ps2cho · #11 10-04-2010, 01:03 PM

Went right for the EHA...and noticed it was wet. I checked O-rings and they looked OK. I removed it, cleaned all the surfaces and retightened the bolts (need new bolts too...these are getting stripped)-- But upon reinstallation I noticed this!

It seems to be weeping out a hole in the rear

Is this a 'weep' hole? Either way, looks like I need to replace it. It was quite handsomely dripping out when the car is running. Guess this could be it?
Although maybe I don't understand how that would cause an overfueling situation? I would have thought it would make the car run very lean?

Rahulio1989300E · #12 10-04-2010, 03:30 PM

When EHAs go bad, it seems that they can cause all sorts of issues including lean and rich conditions...

I think if you replace this EHA with a good used unit or a new one, you will have solved the problem... but we shall see.

tinypanzer · #13 10-04-2010, 03:32 PM

Well, if the EHA is failing to properly adjust upper/lower chamber pressures, then I suppose it could wind up being either rich or lean, depending on which "direction" it was failing.

I would still check the temp sensor by measuring its DC resistance, and comparing it to the temperature/resistance chart. The symptom you have seen with a bad temp sensor is what happens when it fails completely. If its value has drifted, that might give different results.

It's also possible that something got disconnected when the work was done. Also, on an M103, there is always the possibility of vacuum leaks even in spots you have already fixed.

ps2cho · #14 10-08-2010, 03:44 AM

In the case of buying a new EHA, does it require adjustment to set to factory settings or is it plug'n'play?

--
In regards to engine temp sensor...
Do you measure resistance with it at operating temperature? Compare it to the cluster sensor adjacent to it?

I replaced the engine temp sensor about 4 months ago, but I'll check it just in case also. Regardless though the EHA is a problem for sure.

ps2cho · #15 10-14-2010, 03:08 PM

Going to put it in right now and report back later with the results.

Posted pics so you guys have the part #'s now for the screws. Although the dealer will not sell them separately, it may be possible to find them elsewhere now!