Gradual loss of power, resting restores it - PeachParts Mercedes-Benz Forum

tmohler · #1 12-17-2008, 10:35 AM

Hi all,

I was driving from Amherst MA to New York yesterday. Car (1987 300TD with 186,000 miles) was running fine the first half an hour or so. I began to slowly lose power going up hills. At first, not too noticeable then by another 30 minutes, I crept up the last hill at 30MPH with my flashers on.

I got off the highway, car ran fine under 40MPH. Oil pressure fine, temp gauge fine. I checked the oil level, fine, checked coolant level, fine. I sat and thought about my options and decided to find a mechanic. When I restarted the car, she went fine above 40MPH again and I decided to try and make to New Haven. After about 20 miles as I was coming to New Haven, same thing started happening again.

I eventually made it back to NY driving for 20 miles and then sitting with the engine off for 20 minutes.

My first thought was a fuel problem, but why would it only show up after 20 miles? Second thought was a turbo issue but by the end of the 20 mile run, I couldn't maintain speed even on level ground.

My only other possible clue is that when I had to put the pedal down, I heard a distinct hissing sound coming from the dash.

Car is at my indy mechanic right now. Any thoughts?

Thanks
Tim

vstech · #2 12-17-2008, 10:40 AM

classic fuel filter plugged symptoms. change the spin on filter and the clear primary filter.
done.

tmohler · #3 12-17-2008, 10:54 AM

I will certainly do this.

The only prior time I had a fuel filter problem was long ago on a gas-engine car and there the problems seemed to stay constant - they didn't get better when I stopped the car.

Is it normal for the symptoms of a fuel filter to be intermittent?

Thanks
Tim

Bio300TDTdriver · #4 12-17-2008, 11:29 AM

Quote:

Originally Posted by vstech

classic fuel filter plugged symptoms. change the spin on filter and the clear primary filter.
done.

X2, he's right.

Chad300tdt · #5 12-17-2008, 12:09 PM

Do the w124's have a tank screen like the w123's do? That would cause these symptoms too. As the car sits the sludge would drop and as you drive the sludge would collect on the screen again.

How much fuel did you have in the tank?

Bajaman · #6 12-17-2008, 12:15 PM

Whats the temp like? Could be viscous fuel trying to get through the filter. Resting would allow the fuel in the filter to get warmed by the engine and flow better. This could also be in conjunction with a plugged filter.

Bio300TDTdriver · #7 12-17-2008, 12:15 PM

Yes it has one of these. That could also contribute to the problem. The primary and secondary filter are an easier place to start though.

Quote:

Originally Posted by Chad300tdt

Do the w124's have a tank screen like the w123's do? That would cause these symptoms too. As the car sits the sludge would drop and as you drive the sludge would collect on the screen again.

How much fuel did you have in the tank?

Bio300TDTdriver · #8 12-17-2008, 12:21 PM

That might be the case if he is using a high blend of biodiesel, but probably not petroleum diesel. At least not in MA and NY, perhaps in Minnesota.

Quote:

Originally Posted by Bajaman

Whats the temp like? Could be viscous fuel trying to get through the filter. Resting would allow the fuel in the filter to get warmed by the engine and flow better. This could also be in conjunction with a plugged filter.

bob_98sr5 · #9 12-17-2008, 01:19 PM

tim,

maybe unrelated, but on monday, it rained here in LA (imagine that). i did a little italian tuneup up a long, hilly stretch when my power going up the hill just decreased very quickly. it would not push 60 easily. turned out that my air filter had ingested a lot of water. the thing was completely soaked and heavy.

give it a look

tmohler · #10 12-17-2008, 01:47 PM

Quote:

Originally Posted by Bio300TDTdriver

That might be the case if he is using a high blend of biodiesel, but probably not petroleum diesel. At least not in MA and NY, perhaps in Minnesota.

Just before the drive, I had put 5 gallons of 100% biodiesel in a 3/4 full tank of petroleum diesel. It could be the fuel was bad/gunky but I don't think it would gel, since the outside temp was about 35 degrees and a 25% blend should be OK at that temp, I would think.

It did seem like the position of the car made a big difference. As I started to go downhill, the RPMs would start going back up again as though the car had started to get more fuel. Chad's comment about the sludge re-positioning might make sense.

Tim

barry123400 · #11 12-17-2008, 02:21 PM

Could be the fuel tank vent is pretty well obstructed as well. Although twenty minutes is not quite enough time to form a vacuum in the tank strong enough to starve the engine.

When stopped slowly the air leaks back in to restore a lesser vacuum condition. Proof is usually made by running with loose fuel filter cap or a rag stuck in the fill pipe. Yet if the rest period to restore to operation is the approxamatly same time it runs along the road it is a definate possibility.

LUVMBDiesels · #12 12-17-2008, 03:02 PM

I think it could be one of two things.

1) the tank vent. If you run her until she gets sluggish, stop and open the filler cap and you hear a whoosh of air entering the tank, the vent is clogged.

2) The shut off switch in the dashboard is leaking. This actually happened to me. The car would run fine for a while, then slowly run down like an old watch. eventually it would stall out. The vacuum switch attached the the key lock was leaking and vacuum would build up in the shut off valve on the IP until it was strong enough to kill the engine. Since it was a slow leak, the engine would run weaker and weaker until it just quit.
When you are experiencing the slow death syndrome, open the hood and pull the vacuum line going to the shut off valve. If the engine perks up you have found the problem.

tmohler · #13 12-20-2008, 09:45 PM

So I changed both fuel filters and the air filter. All were pretty well plugged up. Since I changed the primary filter 6 months ago, something must have happened fairly recently, probably a bad can of biodiesel? In addition, the transmission fluid was nearly a quart low and was brownish, although not burned-smelling. Hope I didn't damage anything running with a plugged filter. Since I still have half a tank, I am assuming that I might re-plug the filter and must keep an eye on it. I was wondering whether adding a gallon or so of RUG would help "de-gunk" whatever might be left in the tank? Tim

Bio300TDTdriver · #14 12-20-2008, 09:50 PM

Please don't burn RUG in that car, there aren't that many of them left.

How recently did you start blending in the biodiesel? Also if you are blending 3 gallons of biodiesel, put the biodiesel in first and then the petroleum diesel.

Quote:

Originally Posted by tmohler

So I changed both fuel filters and the air filter. All were pretty well plugged up. Since I changed the primary filter 6 months ago, something must have happened fairly recently, probably a bad can of biodiesel? In addition, the transmission fluid was nearly a quart low and was brownish, although not burned-smelling. Hope I didn't damage anything running with a plugged filter. Since I still have half a tank, I am assuming that I might re-plug the filter and must keep an eye on it. I was wondering whether adding a gallon or so of RUG would help "de-gunk" whatever might be left in the tank? Tim

el sea · #15 12-20-2008, 09:52 PM

This has all of the signs of bad fuel. When we clean tanks we are finding stringy black goo from the new bio-blends. In gas tanks we are finding oatmeal like crud from the ethanol fuels.

Luther