Transmission on 1984 300SD slipping - from Mia J.

There appear to be two separate issues:

Why is the engine not running as it should? and Why is the transmission fluid level so high?

Your description suggests fuel starvation is the cause of the engine's stalling and poor running. This isn't starvation from a clogged fuel filter (but that may be contributing to it) but rather could be from a clogged or blocked fuel tank vent. Remember, as one drives fuel is withdrawn from the fuel tank, air is normally vented into the tank to replace the withdrawn fuel. If the air can't enter the tank a vacuum will be created and the car will act as if it were out of fuel. The easy test of this hypothesis is to simply open the fuel filler cap at the next stall and see if there appears to be a vacuum holding it onto its seat. I believe the tank's air vent is on the tank's driver side; it should be checked if there is a vacuum being created inside.

The other possibility has a similar result but from another cause: bad fuel, not diesel #2 but veg oil/grease that's congealed in the fuel lines and or filter and needs engine heat to melt. In that case I suggest Biodiesel 20 or 100 only from a station and some new fuel filters - there are two.

The transmission should be checked with the engine running, in Park, and after sufficient driving for it to be hot. The correct level is BETWEEN the H and L marks on the dipstick. The engine must be running so the torque converter is filled, otherwise the fluid will appear to be overfilled. If it IS overfilled, the excess ought to be drained; overfilling is as bad as under filling.

Given the confluence of events I'd discount the transmission's "acting up" because when these 5 cylinder diesels misfire they shake like hell and one thinks "it's all over" for this engine. Fix the engine's fuel supply and it will be fine.

Remember, it's usually the simple stuff.

I haven't had any similar problems yet (maybe I just haven't driven far enough) but I did notice recently when filling up that the fuel cap came off with a hiss ('88 w124 petrol, 116.000 km). So if I understand you correctly I should get this fixed ASAP? Is the vent easy to fix diy?

I read a lot about problems caused by dirty fuel tanks. Perhaps I should get mine cleaned as a precaution. Is that a standard operation?

Ximinez

T.G.,

I think fuel starvation idea is correct. Now that I staled again, I noticed tranny was shifting without a hitch. However, when I popped the fuel cap open, I got only faint air noise, nothing distinct.

The car starts up and drives beautifully for first 15-20 minutes. I noticed when I go on any hill, I experience loss of power, and eventually the car stalls if I don't keep foot on gas. Also, I can't seem to get it up to 70 even though gas pedal is almost floored. I didn't keep trying.

You mentioned fuel quality - I added cetane boost/injection cleaner during my last fill-up, used it before, never caused a problem. Just recently diluted what was in the tank with 10 gal. of good diesel, stalling problem persists.

In general, it runs smoothly, shifts smoothly, temperature and oil pressure gauges normal, but sometimes (on a hill, most often) when I try to maintain constant speed, it feels like I was robbed of 4 cylinders and drive on just one.

Do you have more info on checking fuel lines? I studied my manual last night (the pdf where every "job" has a multi-digit number), but the only thing I got out of it was how to replace fuel filter and pre-filter, which I will do soon.

Yours,

Mia

Fuel starvation is the cause.

Check the tank's air vent; there should NOT be any pressure / vacuum in the tank. The engine's fuel supply pump (separate from the injector pump) cannot overcome such problems.

If the vent is clear replace both fuel filters. Clean the fuel pick-up screen and flush out the fuel tank if you find algae in the filters. Algae is able to grow in water and diesel fuel. Commercial algacides are available to kill the algae. Otherwise, don't use fuel additives, they're generally not needed.

As an aside, and for future reference: Very very rarely, the diesel #2 itself is bad, probably from gasoline dilution during distribution. The engine will be noisy, run poorly and its power will be reduced. The choice is to dump that fuel or dilute it with fresh #2 until it's consumed. These old diesels will NOT be harmed by some gasoline so think of it as a bad meal - all will pass.

Hope these suggestions get your SD back in action.

A car with a gas engine has an entirely different fuel system, including an electric fuel pump and a sealed fuel system. What you describe is NORMAL for a gas driven car.

Don't look for things to be problems, drive your car and enjoy it.

T.G.,

My mechanic is agreeing with you. Would it be reasonable to buy primary and secondary fuel filters from Autozone, or do I need something specifically better, like OEM stuff?

Mia

The "experts" will swear to OEM quality and so will I in most instances, however, in this instance I believe you may soon have to change the fuel filters again, so here, and for now, a store filter will be OK. After the problem is resolved use OEM quality filters - for everything. Seem expensive but last longer and really are better; they can be bought on-line for reasonable amounts. After changing the fuel filters the fuel system needs to be "primed" with the hand pump that's next to the small fuel filter. This will expel most of the air so the engine will start. Check that the rubber seal ring in the pump's screw down handle is intact otherwise air can seep into the fuel system and cause starting problems. The hand pump is not repairable; on-line for about $20 if needed.

That's a relief ;)

I still have a stalling problem as described earlier. Dirty fuel lines might play a part in that. The car goes back to the dealer today to have that fixed.

Ximinez

Normal, don't worry.