Fuel contaminating my oil

[Fireball]

Hi all, I hope you can help.

I have a 250sl, I replaced the fuel pump recently and change the oil. I have driven the car for a short distance and the oil level on the dip stick has risen and smells of petrol. Is there any obvious reasons for this. Dumped the contaminated oil out, filled to the line with new oil, run it a short way and again oil level has risen. Any ideas what might be going on?

[Fintail]

Sounds as though running far too rich.

Regards John

[scoot]

I have a 250sl, I replaced the fuel pump recently and change the oil. I have driven the car for a short distance and the oil level on the dip stick has risen and smells of petrol. Is there any obvious reasons for this. Dumped the contaminated oil out, filled to the line with new oil, run it a short way and again oil level has risen. Any ideas what might be going on?

Just to clarify, you replaced the fuel pump at the rear of the car, not the injection pump, correct?  And the dipstick with the fuel smell is the engine oil dipstick, not the injection pump oil dipstick, right?  If fuel is getting in the injection pump oil it's something entirely different than running rich.

[Benz Dr.]

I had this happen once and it was the IP.

[Fireball]

Yes it was the fuel pump at the rear of the car and the dip stick is for the engine oil

[Fireball]

I had this happen once and it was the IP.

What is the IP ?

[Fireball]

Oh, I get it the injection pump.

What was wrong with your pump and how did you fix it?

[Dave H]

Benz Doctor ( Dan ) will be correct in saying its the IP at fault.

Its also worth looking at the following, the link  explains another reason the oil in our cars ends up stinking of petrol.


https://www.youtube.com/watch?v=PLedjKWbU5w

Hope the above link works

[CCollum]

Happened to mine earlier this year, and it was the fuel injection pump too. Mechanic said it was time to send off for a rebuild.

[Pawel66]

If I may add (and please correct me if I am wrong, for my learning): I was running quite rich for quite some time (2000km) and I have never had neither oil level raising nor oil smelling petrol. I would have thought that running rich may result in these symptoms over a bit longer runs, provided that your head gasket and compression are ok. If you say you took it for a short run after oil change and it increased its volume and started smelling - this might have been something much more "direct", like IP, as more experienced members are suggesting. Maybe if you replaced the pump, you got higher pressure or not enough return flow and something did not hold in the IP, if I were to speculate.

Could be a good time to look at dumper diaphragm if this step was skipped for years... all filters you probably did as you replaced the pump.

[Benz Dr.]

I guess something breaks inside the injection pump which makes it go really rich. Other possibility is a leaking cold start valve although most of that fuel would get burned ( poorly ) while going through the engine.

My money is on the injection pump.

[Fireball]

If I may add (and please correct me if I am wrong, for my learning): I was running quite rich for quite some time (2000km) and I have never had neither oil level raising nor oil smelling petrol. I would have thought that running rich may result in these symptoms over a bit longer runs, provided that your head gasket and compression are ok. If you say you took it for a short run after oil change and it increased its volume and started smelling - this might have been something much more "direct", like IP, as more experienced members are suggesting. Maybe if you replaced the pump, you got higher pressure or not enough return flow and something did not hold in the IP, if I were to speculate.

Could be a good time to look at dumper diaphragm if this step was skipped for years... all filters you probably did as you replaced the pump.

Interesting, I replaced all filters after fuel pump replacement. What is the dumper diapham?

Sorry for my ignorance.

[rgafitanu@gmail.com]

The fuel damper is located after the IP on the return to the fuel tank. It contains a diaphragm that absorbs sudden surges of fuel pressure.

[Benz Dr.]

The fuel damper is located after the IP on the return to the fuel tank. It contains a diaphragm that absorbs sudden surges of fuel pressure.

It does that but it won't cause this problem. I would NOT under any circumstances, drive this car! Heavy fuel dilution can do extreme damage to the internal parts of your engine.

[Tyler S]

The 250sl uses the R18 injection pump. This pump has its own oil reservoir with dipstick. However it also has an oil feed line from the crankcase to lube the Injection Pump pistons. This is separate and not connected internally. If the injection pump and engine oil pressure are in marginal shape and the fuel pump is rebuilt or replaced, the extra pressure is enough to overcome the incoming oil pressure at idle or when the key is on/fuel pump running. Later versions of the R18 had a check valve added to this oil line to prevent this problem.
To check you can disconnect the oil line banjo bolt from the side of the IP and watch for fuel. If fuel starts leaking with the key on, you need an IP overhaul or at the very least a check valve.

[Benz Dr.]

I added a check valve to my oil line so this could never happen again because it did once and it was engine rebuild time after that.

[Pawel66]

It does that but it won't cause this problem. I would NOT under any circumstances, drive this car! Heavy fuel dilution can do extreme damage to the internal parts of your engine.

Yes, certainly, fuel dumper is not the reason here - I meant to look at it on the "once you are there" basis. I smell this will end up with IP rebuilding - removing and waiting - may as well look at related items. Spring and/or diaphragm are not expensive.

[stickandrudderman]

Whilst it is LIKELY that it is the IP, you should first check the CSV. If it is constantly open it can give the same symptoms.
You will find that the diluted oil causes poor ring sealing and so smoking, loss of compression and poor performance all follow.
Plenty of information on here about CSV and how to check it.