I hooked up what I thought was the fuel feed and return line, but the liqui moli was being sucked up and returned as if someone was running a faucet.

I had the return line in the same bottle but I took it out because the liqui moly turned black.

I thought I would destroy the fuel filter and

what went wrong?

Sounds right to me...

Remember the system constantly circulates the fuel. At idle most of the fuel goes back down the return line to the tank. The circulation cools the injectors and the injection pump.

Yes the purge will turn black. It is all the junk being liberated out of the fuel system. It is being trapped by the large cannister filter on its way back into the injection pump.

Be sure to change the filters when you are done. Use clean diesel, another can of purge, or ATF to fill the new filter before installation to make priming/starting easier.

Also helps to have your year and model of car in your signature. There are some differences in the engine fuel systems that are model dependent.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

The lift pump moves a LOT of fuel, far more than the injection pump can ever inject. You will always have a fairly strong return stream if the lift pump is healthy.

All of the black crap USED to be in your fuel system. This is why you're running a purge. As stated above, when the purge is complete, change your fuel filters.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Oh ok, so it's supposed to do that.


I have an 86 SDL btw.

It actually runs a little better and quieter now.

I think it needs one more treatment.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

You want to get as much junk out of the system for the dollar and effort expended. Start the car and get it running on the diesel purge. Turn it off . Let it sit a few hours and repeat this several times.


So the solvent can really do as much as possible in dissolving built up crud. Just starting the engine and letting the engine consume the solvent allows no time for soaking. Also switch to the diesel purge with at least a warm engine. Solvents are far more active at elevated temperatures.


Chemicals in general are pretty much inactive below 70 degrees in comparison to higher temperatures. Generally can be considered as an almost somewhat universal law. directing a heat lamp at the pump is another way to elevate the temperature.

When the engine is warmed up chances are that the injection pump is to some extent. There are no parts in these earlier injection pumps that the solvent can damage.


Directing a heat lamp on the injection pump for awhile is another alternative before starting. But the flash point of the solvent may be low so stop using the lamp before getting the solvent into any proximity.

None. Also you can take this even a little further. After the first soak turn the engine over with someone holding the injection shut down lever in the off position. This should put most of the dissolved junk out the return line. Instead of pushing at least some of it through the injectors. All the excess junk of course will return to the bottle.


The fuel filters will catch it before it gets into the injectors on the next pass. Or at least less of it when you run the engine again. The amount of built up crud in the injection pump over the years can vary widely. In your case it sounds like your solvent went dark fast.

I suspect that most of the black stuff was dissolved out of your fuel filters. It would make more sense to me, to change the fuel filters, get the engine running again, and then do the diesel purge. That would keep much of the black junk from being flushed into the injection pump and injectors.

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'97 E 300 D

You may be right of course. But some people do the purge and the fluid in the container that is returned remains pretty clean.

On the law of averages the secondary and to me the most important filter is not changed out frequently enough. There has been a tendency to leave them in service perhaps for longer than they should be.

Some believe that even until the engine shows signs of fuel starvation. It is a false economy to do so. Too many reports have appeared over the years. Basically claiming their fuel milage increased after they changed the filter on these engines. Logic would seem to apply that a starving engine would burn less fuel. Also since they were not looking for this it would have had to be at least a mile or two better fuel milage per gallon for them to have even noticed it.


The archiles heel is as the filter becomes more obstructive as time goes on. It reduces the available fuel pressure in the base of the injection pump. So operationally speaking chances are it is cheaper to change the fuel filter periodically than almost never. And remember it is the only water trap you have unless you install an aftermarket one.


Also as the filter starts to present a greater pressure drop or resistance . The chances increase of forcing more dirt through the element. Based on the pressure differential is increasing.


Personally I like spending money where the benefit is greater than not doing so.

I don't disagree with you conceptually, however your strategy is only valid if you don't measure. 99% of owners do not have a fuel pressure gauge so they are forced to operate on a blind-worst-case-scenario. Bad fuel is way too common so it makes sense to change often even when the filter is still perfectly good - because you just don't know. As a counterpoint - I've got a fuel pressure gauge on my project car and have well over 110k miles on the filter with no discernible pressure drop. I'm extra meticulous to avoid bad fuel and it has paid off. I did replace the plastic filter once in this time period because I replaced my rubber hoses with stainless steel braided lines and needed to modify the attach point. I cut the filter open (roughly 80k miles) and found it to be almost "as new".

Again, not arguing with your logic and advice, but pointing out that clean fuel makes all the difference and there is no substitute for hard data.

By the way, it is very difficult to get meaningful pressure data on these diesels - i use a snubber, a damper and software averaging to get a meaningful reading.

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Current Stable

• 380SL (diesel)
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