By James Wong


I guess this is not a really surprising photo but there’s a bit of story behind this.

I was on medical leave whole of one week recently and you know what people do during times like these – their minds wonder and they get restless. I had a bottle of Liqui Moly’s Valve Clean which I previously used on my S204 (without any issue) lying around in my room. Then an idea popped up: “Why not put this in the SL? Seeing as how old the car is there must be a lot of cleaning to do.”


So in went the liquid, and ever since the car ran like it had a whole battalion of riflemen in its exhaust pipe. The misfires, or pops and bangs, were so loud they were like a 21 gun salute. At first I reasoned that it must have been dirt getting cleared and it’ll be OK after a while. Little did I know it got worse and worse the more I ran the engine! Plus the engine usually died after 20 minutes and I had to wait for another day to start it again.

I got sick of doing it so many times one day I decided to get the car out on the road to clear the gunk once and for all. Sadly, the gunk won! After embarrassing popping and banging, the car basically died while going uphill (thank you SL!) and refused to start again.

I then had to push it uphill, thankfully with some reinforcements – dad and brother-in-law – to a petrol station to avoid incoming high speed traffic. A tow to the workshop ended the evening.

In the end, the workshop determined that the Valve Clean liquid kicked up residue dirt that lay dormant in the fuel tank for many years. The dirt then – horrifyingly – went into the engine and struggled to combust together with fuel. The solution was emptying the whole fuel tank, letting the residue settle and then return a top layer of clean fuel back into the tank.

It seems to run fine now, except the car seems more prone to dying than usual – as if its resolve to keep alive weakened somewhat. I’m unsure what the issue is, but will monitor as I go on a long drive this weekend.

Other observers speculate that it’s the primitive mechanical fuel injection system that is unable to deal with the change in fuel characteristics thanks to the valve clean liquid. Others say engines will form their own natural tolerances and will react adversely if it gets a clean it doesn’t really need in the first place.

Whatever it is – to anybody thinking of using Liqui Moly Valve Clean for their classic cars – beware!

I even asked Liqui Moly what the issue was, and they didn’t give a great answer either: “Our Liqui Moly Valve Clean is a cleaning additive which is suitable for new and classic cars. Problems with Valve clean in classic cars are unknown to us. If you use the correct dosage, the ignitability of the gasoline does not change.”

I will continue to liaise with them and if there are any updates, I’ll post them here.


One Response to “Liqui Moly Valve Clean: beware!”

  1. Roland Norburg

    Why would you pour whatsoever in the car, especially if it’s an old-timer without a guarantee for how contaminated the car is… Even worse: Valve cleaner and Injection cleaners work quite well nowadays (as you had to learn the hard way), why was the SL so contaminated in first place? Additives unfold their true power when added to cars that consequently and constantly over time have been treated nicely. Yours doesn’t seem to have seen a valve cleaner too often before…


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