Bizarre idea for 'cleaning' an engine .. how much damage?
 

Just heard of a the bizarre idea that running a 50/50 tank of mixed petrol and diesel through a modern petrol engined is a good way of cleaning the valves?!? Worse than that, a friend of a friend is determined to try this to 'fix' his badly running car.

This sounds like complete and utter madness and a surefire was of killing ones catalytic converter at the very least. Anyone care to speculate on just how badly this would f*** the car up?!

smoke like a trooper if it ran at all.

optimax does a good job of cleaning.

That was the first thing that came into my mind too. Remember seeing those bore-scope type pictures from when Evo used it on their Fast Fleet.

Red-ex do a "one shot" injector cleaner...

Would be a better bet!

I used to run my Nissan on a mix of diesel...anything above 60/40 Ptrol/diesel it would be a pig to start....but once running was fantastic at getting tailgaters or people blinding me in the mirrors with their fog lamps to back off to a nice safe non-intimidating distance. :D

Can't say it did anything for engine internal cleanliness...it made it worse...as the engine missfires and puts out unburnt fuel through the exhaust I'd expect the exhaust valves to be very choked up with carbon and sticky black tar.

Best combustion chamber cleaner known to man is Water+Antifreeze. Yes...it's true, if you've ever seen a engine with a cracked cylinder head or a blown head gasket (water entering combustion chamber type failure NOT to be confused with oil to water type failure). The cleanest combustion chamber is always the one that's had the water entering it. :)

Alternative, use an engine de-coke or Engine carbon cleaner (not Fuel system cleaner - but you may want that too). Run the car out of petrol (or drain it), and mix it with 1 gallon of fuel and run it til it uses it all up vary speeds and load. Or find a vacuum line and suck it up neat SLOWELY bit by bit. Giveing the engine a Good revving every 5mins (hold 4000rpm for 30secs and a few full thottle blips). Contiune until the bottle is empty, then go for a blast (not too hard, just keep it under moderate to hardish load).

Or, just a half hour blast down the motorway everyday with the engine doing 4000+rpm continuously. Do this for a whole week. And assuming the fueling system (sensors etc) and ignition system (plugs, leads etc.) are all in good condition. The engine will be as good as all the above :)

Proove it :p Or are you just taking Quentin's word for granted? ;):D

You will end up with a clean engine...it'll be a new one but clean all the same :D

taking the word of a mechanic who works for a very well respected engine specialist. he said he could tell an engine that had been run on supermarket cheap unleaded vs decent brand stuff.

he knows a damn sight more than me about engines than i do so i take his word for it.

Perhaps its to do with the water content (just clicked with what I put in the previous post :D )....

I did have a tank of water after filling with optimax...suffice to say the car didn't go too far...but I suppose it would be quite clean :D

Not wanting to poop on Ali-B's post, but water and anti-freeze is an absolute no-no. I would strongly advise against any kind of unsuitable chemical to be alowed into the combustion chamber. If nothing else, you don't know if you get any (probably carcinogenic) gasses leeked into the cockpit. Please for the sake of God don't put antifreeze in your tank. Its colour-coded blue for a reason. As for water, need I state the obvious that it doesn't mix with fuel? You'd end up sucking pure H2O from your reservoire at some point, and needless to say, not much BHP can come from that. Interesting little tidbit of info though, Ali-B. Thanks for that one.
As for running diesel, there are 3 issues: 1) Diesel has a lower caloriffic value than petrol, so you'll be making less power. Possibly not evenenough to keep you from stalling as Ali-B mentioned in his Nissan. 2) Diesel has a lower octane rating than fuel. Lower than the lowest standard cars are built for. (around 87 unless i'm talking out of my rear?) This means that detonation will be a lot more likely, and/or missfire. 3) Diesel is a slow-burning fuel, meaning that it doenst have time to burn in revs higher than 4000. Racing diesels only rev till 5-6000. Thus, assuming your not very smart friend runs diesel, most of it will be burning after the exhaust valve opens. This will in effect cake the valve with lovely carbon deposits, and may even seal them open. If this happens, your head will explode as the combustion will be taking place in an unsealed cylinder. It will also practically 100% guaranteed murder your catalyst, and I wouldn't be surprised if it contributed to a new hole being burned in the exhaust somewhere, due to the unnaturally high gas temperature.
All in all, I think Diesel in a petrol engine is probably one of the worst ideas known to man. The two engines have only reciprocating pistons in common and little else. Diesels comonly have 3-4 times the stroke of petrols, and normal compression ratios for them are inthe region of 21:1. In contrast to 10.5:1 for the petrol, naturally aspirated, that is.

Please for the sake of the environment and your mate's bank account, tell him to stop the wonky ideas, and go pay Halfords the (whole amazing sum of) 4.99 that a bottle of valve-cleaner additive costs. Dump it into a fresh loading of Optimax from a station that has it's stock regularly refilled. Let the engine warm up for a good 30 minutes, pop second gear and drive right under the rev-limiter for a good 10-15 seconds a few times. Let it cool down before switching off. If that doesnt work, it's not dirty valves that is his problem.

Good luck

No worries :D Anyone stupid enough to do it wouldn't get any sympathy from me ;)

Also Red, Orange, Pink Green and even clear! :D

This would make very little effect. As 300ml into 50litres is so diluted that any diference or noticed improvement would only be caused by the octane boosting of the ingredients (usuallyToluene) and nothing much else. Professional Fuel system and combustion chamber cleaner devices work by bypassing the fuel tank and running the engine off a small 1/2 Gallon tank of fuel with the cleaning addatives mixed in, until it is empty. Add the same addatives to a nearly empty fuel tank has the effect as these machines.

LOL that old chestnut - Where's Mycroft ;)

Ali-B

Unless I missread, the bottles read "one bottle per 50liters fuel" so obviously the concentration is enough. Either way, it's a lot more efficient than getting a garage to bypass your fueling. Although i must admit it does sound more likely to result in a successful cleaning.

However, I still scratch my head in wonder, how did this gentleman suddenly decide "dirty valves" is the reason his car is running badly?

Most bottles available off the shelf to the general public actually do indeed say add to a full tank. But these are usually stocked in the snake oil section of Halfrauds.

Experience dictates that no off-the shelf can of magic will do what the buyer wishes and cure a bad running engine, especially in such weak dilution.

In the garage environment when fault finding a bad running engine, it is extremely rare to ever need a fuel system clean or a decoke. And the majority of cases the cause is more often than not faulty/failing components, be it ignition or EFi related. The only vehicles I know of that ever require any form of cleaner are of the early 80's era with large capacity 8+ cylinder engines and open-loop EFi or Jetronic fueling systems that endure being used in urban driving conditions.