I thought I’d do a case study of injector cleaning on the Stilo as I had an ideal opportunity to take some readings before and after.

I’d guessed my injectors were clogging up, no driveability problems but I’d taken some data readings over a 6 month period and knew my long term fuel trim figures LTFT were going high. What does that mean? It means the ecu is having to leave the injectors open for longer to squirt enough fuel in so that correct combustion can occur as recognised by the lambda sensor. You could say it was putting in 30% more “fuel on” time in order to get the right mixture. Not more fuel necessarily, it’s just the taps are partially blocked and have to stay open longer. With me?
The fuel spray pattern at the injector Is likely to be affected too of course resulting in not so good combustion

Short term fuel trim STFT, as you might imagine, is a fine adjustment over a small range. Once it goes beyond a certain amount then the LTFT racks up or down a notch to get closer to ideal and the STFT makes fine adjustments again on that

Now high long term fuel trims can mean other things too like inlet air leak providing extra air or an incorrect lambda will fool the ecu too but if you’ve investigated those then it’s time for unclogging those injectors

Ok so here’s my LTFT figures before the slimming plan magic formula

fuel trims b4 reset graph.JPG (click to enlarge)
Look at the blue line showing LTFT in the top graph, range is shown on the right. Pretty awful at +40% more “on” time
STFT on the bottom graph here

I decided to use Comma oil petrol injector cleaner. Never used it before. Only £6 and it said it could be used with up to 40 litres of fuel but I waited until I had only ½ tank of fuel for a good mix. Simply pour the lot in the fuel tank.
“OK” I thought “Go for a drive now and give it time to do its job”. No need to thrash it by the way, injectors are simple on off solenoid valves
Anyway, I was astonished. It was noticeable before I’d got to the end of the road! Smoother pick up and better running at low rpm.

But to make sure I wasn’t fooling myself I looked at the LTFT and STFT now.

fuel trimsw imm after treatment warmer engine 2.JPG
Now the STFT was saying “Whoa! Way to much fuel going in now” LTFT is on + 45% and STFT is compensating frantically, pulling it back and going down to -60%

So after an hour or so of driving I reset the fuel trims to save time and had fingers crossed.

fuel trimsw imm after treatment reset warmer engine 6.JPG
What a difference. LTFT now on + 4% and STFT nicely oscillating over small amounts and quite happy at that

So there you go. A magic potion that is proven to work and cheap as chips. There’s also a Comma diesel injector cleaner. All available from your local Halfords