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When EFI was new, injector clogging from varnish was epidemic. Stuff that literally looks like and acts like varnish would precipitate from gasoline in an engine left to cool down after it got good and hot. It clogged injector nozzles so they didn't make the required fine spray, but instead shot out an erratic stream, or nothing. The engine mfgrs never anticipated it, and in fact never saw it, because in all of their emissions-over-a-lifetime testing, they never shut the engines off and let them 'hot soak'. Now, they do.
The effect on driveability was distinct, but subtle; at idle or WOT, the engine ran fine. But at near-closed throttle, just gently keeping up with traffic, the engine would act like it was either on, or off. On a carbureted car, you get exactly the same effect from clogged transition jets.
The solution for EFI or carbureted cars is the same; Marvel Mystery Oil. A pint or so to start cleaning, and about 4 ounces per 12 gallons of gas thereafter to keep things clean. I have _never_ had any other additive I've tried make such a dramatic difference, and so quickly. MMO usually cleans things up, as if you've just had a valve job, within ten miles.
Because of the varnish fiasco, mfgrs now do hot-soak testing, and injectors made in the last ten years are much more resistant to varnish clogging, and gasoline formulas have changed, so if you drive a new-ish car, you may never see the problem. Older cars may still benefit from MMO.
I have found _NO_ additive other than MMO that made _any_ detectable difference in my cars.
Originally Posted by dom
