M103 / CIS soft-response to light throttle - dirty injectors

[babymog]

I am posting this because it is a problem I've had in EVERY CIS car I've owned, which is probably well over a dozen, most of them were new.

I have ALWAYS run these cars exclusively on premium, Mobil or BP/Amoco. I've found these to run the best of the major brands.

It is a common and well-known problem for US fuels to foul injectors and has been discussed in many SAE and other documents/tests. Not a new problem, Bosch has had this problem since the mid-'70s in different vehicles, including the early (Bosch licensed) GM EFI cars so not limited to CIS, just more obvious in CIS.

Remember that the CIS sprays fuel all of the time, thus the Constant Injection System / CIS. Because of this, the fuel is spraying on and soaking the intake valve through 720degrees of rotation, even though the air is moving/valve open less than 1/4 of that time. The valve gets soaked, any carbon on the valve gets soaked in like a sponge, so it dribbles into the combustion chamber more than it would with a clean valve stem.

The bigger problem is the injection pattern, especially at low flow rates (cruising and idling), and that tends to manifest itself as an occasional miss at idle, a stumble when accelerating off-idle, and soft response from the accelerator when cruising to the point where it seems to accelerate slightly when you let OFF of the pedal slightly better than you push it down slightly.

I've been driving these CIS cars since the mid-'70s, and it has always been a problem. Audi would pull intake manifolds and walnut-shell blast the valves and pull the injectors for cleaning/replacement. Mercedes replaced all kinds of stuff in my '91 when it was fairly new, I even paid an indy to replace parts Mercedes wouldn't, such as the entire fuel distributor. My CIS-E Jetta was in service 13 times in the first year to address stumbling and stalling problems. ALL WERE SOLVED WITH CLEANING INJECTORS!!! This is not a new problem.

I have also tried the factory labeled (Chevron Techron) cleaners and private label, no change. The cleaners are basically solvents, and think about pouring 6oz of a solvent into 18gallons of (already solvent) gasoline, ... other than causing lines to swell and polluting your engine oil, there isn't much effect.

Now here comes the pitch, and I have NO CONNECTION WITH THIS COMPANY other than being a satisfied customer for years.

For about $6-$8, and it's getting harder to find, RedLine sells a detergent-based cleaner called Complete Fuel System Cleaner SI-1. I first poured it in my '91 300TE around 1995, had tried all of the others and spent thousands trying to get it to run like new. The stuff worked, I remember seeing 72miles on the trip odometer when I was talking to a friend, and noticing that a little pedal = a little more power just like when it was new. The performance seemed to drop off again a little when the tank was refilled without the treatment, but running the "maintenance dose" (something like 1oz-10gallons) regularly kept the car running properly. I have run it in all of my gasoline cars periodically since with good results (from Ford to Aston-Martin, Land-Rover to Mercedes, etc).

Now I'm not claiming that this stuff will fix all of your problems, and a lean-running car will run lean until you fix the cause etc., but for those of us who would like the car to have a little stronger response and have sorted everything else out, try the stuff, it's cheap and harmless.

Go ahead and flame away, yeah it's snake oil etc., but I've found it useful enough to spend my 10minutes here trying to help someone else.

[appatula]

I'll have to give this one a try, appreciate the backround writeup as well.

[Cal Learner]

how does this additive affect that? Or is your point that its beneficial effects are in helping clean up the carbon deposits on the valve stems and, hence, better fuel input into the combustion chamber? Are these effects noticeable with the very first can of the stuff, or does it take a few tankfuls to notice?

[babymog]

My experience is that the first bottle improves the part-throttle response. Whether it is the carbon or cleaning up the injectors to improve the low-flow-rate pattern, I suspect the injectors but I've never done any kind of test other than just running the stuff.

I've pulled injectors from several cars in my self-indy days, it is very common to see the CIS injectors put out a stream at low output, but when the sensor plate is lifted for more flow, the pattern clears up to a nice atomized mist. A good clean injector should create an atomized mist from the very first pop, even at idle flow rates, and without it you lose power and efficiency. Replacing injectors is what I used to do, it gets tedious and expensive to put them in every 60,000miles or less (when you're as picky as I am about engine NVH issues).

I don't know how much cleaning of the stems it does, once the symptoms go away I just drive them happily. Italian tune-ups and long high-speed runs are the norm for my driving so theoretically a good running engine shouldn't carbon in my cars.