308 fuel injection system question

The best info on the CIS aka K-jet system is in Mercedes manuels. This is the same system used by Mercedes for years. Call me and I can copy and send you some info and answer any questions you have.

Larry Fletcher
CIS Flow Tech Llc
251-929-3771

 

The CIS can get clogged over time, and those fuel-injector cleaner bottles don't really clean them. Either hold down the accelerator halfway to release the fuel when starting, or replace the injectors.

 

The Ferrari WSM for the Mondial / 308 has very good info on troubleshooting the CIS. If you need to know "everything there is to know about CIS", Bosch has published many books and manuals on their FI systems that make any other repair manual toilet paper by comparision

 

How does holding down the accelerator release fuel? I think if your idle bypass is set up correctly you should have no need to touch the throttle to start. This isn't a carburator, there is no accelerator pump that squirts when you floor it. On all of the CIS cars I've worked on the throttle cable did nothing except open the throttle body plate(s).

 

I may have misunderstood the reasoning behind this, but it is what a mechanic/friend once told me. Here is the scenario... After sitting for at least a day or so, the car starts up with no problem, but after turning off the engine for only a few hours, it is hard to restart. It cranks, but doesn't turn over for at least 15 minutes. He told me that on CIS cars, fuel injectors can get clogged, and to rectify the problem - depress the accelerator halfway (here is where I believe he said it releases the buildup of fuel/air mixture into the engine) to get it to start. I really don't know enough about it, other than what my mechanic/friend told me, however this method works. He also said the fuel injectors probably needed to be replaced to correct the problem. It was a generalization without looking at any particular car.

 

it might be my understanding of this but if i think i know what you've said, sorry to say but that's bs. your mechanic appear to know nothing about the k-jetronic system...

 

Ok. So, now I'm wondering if that problem would be from an improperly set idle bypass as Brian stated, or something else. Any thoughts?

 

It starts better because depressing the throttle opens the butterfly and allows considerably more air to pass through the plenum (and by the poorly operating injector fuel cone) as the engine is being cranked - this allows for some marginally improved fuel atomization in the cylinders....

 

actualy, what is trhe problem again ( seriously i did read back but could only find a question on clogged injectors)

anyway to know what happens first you should determine the state of the engine: cold or hot.

If cold the airflow passing the measuring plate wil be relative big as the resistance for movement for this metering plate is low (the WUR regulates this resistance by lowering the presure on top of the plate (by means of an small plunger)
If cold the extra cold start fuel nozzle will drop some extra fuel ( only shortly and a relativly small amount) to enrich the mixture.
So starting a half trottle will increase the opening of the metering plate even more creating actually a leaner mixture. I think starting will not needs you to troltlle at all, IF only the cold start equipment works properly.

Esspecially the electrical connections to all cols trat parts( electrics on the wur are not needed for cold starting) and the extra fuel nozzle and timeswitch ( are important with cold start) can play tricks
If you do need some/half/full trottle at a cold start i would check the metering-plate height.

If Hot.

as the proper fuctioning WUR sets a relative high pressure to the metering plate this plate will find more restistance in moving ( giving less fuell to the air > leaner mixture adequete for normal driving
giving half/full trottle is not nessecary either, again IF all parts funcioning properly.
In this case however al hot start-parts with expecption of the wur are not doing anything

My QV gave me a hard time starting hot and did only whewn given full trottle ( only just after shut down, no problem after an hour or so) Eventually the nozzles "pissed" the overpressure into the cilinders causing ( do nnot know the term in English but they soaked the champers)

Clogging of thre injectors is possible, esspecially with the older steel ones. cleaing them is not really simple and not always possible. New ones are rather cheap however ( find them for less than 20 bucks in the net)

Cleaning the nozzles by rinsing them with petrol seems stupid, as petrol runs trought them constantly..


Above reflects my understanding of the CIS system and could be not complete ( or even wrong... )

 

Steven, I believe the question that was being queried is what's going on with the warm start problems with CIS cars. Specifically, why is it helpful to partially depress the throttle when trying to re-start the engine when it's warm.

My answer above was directed specifically to this point of inquiry.

If you're looking for a "root cause" of this problem it likely is related to dirty injectors. Re-starting is difficult because there is no auxillary devices helping in a warm re-start procedure so you're really depending on decent fuel atomization from the injectors - when they're dirty then proper atomization is compromised.

 

Another well-known cause of 'warm restart' problems is a bad fuel accumulator. The accumulator's purpose is to keep the fuel system pressurized so that fuel will flow immediately when the engine is cranked. The accumulator works just like a water well's 'captive air' tank. If the accumulator has an internal leak, the pressure bleeds off in 10-20 minuites, resulting in low fuel line pressure which causes a very lean mixture while the pump is bringing the pressure up again.

If it's a US Spec car, pull the black hose off of the nipple on the accumulator's back. If any fuel comes out, the accumulator is bad. On Euro cars, there's no hose on the nipple so you have to look for the nipple dripping gas after the engine is turned off.

Accumulators are around $100, so not prohibitively expensive.

 

Although i'm aware of this accmulator problem i just never understood the " why"
as you say the pressure bleeds of in a 10-20 minutes anyway so when cold there is no pressure either and this will not cause any bad starting. Furthermore the engine is still hor after this period.
Why is the pressure important with a warm engine (and not when it's cold)

willing to share?

 

I had a warm start problem when I bought my car. Shortly after buying, the fuel accumulator spit gas out the breather tube, so I replaced it. The warm start problem went away. I don't understand why this is. On my car the blue safety switch is bypassed, so the pump runs whenever the ignition is on. If it is just a fuel pressure issue the pump should have the system pressureized in a second.

My best guess is that the bad accumulator drops the pressure to the point where the pintles no longer hold the fuel back and let it leak into the intake. The hot start problem is actually flooding, perhaps? Maybe that's why there are reports that openeing the throttle help it start (dries out the plugs)? It seems like on a hot enigne it should evaporate sitting in the intake, but maybe it is a lot of fuel.

 

The injectors depend on certain threshold pressure to atomize the fuel properly. Raw/liquid gas is moderately flammable, whereas properly atomized fuel is down right explosive. The fuel accumulator maintains the line pressure so that the spray cone coming from the injector is properly atomizing from the first turn of the starter... So, as is suggested above, the pressure accumulator can be a critical factor in warm starts.

 

I bought the Bosch manual when I bought my fuel metering kit. I found the Bosch manual very easy to understand and with a lot of photos. Between the manual and sitting on the phone with Larry for an hour or so I was able to trouble shoot my CIS. I was having a hotstart problem. I replaced the accumulator during trouble shooting but my problem was an open checkvalve at the fuel pump. When hot, the fuel system would not stay charged when it was shutdown and sitting. This was causing a vaporlock in the fuel line and I don't care how long you sit there with the gas on the floor, spinning the starter....it won't start! Once I got the gauges on it (like Larry had been telling me for 2 weeks) we nailed down the faulty checkvalve. Replaced the valve and Varooooom...no more hotstart problems. Joe

 

yeh but then again i do agree with Brian.
the soon you start the engine the pressure will be there in a second.
I do think too that bad nozzles will cause flooding (I actally saw it, just after shutting down the gas was peeing out...)

so still puzzled about the reasin the accumulator can cause a bad hot start. (as said, i do know it does!)

Maybe Larry can/is willing to fill the gap in my knowledge?

 

From the first turn of the starter, I agree. But after the fuel pump comes on, shouldn't all the necessary fuel pressure be there pretty darn quickly? I think those of us with hot start problems are talking about cranking for a long time. In my case it was more than 20 seconds of cranking usually. Even with the safety switch, at some point wouldn't the switch have bounced enough for a bit of pump running and the pressure would come up? (In my case the pump was running right from the start.)

 

Generally, I would think you're right Brian but it's going to take more than one crank. If the pressure has been lost then the CIS is going to have to rotate a few cycles to allow the (now pressurized) supply line to equalize pressurize for each downstream pipe/injector. And, until that pressure gets to the threshold amout at the injector, that injector is either not going to flow at all, or, flow a poorly atomized cone. I believe the reason why it helps to crack the throttle while doing this is that more air flows through the plenum and past the injectors and helps clear the condensed fuel in the area.

That's just my $.02.