Newbie after kjet help please - PeachParts Mercedes-Benz Forum

2.8jonny · #1 07-29-2010, 11:52 AM

Hi all,

I have often observed this site as an outsider with great interest and hope that you wont mind me leaning for help the second i joined!!

I have a k jet without lambda fuel injection system factory fitted to a European Ford Capri 2.8 V6. I know this is not a manufacturer you guys deal with but i'm hoping that the fueling system we share may sway you towards reading on.

My engine has had extensive top end work done,
Full big valve race heads skimmed by 1.5mm with a comp ratio of 9.5:1 (std 9.2:1)
Plenum and inlet are flowed and port matched origionals
Exhausts are matched, tubular, high flow after market units
The cam is a very mild fast road item
The capacity of the engine remains as factory ie std bore and stroke.

When i fitted all of this in order to achieve a smooth idle i had to lean out the idle mixture with the CO screw a fair bit.

Some weeks went by and with a substantially faster but more highly strung engine i went to the local rolling road.

The engine made substantially more torque and power than standard BUT the fueling was very lean. With CO, control pressures and system pressure at factory she ran lean at all but full throttle. Even at full throttle she was lean until 4500rpm.

I have made the WUR adjustable as many web sites suggest but tapping the cold control pressure plunger and uncovering the hot control adjusting allen screw in the base. Still the full throttle mix was lean at low revs. So I made the brass seat that the vacuum enrichement diaphragm falls on to adjustable by pressing in a machined alternative that can be adjusted simply with a screw driver.

So then i had an adjustable WOT (wide open throttle) adjustment that meant i could set the afrs where i wanted at full throttle.

Still no amount of fiddling with the hot control pressure would give me a a rich enough afr at anything but WOT. Any gains made by dropping the control pressure were lost as the idle ended up so rich that when brought back to a usable level with the CO screw any gains from the control pressure drop were lost!!

Next i experimented with Audi 5 cylinder air metering units, to see if there cone profile would be better suited to my engine but it ran rich. I then turned new proiles in to the audi units with some success but not to a satisfactory conclusion.

Sadly i am on rather a junk yard budget and i only have a narrow band lamda set up to measure the fueling but i should at least be able to get i to run near to stoich 14.7:1, shouldn't i? I should also mention that i have made a test rig with pressure guage to monitor the controll and system pressure.

I have tried swapping many different parts on the fuel injection system including the distributor with no improvement.

So my questions after that introduction are, has anyone experienced similar leaning problems on a modified engine using k jet and what was your sollution? Or does anyone have any idea what went wrong and how i can resolve it?

Many thanks to anyone who takes the time to read my post.

Jon...

ps2cho · #2 07-29-2010, 12:01 PM

You may want to purchase the book Bosch Fuel Injection and Engine Management. It has some great information that may help with the swap. Tons of DIY diagnosis procedures.
http://www.amazon.com/Bosch-Fuel-Injection-Engine-Management/dp/0837603005/ref=sr_1_1?ie=UTF8&s=books&qid=1280419216&sr=8-1

david s poole · #3 07-29-2010, 01:32 PM

what you are going to have to do is adjust the vacuum to the warm-up pressure regulator so that a slight increase in throttle opening will cause the vacuum to drop and therefore the control pressure as well.this if you will is the kjet equivalent of an accelerator pump.

2.8jonny · #4 07-29-2010, 04:20 PM

Ps2cho, thanks for the info, I already have access to an online copy of that book. So i think that base is covered. Thanks again.

David S, could you elaborate on your suggestion?

I fear that if i were to be able to get the vacuum enrichement to operate on the WUR at a lighter throttle, i would compromise heavily on fuel economy!

I did consider this option, my plan was to use a solenoid operated by a throttle switch to purge the vacuum line.

On the rolling road my afr's were 17:1, way lower than they should safely be, that said i only noticed a slight surging at light throttle as a result!

Thanks

Jon

david s poole · #5 07-30-2010, 06:44 PM

if you have access to a gauge to read the control pressure,you will be able to modify the height of the main plunger in the warm up regulator so that warm pressure is lower than 3.2bar.the lower the control pressure the richer the engine will run.

Gurunutkins · #6 07-31-2010, 01:26 AM

david s poole has it dead to rights! by changing the compression ratio you have increased the vacuum at idle if all else is still standard size - this may have been further exacerbated by an increased scavenging effect with your flowed inlets and high flow exhaust. by increasing the vacuum at idle you will throw off the WUR and lean out the engine quite a lot. you might want to check the vacuum and try to reduce it to 16 to 17 inches at idle cold with a small inline venturii so you can bleed a little air into the WUR supply.
good luck
barri

2.8jonny · #7 07-31-2010, 03:39 AM

Hi gurunutkins

By reducing the vacuum at idle would i not reduce the control pressure and then exagerate the problem by having to lean the idle further to compensate for the control pressure drop?

My idle mix is ok as is my WOT mix its just everything in between thats lean.

The inline venturi idea interests me greatly but could you explain what you mean a little further? Do you mean in the WUR line? Where would i get such a venturi?

Thanks for all coments so far, you have all been more helpefull than you realise.

This is a saga that has gone on now on and off for 3 years and due to tight time restraints has never had my full attention, i would like to sort it now as this winter the car is being laid up for at least a couple of years.

Jon

Gurunutkins · #8 07-31-2010, 09:57 AM

hi 2.8
not really - i suggested doing it while cold as the control pressure is kept low artificially by the temp switch in the line to reduce the vacuum to the wur to richen up the mixture as a cold start mechanism (like a choke) - hence the name wur - warm up regulator

if you know the vacuum at cold idle - say 16 to 17 inches and the wur is running at say 14 inches (these are just off the top of my head) then when the engine is running at normal hot idle (in your car with the mods I would guess high) say 19 inches then you need to reduce the line vacuum to the wur by 2 inches of pressure.

As you lower the vacuum you enrichen the mixture from the wur by decreasing the control pressure and allowing the air restriction plate to drop lower. the higher the control pressure the slower the air restriction plate drops and the less fuel you inject. the injection fuel pressure is kept constant by the pump system and the regulator in the fuel distributor. you need to reduce the cobntrol pressure in the distributor to richen up the mixture and then reset your idle mix to get it back to where you want with the idle mix screw which controls how much of a fulcrum there is on the air restrictor plate

hope that makes sense
cheers
Barri

2.8jonny · #9 07-31-2010, 02:50 PM

Hi again.

I am covering some old ground here but thought i'd run the idea past you guys.

It would seem that my engines fuel requirements are now so far from the factory fuel curve that even with major fiddling with the control pressure i still get an unstaisfactorally lean fuel mix at all but idle and WOT.

I tried this idea once before with some success on a dark and raining night, unfortunately a stray fuel line rubbed through on a pulley and haulted proceeding prematurely. I think it was around then that i shelved the whole project and abandoned the car for some time!!

I intend to run 2 cpr's in series, both connected to the plenum by vacuum enrichement lines.

The first will have a high control pressure used for idle where i currently have more than enough fuel, the idea being i will need to add more fuel with the CO screw to compensate. It shall have no seat for the diaphagm in the lower chamber to fall on to when there is a vacuum drop, so should send the control pressure very low and the mixture extremely rich (if connected singularly).

In the vacuum line i will run a solenoid with air purge, this will be open while the throttle is shut ie at idle but as soon as the throttle is opened the valve controlled by a switch at the throttle linkage will shut the vacuum line sending the control pressure very low.
At this point the second cpr will come in to use.

The second cpr will have a control pressure set more in tune with that required by the engine off idle in mid range. This unit will have its vacuum line 'T'ed off before the solenoid and will there for have a constant vacuum feed until the vacuum genuinely drops for WOT.

I know the concept seems sound to me and that in a few pictures i drew first time around i looks quite good. I even know that in practice the control pressure did exactly what i expected it to.

So what do you think am i making a simple situation more complicated?
I even still have an extra plug spliced in for the warm up heater in the second CPR.

I should also mention that my air metering unit is of the upflowing variety, unlike most mercedes ones as i understand.

Jon

mercmad6.3 · #10 07-31-2010, 10:15 PM

You may have to install a damping chamber to act as reservoir to allow less engine vacuum at run position. Have you tried opening the spindle slots in the fuel distributor ?

2.8jonny · #11 08-01-2010, 03:15 PM

Hi Mercmad6.3

I have read a little about the vacuum capsule/resovoir idea elsewhere but never found any definative answers as to what it actually did. Are you suggesting that it could lower my idle vacuum?

Also no i haven't dared look in to the fuel dissy so far as i was told its full of black magic! I guess it's worth a look, are you talking about tapering the slots? If i remember right are they not .2mm wide, would it not be very hard to retain even fuel distribution between cylinders if i were to alter them?

jon

mercmad6.3 · #12 08-01-2010, 05:35 PM

hi,theFuel distributor is a very basic device. In your position i would collect a pile of old ones and open one up. You will see that the flap simply works via a lever that moves the spindle up and down and meters the fuel accordingly. If you are running lean in the mid range (where you drive the most) find a place which looks like it may be the mid range position of the spindle . Use a sanding disc on a dremel to open the slots a few thou. and try it on the car.
The ideal situation is fit a oxy sensor in the exhaust and measure the CO levels to calculate your fuel ratio on the fly.
Why i suggest you try and collect a few fuel distributors is because what you need to do is normally done a Dyno and the amount of fuel /air ratio can be determined immediately . I understand your budgetary constraints (to use political speak) so you could try a few electronic stores for a kit to build your own Fuel ratio meter of pick up a used Garage item and adapt it .

2.8jonny · #13 08-05-2010, 03:00 AM

I stripped a spare metering head last night to have a look at the possibility of modding the fuel slits within.

On inspection i felt this is a job that goes a little beyond my tallents, as the slits are as thin as a hair!! There is no way i could accurately open them up with my set up here. Even if i did i would never maintain uniformity between different ports.
I do by the way have a diy fuel monitoring guage in the car that despite being narrow band have proved itself to be quite accuate on the rolling road. So despite not being perfect i at least know when i am seriously lean of rich. On a recent trip driving very conseratively with control pressure set to the slightly lean region resulted in excellent MPG.

Interestingly enough the guy at the rolling road suggested a firm here in the UK that could modify the slits for an altered fuel curve but at a very serious price!

I have re-tried my two WUR with throttle switched solenoid and the results were initially good once i had a control pressure that worked for both, i could acheive a nice idle on WUR 1 with a control pressure of 63ish psi then Wur 2 was set eventually at 50ish psi. This set up resulted in a nice transition between the two with a good steady idle, driving at light throttle resulted in a nice slightly rich afr but as you say climbed a hill the afrs dropped right in to the lean teritory and a little pinking was heard!!

I spent time massaging the pressure differences and could not seem to get rid of this high load mid throttle leaning.

I turned a different cone profile in to an audi unit a couple of years ago and after several attempts managed to get a steady afr in the 13:1 region at all but the same region ie high load mid throttle at which point it ran lean as above.

My factory cone profile has only one angle change and that is right at the top where essentially if they didn't change angle then the top of the unit would have been machined off, so with this info my turned out unit had a similar contour.

I have also turned a second profile in to another unit, this is the one i use most of the time. This one features an angle change within 10mm of the disks rest possition and resulted in much more consistent mixtures until you go over 2800 rpm at which point it would run rich. At this 2800 mark i think that the second angle that i machined in was a little steep and as a result rich.

So with the knowledge that i cannot influence the mixture across the rev and load range sufficiently with control pressure and idle CO settings and that different cone angles give better fuel curve characteristics than the standard unit. I cant help but feel the best course of action is altering the cone angle further.

To sumarise, i think that my new engine has fuel needs so different from the factory one that the cone profile is totally incorrect. Unless you had an ecu or lambda type control at all but WOT there is no way that retaining this part will result in a decent result.

Jon

mercmad6.3 · #14 08-05-2010, 05:08 AM

The shape of the cone is an area which shouldn't need altering as the air flow requirements of the engine are determined by the amount of throttle. Unless you fit a substantially larger throttle body you will only move the same amount of air as the engine did before you modified it.
What you are trying to achieve is the same stochiometric ratio across the air flow range . You can't do that without modifying the spindle.
I have seen a Volvo race engine here which developed 700HP naturally aspirated with the standard aircone. Another is a mercedes 6.9 which used the standard Cone but the larger throttle body from a 560*. The result using the standard injection was a massive increase in the already impressive torque.
Have you spoken with any laser cutting firms? A laser can cut to tolerances in the micron range. And...I'm not talking about water jet or plasma either,both of which can't cut to such a fine degree. I don't understand why some firms want your left bollock to do a simple cut.

*http://www.youtube.com/user/SELfor50#p/a/f/1/_QOqPw-h6L4