300SEL 3.5 idle speed...still fighting

[werminghausen]

Originally Posted by vandor

> AFT for idle was in the 12.8/13 range… lean but acceptable.

You mean AFR (Air/fuel ratio)? The chemically 'perfect' ratio is 14.7.
If the number is less than that the mixture is rich. When I was tuning high performance engine we wanted between 12 and 13 at full throttle, and leaner than that at idle and cruise.
I know the conditions at your location area bit extreme, but I wanted to let you know as a point of reference.
bye,

Csaba
72 280SEL 4.5

Thanks Vandor
for my engine 14.7 as an average is too lean (best for me is around 13 -13.5 for cruise and 10 at full throttle)!
My numbers are 14.5 at cruise speed of 60...slightly higher at 50 (14.8-15) then 13.5 range above 13.0 is the lowest at some point (maybe 75mph) richer idle is much stronger (around 13). This translates actually in a hot start AFR of 14- 15... Any higher AFR than that and I have a problem with idle and start up.
This is just my experience so far...still learning.
Martin

PS actually I tested even richer mixtures and the richer the better the engine runs. With 33psi fuel pressure this thing is just flying but AFR is at 10 cruising at 80....

[Graham]

Originally Posted by werminghausen

It was testing the installed sensor at 210 Ohm at approx 35C (95F) and I was testing a spare sensor I have at 180 Ohm at same temperature.
I was swapping the sensors and installed the one with lower Ohms... How does this affect AFRs? AFRs are slightly higher (leaner mixture) with a sensor with lower Ohms?
Martin

180ohm sounds about right at 35C.

This sensor causes only a minor adjustment to AFR. As temperature goes down (resistance increases) mixture is enriched (AFR goes down.

But this adjustment only happens at low temperatures (below 70F/21C). In Oman, it should make no difference unless it is way off or disconnected.

Hole in spare cap is good as a test. I looked on epc to see if there was a vent valve, but could not find it. There must be a vent of some type on the tank, and it may be blocked. If you have shop manual it should be in section 47.

[werminghausen]

Originally Posted by werminghausen

Thanks Vandor
for my engine 14.7 as an average is too lean (best for me is around 13 -13.5 for cruise and 10 at full throttle)!
My numbers are 14.5 at cruise speed of 60...slightly higher at 50 (14.8-15) then 13.5 range above 13.0 is the lowest at some point (maybe 75mph) richer idle is much stronger (around 13). This translates actually in a hot start AFR of 14- 15... Any higher AFR than that and I have a problem with idle and start up.
This is just my experience so far...still learning.
Martin

PS actually I tested even richer mixtures and the richer the better the engine runs. With 33psi fuel pressure this thing is just flying but AFR is at 10 cruising at 80....

Vandor and those who know... please comment

Update: readings per today during steady cruise
50mph - 15.0 (AFR)
60mph 14.0-14.2
70mph 12.8- 13.0
80mph 13.0-13.2
90mph 13.5
idle 12.7 (after driver- this number will go up after 15 minutes and hot start ...maybe 15range

[Graham]

Originally Posted by werminghausen

Update: readings per today during steady cruise
50mph - 15.0 (0.3%CO)
60mph 14.0-14.2 (1.6-1.2%CO)
70mph 12.8- 13.0 (4.35-3.8%CO)
80mph 13.0-13.2 (3.8-3.3%CO)
90mph 13.5 (2.6%CO)
idle 12.7 (4.6%CO) (after driver- this number will go up after 15 minutes and hot start ...maybe 15range

Your AFR figures look fine, but if anything on lean side. These cars need to run on rich side. The idle %CO is above spec, but I have mine set that way too. It seems to help with the hot starts.

The emission specs for the 117 engine according to the MB Technical Data Manual below show quite wide acceptable range. Generally richer settings give more power and leaner settings better fuel consumption. I have inserted approximate equivalent %CO in RED against your AFR figures.

Full load shifter position S 3,000 rpm = 2% - 5%
Upper partial load position S, 2,500 rpm 300 mm Hg vacuum 0.1% - 0.5%
Lower partial load 3rd gear shifter position S, 1,500 rpm 300 mm Hg vacuum = 0.2% - 0.5%
Idle neutral, idle speed, oil at operating temp = 0.5% - 2.0%

[werminghausen]

Originally Posted by Graham

Your AFR figures look fine, but if anything on lean side. These cars need to run on rich side. The idle %CO is above spec, but I have mine set that way too. It seems to help with the hot starts.

The emission specs for the 117 engine according to the MB Technical Data Manual below show quite wide acceptable range. Generally richer settings give more power and leaner settings better fuel consumption. I have inserted approximate equivalent %CO in RED against your AFR figures.

Thanks Graham...
for confirming that I am not out of my mind.
I had the car running richer and it runs better...it flies ..but the amount of gas....
So I would consider this a good compromise. Anytime able to turn the fuel pressure up.

It is amazing but with the same weather I have no issue any more. Idle is stronger and I can get the idle speed so that it works in the morning at start up (no hunting) and the very hot idle works...including hot start....very happy again. I think the insulation of the fuel rail does the job plus the richer adjustment (for an unknown reason the car was running leaner than I adjusted in February).
Also I am watching the gas tank vacuum or pressure ( I drilled a little hole into the cap but I doubt that it does something. So I need to look into the tank ventilation. Not sure where it is blocked.
How does negative pressure/vacuum occur? Well I think that the tank is getting empty and will cause vacuum...
How does positive pressure occur? nI guess by evaporation of fuel....
So I think I must have had positive pressure if the tank was boiling before:
Graham you asked me and I said it was vacuum...but this sound not logical.
I was thinking today at the gas station that I had positive pressure?

Martin

[werminghausen]

Hi Graham,

I was checking the 2 vent lines from fitter tube to the passenger side vent container in the trunk... the 2 plastic lines right at the border to the filler tube (rubber sleeve there) were kinked and I think no gas passing through. It was easy to repair in 5 minutes...will see if the pressure in the tank is gone...I think I had positive pressure...I was saying vacuum before but I think it was overpressure. I'll report back.

I case of positive pressure...what is the reason for that (heat? evaporated fuel?) and what is it doing in there?

Another question is the high pressure (33psi) in the fuel rail I am running right now: Someone was saying that the high pressure causes the injectors to open not very well and there could be a potential problem at idle and top end power.
Any thoughts?
In my car the high pressure is the only way I can survive here but who knows.
Martin

[werminghausen]

I was testing pressure in tank this morning and there is still positive pressure in ...so I guess a valve must be closed and not opening?
Can't be that complicated. I can see a round black cylinder with lines in and out. I believe this is the box with the valves?

[Graham]

Martin,
The reason pressure builds in tank, is that fuel has a higher vapour pressure at higher temperatures. Leave a closed gas can in sun and you get same affect. But our tanks have a vent system that at least in later cars goes to engine crankcase which in turn vents to intake. So there should not be much increase in pressure. But neverthess, on my car I get a bit of a swoosh when I open the filler cap!

The M117 engine manual shows the vent system in section 14. It shows same system for 71 and 72. 72 system is described at end of this link.

Regarding the injectors not working properly at 33psig. In theory that could be true because each time they are pulsed, more fuel flows through injector. We compensate for this using screw on ECU for idle (and MPS for load) to get back to proper AFR/%CO. It is true that ECU circuitry may then have more difficulty providing smaller or fewer pulses. But, those of us who have actually set the car up this way have not seen any problems. In fact a top rate mechanic who worked on D-Jets in their heyday, told me that this is what they did in hot climates with fuel they then had. If it works for you, what the heck!

[werminghausen]

Originally Posted by Graham

Martin,
The reason pressure builds in tank, is that fuel has a higher vapour pressure at higher temperatures. Leave a closed gas can in sun and you get same affect. But our tanks have a vent system that at least in later cars goes to engine crankcase which in turn vents to intake. So there should not be much increase in pressure. But neverthess, on my car I get a bit of a swoosh when I open the filler cap!

The M117 engine manual shows the vent system in section 14. It shows same system for 71 and 72. 72 system is described at end of this link.

Regarding the injectors not working properly at 33psig. In theory that could be true because each time they are pulsed, more fuel flows through injector. We compensate for this using screw on ECU for idle (and MPS for load) to get back to proper AFR/%CO. It is true that ECU circuitry may then have more difficulty providing smaller or fewer pulses. But, those of us who have actually set the car up this way have not seen any problems. In fact a top rate mechanic who worked on D-Jets in their heyday, told me that this is what they did in hot climates with fuel they then had. If it works for you, what the heck!

hi Graham,
thanks so much! Valve system 72 (picture) seems to be mine. So...if a swoosh is normal..then the valve in the saucer opens at a certain pressure...maybe couple psi.... and so must do the line to the crank case...
Who knows what the holding pressure might be?
D-jet: higher pressure definitely works.. here it has 40C early in the morning ..I just want to understand what is happening in real world. I doubt that there will be damage to the system or wear...but there are some basic believes that the wisdom of Mother Benz demands 29psi.