manual how to adjust the 3.5 engine (ignition and A/F ratio) - Page 4 - PeachParts Mercedes-Benz Forum

Graham · #46 01-29-2013, 09:35 PM

Quote:

Originally Posted by Mike D

Not quite. Vacuum measured at right below the throttle bottle will be a bit less than what is measured further down the plenum. Compare it to a funnel. Pressure is highest at the narrowest part of the neck and less at the widest part.

Mike, with throttle plate closed, there is no flow through throttle body, therefore no change in pressure. All air flow at idle goes through aav & idle valve directly into manifold.

werminghausen · #47 01-29-2013, 11:19 PM

I believe that the vacuum rise (by 3 or 4 ") ..when I pull the vacuum line of the distributor....is through the change in ignition timing (from 3 ATDC to 9 BTDC) along with raised idle speed. I can pull the distributor vacuum and close the open vacuum line and see if it makes a difference... I doubt it.

Martin

Graham · #48 01-30-2013, 12:05 AM

Quote:

Originally Posted by werminghausen

I believe that the vacuum rise (by 3 or 4 ") ..when I pull the vacuum line of the distributor....is through the change in ignition timing (from 3 ATDC to 9 BTDC) along with raised idle speed. I can pull the distributor vacuum and close the open vacuum line and see if it makes a difference... I doubt it.

Martin

Martin,
You are right. I never thought of that. You can set timing like that. Rotate distributor to advance timing until you get best vacuum, then just back of slightly. I tried that once and it seemed to get timing about right. I have an article about that somewhere.

Graham

werminghausen · #49 01-30-2013, 12:28 AM

Quote:

Originally Posted by Graham

Martin,
You are right. I never thought of that. You can set timing like that. Rotate distributor to advance timing until you get best vacuum, then just back of slightly. I tried that once and it seemed to get timing about right. I have an article about that somewhere.

Graham

Hi Graham
That is correct. I tried both methods (I mean according the MB manual...set 6 ATDC) and the vacuum method... they are in a close range...would be great if you could send the article... I learned to back off 1-2 degrees at distributor after setting best vacuum at idle (w/o vacuum connection at distr.) then install vacuum line to distr.
With this method I ended up at 3 ATDC (instead of 6 ATDC according manual)I believe the late timing for the US version (6ATDC) is just for the exhaust data..in fact not so great.

Another thought on the fuel pressure....as my pressure is absolutely stable no matter the driving conditions...If flow would be questionable...perssure would back off slightly making the injectors inject less fuel. But since the pressure is so stable I don't think that there is any issue.

I'll increase pressure slightly and then look at AFR numbers again...
If AFR is going down towards rich... then I'll try the spare MPS on my shelf.

Last thought on mixing air with bung at the current exhaust position:
I have looked at the end of the exhaust pipes (where I have the bung welded): per exhaust there are 2 pipes, an inner and an outer pipe...why is that?
Could it be that the cavity between the 2 pipes is drawing air in (by some Venturi effect?) For what reason did MB do this? Can it be that the exhaust gases get deluded with air in order to show better numbers. Do European cars have the same double tube wall?

My thought was to close the void between outer and inner tube at the end and then see if AFR numbers change for the better.
Martin

Mike D · #50 01-30-2013, 06:51 AM

You're right Graham. I forgot which system we were discussing.

Mike D · #51 01-30-2013, 06:55 AM

Martin, quit "chasing your tail" and move the sensor.

werminghausen · #52 01-30-2013, 07:56 AM

Quote:

Originally Posted by Mike D

Martin, quit "chasing your tail" and move the sensor.

You might be right...I'll need to cut the bung out and weld to a better spot.
Just lazy

werminghausen · #53 01-30-2013, 10:07 AM

What concerns me most of all is the unstable idle and this weird powerless of this 3.5 engine. Could this be failing trigger points? or gunk in the distributor point?
What else causes these symptoms together with the too high AFR?

I am really puzzled...

Graham · #54 01-30-2013, 12:34 PM

Quote:

Originally Posted by werminghausen

What concerns me most of all is the unstable idle and this weird powerless of this 3.5 engine. Could this be failing trigger points? or gunk in the distributor point?
What else causes these symptoms together with the too high AFR?

I am really puzzled...

Martin,
On my SL, there is no double pipe at exhaust - Is it just that you have separate tailpipes that fit over the muffler outlet pipe? Any any event, moving is best bet.

Regarding unstable idle and lack of power. You should make sure teh trigger points are free of oil. (They never burn, so don't use any abrasive to clean. I sprayed with a degreaser and ran a piece of paper or thin card back and forth. The ingnition points are another possibility. Maybe dwell meter is inaccurate? I would just set at 0.014" for a start. Dwell then is usually about right.

Have you tried setting ECU knob in centre of range and watching if AFR varies as you adjust it? You say idle is bad, but not if you have tried adjusting idle AFR with ECU knob.

werminghausen · #55 01-30-2013, 01:34 PM

Quote:

Originally Posted by werminghausen

You might be right...I'll need to cut the bung out and weld to a better spot.
Just lazy

I removed the old bung and start putting it more upstream. Here are pictures.
Do you approve the new location? I am still not willing to remove the exhaust.. I fear if I do lots of old rust will come down..I spare removal until the time comes. I think I can weld with exhaust in place.
However this ****ty drill is dead after one hole in the exhaust...need to find a way to get this hole open.
Any comments?

Graham · #56 01-30-2013, 04:40 PM

Quote:

Originally Posted by werminghausen

I removed the old bung and start putting it more upstream. Here are pictures.
Do you approve the new location? I am still not willing to remove the exhaust.. I fear if I do lots of old rust will come down..I spare removal until the time comes. I think I can weld with exhaust in place.
However this ****ty drill is dead after one hole in the exhaust...need to find a way to get this hole open.
Any comments?

Different car than mine, so I am not sure where that is. But anywhere further back should be a help.

Good Luck with it. Once you get the sensor relocated, do the test at idle using the ECU screw. It would confirm you have mixture control at idle.

Car is packed and we are hitting the road first thing tomorrow. Limited email access for a few days.

Tomguy · #57 01-30-2013, 05:11 PM

Quote:

Originally Posted by werminghausen

I removed the old bung and start putting it more upstream. Here are pictures.
Do you approve the new location? I am still not willing to remove the exhaust.. I fear if I do lots of old rust will come down..I spare removal until the time comes. I think I can weld with exhaust in place.
However this ****ty drill is dead after one hole in the exhaust...need to find a way to get this hole open.
Any comments?

Looks much better, you should have no air mixing in down there!

I have a Dewalt 18v drill. The 10+ year old battery lasted ~5m if I was lucky. I took it apart & found out the cell size. I then ordered replacements from eBay & soldered them together to rebuild it myself. Cost was $25 IIRC, and now it lasts a LOT longer under MUCH heavier use before needing a recharge!

werminghausen · #58 01-30-2013, 10:37 PM

Quote:

Originally Posted by Graham

Different car than mine, so I am not sure where that is. But anywhere further back should be a help.

Good Luck with it. Once you get the sensor relocated, do the test at idle using the ECU screw. It would confirm you have mixture control at idle.

Car is packed and we are hitting the road first thing tomorrow. Limited email access for a few days.

I have a 1971 300SEL 3.5. Do you think there are different models?
I will do the testing with Idle control screw as you suggest and report back.
Safe travels and thanks so much for all your help.
I am still puzzled about the unusual behavior of this engine.

werminghausen · #59 01-31-2013, 02:17 AM

Update: I welded the 'new bung'. Seems to be a much better spot.
I started the engine.. engine is backfiring a lot, well I don't have the cold start injector installed.
The vacuum is jumping with every backfire.
However the AFR is reading much better with around 14- 14.5 now...engine not quite hot. some pics of old and new spot.

I'll prepare now the following: I'll try and clean the trigger points as well as
ignition point...then do another road test, followed by the ECU idle adjustment.
What do you think is the reason for backfire and unstable idle ?

werminghausen · #60 01-31-2013, 03:39 AM

Good news..
guess what I found: I opened up the valve cover on the left side (cylinders 5-8) and checked the oiler rail...as I had bought the wrong size plastic parts (bought 10mm instead of 8mm plastic for fuel rails). So I installed the old ones...grrr for the time being..need to change as soon as I get the right parts...
I was hearing a lot of mechanical noises from the left side. I thought maybe I am toasting the cam bearings when the rails slipped off. But this was not the case fortunately...But what I found when I checked the fire order of cylinders...I must have confused 7 and 8... (firing order was the 15476382 instead of 15486372) so this was the reason for the weird performance and backfire...unstable idle? 7 and 8 are 270 degrees apart...
What exactly are the symptoms of this wrong firing order?
I need to work now but will catch up later.