M116 running rough warm - PeachParts Mercedes-Benz Forum

chazola · #1 06-08-2007, 07:30 AM

Hello, trying to troubleshoot a problem with my W108 M116- runs fine until properly warmed up then starts to miss and really sound unhappy at idle in drive (waiting at lights etc). Checked all usual ignition things then yesterday tried disconnecting the air-flow meter sensor in the air intake and it ran much better.
can you run the engine ok with this disconnected or is it vital to the injection system mix?

northshore · #2 06-08-2007, 07:52 AM

Quote:

Originally Posted by chazola

Hello, trying to troubleshoot a problem with my W108 M116- runs fine until properly warmed up then starts to miss and really sound unhappy at idle in drive (waiting at lights etc). Checked all usual ignition things then yesterday tried disconnecting the air-flow meter sensor in the air intake and it ran much better.
can you run the engine ok with this disconnected or is it vital to the injection system mix?

The air temperature sensor you disconnected has made the car efi control brain pump more gas into the engine. You were running lean and now you are running richer. You most likely have false air leaks getting into the dual plenum manifold underneath where the top and bottom of the intake manifold are connected. You can compensate this lean running condition by popping the black plastic cap off of the barometric pressure sensor and with an allen wrench turn counterclockwise one turn to start until the engine runs smooth. Remember to reconnect the air temp sensor it is not an air-flow meter it mearly changes the resistance to the fuel injection brain.

Tomguy · #3 06-08-2007, 08:13 AM

Quote:

Originally Posted by northshore

You can compensate this lean running condition by popping the black plastic cap off of the barometric pressure sensor and with an allen wrench turn counterclockwise one turn to start until the engine runs smooth. Remember to reconnect the air temp sensor it is not an air-flow meter it mearly changes the resistance to the fuel injection brain.

I wouldn't do that!
FIX ANY VAC LEAKS. Do NOT use the MAP to compensate for them. It is true that you probably do have a few vac leaks, and you're running rich, but it's always better to fix the leaks than to adjust your map.

If you would like to adjust your IDLE mixture, there are two ways to do it without affecting your running mixture. One is the idle air speed screw used primarily to control engine RPMs at idle. The second is the idle mixture adjustment on the ECU.

northshore · #4 06-08-2007, 09:22 AM

Quote:

Originally Posted by Tomguy

I wouldn't do that!
FIX ANY VAC LEAKS. Do NOT use the MAP to compensate for them. It is true that you probably do have a few vac leaks, and you're running rich, but it's always better to fix the leaks than to adjust your map.

If you would like to adjust your IDLE mixture, there are two ways to do it without affecting your running mixture. One is the idle air speed screw used primarily to control engine RPMs at idle. The second is the idle mixture adjustment on the ECU.

From the problem he described he has a lean condition caused by leaking connectors between the manifold halves. The manifold pressure regulator adjustment easily corrects this drivability problem until one finds the time to pull the intake manifold. After the replacement rubber connectors are in place the the original adjustment on the manifold pressure sensor can be restored with absolutely no damage done. I have been to lazy to fix my own leak and my car has been running fine for many years after the adjustment. By the way the car described is running very lean because of the false air and is currently running richer because the air temperature sensor is now disconnected. The car he is describing most likely is running so lean that it has no power at all and adjusting the idle mixture on the brain and or idle air bleed screw on the intake will not help to restore any power.

Tomguy · #5 06-08-2007, 09:02 PM

Actually, an intake manifold leak makes the engine run richer because the higher manifold pressure from the extra air (less vacuum) makes the map say "Engine is under load and is sucking in more air, so I'll dump in more fuel" - but the engine isn't under load!

Add to this rich condition the fact the temp sensor is off and it's even richer! And one turn counterclockwise on the MAP is about 5 gallons worth of fuel per mile.

chazola · #6 06-09-2007, 06:17 AM

ok... I can't work out if you've both managed to agree or not, but anyway...

where is the MAP adjustment screw I need located on the 108? ( I have the Euro version of course so not sure if it may be different from you guys...)

Tomguy · #7 06-09-2007, 09:44 AM

chazola: before mucking with the MAP, have your exhaust gas analyzed. If you richen your mixture when it doesn't need to be richened, you can cause WAY too much gas to be dumped into the engine, which means gas will dilute the oil on your cylinder walls and score them. Hey, it's your engine!

It's much easier to adjust the idle air speed screw. Set your parking brake with the car in drive, and turn the large screw on the front of the engine (right by where the vent hose from the left, UK Passenger cam cover goes to the idle air circuit). Turn it to the left a half turn, it should smooth you out a lot. If turning this screw doesnt seem to fix your idle, you need to clean out your aux air valve.

Adjusting the map will also result in adjusting your mix at highway RPMs (throughout the whole engine band) and most 3.5's and 4.5's already puff black smoke when you floor them and they kick down due to the fuel being dumped in. Adjusting the map would exacerbate that 10 fold.

chazola · #8 06-10-2007, 07:23 AM

Hi Tom.. cleaned out aux air valve and tried adjusting idle speed screw and still get symptoms.

I had a look at the adjustment screw on the main ECU and by the looks of it it's been fiddled with a fair bit in the past (i've only had the car 2 months)... at present it's around mid-way through it's travel... do you by any chance know the number of clicks from fully anti-clockwise it should be at factory setting?

Tomguy · #9 06-10-2007, 08:49 PM

chazola: There's no real factory setting. You use the idle air speed screw to properly balance engine vac and idle speed, then you use the mix screw on the ECU to set idle emissions to 0.5% to 2.0% CO. Adjust the screw until you feel your idle is smooth in drive!

mercmad6.3 · #10 06-11-2007, 06:21 PM

Warm idle miss....AIR LEAK.
To put it simply with out f@#$%&g about with the MAP and ECU remove the injectors,get them cleaned,Replace the seals and short hoses.Remove the intake manifold,replace all vacuum lines,especially the line to the MAP and the brake booster, and the rubber manifold ring halves and the gaskets.
Replace the cold start injector and check the condition of the distributor trigger points.They're expensive but better than wasting fuel driving around smoking or missing and washing down the bores.
Check the condition of the Plug ends (and cap ends both are resistors) and if they need replacing because of high resistance ,DO SO and change the fuel filter at the same time.
Check the plugs which could be shot also .
When was the timing chain last changed?.May be the time to do that has arrived too.