1976 280E Hot Idle Problem

[pookybbb]

Hi All,

I was wondering if anyone could help my '76 280E? She starts and runs like a dream most of the time, but occasionally, after she has been stopped and started continuously, she runs rough and won't idle. This only it seems to happen when the engine is warm. To over come it I must keep the accelerator slightly depressed, I can hear the engine is running slightly rough, then after about 30-60 seconds it seems to clear and all is normal again. All the usual engine services were recently done (problem existed before then anyway). Can anyone suggest anything that might cause this.

Cheers

[280EZRider]

I would check the easy stuff first like a possible vacuum leak anywhere from the intake manifold and, because your problem happens when the engine is warm, see if the spark plugs tell you anything.

[Robert Squires]

* Pookybbb, sounds like you have an occational over-rich idle. You need to determine whether that means too much fuel or too little air.
* Since you have an "E", you have an early K-Jetronic (CIS) fuel injection. Have you checked fuel pressures? There are 3 pressure specs, line pressure, control pressure and rest pressure. The control pressure, particularly when the engine acts up hot, would be useful here. Of course, if the line pressure spikes occationally (pressure regulator) you could have the rough idle too.
* Also, the (thermostatic) auxiliary air valve regulates idle air flow over engine temperatures. It has to work right for a smooth idle since it's adjustments are in concert with the control pressure (fuel mix). Since cracking the throttle just a bit smooths out the idle, the "air slide" might be sticking a little at the hot-end of its range of movement (restricting air flow).
* Knowing what wouldn't be the problem is often as useful as knowing what could be a problem. A vacuum leak wouldn't create your problem. That is, cracking the throttle a little would make the problem worse. And an arcing plug wire wouldn't really be affected much by a throttle change.
* On the early cars there's no engine management. In fact I think that '76 was about the year that the points-and-condenser ignition gave way to the solid-state ignition. Make sure that your dwell and idle timing are spot-on. While you're at it, look at any timing changeover and the timing advance. Sounds pretty involved but it actually only takes a little while, after which you know where you stand. This used to be part of the "tune-up", a necessary but somewhat involved series of steps covering the ignition settings, the valve clearances, and finally the fuel settings. Today the "tune-up" is little more than left-over vernacular, a plug change and a code scan. Good luck.

[alabbasi]

Let's double check this as to me a 76 model would it could be a W114 which would have D-Jetronic.

My 280SEL 4.5 with D-Jet does this every now and then. After revving it a couple of times, the computer figures out what the idle should be and it's fine.

Originally Posted by Robert Squires

* Pookybbb, sounds like you have an occational over-rich idle. You need to determine whether that means too much fuel or too little air.
* Since you have an "E", you have an early K-Jetronic (CIS) fuel injection. Have you checked fuel pressures? There are 3 pressure specs, line pressure, control pressure and rest pressure. The control pressure, particularly when the engine acts up hot, would be useful here. Of course, if the line pressure spikes occationally (pressure regulator) you could have the rough idle too..

[Adenauer]

I am fairly certain that '73 would be the last year for the D-Jet. '74 began the use of the K-Jet system, I believe??

[Robert Squires]

* W114 was never an "E" model (back in the day, E was for Einschpritzung (sp?) = fuel injection). That is, the W114 was always carbureted and, with the M110 six cyl, was called the 280, not the 280E.
* I thought Pookybbb's 280E was a very early W123 car with K jetronic. Late 76 or early 77 was a model change year; both the W115 and W114 were replaced by W123. The W108 had already been replaced by the W116 in '73. * This would be a great time for Pookybbb to recheck his production date (mo/yr) and give a chassis number (first six digits will do). Is it a 123.033 after all?

[alabbasi]

W114 was available with a 185 hp M110 engine with D-Jetronic and sometimes even a 5 speed gearbox. But not in the US.

Here is a nice article on a compare

http://www.autospies.com/news/MB-Classic-280E-W114-vs-E500-Limited-W124-vs-E63-AMG-W211-11096/

[Adenauer]

*1973 W114.072 M110.981 (high compression) 280CE
*Number 5224 of 11518 such coupes produced
*(not for the US market though this one was purchased new on the European Delivery Program. It lacks everything that a US-spec 114/8 would have been built with for the US market with the exception of the instrument cluster - which was swapped by a US dealer when the car was finally brought here by its original owner).

The fuel injected sedan was 114.062, 280E

[alabbasi]

Seeing as we are on this topic

This is my 250CE. I bought it when I was 19 and living in London. I still own it.
It has a 2.5 liter M114 engine with D-Jetronic fuel injection and 170hp.

This picture was taken in 1996. It now needs restoration. I plan to bring it here and bring it back sometime soon.

[Adenauer]

What a beautiful coupe in RHD, no less!

What is the interesting, horizontal bar across the width of the lower grille?

[alabbasi]

It's for mounting grill badges.

[Adenauer]

Any idea where (overseas) I can get a line on headlight wipers for a 114...

Motors -
LEFT 1148201342
RIGHT 1148201442

Arm/blade -
BOTH 1158203644

[alabbasi]

I have only seen those once and that was in a magazine review. I bet money that you can get them from Mercedes Benz in Europe or through the classic center.

[Adenauer]

I'll be working on it.

Thank you. And thanks, for loading the shots of your 250!

[Adenauer]

Back to the topic.

You've got a 1976 114/8 280E, W114.062 chassis and either an M110.981 or an M110.991 engine. In any case, the engine in your car uses a BOSCH D-Jetronic electronic fuel injection system. This system is about the most basic and in my opinion, reliable, of the many varities of BOSCH injection systems (electronic). There really isn't much to the system. To familiarize yourself with the components on your M110, here's a list injection components - starting with the fuel tank:

FUEL
- Fuel pump
- Fuel filter
- Diaphragm fuel damper
- Fuel pressure regulator
- Cold start valve
- Injection valves

AIR
- Auxiliary air valve
- Temperature sensor 1 (intake plenum)
- Idle adjustment screw
- Pressure sensor
- Throttle butterfly valve

WATER/ELECTRICAL
- Temperature sensor 2 (coolant)
- Thermo-time switch

ELECTRICAL
- Battery
- ECU (electronic control unit)
- Throttle position sensor (potentiometer)
- Pressure sensor (same part as above)
- Ignition distributor

The very first consideration is to ask yourself, "Is this car's fuel delivery system in its original configuration as when delivered new." Some may question my logic there but the rationale is that you'd really be surprised to pop the hood of a 30 year old Mercedes to find that one or more essential components has been removed or bypassed - handicapping the parent system. Because you indicate that the car runs well for the most part, I will assume an intact set-up. So...

1. Because the fuel delivery system requires voltage to operate, with the engine running, check to see that you've got at least 13 volts DC at the battery. This is just a matter of course and I make it a habit of doing that whenever troubleshooting a system that draws any current.
2. You've got two fuel filters in this car. One is above the right rear axle and above the fuel pump - surrounded by a sheet metal shield. The other, you will see as what looks like a plug in the very bottom skin of your fuel tank. It is in fact a drain plug but it has a built-in fine mesh screen that filters fuel from inside the tank. I'd venture a guess that the in-tank screen is plugged-up with years of accumulated debris from the bottom of the tank. Change both filters. While the tank filter is out (you will have to remove it when the tank is, of course, empty) and the main filter above the pump has been removed, remove the fuel line where it attaches to the fuel pressure regulator at the engine. Flush the lines with fresh fuel until it flows free and clear.
3. Re-install these pieces.
4. Give the car a proper tune-up per specs. I can post the numbers if you'll need them. This would include spark plugs (and correct gap), wires, a new set of ignition points, adjusting the timing, adjusting the valves, and changing the air filter.
5. There are two most common causes for what you describe - surely others as well but the following would be where I would dive-in first.
- The throttle plate/bore area accumulates years of built-up, caked-on particulate matter from within the intake plenum. Since the throttle plate, when new, closes to within a VERY slight gap with the throttle bore, any amount of built-up deposits around the circumference of the bore will in effect, reduce this tolerance; this gap. That has the net effect of simply backing out the adjustment stop screw and closing the throttle to the stop - which, would either cause the engine to stumble or kill it alltogether. You will have to remove the 90 degree rubber boot from the air breather housing to change the air filter anyhow. So that is a great opportunity to have a look at the throttle bore area. A word of caution...be very careful when removing the rubber boot from the air breather housing and throttle plate housing! The rubber is old and not as pliable as when new. It will be esy to accidentally tear the boot. If the boot is torn beyond where the clamping surface is, you'll introduce unfiltered air into the engine. With the boot removed, remove the throttle linkage rod (a flat strip of metal, really) from the throttle lever on the driver's side of the throttle housing. Using a flashlight while manually opening the throttle (you won't set any trouble codes on this car) with a free hand, have a look down into the throttle bore. If you see a build-up of black crap (for the lack of a better word) that almost makes a perfect ring around the throttle bore where the throttle plate would rest when released, that would contribute to the stumbling you have experienced. Prop the throttle to the wide open position and lightly spray Easy-Off oven cleaner into the bore - soaking the throttle plate and shaft as well. Allow the Easy-Off to soak for a couple of minutes then scrub with a soft toothbrush. The Easy-Off will immediately remove the black and you will see fresh aluminum. Try not to spray any more than needed to do the job. Now, using old, lint-free rags, wipe out the throttle bore area. it will shine. Follow-up with a shot of carburetor cleaner to remove any remaining residue. Place an old towel below the opening of the throttle bore to catch any drips from the Easy-Off and carb cleaner. Now it's time set the base settings for the throttle. You'll see a 10mm stop bolt below the throttle lever. It will have a 10mm nut at the top against the throttle housing as a stop nut/jamb nut. Loosen the nut several turns. Now, looking down directly from above so that you are looking at the very end of the 10mm bolt, loosen the bolt while watching the throttle lever move. As you loosen the bolt, the throttle will close until it can no longer move. Do this slowly so that you can actually see when the throttle lever stops moving. Do not touch the throttle plate inside the bore while doing this as you do not want to add any "artificial" feel to what you are doing. Rely on your sight alone to watch the throttle come to its stop. With the throttle at the closed position, turn the bolt in now by a half turn and tighten the lock nut slightly. You have just set the base throttle position. Now it's time to locate the idle adjustment screw. On top of the intake manifold, you'll see the bellcranks for the throttle linkage. Just ahead of them by about an inch, is the fuel pressure regulator. Ahead of the pressure regulator by about another inch and below, is the idle adjustment screw; about 13mm in diameter and slotted for a large blade, flat screwdriver. Turning this screw clockwise reduces the amount of "bleed air" and the idle speed drops. Counterclockwise increases idle speed. I am guessing you don't have access to a CO% concentration meter so this is what will get you moving without. Turn the screw clockwise until it stops. Do not force the screw any further. Now put the air breather back together - again, being careful with the rubber boot. Reattach the throttle linkage and start the engine. The engine may not want to idle at first because you have just used a bit of cleaner in the plenum. Rev the engine for a moment to clear it then take your foot off the accelerator. The engine should die. In quarter turn increments, turn the throttle stop bolt "IN" after first, loosening the lock nut. Tighten the lock nut after each adjustment. Do this until the engine will start and idle without quitting. Once the engine is running, allow it to reach normal operating temperature. Now, go back to the idle adjustment screw and back the screw out counterclockwise until an engine speed of between 750 and 800 RPM is obtained (whether manual transmission or automatic makes no difference). Having a hand-held tachometer would be nice but if you have a good ear and a "feel" for that RPM, you can do without. Take the car for a test run - a short Italian tune-up by flooring the accelerator momentarily will evacuate any leftover Easy-Off and carburetor cleaner. Run the car at normal operating temperature for a few minutes - at moderate speed then slowing to a stop a few times - noting the behavior at idle. Make any adjustments using the idle adjustment screw. The following morning will be the cold test for how well you did on setting the throttle plate adjustment. If, when starting the car the next morning, it tends to want to die, turn the throttle stop bolt in another quarter turn. At this point, (and really, this is all very simple) you should notice a tremendous difference in how the engine behaves at idle. The next item to consider is the the throttle position sensor. This is the black box on the opposite side of the throttle housing. If the above didn't cure the problem, you'll want to test this component as it has a built-in, temporary fuel enrichment device. But, that's another day. One last thing to do a visual check on is the hose that goes between the auxilary air valve and suction tube. If this hose is cracked or loose, you'll experience idle symptoms. This part is screwed directly to the engine block just about a half inch above the engine casting number. Check the hoses at its connections.

Try the above and post your results.