300E with cold-startup problems/low idle - PeachParts Mercedes-Benz Forum

pawoSD · #1 11-22-2008, 10:26 PM

My newly purchased 1990 M103 300E apparently doesn't like to start cold....it takes either one long crank (8-10 seconds or so) then it begins to catch and finally manages a low idle (around 450-500rpm), you have to let it sit doing that for a couple minutes, then you can give it throttle and drive etc....giving it throttle right away results in it just stalling/shutting back off.

Also, about 60% of the time when just using it/driving around it settles in a very low idle, like 500-550, it never stalls out, but it does quiver a little. Obviously this idle is too low....sometimes it will come back up to about 600-700 and stay there, other times it goes lower. Playing with the throttle in park, giving it little nudges up to 1200 or so then letting off, it goes down to 500 then rises back to 600...seems normal, and it idles very smooth and starts ok when warm.

When at idle with no throttle applied shouldn't the economy gauge be all the way pegged to the left? It hovers about 3 needle width's towards the center. This means leaky vacuum tubing under the air cleaner right?

The air cleaner assembly isn't held in place very well, one of the mounts is missing and it moves a little.

What things should I check? Idle control valve? Tubing? The exhaust smells like its running a bit rich/a bit of unburned fuel. Its clean though, no blue or white or anything. Coolant is clean, oil is clean. Pickup is very good once its been running for a few mins, it takes off VERY quick, and pulls very strong/rev's way up just fine. Its just that annoying low idle and cold start!

It will cruise at 75 on the freeway with the cruise control on smooth as silk and steady. So the ignition/injection system must be ok.....

Help?

pawoSD · #2 11-23-2008, 12:51 AM

Anyone?

rg2098 · #3 11-23-2008, 12:27 PM

My newly acquired 380SEL is doing the same exact thing, down to the vacuum gauge reading and all. All the vacuum lines look like they have been replaced and when I spray around with carb cleaner, it looks like my intake manifold gaskets are failing. You definitely have a leak somewhere.

Ron in SC · #4 11-23-2008, 02:38 PM

Very common problem. Could be lots of things.

Read the posts below they may be helpful.

Removing OVP on a W124

W124- E300 M103 yr1992 – COLD START VALVE ACTIVATION

cold start problem

Idle Control Valve not getting power

Cold start valve: Testing question

300 E very very hard to start only when cold

babymog · #5 11-23-2008, 04:29 PM

I'm going from memory here on locations etc., my '91 has been out of the fleet since ~'96 so if it doesn't seem right, question it.

Start by checking the ignition wires, plugs, and distributor cap & rotor, if any are suspect then replace. Might as well get the maintenance items out of the way.

It's important to define the cold starting issue. Starting when it's very cold out and only shut down for 10-20minutes, starting in moderatly cold (garage or mid-day upper 30s) and the engine hasn't been run for an hour or more (engine at outside temp), etc.

Then when it starts, does it seem to miss for a little while before all cylinders are on-line or pretty much go right to a smooth idle.

I believe that you should hear the fuel pump run (under the car near the rear wheels) a soon as you turn to position 2. It will time-out if there is no ignition signal after a few seconds. If you don't hear it run, the problem is likely a relay and simple to fix.

The cold-start valve (or 7th injector) is supposed to inject fuel to start the engine cold, this is the CIS-E equivalent to a choke. Without it you will have a rough time starting it cold. It requires the coolant temperature sensor to tell it the engine's cold, be sure all sensors are plugged in, and follow the fuel line to the injector on the intake, be sure the electrical connector is plugged in (cold-start injector). Cold enrichment also relies on the coolant-temperature sensor.

Vacuum leaks are a good thing to chase down, I always used small shots of starting fluid to see if the engine revs when I spray to look for leaks. Explosive/flammable, don't spray anywhere it could ignite. Seems that the economy gauge wouldn't be pegged at idle, shouldn't be that much vacuum, but I don't remember where it was on my car so hopefully someone else can tell you if that's normal. If it seems to be running rich, open a vacuum line between the engine and airflow-sensor to see if it runs better, which would support your suspicion that it is running rich. If it runs worse, that indicates it was already running lean. Most cold-start problems are lean.

Leak-down is the CIS's enemy. To combat the possibility of fuel boiling in the hot injectors, there is an accumulator near the pump (looks like a filter) to maintain fuel pressure (~4bar if I remember correctly) for about 20minutes after shutting down the car. If it doesn't work, you'll sometimes end up with no gasoline in injectors after shutting down hot because of the boiling point of gasoline at atmospheric pressure, this will lead to hard starts.
This system also causes complications: if the accumulator and check valve do work properly by maintaining fuel system pressure, and you have a leaky injector(s), the fuel will seep through that leaky injector and into its cylinder. This will have the effect of starting hard, with the leaky cylinder misfiring due to the initial lack of fuel pressure and excessive fuel in the cylinder. This is additionally undesirable because the leaky one will cause some washing of oil from the cylinder wall and additional wear could result.

The injectors in the 103 are held in place by O-rings. These O-rings become hard and shrunken after enough years, and should be replaced. While shrunken, they allow un-metered air to flow past them and lead to a lean idle and part-throttle hesitation. Wiggle the injectors and see if they feel loose. They pull straight out and the O-rings are cheap. They do not seal compression like your 617s, just vacuum.

The fuel system on this car runs at a pretty meager pressure, around 1000psi less than you're used to, closer to household water pressure so they are much less sensitive to disassembly/assembly of fuel lines. Also, it does not have the timing issues of a diesel, it flows all of the time, and fuel will quickly replace air in the system for self-priming.

One thing that you'll want to check is the fuel metering plate. In your system, your fuel is metered mechanically by the airflow lifting a plate connected to an arm, which pushes a plunger/metering-pin into the fuel distributor to increase fuel flow. Fuel system pressure/counterpressure on this pin will adjust how rich/lean it runs. You can remove the air filter system and see your airflow meter (no mass-air-sensor). After the engine has been off for about 10minutes, you can lift the meter carefully (key off) and listen for the faint squealing/singing sound that it makes when fuel passes, that will tell you that there is some residual pressure, and the accumulator/check valve/injectors are holding pressure. There is a proper and more precise test, but I've found this to help me in initial diagnoses.

When lifting the metering plate, feel for any dragging or sticking. I've seen a few that wouldn't start in bitter cold, it was just the gunk/oil on the mechanical part of the air-flow sensor that resisted motion. Cranking wouldn't create enough airflow to overcome the resistance so the meter wouldn't move and send fuel. Some fun stories about starting a car by lifting the plate through the adjustment hole with its dipstick, owner amazed, ...

Cleaning the while airflow meter mechanical portion is a good idea but be careful to not bend or force anything. The plate should be centered in the opening, play in the pivot shouldn't let it touch either side. At rest, it should be in the minor-diameter part of the opening, not above or below where air can get around it.

Let me know also what the number is on the fuel computer (behind the plastic splash-shield rearward of the battery), I have a used spare in the shop and don't remember which engine it's from, you're welcome to it.

The thing to remember about CIS is that it is a Constant Injection System, the airflow varies the flow rate through the injectors mechancally via the plate just described, it is a fairly simple system. the CIS-E variant (yours) is different in that it can vary the fuel rate slightly vs the airflow. Checking the flow rates and patterns etc. on the injectors can be done easily with a stopwatch and six graduated cylinders to stick the injectors in.

Hopefully you've already found your problem, if not I hope that this will help you to get started.

Ron in SC · #6 11-23-2008, 04:38 PM

Quote:

Let me know also what the number is on the fuel computer (behind the plastic splash-shield rearward of the battery), I have a used spare in the shop and don't remember which engine it's from, you're welcome to it.

I'd take babymog up on that offer. Easy to swap. Also that turned out to be the cause of my very very hard to start and poor idle situation when cold.

babymog · #7 11-23-2008, 04:40 PM

I might have a spare OVP relay also, but that'd be a loaner. I'd need the number again, I have a few from past/present cars.

pawoSD · #8 11-23-2008, 09:31 PM

I already made an order online earlier today for essentially all vacuum tubes on the engine, some are visibly in bad shape....so why not replace them. I also bought a new OVP relay....since it can cause so many issues....if its not bad then I'll have a spare.

Cold start issue on this car is defined as: Real cold outside and it hasn't been started overnight, or for a long enough period that its back to outside temp. Starting the car consists of cranking for about 10 seconds, the first 4-5 it doesn't do much but rotate, then it begins to catch and sputter, then by 9-10 seconds its going on its own and you can let off the starter. However, you have to let it do its thing for a couple mins (1-2 mins) as its idling around 450-500 and if you give throttle it dies instantly. After those 2 mins of low idle, it will respond to throttle and is fine, and you can't stall it....despite a very low idle. In driving when warmed up the idle varies anywhere from 500 to 700, in park all warmed up it stays around 600-700 nice and steady.

When the car is warmed up (has been run within the last hour or so) it will start pretty reliably, might take two tries, but only a couple seconds of cranking. Then it runs but idles low (500) as usual....but will not stall. It has never shut off or stalled while driving.....it does feel like it misses sometimes.

When its real cold starting it does feel like its missing on a cylinder or two, and it shakes (but not every time), this smooths out within a minute or two.

When I get some time to actually disassemble it a bit over the next couple days I'll be able to test some stuff and give a good visual inspection to all components. I'm sure it needs hoses/tubing replaced, some are bandaged with tape.

The fuel pump runs properly for a couple seconds and then shuts off when the key is put to run.

When running warm it runs very nice, and it has good performance....it will rev all the way to red line and pull hard.

ARNOLDLewis · #9 11-27-2008, 01:13 AM

im not an expert and im just shooting from the hip here but... cold start injector, or CSI circuit?

mak · #10 11-27-2008, 08:43 AM

I had the same problem in my 300Se.
it was rectified by replacing the Over Voltage relay . The old one had developed a cold solder joint. i resoldered it and that became operational too!
Perfect cold starts and warm up now!
regards
mak

pawoSD · #11 11-27-2008, 10:07 AM

Sweet, since I am lazy though I just ordered a new one.

I'm going to put it in first, and see what happens.

I'll still replace the rubber parts, but it'd be awesome if that fixed the start issue. I re soldered a Klima before successfully....I'm good with a soldering gun, I just figured it'd be better to replace a critical relay rather than try to fix it.

pawoSD · #12 11-28-2008, 11:48 PM

Today I went to the dealer and picked up a set of new Bosch non-resistor spark plugs for $22....got a new micro-torque wrench at sears half off! awesome.

And then went to work swapping them out. The plugs that were in the car were Champion RESISTOR Junktastical plugs, filthy and gross. Got it all set, got in and turned the key, fired off INSTANTLY and idled SMOOOTH. Problem solved!! I can't believe it was just the junky spark plugs. I am very happy.

It drives awesome, way more power, silky smooth idle, truly awesome. The wife is quite happy too, because she got to "take possession" of it today.

Now I can sell off the stupid Rav4 and collect some $$

Once my order of stuff gets here I'll replace the air filter and some vacuum tubes and such.....but it seems to be running good for now. Also, I'll have to get a new thermostat, it only gets up to about 65C or so....

johnathan1 · #13 11-29-2008, 02:05 AM

I was JUST about to tell you to change the spark plugs lol! My 300SEL used to be VERY hard to start when cold...sometimes I thought I might be stranded even. Changed the plugs...no more problems!

One must wonder why your plugs were bad though...my M103 kept fouling plugs because of excessive oil consumption, and would lead to this really hard starting problem, which, if let go would turn into a misfire and hesitation.

I would keep a close eye on your oil consumption.

ps2cho · #14 11-29-2008, 02:09 AM

^ If the plugs are fouling it may be time for the valve stem seals! See how long those plugs last til they are coated in oil again. A good stiff brush can rid of most of the burned oil on the plugs until the real issue is resolved so you don't have to keep buying new plugs every time.

johnathan1 · #15 11-29-2008, 02:13 AM

Yea, I was changing my plugs about once a month or so...