94 S420: Car won't start after being driven a while
 

Hey all,

I assume I am going to hear a lot of responses having to do with vapor lock, but...

Car starts fine in the morning.
I drive it 5-10 miles, get the engine decently warm
outside temperature is low 70s, so not scorching
stop the car
come back for 5 minutes, try to start the car
most of the time it will start but ONLY if i press the gas pedal while cranking
if i don't push the gas pedal (and even if i do) sometimes the car doesn't start
the starter cranks like mad, but there is just no sense of any turn over

usually, also, the car feels like it will want to cut out on me on these warm starts when I put it in reverse (not drive though) - no clue if that is related to the starting problem, but it's partly why I wrote a new thread instead of looking up vapor lock threads

any thoughts on what is wrong, and more specifically, HOW to FIX it!

I'd check the engine temp sensor that activates the cold start injector, if not the injector itself to make sure it's not leaking.

Thanks for your response. I'm not experienced with car stuff, so can you explain how a cold start injector issue would cause problems during a warm start?

Thanks in advance.

How's your engine wiring harness? '94 is one of the problem years for deteriorating wiring harnesses, and they can cause a whole mess of havoc.

Is there any way for me to check this myself?

And this would cause a "engine starts fine cold, but doesn't like to start when it has been running for a while" issue? Always interested in learning more, so please feel free to elaborate!

Pull the breather cover off of the engine and look at the wires - the insulation on the wires will be cracked and exposing copper if the harness is bad. The problem is simple - wires short out to one another and create false readings in sensors or wrong actuations. It can create a multitude of different problems depending on what's touching what. For instance, we had a '94 and a '95 E320 (both with the old M104 inline six) in recently. Both of them had misfiring coil packs and check engine lights and other problems (with variable valve timing units, air pump clutches, etc.) due to bad wiring harnesses. Replaced both wiring harnesses and the cars were back to flawless functioning of the engine.

Could be the crankshaft position sensor, they are known to fail intermittently, especially after a short warmup.

Umm.... If I am to take this literally, that means that the starter isn't engaging the engine, which I assume is not what you mean. If you do mean this literally, than the diagnosis is a bad starter solenoid or bad wiring to the starter causing insufficient current and voltage. But I think you meant to say that it turns over, but does not fire, is that correct?

There are many things that could cause this, and I could irresponsibly pick a part and tell you to replace it, but that would be easy and wrong.

In my opinion, your symptoms are good cause to conduct a fuel pressure test, or multiple tests under different circumstances.

My instinct tells me that this is a fuel delivery issue, and because there are so many iterations of that (pump, filter, injectors, etc..) you should first confirm that you are having a fuel delivery issue, and under what circumstances.

Also, just to rule out a plugged up tank venting system, try opening your gas cap next time this happens and see if it "cures" the problem.

What I am getting at is the diagnosis is: tests. You must conduct tests to figure out what is wrong, and fuel pressure is a good place to start.

thanks all for your help!

I am seriously not good at explaining car things as I have no expertise in the area - it makes the sound that all engines make when you start the car up, right up until the point where the engine goes "vroom" and the tachometer indicates engine activity. if that made any more sense ;)

I appreciate your opinion that tests are the first order. I live in a town without a lot of import mechanics. We have a Mercedes Dealer but I'm not sure they have a service department (and if they do it's probably going to be ultra-expensive as they are the only game in town). Can "fuel pressure tests" be conducted on this car by any old mechanic, or does it require specialized MB equipment and/or expertise?

Thanks again!

Yes, it will still crank if the crankshaft sensor is if faulty. It will disable the
ignition system, but a press on the accelerator will not cure that. Therefore
my best bet is a vapor lock. Heres a simple movie I made about
measuring the fuel pressure to see if the regulator is faulty.

http://www.pvv.org/~syljua/merc/FuelPressureME.mov

You can do the same, and that you should look for in your case, is the last
step, if the pressure falls rapidly after you turn off the engine.
The regulator is not the only reason for this problem, it could also be caused by leaky
injectors of return valve in pump, but the first step is to see if you have a fuel pressure problem.

Ok?

br,
syljua

warm starting
 

I agree with Woolridge,pull off hose on fuel pressure regulator
and see if any gas come out if it doesn't,replace the crankshaft
sensor.Mine had 110,000 miles when it started to go,drove me
crazy with hot starts,warm starts,replace the ccs and never
had another problem with it.

I think that is a rather inadequate test to warrant replacement of a ~$150 part. Granted the CPS may be the problem, but the test you suggested does not determine that one way or another.

To the OP, if you're made of money then by all means feel free to throw parts at the problem. The CPS may even be the problem...... But just know that diagnosis is the preferred route.

I would certainly want to rule out fuel system issues before plunking down for a new CPS. But if time is money to you, then perhaps the shotgun approach will work for you.

Did you check the engine wiring harness?

If you dont get it fixed, I will trade you my 1988 300SEL in perfect working order, as I have been wanting a W140, despite people saying me keep my W126. ;)

Seriously though, keep us posted.

As requested... an update!

I didn't drive the car much over the winter, and the few times I did, I ran into a new problem: if I let the car idle for 1-3 minutes, and then put the car back into drive and drive off, you could feel the car shake a bit when you accelerated, and the next time the car came to a stop (at a light or an intersection) the car quit on me.

so, i decided to bite the bullet and pay my local dealer the $ to diagnose. Apparently they looked a bit at the fuel system found it not to be the issue, and then "got it to fail real hard" and found the engine code to be the crankshaft position sensor.

so, the problem now is that the dealer wants a ton of money to repair it. ~375.00 (on top of the $150.00 spent on diagnosis). So, I call up our local mechanic (who definitely does not specialize in mercedes - i originally took it to him to get it diagnosed and he point blank said that he didn't have the computer gizmos to properly read the codes) and the part will cost $100.00 and the labor will be $65.00 (shop rate is 65.00 per hour)

so my new question:

is $100.00 for the part and 1 hour of labor sufficient to perform the work?

most importantly, however, is this something that someone who is not a mercedes-trained mechanic can properly do?

(i'm not a car buff so I don't know how varying under-the-hood configurations and set ups make a task difficult or easy or impossible for someone who hasn't done it before)


thanks for everyone's help here!

Here it is for $59.07...
 

Here's a genuine Bosch cps for $59.07. It should take about 15 minutes to install...

http://www.*********.com/gbproducts/WC/6436-01020192.html?utm_source=google&utm_medium=ff&utm_term=1994-1999+Mercedes+S420+Camshaft+Position+Sensor+Bosch+94-99+Mercedes+Camshaft+Position+Sensor&utm_content=YN&utm_campaign=*********+Google+Base