Help with P0300 2001 SL500

[2001SL500R]

I have had this car for about two months now and still have a nagging check engine light. It only comes on occasionally, but it has a consistently poor idle. The car otherwise runs very well. It pulls hard and runs smoothly above idle. It currently has 104k miles. I did the 100k and replaced all the plugs, new MAF, did all the fluids. The car and engine compartment are very clean, this is a socal/arizona car. Any ideas? It feels like it has a poor idle control but shouldnt the computer spit out an EGR code or something a little more definitive? It is so annoying that other than this idle issue I have a beautiful strong running car. Thanks in advance for any help! -Doug

[ILUVMILS]

How old are the ignition wires?

[2001SL500R]

I did not change the ignition wires. They APPEAR to be fine. I went around the motor while it was running and molested the wires to see if there was any continuity issues and did not appear to be. I have also read a few threads suggesting to change them without much success

[2001SL500R]

Any more suggestions? If feels so much like a fouled IAC valve but this car doesnt have that I dont think? Part of the problem is that it is pretty minor, it is just idling maybe 75-100 rpms low with a slight burble and occasionally sets a CEL for P0300, so cant isolate one cylinder. Thanks again!!

[atikovi]

Originally Posted by 2001SL500R

They APPEAR to be fine. I went around the motor while it was running and molested the wires to see if there was any continuity issues

Did you test them under load or at 3,000 rpm? They're 15 year old wires for heavens sake.

[2001SL500R]

Originally Posted by atikovi

Did you test them under load or at 3,000 rpm? They're 15 year old wires for heavens sake.

With all due respect, I am trying to graduate from just throwing parts at it. ; )

I have read other threads that mentioned ignition wires under similar circumstances with no luck and the car pulls smoothly and strongly over the road. The issue is at idle.

I do appreciate your help though. I am not against doing that, but trying to find something definitive if possible.

[atikovi]

Nothing to do with just throwing parts at it. Just common sense to replace 15 year old plug wires as a matter of course if you plan to keep the car a while.

[engatwork]

Have you been able to do a smoke test on the intake?

[2001SL500R]

No I have not smoked it, but I did spend a few minutes poking around today and see there are a several questionable vac lines so I am going to redo those.

[2001SL500R]

I replaced several vac lines this morning and got a small improvement. I think my EGR valve might be on its way out. I still have a perceptible burble. Maybe I am chasing a ghost but I am used to having a V8 Mercedes that I cant feel when its idling.

[paul roberts]

Get the fault codes scanned and take a look at the values of the engine management. I'm pretty sure this generation of the 119 motor has coil overs, so the plug wires are part of the engine harness. You may want to check the sparkplug boots though. For fun, take a look at the main vacuum line from the brake booster to the intake to see if it's broken. That could cause a massive vacuum leak that would surely feel like a dead miss at idle...

[2001SL500R]

The code was a P0300 random misfire.

Could be a faulty coil pack. Could be plug wire. Still might be a vac leak and possible egr is failing or some other component is not running at 100% but not quite poorly enough to throw a code. I forgot to monitor the O2's when I had the scanner on it but will look at that too.

[dkveuro]

Remember the ECU will kill the fuel injector on a cylinder with dead miss. This will be in memory as code..... being as P0300 is Random Miss Fire I'd look for,
vac' leaks are most common as are wiring harness concerns....try the 'wriggle test' on the wire harness.

Fuel pressure ?

Have you a Mercedes compatible scanner? If so check engine smoothness.

Are you running 100% pure 91 super gasoline ? Any possibility of water in tank?



.

[paul roberts]

The fault code you're getting is too generic. You need to look at actual values. See if anything is near the threshold of a missfire. Your MAF voltage, your spark KVs, etc. If you're worried about your EGR, that's easy enough to check without actuation via scanner. Just pull a vacuum on it at idle and you should stumble. I wouldn't worry too much about plug wires since this year doesn't have any. like I said before, it's part of the wiring harness and the 2001 has NO issues with that like the 94 and 95,if at all, not like the 104s. You might want to save yourself the headache and pay 100 bucks and have it read a little more in depth. Also, it could be as simple as a bad sparkplug boot. Pull them off and look in side for any hairline arc marks. They'll be white.

[ILUVMILS]

2001 SL500 is powered by the M113 V8 and it has plug wires. If they're fifteen years old they're shot........