2000 MB E430 with Code P0300

[HTSS_TR]

My 15 years old with 15x,xxx miles 2000 Mercedes E430 had the code P0300 last night when we drove home on highway.

The engine seems to run smooth at speed 70-75 MPH with normal RPM around 2200-2300. When we were on surface street and at stop light the engine is rough and shake enough that I can felt it inside the car. Acceleration is hesitant and very slow, but after get up to speed of 40-45 MPH and cruising the engine was very smooth again.

The only time the engine was shaking was at idle and Idle was a little low at around 500-550 RPM instead of normal 600-650 RPM

According to engine-codes.com, possible causes:

- Faulty spark plug (s)
- Faulty ignition coil (s)
- Clogged or faulty fuel injector (s)
- Intake air leak
- Fuel injectors harness is open or shorted
- Fuel Injectors circuit poor electrical connection
- Ignition coils harness is open or shorted
- Ignition coils circuit poor electrical connection
- Insufficient cylinders compression
- Incorrect fuel pressure

http://engine-codes.com/p0300_mercedes-benz.html#ixzz3WSSGf4yS

Spark plugs(16 of them) were changed 2-3 years ago at around 130-135k miles with OE Bosch platinum plugs.

Ignition coils and wires are original.

Intake air seems to have no leak, but I don't know how to check for possible Intake air leak. Any idea how to check air leak ?

I didn't use Chevron Techron for 2 years, I usually used it every other year before smog test, but this year I forgot and just had smog test last week. May be use a large bottle to treat 20 gallon tank.

What should I do next to fix/repair the rough idle/misfired ?

TIA and happy Easter Sunday.

 

[Mr Nice]

I've had that code before: a spark plug was not correctly torqued and got loose causing misfire.

 

[Falken]

I have seen that before in Canada usually around the time of seasonal fuel changes, you just get a bad tank of gas and the weather changes.

But, based on your location, look at the usual suspects but don't rule out a bad tank of gas.

A bad tank of gas is one of the trickiest things to diagnose, as people rarely get them.

Did you fill up at a crummy gas station recently?

Note: I had that code (Random Misfire) about 2 years ago, it happened when we had massive temp swings in the early Spring.

 

[mattwithcats]

Toss a red bottle of Heet in the gas tank...

I suspect bad gas...

 

[Olas]

Spray some carb cleaner/deodorant/propane etc at all of the piping and hose between the MAF sensor and the intake manifold gasket to check for vacuum leaks, if the spray changes the idle you found your leak.

If that doesn't work then wait for it to go dark. With the engine running, lift the bonnet and spray a fine mist of water ( from a plant spray or similar) at the coil/distributor/leads/plugs basically all the wiring in the ignition system, if you see any arcing ( easier to see at night) replace the offending component.

Check the plugs for wear and gap, and visual inspect them for signs of fouling/dryness/ash that might point to an injector problem.

Good earths throughout the bay & loom?

Clutching at straws but could a lazy/bad 02 sensor relay bad information in such a way as to cause a misfire?

*edit*
Try a bottle of redline si1 in the fuel, it fixes everything without robbing any snakes of their precious oil

 

[HTSS_TR]

We only bought Costco premium gas the last few years. Costco gas turn around at very high volume, so that there is no possible for staled gas. Bad gas is possible but very unlikely.

I just poured in a large bottle of Chevron Techron fuel system cleaner concentrate in the gas tank.

The spark plugs were installed 2-3 years ago, so the possible for it to get loose is slim. But i will check that if other checks did not fix the code P0300.

 

[The Critic]

You're going to need more data to accurately diagnose the issue. Fuel trims, misfire count from each cylinder, etc. At this point, we're all just guessing.

 

[Trav]

P0300 can be very easy or very difficult to diagnose. Because this engine uses COP a few things can almost be ruled out.
Look for something common to all cyl. Without scantool data its impossible to even make a guess but look at the tube going from the MAF to the throttle body for cracks in the accordion part of it.

Other than that there is no point in going any further without more data.

 

[HTSS_TR]

After added a 20 oz bottle Chevron Techron into the gas tank, started the car to Costco to fill up the engine was shaking when idle went down from 1300-1400 RPM to 700-800 RPM with CEL lighted up after few seconds(after cleared the code the niht before).

The engine was not shaking at Costco gas station after about 3.5 miles drive. Drove around for some errands and the engine was quiet and smooth as it should be. I hope it will stay this way.

Looked like we had bad gas from the same Costco location my wife and I almost always buy gas from. Now, the engine is smooth and quiet(after using Techron) so there is no way I can fill a complaint with Costco. I'm not trying to extort anything from Costco, but I would like to inform Costco about possible bad gas they bought, and whoever supplied bad gas last time should test their gas before delivery, several thousands gallons of bad gas can cause problem for hundreds of car and repair cost can be many thousands dollars.

 

[Olas]

Stick with shell, and regular use of redline or techron.