c_benz_240

Hi All,

I recently had my CEL light come on, and after having the codes read, the problem seemed to be a failed Fuel Pressure Sensor. The code was P0452 (B4/3 - Fuel Tank Pressure Sensor - Short to Ground).

I had an independent shop replace the faulty unit, which was located under the rear seat, with a new OEM Bosch unit. After the repair my CEL light was resolved, but now I am having problems starting my car. When starting the car, after it has been sitting for a while, the engine struggles and then eventually starts. All attempts to start the car following the initial start are fine.

My car did not have a starting problem before the repair. Even when the CEL light was on, it started smoothly. What could be causing the hard starts.

Thank-you in advance.

Sorry, I dont know how to enter my vehicle details in my profile so I will list it here for now.

2002 C240 (Elegance)
Automatic
97,000 Km

c_benz_240

Nothing?

Glyn M Ruck

Check fuel pressure at Schrader valve on the front fuel rail. s/b >3.8bar or 55psi while cranking.
Make sure EECS pipework is all connected properly ~ this should throw a code however.
Check for spark.
Check CPS

c_benz_240

The shop is double checking their work.

They said it might be a leaking gasket, as they reused the original ones because the new unit didn't supply any. Or, it could be a failing fuel pump. It strikes me as odd that the pump would fail right after the FPS repair.

Anyhow, they have ordered new gaskets and will be installing them today. I will ask them to check those items as well and get back to you.

Thank-you

c_benz_240

Okay, I talked to the shop about the possible causes for the hard starts and they don't think it's any of those issues. They think the fuel pump is failing because the problem is intermittent. What do you guys think?

Glyn M Ruck

It is very likely a fuel pump or clogged fuel filter problem. That's why checking the fuel pressure is so important.

For start you need:

Correct fuel pressure, spark at the right time & air.

Start enable fuses 31,52 & 57 must be intact. CPS & temp sensor are interrogated. If sending correct info start is enabled. Air cleaners must be free of excess dirt.

c_benz_240

To narrow the problem down to either the fuel pump, which they believe is the culprit, or the fuel filter, they have asked me to cycle the key through the off position, and position 2 a couple of times before fully starting the car. By doing this the fuel system will have a chance to prime itself. If they car starts normally then there is a high probability that the fuel pump isn't strong enough to push the fuel to the fuel rail on a cold start. What do you think?

By the way, thank-you for all your input thus far.

Glyn M Ruck

The ONLY way to check properly is to measure the fuel pressure while cranking. It's as easy as checking tyre pressure.

pcy

As suggested [3 times] above by Glen, check the fuel pressure. That is the first thing the mechanic should have done.

Since there is a valve readily available at the end of the fuel rail, it doesn't make sense why the mechanic hasn't simply connected the fuel pressure gauge to that valve and test the pressure.

BigN8

I can say this if you are at an indy shop they don't always understand Mercedes.

I have had a shop tell me I need a full fuel system overhaul when like Gyln said it was just a fuel filter. In fact that repair is the reason I am a member here. I thought about what the shop had said then got on line. it led me to a thread where Glyn had post the very same thing he has here. I purchased a very cheap fuel pressure gauge from harbor freight. Checked the pressure to find it to be low. now swapping the fuel filter was 30 minute job and was less then $50. So I did that first, thinking I should change it anyway even if i need a new fuel pump. car has run great for over 20k miles and i have been a Glyn believer ever sense.

c_benz_240

Thanks for the advice, and sorry for the late reply.

The shop installed a new BOSH fuel pump, and the car starts normally. Before going ahead with the repair I thought about the fuel filter, but the problem started after I had the Fuel Sender Unit replaced and only on a cold start. With that in mind, and the fact that the fuel pump was the only other component touched during the Sender repair, I concluded it was probably a failing pump. The pump use to make a groaning noise when the key was in position 2, now it's practically silent.

Now I have to deal with a clunking (which I think is the shock), and weird hollow sound from the left rear suspension. If you have any insight on that please comment in my other post, it will be greatly appreciated.