My car is having issues starting lately. It seems to start better on cold vs. warm, but either way it takes a while for it to actually start.

I think it is the fuel pump - but wanted to pass it by here before I start investing in parts.

I purchased a fuel pressure gauge to see if fuel delivery could be the issue. What happens is I never really see the Fuel rail pressure up till it starts cranking. When it does start, the pressure raises to about 30 PSI, but when I stop it drops down to 0 PSI within a second. My thought is that fuel is having problems initially making it to the fuel rail, but once it does, it is fine. As best as I can research, the check valve that keeps the fuel from flowing backwards is in the pump - is this correct?

Also open to any other ideas as well.

 

sounds like a fuel pressure regulator issue. If ur car is hot ( you've had it running) and u shut it off and 10 mins later come back and it has a hard start then it's a pretty good sign that it's the fpr.

 

Two things come to mind:
1-CPS (crank positioning sensor)
2-Battery's state of charge
1- If you have trouble starting when engine is warm but not when cold it is a clear signal for a CPS replacement. They are less than $40US and can be done DIY in less than ten minutes from the top back of the motor.
2- These cars require a full charge to run and start properly. Average age to replacement of the standard battery is 4-6 years and for the black gel battery is from 5-9 years.

 

1 - I really need to pull that CPS out - but it was replaced by a mechanic when the issues first starting showing a couple weeks ago. I tried to pull it out, but they put that screw on really tight and I can't seem to get the socket on it well enough to torque it off. I really just want to verify that they put a Bosch part in before I get another one.

2 - Battery was purchased last Christmas. I think the issue is a lot of cranking and not much driving lately. I have it on a trickle charger now - and since then the battery/Alt screen has not shown up again. I'll be leaving the Trickle charger in place till the car is up and running again.

For what it's worth - it is having a hard time cranking even cold now.

EDIT: Is the fact that the fuel pressure drops down to zero when the car is shut off reason for concern? I thought the pressure had to remain for a while - hence the reason to release the pressure before anything is done (fuel filter change, pump, etc...). In my case - all I need to do is shut off the engine and the pressure is gone.

 

1-Was it an independent shop that put the sensor in? They should never be torqued in place, a little more than hand tight does the job. The sensor could have been damaged if put in that tight. Go back to the source that did the work and ask for a replacement.
2-Battery will go down over time sitting dormant since electronics are still on in the vehicle. I store mine with a trickle charger for the winter and start up in the spring is no issue. Short runs around town of less than 30 minutes will not allow the battery to fully recharge.
2b- Cold weather draws more amps to start a cold motor than summer start ups. Maybe a block heater would be in order where you live.
3- If you have checked all visible fuel lines for damage or crimping and replaced the fuel filter then I highly doubt it is the pump. That is not a common repair need for these cars. Battery protocol is important though especially if not driven much or only very short hops around town.

 

Definately the CPS>>!! I had the same exact symptoms as your car..

 

The fuel rail thing is bugging me enough to think it is the pump - I really don't understand how else it would drop from 30PSI to 0PSI in less than a second... Priming the pump does nothing - as soon as it is done priming it drops to 0PSI. The only time I'm registering pressure is after a long crank. As soon as I let off (assuming engine did not catch) pressure drops back down to 0PSI.

1 - I'll work on getting that sensor out - but to be honest - the issue wasn't even phased when the swap was done. I was across town at the time looking at a used R500 when it happened, and the used car dealer recommended the mechanic. Quite honestly - I'd rather pony up another $100 for the part at the local dealer than drive back to this particular mechanic - my loss. I did see them swap the part out - they let me sit there and watch. I really felt taken when I saw how easy that swap was....

2- I live in Dallas - the battery is less than 5 months old. Since the car started having issues (and leaving us stranded) we have stopped driving it (over 2 weeks now). The battery is just dropping it's charge due to all the cranking we have been doing along with not driving to charge it back up.

3- All visible fuel lines look fine - no crimping or hard turns or anything.

It may be a waste of $$ - but I have ordered a new pump and filter (hopefully they will be here for install on Sunday). Once the car starts up - it is fine. The only issue car has is getting it initially started. Idle may be rough at first but once it is up - it runs fine.

I'll report back - if the issue still persists after the pump/filter swap - I'll look into the CPS again. I just can't imagine that sensor being the culprit when I can't seem to keep pressure in the fuel rail.

 

it's ur fuel filter is bad because ur fuel have a built in fuel pressure regulator inside the fuel filter...because you said that the pressure drop right after you turn off the ignition right!!!

 

just for the record, ckp is crankshaft sensor, cps is camshaft sensor. atleast as far as benz goes.

If ur ckp was bad ur car would shut off while u were driving, when u attemped to restart it wouldn't. If u let it sit for about 5-10 mins it would start again.

So i'm still going with fpr.

 

If I read correctly - the Fuel Filter is the fuel pressure regulator. I'm replacing both so hopefully this will be the catch all/cure all.

The car - as best as I can recall - has never shut off on its own.

I honestly think the fuel pressure is fine - it's the check valve that keeps the gas from flowing back into the tank that is bad. Now that I think about it - I can only imagine how much crap has been dislodged from the filter.... As best as I can tell - that check valve is located in the fuel pump. That check valve would prevent the pressure from dropping down to 0PSI when the engine was shut down.

 

u are correct, the fuel filter is also the pressure regulator. The pump has a built in check valve. When u do one i would do the other anyway. Ur problem should be gone after u replace both. It's easy, couple 10mm bolts for the side belly panel, a t20 torx and 4 clamps. The small hose in the middle of the filter on the left side doesn't need a clamp. It's only a vent hose.

 

Finally got time to get the filter and pump installed.

Filter arrived yesterday - a true Bosch filter this time.


Pump is supposedly a Bosch...


But when I unwrap it, it's actually a Pierburg... Same as the one I pulled out.


Anyways - I swapped them out, and made a huge mess in the process. Gasoline everywhere.... Garage will smell all night now.

Result was I instantly noticed an increase in pressure. Before, if I got the engine running, it would only measure 30PSI. Now - I saw it spike to 60PSI and settle down to 50PSI. Better yet - when I shut down the engine, instead of dropping down to 0 PSI, this is what I saw:


Result was it took 2 cranks initially to get the car started. I depressurized the system again to remove the fuel pressure gauge, and it only took one longer crank to get it started after that (about 4 seconds). I drove the car around for 5 minutes to get the engine warm, and then parked it into the garage. I let the car sit for about 15 minutes, then went back out to start it up, and it started right up (about a second).

I do believe my issue was the pump, and I do believe by swapping it out that I have resolved my issue.

Little tips I learned from swapping the pumps out - the electrical connections are sized differently, so can't put the positive connection on the negative terminal of the pump, it won't fit. It was pretty cool cause I couldn't really tell which one was the positive cable otherwise, as the orange and red cables looked the same from under the car.

Other think, the big hose connecting to the pump probably goes straight to the tank. I thought it went to the engine but after removing over a gallon of gas I determined that was a wrong assumption on my part... (Oops). I ultimately resorted to the dropping of the old pump and putting the new one in as fast as possible, and spilling a little gas in the process.

Otherwise, I can now confidently say that is you are not measuring pressure, or building and retaining pressure at the fuel rail - perhaps the fuel pump is the culprit.

 

just wanted to follow up as my car has recently scared me twice.... both times the engine turned over just fine when i turned the key. the first time it turned over for about 4 seconds then started. the 2nd time it turned over until the computer eventually stopped the cranking process..... i got worried, tried again, and it fired right up....

sound familiar?