Problems after fuel filter replacement

Post by **misha** » 18 Sep 2019, 13:17

A month ago i replaced fuel filter because it was time for it.
Replacement was Bosch filter.
From the 1st day after replacement i have problems with starting when car sits overnight.
The problem is that car is cranking much longer and sometimes won't start at 1st attempt.Second try...it starts instantly.

Before replacement car was always starting on 1st-2nd crank no mather how long it sits.

It seems that priming few times before starting doesn't improve things...so i ruled out bad check valve.....also no leaks.

The only thing that was touched was fuel filter and the problem was there in the first morning after replacement.

Note that on 2nd try it start instantly!

Anybody had similiar experience?

Post by **jreed** » 19 Sep 2019, 05:31

I haven't had a similar experience, but I also haven't tried the Bosch brand fuel filter. The current filter I'm using on my '97 850 is a Mahle brand. Before that, I used a Volvo OEM filter. No problems with those.

Post by **erikv11** » 19 Sep 2019, 07:35

Nor do I have any suggestions but I've always used Bosch fuel filters, probably put 8 of them on P80 cars.

Post by **misha** » 19 Sep 2019, 10:22

In the past i used Knecht(Mahle) and Delphi(should be Ac Delco in U.S).
This is the 1st time i used Bosch fuel filter in 850.
Although... i have it on Xm and have no problems with it.

The thing which drives me nuts is that problem appeared from the 1st day i installed it.
I can't re-install the old one just to see if problem will go away because i threw it away.

If there is a pressure leak overnight at "quick connections" at fuel filter i assume that there would be a leak.But there are no leaks or fuel smell.

Post by **jreed** » 20 Sep 2019, 05:14

Have you tried testing for fuel pressure at the fuel rail? It could be there is an air bubble in the fuel rail that prevents starting the first time. If you open up the Schrader valve at the end of the fuel rail, you could see if fuel spurts out right away or if there is an air bubble. Be sure to wear safety glasses, gloves and have a rag handy to capture the fuel. Good luck!

Post by **misha** » 20 Sep 2019, 05:57

Fuel system is self-bleeding.
Fpr is there to maintain fuel pressure and excess fuel is returned into the tank via return line.
So....no air lock is possible.
Even if there was some kind of air lock at first....it was eliminated after priming before start.
No driveablilty issues,idle is smooth and even,all hp is available in every rpm range....problem is just 1st start after it sits overnight.
2nd attempt....it starts instantly.

Post by **Chuck W** » 20 Sep 2019, 06:55

You know more about the Motronic M4.4 than I do, I imagine, but is there a chance that the ECU adjusted starting parameters based on fuel delivery with the old filter? Would an ECU reset help?

Post by **misha** » 20 Sep 2019, 11:35

It shouldn't.
M4.4 don't have insights or control on fuel pressure like Me7 cars.

Post by **Chuck W** » 20 Sep 2019, 11:56

I guess I should've specified fuel trim and not necessarily fuel pressure.

I've only recently started messing with M4.4 tuning/flashing, so I'm still learning details here and there.

Post by **misha** » 20 Sep 2019, 14:42

No... fuel trim should not be affected with fuel filter replacement.
Even if it was.....after a month it should be addapted...since fuel trims are changing while driving and stored every time you turn the engine off.
I'll investigate those "quick connections" at fuel filter.
Those are the only items which were disturbed in the process of fuel filter replacement.