So i've been trying to chase down a long start/occasional surging idle (very subtle change in RPM, but noticable to a perfectionist) condition on my 2002 w215 amg swap

in the past year or so, the car has taken longer than normal to start. it usually would crank for one or two seconds before firing up. lately it has been taking upwards of 4-5 seconds of cranking before starting, which frankly is a little embarrassing for a car that was about $100k MSRP

one thing that started happening at the same time is i noticed a gasoline odor from the outside of the car...and eventually i even started seeing a fuel drip/leak from the fuel pump area

so i changed out the fuel filter/regulator and also the fuel pump. both with aftermarket non OEM parts.. i have plans to upgrade the fuel pump/regulator in the future anyways so i figured there wasn't a huge use in buying OEM parts just to replace them soon for my turbo build. the leaks stopped but the long start condition still persisted.. which led me to believe i had a leaking fuel injector, bleeding off pressure

so i tested the resistance of each injector, i found cylinder 8 resistance was slightly lower than the rest (all read 17.5 ohms, cylinder 8 read about 17.3) so i thought i had found my issue

i ordered a refurbished injector and installed it. annoyingly to my surprise, i re-tested resistance on the old injectors and they all read good, all read right around 15 ohms. wtf!

so i changed out the suspect injector anyways, replaced o-rings and pintle caps. find my thread in the w215 non AMG forum for diy threads and insights on this issue. please heed my warning and replace not just your o-rings but also you pintle caps BEFORE 100k miles or they may break off into your cylinder head and possibly your cylinder. i spent hours vacuuming potential debris from my cylinder since that was the only cylinder with the intake valves open unfortunately, but i digress

so after replacing the suspected injector, i still was losing fuel pressure (verified with a fuel pressure tester i rented). not super quickly, but the car would steadily drop to about 10 psi fuel pressure after about 15 minutes which is unacceptable. keep in mind this pressure drop is with IGNITION OFF, during idle the fuel pressure was healthy

at this point i decided to try another test, i realized if my injector truly was leaking, i would be able to see it leaking by pulling up my fuel rail, and if the injector truly was leaking, i would see it dripping as the fuel pressure bled off...

so i pulled the rail and watched the injectors, as the fuel pressure bled off, not one injector was leaking fuel. another "wtf moment"

so at this point i feel better that its not really my injector leaking, but instead this is an issue with either my stop valve in the fuel pump, or my pressure regulator. keep in mind we do not have the fuel rail "dampener" you guys have on the m113k motors on the fuel rail so that is not suspect

so lately i have been priming the fuel pump twice, before starting. now it always starts right up as it has the proper fuel pressure. but i still have a varying idle condition so i wonder if my fuel pressure is not being properly regulated, AKA dumping in too much fuel at times maybe? but wouldn't my ECU adjust? maybe not since i still have the cl500 ecu and fuel maps....

also worth noting i have been trying to track down a pesky set of p0131, p0137 and p0151 codes...indicating either my o2 sensors are dying (not unreasonable at 120k miles) or that my air/fuel is off

keep in mind, i didn't always have these issues with my cl55 swap, i believe these issues came about after the car had to sit for about 8-10 months once. but for years the swap ran great with no engine lights etc

just wondering if anybody has heard of these same sets of issues, fuel pressure bleed off + o2 sensor check engine light codes...

 

Did you end up solving this issue ?

 

it’s usually the fuel pump, I’ve helped a few people with this same advice since I finished my repair. The cheap aftermarket fuel pumps would lose pressure, the expensive OEM grade pumps did not have issues

you can diagnose the fuel injectors leaking by lifting the fuel rail while the system is primed. Only need to unbolt the rail and lift the fuel rail a few inches until the bottom of the injectors are visible .