Hot start issues on M275
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Newbie
Joined: Dec 2025
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2012 S65 AMG
Hot start issues on M275
I am having an issue where cold starts are fine, but hot starts (sitting for 10 min-1 hour or so) and the car will still start normally, but within a second or 2 stumble and die. Upon restart it starts normally. Reading around it seems the likely culprits are either the crank position sensor or fuel pump. Anyone know of any way to DIY test either before throwing parts at it?
MBWorld Fanatic!
Joined: Feb 2012
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From: Phoenix
222 S-65
My 221 S-65 would do something very similar to what you describe. Fuel in the rail boils with a hot soak. The fuel pump delivers X quantity of fuel to make Y pressure. Mercedes solution involved a software update. Don't ask me to quote which one.
I found that gasoline without Ethanol was much less likely to do the hot start blub-blub-blub-blub-stall thing. Then restart and run fine. Over the years I have encountered cars available for sale that the dealer got to buy back for this very issue. Branded title= lower pricing on that used car.
I found that gasoline without Ethanol was much less likely to do the hot start blub-blub-blub-blub-stall thing. Then restart and run fine. Over the years I have encountered cars available for sale that the dealer got to buy back for this very issue. Branded title= lower pricing on that used car.
Senior Member
Joined: Feb 2014
Posts: 268
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From: DFW
2006 C55
Yep. Fuel boiling in the injector rail. Well known issue but highly dependent upon local fuel quality. In DFW the winter blend fuels are an issue.
Updating the software forces higher fuel pressure and eliminates the issue.
Updating the software forces higher fuel pressure and eliminates the issue.
Thread Starter
Newbie
Joined: Dec 2025
Posts: 8
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2012 S65 AMG
I'm in Florida so I'm not sure what they do with fuel here, but I always get premium from Costco. I got the car last May and don't remember it doing this until a few months ago, so maybe it is some winter blend they do down here too. Any idea if MB will update the software for free?
Super Member
Joined: Sep 2021
Posts: 541
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From: Ann Arbor, MI
2004 SL600
If your car cold starts and runs fine 99% of the time, you can be sure the crank sensor is fine.
As others have mentioned this is a fuel problem such as fuel boiling in the fuel rail.
Here are my suggestions:
1. Take off the engine cover and hang it in the garage or living room. Sure they look pretty but engines cool better with them all off. IMHO the fuel rail temperature is greatly affected by the engine cover.
2. Buy a cheap fuel pressure gauge and screw it onto the front of the fuel rail. If not familiar, search Amazon for "BETOOLL 0-140PSI Fuel Injector Injection Pump Pressure Tester" and pay $30. Or rent one free from the local auto parts store.
The fuel pressure for the M275 is around 56 psi. (AI says "54-61 psi").
If the pressure is out of that range, you may have a problem with the pressure sensor, the fuel pump control module or the fuel pump itself. The pressure sensor is easy to replace, the other parts not so easy.
If you can report the fuel pressure to us, we will try to guide you further.
FYI - I has a stubborn "misfire code" problem. Checked the fuel pressure and it was 110 psi. Replaced the pressure sensor and that corrected all.
As others have mentioned this is a fuel problem such as fuel boiling in the fuel rail.
Here are my suggestions:
1. Take off the engine cover and hang it in the garage or living room. Sure they look pretty but engines cool better with them all off. IMHO the fuel rail temperature is greatly affected by the engine cover.
2. Buy a cheap fuel pressure gauge and screw it onto the front of the fuel rail. If not familiar, search Amazon for "BETOOLL 0-140PSI Fuel Injector Injection Pump Pressure Tester" and pay $30. Or rent one free from the local auto parts store.
The fuel pressure for the M275 is around 56 psi. (AI says "54-61 psi").
If the pressure is out of that range, you may have a problem with the pressure sensor, the fuel pump control module or the fuel pump itself. The pressure sensor is easy to replace, the other parts not so easy.
If you can report the fuel pressure to us, we will try to guide you further.
FYI - I has a stubborn "misfire code" problem. Checked the fuel pressure and it was 110 psi. Replaced the pressure sensor and that corrected all.
MBWorld Fanatic!
Joined: Feb 2012
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From: Phoenix
222 S-65
I encountered enough failed crank position sensors in the two-elemon cars that I kept a sensor, ratchet and socket in the glove box. I have not had a crank position sensor fail in a 275 or 279 powered car.
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2023 Rover
Gents, same thing here! Changed the fuel pump, fuel filter, new crank sensor, new plugs, new coilpacks, and now I know it's a heat soak issue. I put a fuel gauge on the car, and it's at 55PSI while running, drops to 50 when you turn it off, and drops to 35PSI in about 30 minutes of just sitting there.
I fill up with premium 91 octane here in California. The car is tuned by Weistech, but for 91 as well. I'm going to take the engine cover off and see how the car behaves tomorrow.
@mrvedit, what should I try next? Would you happen to have the Mercedes spec for fuel pressure loss?
I fill up with premium 91 octane here in California. The car is tuned by Weistech, but for 91 as well. I'm going to take the engine cover off and see how the car behaves tomorrow.
@mrvedit, what should I try next? Would you happen to have the Mercedes spec for fuel pressure loss?
Last edited by anton28; Mar 26, 2026 at 12:21 AM.
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Super Member
Joined: Sep 2021
Posts: 541
Likes: 117
From: Ann Arbor, MI
2004 SL600
@anton28 asked for my fuel pressures and I therefore just did a test. SL600 with reasonably new pump, filter/regulator and fuel pressure sensor.
After sitting for 3 days: 0 psi
Ignition on: 47 psi - built within a second or so
Ignition on for 5 mins: 41 psi
Engine on: 57 psi
Engine off: 50 psi
Engine off 5 mins: 48 psi
Engine off 30 mins: 43 psi
By "same thing here" do you mean engine starts fine but then dies shortly thereafter? Is engine cold or hot when started?
If easy to replicate, have someone monitor fuel pressure when it stalls. (I guess it could remain at 55 psi even if air got in the system due to heat soak, but if it indeed drops low we have usable info.)
After sitting for 3 days: 0 psi
Ignition on: 47 psi - built within a second or so
Ignition on for 5 mins: 41 psi
Engine on: 57 psi
Engine off: 50 psi
Engine off 5 mins: 48 psi
Engine off 30 mins: 43 psi
By "same thing here" do you mean engine starts fine but then dies shortly thereafter? Is engine cold or hot when started?
If easy to replicate, have someone monitor fuel pressure when it stalls. (I guess it could remain at 55 psi even if air got in the system due to heat soak, but if it indeed drops low we have usable info.)
Super Member
Joined: Sep 2021
Posts: 541
Likes: 117
From: Ann Arbor, MI
2004 SL600
On a GM forum where I'm active some members had to replace their crank sensor several times before they got a working one. Part of their problem is the high percentage of counterfeit ACDelco sensors on Amazon.
No idea if Amazon is filled with counterfeit Bosch sensors too, but don't see why not. For auto parts on Amazon I always make sure the seller is Amazon itself and not something like CheapoAutoParts and even that is no guarantee.
In short, maybe try another crank sensor, this time from the MB dealer.
No idea if Amazon is filled with counterfeit Bosch sensors too, but don't see why not. For auto parts on Amazon I always make sure the seller is Amazon itself and not something like CheapoAutoParts and even that is no guarantee.
In short, maybe try another crank sensor, this time from the MB dealer.
Senior Member
Joined: Nov 2006
Posts: 428
Likes: 11
2023 Rover
@anton28 asked for my fuel pressures and I therefore just did a test. SL600 with reasonably new pump, filter/regulator and fuel pressure sensor.
After sitting for 3 days: 0 psi
Ignition on: 47 psi - built within a second or so
Ignition on for 5 mins: 41 psi
Engine on: 57 psi
Engine off: 50 psi
Engine off 5 mins: 48 psi
Engine off 30 mins: 43 psi
By "same thing here" do you mean engine starts fine but then dies shortly thereafter? Is engine cold or hot when started?
If easy to replicate, have someone monitor fuel pressure when it stalls. (I guess it could remain at 55 psi even if air got in the system due to heat soak, but if it indeed drops low we have usable info.)
After sitting for 3 days: 0 psi
Ignition on: 47 psi - built within a second or so
Ignition on for 5 mins: 41 psi
Engine on: 57 psi
Engine off: 50 psi
Engine off 5 mins: 48 psi
Engine off 30 mins: 43 psi
By "same thing here" do you mean engine starts fine but then dies shortly thereafter? Is engine cold or hot when started?
If easy to replicate, have someone monitor fuel pressure when it stalls. (I guess it could remain at 55 psi even if air got in the system due to heat soak, but if it indeed drops low we have usable info.)
What I mean by “same thing here” is that the more I drive the car throughout the day and the hotter the motor gets, the more it starts stumbling on startup—and eventually it won’t start at all toward the end of the day. I put some miles on it yesterday, and even though I couldn’t get it to completely not start, I did get it to misfire on startup.
What I did next was relieve the fuel pressure through the nipple at the fuel rail, and I’ve got to tell you—I’ve never seen a fuel line purge like that for as long as it did. There was a ton of vapor that purged along with the fuel, which confirms the fuel is boiling in the rail. This is exactly how old carbureted cars behave when the fuel line runs too close to the exhaust—classic vapor lock. After that, I tried starting it again, and it fired right up, just like it does when the car is cold
@JohnLane mentioned Mercedes had a solution involving a software update, so I reached out to Paul at BenzNinja. He was familiar with M275 issues like this and said MBZ addressed it with an ECU update, so we went ahead and did that today. Unfortunately, the update wiped out my Wistec tune, so I sent the ECU back to have it reflashed.
Once the car is back on the road next week, I’ll report back on how the MBZ software update affects things on my end.
If, for some reason, this doesn't fix the issue, I'm going back to my hotrod roots and already thinking about a fuel purge system that vents through a charcoal fuel canister. More to come.
MBWorld Fanatic!
Joined: Feb 2012
Posts: 2,667
Likes: 1,238
From: Phoenix
222 S-65
The issue being that as fuel pressure drops + hot soak it all turns to vapor in the rail. No fuel return to the tank says the pump delivers a quantity of fuel to make the desired pressure. Pressure doesn’t know there is essentially an empty fuel rail to fill. Would be interesting to monitor fuel pressure in a 279 powered car that doesn’t have the issue. At some point I will plug into my car and do so.
Anton no doubt the software update alters fuel pressure.... likely just as it is turned off. Or are they stopping fuel from returning to the tank (to hold rest pressure high enough to keep it from boiling) via the pump?
Anton no doubt the software update alters fuel pressure.... likely just as it is turned off. Or are they stopping fuel from returning to the tank (to hold rest pressure high enough to keep it from boiling) via the pump?
Last edited by JohnLane; Mar 27, 2026 at 07:34 AM.
Newbie
Joined: Jan 2026
Posts: 14
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From: İstanbul
W220 s500 w215 cl55 w639 viano w218 cls350
There may be a boiling of gasoline on the fuel rail, but this only causes a long starting time. Even if the crank sensors start when it heats up, it does not work. When the engine is hot, pour some water into the crank sensor and try to start. Observe whether it works when it cools down. Your fuel pressure is at the ideal level.
Senior Member
Joined: Feb 2014
Posts: 268
Likes: 47
From: DFW
2006 C55
There may be a boiling of gasoline on the fuel rail, but this only causes a long starting time. Even if the crank sensors start when it heats up, it does not work. When the engine is hot, pour some water into the crank sensor and try to start. Observe whether it works when it cools down. Your fuel pressure is at the ideal level.
It's not just a long start. Sometimes the car will fire then die. Usually, it'll fire up just fine on the second start attempt.
Updating the software helped mine.
MBWorld Fanatic!
Joined: Feb 2012
Posts: 2,667
Likes: 1,238
From: Phoenix
222 S-65
There may be a boiling of gasoline on the fuel rail, but this only causes a long starting time. Even if the crank sensors start when it heats up, it does not work. When the engine is hot, pour some water into the crank sensor and try to start. Observe whether it works when it cools down. Your fuel pressure is at the ideal level.
