SL/R129: SL320 M104 Hard Hot Start
Thread Starter
Junior Member
Joined: May 2009
Posts: 25
Likes: 3
From: Montreal, Canada
2017 GLC43 AMG
SL320 M104 Hard Hot Start
Have a problem starting the car when it is hot/warm and need some help.
When I say warm it is after shutting the engine down, not air temp. Starts great and runs great when I fire her up in the morning or if it has been sitting for a several hours and has time to cool down. Cranks over for several revolutions with nothing firing, usually holding the accelerator down she fires up a little rough but smoothes out in no time and then runs great. This car is well taken care of, mint condition with 38,000 miles on the counter. Plugs, filters Maf/map sensor are all like new. Anything come to mind?
When I say warm it is after shutting the engine down, not air temp. Starts great and runs great when I fire her up in the morning or if it has been sitting for a several hours and has time to cool down. Cranks over for several revolutions with nothing firing, usually holding the accelerator down she fires up a little rough but smoothes out in no time and then runs great. This car is well taken care of, mint condition with 38,000 miles on the counter. Plugs, filters Maf/map sensor are all like new. Anything come to mind?
Senior Member
Joined: Apr 2019
Posts: 428
Likes: 96
From: Houston
001 s600, 94 sl600
Hard starting when hot soaking
Have a problem starting the car when it is hot/warm and need some help.
When I say warm it is after shutting the engine down, not air temp. Starts great and runs great when I fire her up in the morning or if it has been sitting for a several hours and has time to cool down. Cranks over for several revolutions with nothing firing, usually holding the accelerator down she fires up a little rough but smoothes out in no time and then runs great. This car is well taken care of, mint condition with 38,000 miles on the counter. Plugs, filters Maf/map sensor are all like new. Anything come to mind?
When I say warm it is after shutting the engine down, not air temp. Starts great and runs great when I fire her up in the morning or if it has been sitting for a several hours and has time to cool down. Cranks over for several revolutions with nothing firing, usually holding the accelerator down she fires up a little rough but smoothes out in no time and then runs great. This car is well taken care of, mint condition with 38,000 miles on the counter. Plugs, filters Maf/map sensor are all like new. Anything come to mind?
I had for several years a '71 280 SE with Bosch mechanical injection,and high performance Euro Spec 2.8 L straight 6, and it had a thermostatically activated starting valve that squirted extra fuel into the intake manifold on cranking. It had well over 100,000 miles and slowly developed harder starting due to wear in the injector pump cylinders (tolerances were many times smaller than for diesel injecting pumps and the Bosch reman shop in South SanFrancisco could not work to ultra tight tolerances. He advised to 'run it' as delivery curve was spot on except at the slow cranking speed of the very high compression engine). I had the block and head including cam and rockers regrind, meticulously restored to as new with new bores and pistons and rings, and new valve guides. All the bearing surfaces measured within new manufacturing limits so replacement bearings were original specs -each individual shell selected for each journal to minimum spec clearances. I eventually installed a manual spring- loaded toggle switch so one could just pull the toggle a second B4 cranking and it fired by 3rd piston when hot and 1 or 2 when cold.
Thread Starter
Junior Member
Joined: May 2009
Posts: 25
Likes: 3
From: Montreal, Canada
2017 GLC43 AMG
It sounds like fuel vaporization in the fuel lines--due to a leaking check valve at the pump--allowing fuel pressure to drop on standing. Used to be common when carburetors were the means or the mechanical fuel pumps had a leaky check valve and would not hold any pressure on fuel. You have to do a test--attaching a fuel pressure gauge at the engine and watching the system pressure when shut down hot over 30-60 min to see if it drops. IHMO is it at least 75% probable that is the problem.
I had for several years a '71 280 SE with Bosch mechanical injection,and high performance Euro Spec 2.8 L straight 6, and it had a thermostatically activated starting valve that squirted extra fuel into the intake manifold on cranking. It had well over 100,000 miles and slowly developed harder starting due to wear in the injector pump cylinders (tolerances were many times smaller than for diesel injecting pumps and the Bosch reman shop in South SanFrancisco could not work to ultra tight tolerances. He advised to 'run it' as delivery curve was spot on except at the slow cranking speed of the very high compression engine). I had the block and head including cam and rockers regrind, meticulously restored to as new with new bores and pistons and rings, and new valve guides. All the bearing surfaces measured within new manufacturing limits so replacement bearings were original specs -each individual shell selected for each journal to minimum spec clearances. I eventually installed a manual spring- loaded toggle switch so one could just pull the toggle a second B4 cranking and it fired by 3rd piston when hot and 1 or 2 when cold.
I had for several years a '71 280 SE with Bosch mechanical injection,and high performance Euro Spec 2.8 L straight 6, and it had a thermostatically activated starting valve that squirted extra fuel into the intake manifold on cranking. It had well over 100,000 miles and slowly developed harder starting due to wear in the injector pump cylinders (tolerances were many times smaller than for diesel injecting pumps and the Bosch reman shop in South SanFrancisco could not work to ultra tight tolerances. He advised to 'run it' as delivery curve was spot on except at the slow cranking speed of the very high compression engine). I had the block and head including cam and rockers regrind, meticulously restored to as new with new bores and pistons and rings, and new valve guides. All the bearing surfaces measured within new manufacturing limits so replacement bearings were original specs -each individual shell selected for each journal to minimum spec clearances. I eventually installed a manual spring- loaded toggle switch so one could just pull the toggle a second B4 cranking and it fired by 3rd piston when hot and 1 or 2 when cold.
Thread Starter
Junior Member
Joined: May 2009
Posts: 25
Likes: 3
From: Montreal, Canada
2017 GLC43 AMG
SOLVED!
Took some time but eventually after replacing x amount of parts you finally nail it. Was not the crank position sensor by the way. Only reason I mention this is it was mentioned numerous times on various forums that this was most likely my problem. My problem threw no codes FYI. Read about the fuel regulator on a forum discussing another model Mercedes with the same engine so I said what the heck, give it a try. Bingo, starts instantaneously all the time now. Must assume it was simply leaking and allowed fuel into the intake when hot creating an over rich mixture. Bought this one, works like a charm. Very easy to change, sits on top of the engine right hand front side, stares you right in the face. Takes all of 5 minutes to swap out. This part also is in the SL500 and limited SL600's, take a look at the link where it lists all models and years where it is used.
Bottom line, if it hesitates to start when warm, does fire up after a few rotations of the engine as opposed to a touch of the key and then runs rich for a few seconds until the richness is burned off but starts perfectly when cold, the fuel pressure regulator is most likely the problem.
This video is really helpful.
Took some time but eventually after replacing x amount of parts you finally nail it. Was not the crank position sensor by the way. Only reason I mention this is it was mentioned numerous times on various forums that this was most likely my problem. My problem threw no codes FYI. Read about the fuel regulator on a forum discussing another model Mercedes with the same engine so I said what the heck, give it a try. Bingo, starts instantaneously all the time now. Must assume it was simply leaking and allowed fuel into the intake when hot creating an over rich mixture. Bought this one, works like a charm. Very easy to change, sits on top of the engine right hand front side, stares you right in the face. Takes all of 5 minutes to swap out. This part also is in the SL500 and limited SL600's, take a look at the link where it lists all models and years where it is used.
Bosch 0280160587
https://www.eeuroparts.com/Parts/535...ar-0280160587/Bottom line, if it hesitates to start when warm, does fire up after a few rotations of the engine as opposed to a touch of the key and then runs rich for a few seconds until the richness is burned off but starts perfectly when cold, the fuel pressure regulator is most likely the problem.
This video is really helpful.
Last edited by Midnightsun; May 4, 2021 at 11:40 AM.
Member
Joined: Mar 2008
Posts: 79
Likes: 10
From: San Diego, CA or Sequim Washington USA
'95-E320 COUPE, 2000 SL500, 2002 ML320
Thanks for your post and video. I have a 95 E320 coupe with replacement wiring harness, both upper and lower. I'm hunting down a very similar hot start issue. My hot start issue is when the car was driven and then parked outside in the sun for more than 3 hours, but if the car is restarted in a hour or so I starts just fine. My fuel system pressure holds at 50psi without dropping over 3 hours. Temp sensors checked good, ignition system checked ok. The local mechanic is suggesting the throttle body would be the issue. Dose this make sense? Any other suggestions?
