SL/R107: 1975 450 SL hot start
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Joined: Aug 2019
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1975 450 SL
1975 450 SL hot start
So I have been troubleshooting the dreaded intermittent hot start issue on this beast of a 1975 450SL for quite some time. I’m sure there are other forums about the vapor lock on this particular year but not totally convinced that is causing my symptoms as follows:
1. Car always cranks hard
2. Oil pressure appears low at first, As the engine temperature slowly decreases, the oil pressure increases when cranking. Finally the engine starts and oil pressure is pinned all the way and drives perfectly.
2. It’s very intermittent and worst on extremely hot days/when parked directly in sun for long period)
3. Car ALWAYS starts when the engine temperature cools down enough (sub 150 degrees or less typically, I usually leave the hood open for awhile and it cools down enough)
2. Using Non ethanol gas does seem to help (Should have higher boiling point in fuel lines, I wrapped the fuel lines in heat repellent insulation as well) Still occasionally have the issue
5) Car always runs smooth when it finally starts
Would vaporlock cause the oil pressure to be low? Could the oil be thinner when hot and therefore less pressure and is there some reason the car would not start below a certain oil pressure? Has anyone had any success with some kind of bleed valve to help relieve vapor lock?
Sorry for the long response and all the questions but just trying to paint the full picture. Any advice on the best way to make this a more reliable start would be greatly appreciated. Happy Driving!
1. Car always cranks hard
2. Oil pressure appears low at first, As the engine temperature slowly decreases, the oil pressure increases when cranking. Finally the engine starts and oil pressure is pinned all the way and drives perfectly.
2. It’s very intermittent and worst on extremely hot days/when parked directly in sun for long period)
3. Car ALWAYS starts when the engine temperature cools down enough (sub 150 degrees or less typically, I usually leave the hood open for awhile and it cools down enough)
2. Using Non ethanol gas does seem to help (Should have higher boiling point in fuel lines, I wrapped the fuel lines in heat repellent insulation as well) Still occasionally have the issue
5) Car always runs smooth when it finally starts
Would vaporlock cause the oil pressure to be low? Could the oil be thinner when hot and therefore less pressure and is there some reason the car would not start below a certain oil pressure? Has anyone had any success with some kind of bleed valve to help relieve vapor lock?
Sorry for the long response and all the questions but just trying to paint the full picture. Any advice on the best way to make this a more reliable start would be greatly appreciated. Happy Driving!
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Joined: Aug 2008
Posts: 689
Likes: 30
Hot start problem on the Djetronic 107s, is very common and due to overly lean mixture. When under hood temperatures are high after car has been sitting in sun, the fuel in the fuel rails is also hot. The fuel pressure is normally about 29 psig. When this passes through the injector into the manifold (that would be at about atmospheric pressure), some of the hot fuel vaporizes. This reduces capacity of the injector and causes less fuel to be injected. As a result, you get a very lean mixture. Winter fuel that contains more volatile components vaporizes more easily. We start to get this in early Fall and it is still around in Spring and is a problem if we happen to get an hot day. Ethanol free gas probably helps.
You can overcome hot start problem by increasing the fuel pressure. (less will vaporize). But if you do, the engine will run rich under normal conditions. I set my pressure at about 32 psig. However, I have a wide band AFR gauge installed and I adjusted mixture (MPS and ECU) to compensate for the higher pressure. I almost never have a hot start problem, but I do see leaner mixtures on my gauge until fresh fuel from tank starts to flow.
There are other things that help. For example, temporarily disconnecting the air temperature sensor will cause a richer mixture and can help. Some owners put in a switch to do this.
Cheers
You can overcome hot start problem by increasing the fuel pressure. (less will vaporize). But if you do, the engine will run rich under normal conditions. I set my pressure at about 32 psig. However, I have a wide band AFR gauge installed and I adjusted mixture (MPS and ECU) to compensate for the higher pressure. I almost never have a hot start problem, but I do see leaner mixtures on my gauge until fresh fuel from tank starts to flow.
There are other things that help. For example, temporarily disconnecting the air temperature sensor will cause a richer mixture and can help. Some owners put in a switch to do this.
Cheers
Last edited by 107123210; Aug 24, 2019 at 09:08 PM.
