What is the Theoretical Cause of the HOT START issue?

[DMC5180]

Since we see the this issue all the time and tend to give the same basic answers to fix it. How about an explanation of the Root Cause of the HOT START issue.

Here is my theory.

10 minutes after shutdown.

You have a Hot engine with circulating Hot fuel normally under pressure.
The system has an internal leak (Accumulator or PPR o-rings or FP check valve).
System pressure fades quickly after shut down (fuel is returned to tank).
Due to loss of Pressure the Fuel Vaporizes within hotter parts of the system essentially causing a vapor lock.
Because the engine is HOT, you normally are without the aid of the CSV to purge vapor in the FD.

Now the fun part, theoretically when you turn the key the pump will run briefly re-pressurizing the system, it's doesn't run long enough to fully pressurize it. You start cranking. the FP is still pressurizing the system and after a few seconds the fuel SHOULD be spraying into the cylinders to fire,but it doesn't fire. WHY? possibly still too much vapor in the system?

An hour and a half later the engine temp is down, fuel temp is down, the CSV is operational and the engine fires right up.

Please shoot holes in and/or fill in the blanks to my theory ? I just want to make sure all the FACTS are in place.

[Spittybug]

I think you are missing one key point.... the lack of good fuel pressure means that the air deflection plate has no resistance and the rush in of air deflects it a long way, tricking the car into injecting way too much fuel into the cylinders. Basically it floods.

[DMC5180]

I think you are missing one key point.... the lack of good fuel pressure means that the air deflection plate has no resistance and the rush in of air deflects it a long way, tricking the car into injecting way too much fuel into the cylinders. Basically it floods.

This is a NEW one I had not considered. Lets see if the other experts agree. I'm trying to build a consensus.

Just thinking here, it requires about 45 PSI to force Open the Mechanical Injectors. So lack of fuel pressure would also mean there isn't enough pressure to open the Injectors too Flood it.

[Chris4099]

It has to do with the fuel lines going to the injectors. They are not part of the looped system. So it doesn't matter how much priming you do with the pump before starting as that doesn't effect the injector lines (they remain filled with vapor). While you then start cranking the engine, the meter plate moves down a little bit allowing pressure to build in the injector lines and start purging out the vapor. Once the vapor is purged, fuel starts coming out and the engine fires up. Because some lines are shorter then others, the fuel comes in sooner on some injectors which is why it may sputter for a few more seconds as not all cylinders are firing. The good news is once it starts running a bit, the meter plate moves down further which increases the rate the lines are purged. This is also why a cold engine has less issues because the CSV makes the engine roar to life and very quickly purge out the vapor. This doesn't always happen which is why hot start issues can also cause the engine to quickly die when cold starting and require a few attempts at restarting (at least when I've had a bad PPR o-ring or accumulator).

FYI, this is referred to vaporization per the tech manual, not vapor lock (which is a carb issue).

[Bitsyncmaster]

It has to do with the fuel lines going to the injectors. They are not part of the looped system. So it doesn't matter how much priming you do with the pump before starting as that doesn't effect the injector lines (they remain filled with vapor). While you then start cranking the engine, the meter plate moves down a little bit allowing pressure to build in the injector lines and start purging out the vapor. Once the vapor is purged, fuel starts coming out and the engine fires up. Because some lines are shorter then others, the fuel comes in sooner on some injectors which is why it may sputter for a few more seconds as not all cylinders are firing. The good news is once it starts running a bit, the meter plate moves down further which increases the rate the lines are purged. This is also why a cold engine has less issues because the CSV makes the engine roar to life and very quickly purge out the vapor. This doesn't always happen which is why hot start issues can also cause the engine to quickly die when cold starting and require a few attempts at restarting (at least when I've had a bad PPR o-ring or accumulator).

FYI, this is referred to vaporization per the tech manual, not vapor lock (which is a carb issue).

Correct.

The CSV can get it going by getting the engine to fire a little which causes the FD plate to move more and let the vapor purge from the injector hoses. It runs like crap until all the lines are cleared of vapor.

It takes 1.5 seconds of prime to get full pressure (45 PSI same as rest pressure). So you do get full pressure with the pump prime. It just proves that the injector hose vapor is the problem.

[DMC5180]

Correct.

The CSV can get it going by getting the engine to fire a little which causes the FD plate to move more and let the vapor purge from the injector hoses. It runs like crap until all the lines are cleared of vapor.

It takes 1.5 seconds of prime to get full pressure (45 PSI same as rest pressure). So you do get full pressure with the pump prime. It just proves that the injector hose vapor is the problem.


So If I DON'T have a hot start issue, that means the fuel in the injector lines are NOT vaporizing. IF I did have hot start issues it would mean it is vaporizing? Where does the hot vaporizing Expanding fuel go. Wouldn't it need to vent backward into the FD, assuming it can't get past the injectors. I always though the the FD prevented back flowing from the injector lines via internal check valving separating upper and lower chamber pressures.

[David T]

Hot Restart is caused by a lack of rest pressure. When the motor is hot the CSV won't operate to get the motor to run long enough for the rest pressure to get high enough to open the primary injectors. The causes for low rest pressure are typically:
Bad accumulator
Leaky PPR
Leaky check valve on the fuel pump
External fuel leaks.
The rest pressure is a pressure vs time spec. It is not meant to stay high for a long time. Eventually the motor will cool off enough so that the CSV will operate and then the motor will start. The CSV does not require high pressures to operate. That's why the plug swap works. For a much better explanation read the Workshop Manual and another great publication is Bosch Fuel Injection & Management by Probst. Try Amazon or Motorbooks.
David Teitelbaum

[DMC5180]

David,

I mentioned the lack of CSV in the first post due to Hot engine. Dave M. says the prime function will bring rest pressure to full a 45PSI in 1.5 seconds. Assuming one lets the pressure build before engaging the starter. I should fire right up correct.

[Bitsyncmaster]

My guess would be if you press the air plate after you prime the pump with a hot start condition, it would start. It would still take a while to clear all six injector hoses. In the summer, the CSV will never fire after a drive (maybe an overnight rest). I have found with my car, summer sunny day you need to hold rest pressure for 4 hours to not have the hot start issue. You will have a problem about 30 minutes after you loose all rest pressure.

[David T]

The pump prime during starting is not enough to restore rest pressure. If you jumper the RPM relay and run the pump longer that can restore rest pressure but depending on how bad it loses pressure the car may not start. That is why the plug swap is so good. If it starts right up withe the plug swap then it confirms you are not holding rest pressure. Doesn't tell you why it won't hold but at least you know what system and what part of the system (fuel) is the problem. Actually the only way you will know what the problem is, is by the process of elimination. Starting with the accumulator. Next would be the fuel pump check valve and then the PPR. This is assuming you have no leaks but you would smell that!
David Teitelbaum

[Bitsyncmaster]

The pump prime during starting is not enough to restore rest pressure. If you jumper the RPM relay and run the pump longer that can restore rest pressure but depending on how bad it loses pressure the car may not start. That is why the plug swap is so good. If it starts right up withe the plug swap then it confirms you are not holding rest pressure. Doesn't tell you why it won't hold but at least you know what system and what part of the system (fuel) is the problem. Actually the only way you will know what the problem is, is by the process of elimination. Starting with the accumulator. Next would be the fuel pump check valve and then the PPR. This is assuming you have no leaks but you would smell that!
David Teitelbaum

My testing showed about 1.0 seconds to bring the rest pressure from zero PSI to 45 PSI on the control pressure. Did you find your testing was different?

[Citizen]

The pump prime during starting is not enough to restore rest pressure. If you jumper the RPM relay and run the pump longer that can restore rest pressure but depending on how bad it loses pressure the car may not start. That is why the plug swap is so good. [SNIP]

David Teitelbaum

I read this thread with great interest, and I appreciate all the expert knowledge here, but let’s step back from theory for just a moment please, and go to the real world. I’m out stuck somewhere with apparently the hot start issue. What do I do to get started again?

1) Prime the fuel system by jumpering the RPM relay, not by using the ignition key.
2) Execute the “plug swap procedure” AND/OR manually press the meter plate open while starting (latter must require two people).

Is that it?

For us fuel system novices, would you elaborate on the plug swap procedure? What plug is this referring to, where is it, and what tools do I need to carry with me in the car to effect this successfully out in the wild?

...

[David T]

Every owner should know how to do this. You remove the plug from the Control Pressure regulator and the plug from the Cold Start Valve. You take the plug from the CPR and put it on the CSV. (You do NOT have to put the CSV plug on the CPR). You start the car and as fast as you can you remove the plug from the CSV and put it back on the CPR. Then you put the plug back on the CSV and you can drive away. You do not have to push the air sensor plate, prime the fuel pump, remove the air cleaner, etc. It is VERY IMPORTANT you DO NOT leave the CPR plug on the CSV. That will make the CSV run constantly filling the motor up with raw fuel. You can imagine for yourself what can happen. It is better if you have 2 people do it so you can get the plug off the CSV quickly but 1 person working fast can do it. A common quick fix by DPO's (Dreaded Previous Owners) is to wire a switch up to the hot start relay plug near the fuse box so you can fire the CSV from in the car. If you do it at least use a momentary switch so it can't stay on. This is just an emergency procedure, the problem causing this condition should be fixed.
David Teitelbaum

[Bitsyncmaster]

Or you can get my HOT START relay. It bypasses the thermal time switch and energizes the CSV after 1 second of engine cranking. It stops the CSV when you stop cranking the engine so it may not clear the lines as quick as the plug swap.