MFI warm start issue

[Boulder Bob]

years ago I had a problem with my '73S with stock MFI - acted like vapor lock. when cold, the car starts OK. when warm (doesn't have to be a hot day), after sitting for 15 minutes the car won't start (just cranks normally). If the car sits for a few hours, it starts normally. I fixed the problem by replacing the "check valve" (overflow valve 901 110 915 00). I got a used one and and it fixed the problem.

As I understand it, the fuel system from the pump under the car to the injection pump needs to hold pressure and if it doesn't the injector pump will not deliver fuel to the injectors. Is this correct? I also understand that there is another check valve in the pump.

So now the problem is back. Pelican wants $242.75 for this part. Is the valve getting gunked up? Can I soak it in solvent? Can I test it? Is the one at the pump under the car accessable?

Has anyone else had this problem?

Why does letting the car cool off solve the problem?

If there is already a thread on this sorry to be asking, maybe someone can provide the link - it's a tough one to search for.

Thanks.

[RSTarga]

Most of the problem is associated with the ethanol now in our fuel and, in your case with altitude compounding the problem. You can alleviate the problem by holding the accelerator pedal down while cranking. When you hear the engine catch, release the pedal.
The fuel vaporizes in the steel lines when the engine is hot. You need to get colder fuel through the system which can then be injected into the cylinders.
CSI cars have an accumulator that hold pressure, I think MFI just has a check valve so the fuel only flows one way. Save your money and practice starting with your pedal down.

[Flieger]

I don't see what ethanol has to do with anything.

My car does this too. It is because the engine has cooled off enough to need enrichment, but has not cooled off enough that the temperature switch allows the cold start squirters to be activated. Means you need to put your foot down and crank crank crank, or else put a button on the dash to manually operate the cold start squirters. I have done the former and am having TLG do the latter.

[RSTarga]

Max,
In the old days we called this vapor lock. It was always a problem at higher altitudes in high temperatures, ethanol makes it worse because it converts to vapor at lower temperatures.
Injecting fuel through the squirters accelerates the process for getting colder fuel into the cylinders.
This is another reason why MFI has a return line to the tank, the pump is constantly recirculating fresh cooler fuel to the system.

[Flieger]

I know what vapor lock is, but you want vaporized fuel for a cold engine. It is the condensation of fuel on the cold walls of intake ports etc. that leans out the mixture enough that you need a choke to start the car. Liquid fuel doesn't burn, vapor does. It is true the injection pump won't pump vapor, but you want vapor in the chamber. That is why you can't start an engine on alcohol at low temperature- its vapor pressure is too low. Gasoline, being a cocktail of various hydrocarbons, will have lighter fractions like propane and butane that provide sufficient vapor pressure to start at -40. This is the reason there is winter and summer gas. Light fractions are expensive so they save them for winter when it is needed, rather than waste them in the summer when they would just evaporate away.

And anyway, I don't think vapor lock is occuring, at least not in my case. It hasn't happened to me in a while as I haven't shut the car off for only 10 minutes since last summer I think.

Dirty injectors will also exacerbate the problem.

Max,
In the old days we called this vapor lock. It was always a problem at higher altitudes in high temperatures, ethanol makes it worse because it converts to vapor at lower temperatures.
Injecting fuel through the squirters accelerates the process for getting colder fuel into the cylinders.
This is another reason why MFI has a return line to the tank, the pump is constantly recirculating fresh cooler fuel to the system.

[RSTarga]

As you said, the fuel pump will not pump vapor, and the MFI cannot meter it, or pressurize it to inject. Both need liquid fuel to do their jobs.
This problem also occurs with Carburetted engines, how does vapor move through the jets?

[Flieger]

The electric pump can't pump vapor either and on my car it is in the back. The pump doesn't sound any different. It just doesn't strike me as a vapor lock issue. The solenoid on the fuel console can't push vapor either. Yet I don't have to wait for it to cool, just pump more fuel.

[Boulder Bob]

thank you all for your input

my warm or hot start procedure has always been: get in the car, turn the key until the fuel pump comes on, put on my seat belt (giving cool fuel a chance to circulate), put the throttle to the floor, and crank until it catches, which is usually pretty quick, except on a hot day when the car has been sitting and I do need enrichment that maybe I'm not getting. this method has worked well for many years.

I know what it's like when the enrichment isn't working - you crank and crank and eventually it catches and barely runs for a few seconds, then runs well - this is not the the problem I am having - in my situation the car will never start until it sits for hours. Like I said, it did this exact thing 10 years ago and replacement of the valve solve the problem.

Can anyone answer the questions in my original post?

[Flieger]

Well if it takes hours that does sound like vapor lock. Mine was not.

[bliebler]

Could you be flooding it while cranking? I know this is frustrating because my car does the same and I use non ethanol. Flieger may be onto something about the car needing enrichment of raw gas from the squirters. This lack of cold fuel causes you to crank and then flood. Try after a hot run, leave your car in hot garage with engine lid closed for a few hours. open the lid jump wire 12v lead to the valve on top of the fuel filter for a few seconds(try 5sec to really pump allot of raw fuel) start the car. If I do this the car starts right up but smokes at start a little from the raw fuel.