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?

[Mr9146]

I 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.

This is the correct answer.

[edmayo]

Rig a temporary jumper wire from a hot fuse at the 3 fuse panel at the relay board to the terminal of the cold start solenoid on top of the fuel filter. Then when the start problem occurs just momentarily touch the jumper to the fuse. That will squirt fuel through the squirt nozzles. (be careful to ONLY use momentary connections,,,,you can flood the hell out the engine quickly!!) This will tell you if your problem is lack of fuel squirt. Realize above 114 degree engine temp the cold start doesn't work anyway.

[RSTarga]

Ed,
I've had a couple of 6.3 MB that have the same MFI system and suffer terribly from this problem. Ultimately someone came up with a thermo timer relay kit that over rides the hot shutoff of fuel and permits the enrichment circuit to operate at high temps. I think it gives 2 seconds or so of fuel when starting hot.
Is this something that could be made for our cars?

[2.5MFI]

[QUOTE=Boulder Bob;723934]
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.

The fuel system does not need to hold fuel pressure between the MFI pump and electric pump like the CIS system does when the engine is not running. The electric fuel pump in the MFI cars does this. Your check valve/connector (901 110 915 00) on the fuel filter console is the valve that keeps some fuel (not under pressure) in the lines between the MFI pump and filter console after the engine is turned off from draining back into the fuel tank. That valve/connector is the return fuel line back to the tank along with the electric fuel pump via a tee. If that check valve was leaking I think it would just add a few seconds of the electric fuel pump running to build it's pressure up. The actual fuel pressure relief valve is on the electric fuel pump.