very intermittent no starts

only during hot weather above85 degrees,after longer drives(15mi.or more) or numerous short re-starts,car cranks but won't turn over or start but as soon as in gear will stumble and die.After 15-20 minutes starts and runs fine.It's only happened 8or9 times over the past three summers,over about 30,000miles.Fuel pump pressure is ok. getting spark,so i replaced pcm and fuel pump relays didn't that didn't fix the problem.Car is 99taurus 3.0L 97,000miles.

 

Cranking but not starting when hot can be a sign of vapor lock - fuel essentially boiling (or turning from liquid to a vapor) in the fuel lines. When things cool down and the vapor condenses the car will then crank and start.

Some things that promote vapor lock: low fuel pump pressure, leaky injector(s), low vapor pressure fuel, excessive engine compartment temps.

 

thanks for your reply Formula,one more question,what do you mean by" low vapor pressure fuel".

 

I would look more towards electronics that are being overly heat-sensitive, items like ignition modules, etc often start to fail in this manner.

Vapor lock is almost unheard of in modern fuel-injected systems with in-tank fuel pumps.

 

Ditto Mr. Tow Guy
These engines are more prone to ignition type heat related problems

But we need to confirm that the pressure is "OK"
You checked the fuel pressure during the no-start and it was up to specs?
What was it?

Same with spark
Getting spark at the plugs during no-start?

You should have some codes waiting in the computer for you
Can you get those read?
(even an all-clear tells us something)

 

My $.02 I just had a similar starting problem with a Tercel the Coolant Temperature Sensor, commonly confused with the sending unit for the gauge. The CTS sends signals to the computer to tell it when and how to fire the fuel injectors. After checking fuel pressure and spark(I installed inline spark testers on all 4 cylinders and installed a pressure guage to the fuel line so I could check pressure and spark at the time of failure. To verify the CTS I drilled 3 holes in a thin strip of molding, installed the old CTS, a thermometer and a new CTS and set the molding over a pan of water on the stove and heated the water to simulate the engine warming to operating temp, as the temperature increased I measured the resistance with an ohm meter comparing the old with the new, they were consistent most of the way up to boiling, however the old one went crazy once on the way up and several times while it was cooling back down. I replaced it an no longer have a problem. Sorry for being so long winded that was more than $.02. Have a good one. Geo

 

had same symptoms on '94 taurus

it was multifunction relay

make sure you know what the fuel pump sounds like w/
key on engine off. you get about a 2 second burst. listen for that next time.

find service port on fuel rail. With car running fine, shut off car and press on valve stem. Remember spray intensity.

next time you have no start, listen for the pump and try to spray gas as before. If you hear the pump and get the same spray pattern, gas is not your problem.

If gas is your problem, beat on the relay. On '94 it is under emissions labels above radiator.