So I recently found this great Z car mechanic right near where I live and asked him some questions about my ZX, he was really insightful for every question, i had, one of them was about this problem I've been having since I got my car, which is, if I warm my car up, park it, and then go to start it about 20 mins later, the engine would run like crap for about 30 seconds, and if I tried to accelerate, the afm or something would flap closed to prevent backfiring, basically I was getting vapor lock.

He told me its because the gas used when my car was brand new was different, I cant remember how exactly it was different but basically the gas today evaporates at a lower temp then in the old days, so that cooling fan that swoops over the valve cover doesn't activate soon enough to cool off the injectors, he told me a really easy solution to this, there is a wire that activates the fan for like 17 minutes, and if you ground out the female end of it, it activates the fan, so he told me to just ground out that end whenever I think im going to get vapor lock.
I've been doing it ever since, and I haven't had vapor lock at all, soon I am going to make it fancy so I don't have to keep unplugging and plugging it back in, and put a switch on it. it's also useful if you forget to plug it back in and the temp senses it's getting too hot, it wouldn't be able to tell the fan to turn on because the wires not connected and your injectors might burn up. Im going to post pictures soon, to show exactly where the plug is.

 

None of that makes sense...

Gasoline might be produced differently than in 1981, but it's not "that different". Otherwise every car made before 1981 would be having your same problem (I haven't seen all these stranded pre-81 cars on the side of the road if so).

Also, the aux fan isn't activated by FUEL TEMP, it's activated by COOLANT TEMP. This is clearly stated in the Haynes & FSM. Unless water (coolant) has also magically changed it's realm of holding temperatures since 1981...

You "may" be getting vapor lock (I kinda doubt it), but I'm just telling you that those 2 things he told you are 100% bogus.

Vapor lock is usually a problem on carb'd motors, because the fuel pressure is 5-10psi. Fuel injected cars are 35psi+. Now, if you are having a vapor lock problem, I suggest finding the REAL culprit.

 

Unfortunately there are a lot of sensors and electronics even on cars from the 70's and 80's and you need to have good trouble shooting skills to figure out what is wrong a lot of the times. There are a lot of mechanics that can replace the part once they know what's wrong but they just don't have a clue other than a little bit of hearsay they pick up and the mechanic you were dealing with is one of those.

It sounds like a heat issue. Most of what makes your engine properly is the sensors and the control from the ECU. The ECU can from time to time run intermittantly but usually it's the wrong information from the sensors that causes the problems. Check the sensors that would be directly affected by the head (idle control valve, IAT, etc) and those indirectly affected (eg thermal expansion causing poor contact or other false readings). Check the things that directly are affected by heat first because they have a greater effect on the system and they are usually more repeatable (eg every time you let it get hot then sit for 20 minutes it runs rough).

Connectors are easy things and should be checked anyhow. Grounds cause all sorts of issues and are usually an easy check as well. The AFM is a common wear/problem item if you haven't rebuilt it already.

More info would help if you're trying to get help on the forum. Does it run rough at low loads/rpm but OK once you get up to speed, etc?

 

dangit, i had a response that explained things really well but for some reason it didn't post it so ill try and retype what i said.
here, if you don't trust this guys advice you can do some research, on his company, i don't think they have a website, but there's people who have talked about him on the web, just search "tacoma fairlady z motors" in google.
anyway for the fuel, i should have specified that it just comes from a coolant sensor, I know theres no fuel temp sensor, but that doesn't matter lets say (and these are not accurate figures) gas of the 80's vaporizes at like 300 degrees, gas of today does it at like 270 degrees, and the fan activates at a coolant temp of like 300 degrees, if the engine gets to 290 degrees the fuel of today would have already vaporized, but the fan did not turn on because fuel of the old days would not have vaporized yet.
and my car has never died from this sputtering, it just runs like crap for about 30 seconds, until things are cleared out.

 

don't search tacoma fairlady z motors, search university place fairlady z motors

 

From this statement (found here: http://en.wikipedia.org/wiki/Vapor_lock), gasoline used to evaporate at a lower temp...

Here is another good link to review: http://en.wikipedia.org/wiki/Boiling_point

The boiling point increases as the pressure rises... if your fuel pressure is 35psi, it shouldn't be evaporating / vapor locking.

By doing some simple science experiments, you could actually verify if it is vapor lock causing your problem. I would start by using an infrared thermometer & measuring the temp of the fuel rail, also measure the fuel pressure, and lastly, take a sample of fuel at the regulator (it will be in a liquid or gaseous state).

 

if you don't know the anwer then baffle them with BS

 

so it's saying that boiling point goes down as volatility goes up? interesting

 

Exactly. Meaning the less heat (energy) it takes to evaporate, the more volatile it is...

So in this chart (note that temps are in C not F), propane is more volatile than 2-heptene because it has a much lower boiling point:



Long story short... gasoline @ 35psi would have a pretty high boiling point.

From "googling", looks like the boiling point of gasoline has a range (at 1atm) of 100-400*F depending on grade and quality of the fuel:

http://www.engineeringtoolbox.com/fu...int-d_936.html
http://www.elmhurst.edu/~chm/vchembo...oilingpts.html
http://en.wikipedia.org/wiki/Gasoline

There is tons of info on all this... so I say "dig in" and let me know if you find anything stating gasoline these days has a lower boiling temp. And all research aside, I suggest doing the experiments I mentioned above.

 

Don't forget when you turn your ignition to the on position you are bringing your fuel presure up. The regulator will send excess fuel back to the tank which is also vented into the carbon canister. If there is somehow vapor in your line its going to get pushed out before you even start the system. If there is any residual that is on the injector side it will get forced out through the injectors on the first few milli seconds of pulsing. There are plenty of places for the vapor to go so I'm not sure how it would vapor lock..... At worst if the fuel boiled out of the lines it would be a few seconds before the whole system is flushed with new fuel and any vapor sent to the tank or into the intake runners on the back of the valves.....