Weirdest thing: I replaced cold start valve and air regulator to fix a cold start problem, and now the car starts right up when cold and idles nicely at 1100 and slowly drops to that 600rpms smooth as butter. Car runs beautifully during warm up and at full temp. The only problem is if I park the car after it has warmed up and come back anywhere from about 30min to 2 hours later, it stalls at startup, sputters out whilst pulling away, or struggles to survive unless i either hold down the gas a bit for a while and rev it to 1000rpms or just give it a little gas when shifting to drive, which it doesn't like that much either, but at least it doesn't die. sometimes this lingers and i've been stuck at a stop light when it changes because the damn things sputters out. once it warms up again, no problems. Is it a faulty air regulator? I also just found out my oil pump is leaking, which I'm going to replace next, no chance these are related, I assume?

 

Check fuel pressure, timing, air regulator, ignitor, CHTS, and AFM.

 

Vapor lock. Thats why they put the fi fan on top of the motor. It should help if when you park you just pop the hood a bit. How do i know this?? Have same problem as you..
You could also turn your key off and on about 10 times before you attempt to start it, doing this helps the fuel pump push the hot gas back to the tank..
I just did a search on vapor lock, alot of interesting reads.. Someone said just put the pedal to the metal when cranking it while hot. I'll try that next time i run my car for awhile.

 

If you have the fuel pump running (since it recirculates back to the tank) and if the injectors are firing I can't imagine it would take too long to get rid of any vapour in the system....

 

^^^ I also doubt it's much warmer than 40*F in NJ right now... Vapor lock in the winter, on an EFI system? Hurm...

 

I've got vapor lock issues with mine as well, though I am running a turbo. If it sounds like a Subaru for a few seconds until you get going on the road, that's what it is. Make sure your injector fan is working properly, or install one if you don't have it.

 

Lets see he states the car is running fine cold or hot. It is only when he parks it for 30min. to a hour that it acts up.

I doubt that it is his fuel pressure,timing,air regulator, ignitor,chts and afm.
<<< Hmmmnnn..>>> sounds like a pretty generic answer without reading his post.

 

I tried the pedal to the metal while cranking it today after a spirted couple of runs. It seemed to work, you might want to try it.

 

Next time it "vapor locks" on you, will you take an infrared thermometer and measure the temp of the fuel rail? I'd like to know what the rail temp is when it's having this type of problem.

 

I'm not sure if this was directed at me or the op.
I was just trying to help the guy out so he isn't sitting at the gas station or store with the hood up waiting for it to cool a bit. Could they be other underlying problems causing this?? You betcha!! Could be leaky injectors, or partially clogged cat amongst a bunch of other things.
Not trying to get into a pissing contest against you or anyone else. Just trying to help out the op with a problem that is identical to one i'm having also and giving him a couple of options.

 

No worries. I'm just trying to understand if this is even logical - not trying to get into a bun fight.

From the wiki so take it as you will :

"Vapor lock was far more common in older gasoline fuel systems incorporating a low-pressure mechanical fuel pump driven by the engine, located in the engine compartment and feeding a carburetor. Such pumps were typically located higher than the fuel tank, were directly heated by the engine and fed fuel directly to the float bowl inside the carburetor. Fuel was drawn under negative pressure from the feed line, increasing the risk of a vapor lock developing between the tank and pump. A vapor lock being drawn into the fuel pump could disrupt the fuel pressure long enough for the float chamber in the carburetor to partially or completely drain, causing fuel starvation in the engine. Even temporary disruption of fuel supply into the float chamber is not ideal; most carburetors are designed to run at a fixed level of fuel in the float bowl and reducing the level will reduce the fuel to air mixture delivered to the engine.

Carburetor units may not effectively deal with fuel vapor being delivered to the float chamber. Most designs incorporate a pressure balance duct linking the top of the float bowl with either the intake to the carburetor or the outside air. Even if the pump can handle vapor locks effectively, fuel vapor entering the float bowl has to be vented. If this is done via the intake system, the mixture is, in effect, enriched, creating a mixture control and pollution issue. If it is done by venting to the outside, the result is direct hydrocarbon pollution and an effective loss of fuel efficiency and possibly a fuel odor problem. For this reason, some fuel delivery systems allow fuel vapor to be returned to the fuel tank to be condensed back to the liquid phase, or using an active carbon filled canister where fuel vapor is absorbed. This is usually implemented by removing fuel vapor from the fuel line near the engine rather than from the float bowl. Such a system may also divert excess fuel pressure from the pump back to the tank.

Most modern engines are equipped with fuel injection, and have an electric submersible fuel pump in the fuel tank. Moving the fuel pump to the interior of the tank helps prevent vapor lock, since the entire fuel delivery system is under positive pressure and the fuel pump runs cooler than if it is located in the engine compartment. This is the primary reason that vapor lock is rare in modern fuel systems. For the same reason, some carbureted engines are retrofitted with an electric fuel pump near the fuel tank.

A vapor lock is more likely to develop when the vehicle is in traffic because the under-hood temperature tends to rise. A vapor lock can also develop when the engine is stopped while hot and the vehicle is parked for a short period. The fuel in the line near the engine does not move and can thus heat up sufficiently to form a vapor lock. The problem is more likely in hot weather or high altitude in either case.

Gravity feed fuel systems are not immune to vapor lock. Much of the foregoing applies equally to a gravity feed system; if vapor forms in the fuel line, its lower density reduces the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so fuel supply will be disrupted until the vapor is removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by allowing the vapor to cool and re-condense.

Vapor lock has been the cause of forced landings in aircraft. That is why aviation fuel is manufactured to far lower vapor pressure than automotive gasoline (petrol). In addition aircraft are far more susceptible because of their ability to change altitude and associated ambient pressure rapidly. Liquids boil at lower temperatures when in lower pressure environments."

I don't see how our cars with a high presure fuel pump by the tank, a recirculating system with a circular fuel rail would have issues with vapour lock. Once you have the fuel pump running any vaporized fuel will get pushed back to the fuel tank or passed through the injectors wouldn't it?

I know there is a fan to blow across the plenum and everyone assumes it's to prevent vapour lock - is that the case or is that just what everyone assumes?

 

^^^ Exactly.

I've never seen anyone actually prove that the problem was vapor lock on a "280zx hard start" problem. It always turns out to be something else, like a leaking fuel pressure regulator, faulty AFM, CHTS, ignitor, etc (sensors are affected by HEAT / TEMP!).

From a previous discussion... In a nut shell, fuel at 35psi would take 400+*F heat to boil. Unless you are driving in the hottest desert in the world, and have no heat shield between the intake & exhaust manifold, the chances of vapor lock are VERY slim.

https://www.zdriver.com/forums/280zx...locking-33638/

 

Ahh well that makes me feel better. I don't like to go against common belief but it just didn't make any logical sense to me. None of my cars has the injector fan on - first thing to get taken off the car.

 

vapor lock thing is a bunch of hooey. check chts and fpr.

 

if Z does not start after being hot . Open gas cap. It releases pressure and car will start. At least it did for me.

 

I have recently replaced the cold start valve, FPR, inspected all of the injectors and replaced the air regulator. I guess I'll tackle the CHTS next, I haven't even looked at that guy. Fuel pressure reads normal when checked as well. I checked every vacuum tube under the hood made sure all connections were sealed tight.

To further describe the symptom, after restarting the engine after a little "cool down" time, the engine usually turns (usually) starts okay, but then the rpms jump between near 0 and 1000ish, back and forth, like someone is rapidly tapping the accelerator and the engine revs and then nearly stalls, until it finally does stall and dies. Typically there is no problem for shorter stops, like gas-fill ups or running in to a store for a quick purchase, it takes about 20 minutes for the symptoms to occur.

I'll update after I acquire a new CHTS. Thanks for all the conversations and help.

And yes, it hasn't been much above 30 degrees in NJ right now... Although I will admit a heat fault of my own, no heat shield between header and intake, I couldn't get the bugger on when I put after market headers on, I feel like I remember looking into it at the time and that it would be okay. That was a very long time ago, these symptoms are recent (of course I know that doesn't mean they aren't still somehow related).

 

You can test all the sensors through the harness with the FSM and a multi meter. That will get rid of a lot of guess work. Have you checked the AFM and it's connector?

 

ive had some of the symptoms, first fix was the module on the dizzy, when it gets heat soaked it doesnt provide a good signal to fire the coil. next one was my coil wire was corroded and was causing irregular spark and stalling when driving. 3rd time my dizzy shaft had worn out and caused some really weird issues but this doesnt sound like the case here. check your timing as well, make sure its at least 8 deg, i personally like it up near 15+. also your idle seems a tad low, bring it up near 750 when warm.

 

750 RPM for idle? is that where it is supposed to be? I always shot for that 600-650 range... That is where it currently idles when warm, I'll try that out too, I will check the AFM sensor and it's connector as well tomorrow. Still no CHTS replacement... I'm procrastinating on that for some reason, maybe because I'm hoping for a different fix.

 

Thanks for everyone's help. Here is the update, should anyone care. It was too damn cold to work on my car until this month and I finally got around to replacing the CHTS, which appears to have resolved part of my problems. I was having really rough idle when cold. However, the original issue with the rough idle after sitting is due to my air regulator, it is new, but the old one probably worked fine. The issue is it is getting too hot and not cooling down fast enough (this is my theory) due to the header I installed. I think the aftermarket header creates more heat around the intake than the stock and that little piece gets hot and closes up tight even after the engine has cooled significantly. Now that it is spring/summer I doubt this will be an issue until winter again, as the engine won't cool so rapidly, but I insulated the air regulator a little bit and elevated it from the intake so it's not in direct contact. I looked into heat wraps for the header, but it seems like it could cause more harm than good. So I installed a make-shift aluminum foil heat shield, about 6 layers thick between the intake and exhaust, I'll see if I have continued problems. This 81 Z spent the first 28 years of her life in California and now I'm struggling through NJ winters and she friggin hates it.

 

You can wrap you headers and make sure you have a heat shield installed. That will fix a lot of your heat issues. Fricfrac had a post on his s30 with pics of his headers wrapped, looked like a pro job, and he has his heat shields on, some guys remove them thinking it wont hurt, big mistake.

 

here's a picture of the heat shield, did you happen to remove yours ?

 

I would also be checking the coolant temp sensor and thermotime sensor. Keep in mind that all engines heat up after shut-down due to a lack of coolant flow. This extra heat may be causing a borderline sensor to act up.
Also note that the injector rail cooling fan is only designed to come on after shut-down if the engine is "over" a set temp. This is like a back-up to prevent vapor lock and as Nismo stated, it would take some serious temps to vapor lock a 35lb system.
Another thing you can try is running a lower temp thermostat. Going from a 190 to a 180 could make a difference.
As I said, these are just my thoughts.

 

I have purhased this blower and duct for my '78 Z. I need a close up of one installed so I can see what I need to install it. EN1 have a pic for me? Thank you!

 

The problem has worsened. Now the car cold-starts very rough no matter what, summer, winter, very rough. Then, it revs up and down like crazy. Again, once it is fully heated it runs like a champ. I added a heat shield between the headers and intake manifold. I have replaced the fuel pressure gauge, CHTS, AAR, inspected the MAF (Which i just dont think it is anyway since it operates so smoothly once its warm) and checked and rechecked every vacuum tube.

This is driving me crazy. the car is very difficult to drive until it is fully warmed up, at which point there are zero issues. Are there any other parts I can inspect or replace? It has to be something related to temperature since it runs flawless at normal operating temperature, but struggles every second before that (although the warmer it gets, the less variation in the rpm shooting up and down as it starts to calm down). I am pulling my hair out on this one and it is has got me just not driving the damned thing at all, because it often stalls out while driving when I'm still trying to warm it up.

Any last thoughts before I pay some douche 100 bucks an hour to tell me he cant figure it out?