Ok guysm I'm at my wits end trying to figure this out, I have searched here, hybridz, and zcar.com. Several people with hot start issues, several different fixes for each of them, none of which have worked for me. The car starts an runs fantastically,no hesitation, no backfires, no smoke, nothing. When the car is run on the freeway or allowed to get up to operating temperature then turned off, after sitting for 10-15 minutes it will start, but runs on 4-5 cylinders. As soon as the car starts moving and the rpm's reach 2,000, it goes away instantly. Checked the WTS, Thermotime and cold start are disconnected, all fusible link, relays including FI and power relay are good. I know its not vapor lock because the fuel lines are cool to the touch. Any suggestions other than fuel pressure or the fuel pump check valve? those have been checked already

Don't rule out vapor lock due to cool fuel lines. If the lines are obstructed much (eg, dirty filter) it can cause cavitation and little bubbles of vapor in the line that the pump can't pump just like a boiling fuel vapor lock.

Not saying it's for sure a fuel issue, I don't know my butt from a hole in the ground on the fuel injected Zs, but I wouldn't rule it out so quickly without a fuel pressure gauge.

The solution to a problem like this is found by going BACK TO BASICS.

You've got to determine if it's an ignition or a fuel problem.

Start it up and add extra fuel --- starting fluid, brake fluid, anything flammable. Give it lots. If the problem goes away, you've got a fuel or vapor lock problem.

If not,

Unless you have access to an old-style ignition scope, the big ole Sun machines, replace ALL the plugs, wires, rotor and distributor cap. This is called "diagnosis by throwing parts at it."

I'd suggest acquiring a vacuum gauge and seeing what's going on as well, another 20 seconds that will give you invaluable information. If you have a sticking valve guide, valve that's badly out of adjustment, etc, you'll INSTANTLY know with a vacuum gauge. A valve with zero clearance can be VERY temperature sensitive, once coolant is flowing, etc it can close "just enough" for the engine to run properly.

You can have this problem properly diagnosed by lunchtime today.

I think Wade means brake cleaner and not brake fluid. I would stick with starting fluid, myself, or gas from a propane torch.

As for throwing parts at it, on the electrical side, it can be difficult to detect cracks in the distributor cap. It's easy and economical to replace. You can check the resistance of the plug wires to see if they are still in spec. An inductive timing light can give you an indication of a lack of spark, too.

Now, how did you test all of the things you tested? Please give us details. I've wasted too many hours with junior engineers, chasing down problems because they didn't test something right. Therefore, I assume very little about test results.

I'm gonna make an assumption that there no proof that the engine is truly running or 4 or 5 cylinders. When it hot starts, running rough-missing- it feels like it.

In my opinion, if there was a mechanical issues-valves, as mentioned- it would run poorly all of the time.

I have this problem and I followed all of the advice from this forum and the procedures from the FSM.

There is fuel pressure in the rail when it happens- gauge mounted on the rail, with pressure in spec. I have new Bosch injectors, fuel lines, fuel filters and check valve. There is an additional check valve because I do lose pressure after a few hours but others have opined that that isn't an issue.

The valve train is in spec. All of the ignition parts are top quality and new. Other than the sputtering, stuttering rough start-up, the car is a screamer and runs perfectly.

I have popped the hood after stopping, leaving it on the latch, for ventilation, as was suggested by others and it greatly reduced the problem. Just as leaving the car sit longer than 40 minutes seems to help. Some have retrofiited the engine bay with the cooling fan and ductwork from the ZX and reported a solution or improvement.

I believe the issue is connected to todays fuel blend and the summer/winter seasonal blends. My experience is that I have to make minor tweeks to the AFM and distributor timing twice a year. I even have two marks on the AFM toothed wheel and the dizzy scale depending on the fuel time of year.

A long-time and well-respected forum member uses a manually switched fuel pump prior to cranking and claims success.

Good luck.

I'm with you Willoughby. I just responded to a recent thread regarding the same thing and I also have done the same things you have to attempt solving the problem. I recently adjusted (slight) the AFM richer and my problem went away, so far. But I'm not sure if that is a fix or if it is just masking the problem. Only time will tell. Like bryand2, my 77 would do the same thing and it is annoying! Fires right up in the morning, warms up normal, runs great. When it would do this (run like crap) after sitting for 10-15 min I was on the hunt for the problem and never found one. That's when I decided to richen the AFM one full turn of the bypass screw. That was about 2 months ago and I haven't had the issue since then. Again, I'm not saying it will fix it but it seems to have corrected mine. Good luck and post your success when it happens.

Hot injectors + low operating fuel pressures (characteristic of early EFI systems) + modern ethanol fuel (developed at the tail end of the 1970's and not designed for those working pressures) + more volatile winter fuel formulations = flash vaporization of fuel inside your injectors. The ZX engines had injector cooling fans for a reason. I think the only cure is to find a better formulation of gasoline or install a ZX fan. Hood vents also help, although you would already have those. (You could of course remove the rain pan on the driver-side vent to increase ventilation.)

I don't have this problem at this time, although I've had it in the past. I've found opening the hood and letting the engine cool can get you started. I wonder whether it would be useful to put a valve on the return fuel line, so that the fuel pressure could be temporarily elevated to around 45 psi. That might force condensation of the vapor in the injectors. A more elegant approach might be to have a time circuit that would close an electrically actuated valve on the return line for maybe the first 15-20 sec of starting/running.

Of course some of our Z's, particularly the later model ones ('77/'78) have lean running issues due to drift in the ECU. If we were to compensate not with adjustments to the AFM main spring or with addition of resistance in the CTS circuit, but rather with an increase in operating fuel pressure, we might just kill two birds with one stone.

Edited February 23, 201312 yr by FastWoman

A long-time and well-respected forum member uses a manually switched fuel pump prior to cranking and claims success.

I also added a manual switch to the fuel pump so I could circulate fresh fuel prior to cranking, in an effort to defeat what I thought was vapor locking. Made no difference.

Running alcohol-free fuel makes a big difference on mine.

The injector cooling fan seems pretty worthless, imho. What's really needed is a big electric radiator fan to blow out the entire engine bay of hot air, wired on a timer AND thermostat. I eventually

created THAT setup on my Jaguar XKE and had one of the few that didn't lose coolant every time it hot soaked. The continual loss of coolant was the death of Jaguars and Vegas alike, neither of which had a "big enough" coolant overflow bottle.

Edited February 23, 201312 yr by Wade Nelson

From a discussion on Zcar.com:

"You might have noticed that the original poster has a 76 280Z, thus no carbs. He already has a fuel pump that keeps a steady pressure of 30psi. Dollars to doughnuts he doesn't have the problem until AFTER he shuts the car off for a few minutes, and then tries to restart it. That is the typical failure mode for the fuel injected cars.

I agree, your carb car suffered from true vapor lock, which while difficult, can be cured, or eased somewhat. What the poster has is called heat soak. The engine heats up after the coolant stops flowing, taking the temperature of the gas up with it. The fuel lines bolted directly to the head and intake transfer the heat. Vapor forms, and when the car is restarted, it runs badly until WOT clears out the lines. Then its fine."- Zuber

"As long as your car isn't doing this running down the road. Vapor lock attacks cars in motion, hot restart is the term for, uh, em, er... "Hot Restart Problems"

If you are stumbling while going up long uphill grades in high heat, driving in extreme heat, or other similar maladies, check your fuel filter... EFI cars are not really that susceptible to the "Vapor Lock" problems found in CArbbed cars due to the nature of their fuel system design. There is really a lot of flow going through the rail, and the only time the fuel has time to REALLY heat up is when that flow suddenly stops and sits stagnant (like when shutting off the car after a long, hard run) Then you get "Hot Restart Problems". - Tony D

The ZX FSMs describe the purpose of the cooling fans as to cool the fuel supply lines. I've messed with the switch from a ZX and something I rigged up to push water through the head and found that the switch never closed under the conditions my engine saw even though I would still get the hot start problem.

If you have a switch to run your fuel pump before the engine starts you'll hear a lot of bubbling in the tank. It seems like it's well after the rail would have refilled, if it had drained. I think that it's gas vapors, created as the gasoline flows over the hot spots, getting blown through the fuel rail and in to the tank. My guess is that the heat from the exhaust manifolds transfers to the intake runners and head in the vicinity of the injectors and overheats them. There's only a few mm separating the exhaust manifold from the intake runners at the head, and the first cm is exposed, outside the heat shield. You've got radiative and conductive heating all acting on the material that injector body is mounted in. I wouldn't be surprised if the two inner injectors get it worst since they have an exhaust runner on each side.

I might take take my digital contact kitchen thermometer with me on my next drive to see what the temperatures are across the intake runners, just for fun. It would be nice to have a scanner and some logging equipment.

The ever persistant 280 hot restart issue..There are lots of threads about it and lots of things people have tried to fix it. Here's mine...

I put a 10K resistor across the contacts of a normally closed push button switch and then put that switch in series with the water temperature sensor.

With the switch not pushed, the resistor is shorted out by the switch and it's as though it's not there at all. However when you push the button, the switch opens, and now you've now go a 10K resistor in series with the water temp sensor. The resistor adds to the water temp sensor, and presto... Your ECU thinks your engine is very, very cold and supplies the "appropriate" amount of fuel to start a frigid engine.

Worked so well for me that I even disconnected my cold start valve. I just push my "electronic choke" button.

Supporting info? Here's resistance measurements I took last fall from my air and water temperature sensors after shutting down my motor. All measurements are in Ohms.

0 minutes after shutdown: W=250, A=1900

2 minutes after shutdown: W=226, A=1640

4 minutes after shutdown: W=231, A=1530

6 minutes after shutdown: W=245, A=1450

8 minutes after shutdown: W=268, W=1350

Some conclusions?

a) The temperatures of both sensors goes up after the motor has been shut off. No surprise there.

The water temperature seems to crest somewhere between 4 and 5 minutes after shutdown.

c) The air temperature sensor temperature is still rising even after eight minutes of shutdown.

My theory, which is mine, is that the ECU is seeing the temp sensors and injecting a very small amount of fuel because it thinks that's all the engine needs.

On edit - Forgot to mention... This coming summer, my plan is to refine the system a little and use a relay instead of a push button switch. Wire it such that when the key is in "START", the additional resistor will be inserted in series, but once the key moves to "RUN" the resistor would be shorted out and the WTS readings would be normal. That way I won't have to push the "choke" button. It will all be automatic.

Edited February 24, 201312 yr by Captain Obvious

I can agree with the coolant temp sensor possibly seeing hotter than appropriate conditions. Actually, that was the reason I installed a small electric water pump with the ZX fan switch to run it. But as far as I could tell the switch never closed. Maybe I was wrong and I just missed it. I had it installed most of the summer but removed it late summer. Then cold weather and winter fuel came, now I get hot start problems on a regular basis. Maybe I should have left it on with a toggle switch to clear out the hot water.

So close to a solution. A little more supporting data might make things clear.

Edited February 24, 201312 yr by Zed Head

Pump never ran? Any idea what was the threshold temp for the ZX fan switch?

What other supporting data might I be able to help with? I won't be in any position to further investigate until it warms up, but once that happens, I'd be glad to do what I can.