Vapor Lock

[280z]

My early 280z struggles to start up when hot. I think this is a problem associated with vapor lock. does anyone else have ideas about what the problem is or how to fix it?

[mdbrandy]

Interestingly, I have exactly the same problem. Mine's a '78. Wasn't a problem while the temperature never got above 25 or 30 degrees F, but now that temps are getting up to 50+, it has become a problem. After the car is fully up to temp, if I stop it and let it sit for 5 or ten minutes, it starts hard and runs like a tank. If I give it a bit of gas pedal for a minute or two after starting, it smooths out and runs fine. It starts cold fine, and runs fine after that initial hot stumbling start.

I'm not sure if this is a symptom of another problem - I know I need to replace my fan clutch. While driving, temp is fine, but if I idle for 10 minutes or something like that, the temp starts climbing. Fan doesn't lock at high temp, so I know that's a problem. So, I have been thinking about it, and wondering if it is vapor lock from heat soaking after I stop, or maybe the temperature sensors at the front of the engine are getting hotter than they should (namely, the FI temp sensor and not the guage one) after shutdown since there wasn't sufficient fan cooling. That one seems a little off the wall, I guess.

I have wondered about a couple of 280ZX engines I looked at while I was looking for my Z in September. They had some kind of a fan/blower that seemed to take air from the sparkplug side of the engine and blow it on the injectors. Seems to me that would indicate a heat soak type of problem. Does anyone know if this is a known problem?

[280z]

yes you are right, i posted this same problem on zcar.com.

http://www.zcar.com/forums/read.php?f=1&i=447511&t=447511

apparently the 280zx had this fan, hopefully we can modify our older engines with the fan

[2ManyZs]

Both problems sound like a sensor malfunction, either the water temp, thermotime switch, or air temp sensor. They both work in conjunction to control the injectors through the ECM. Vapor lock with FI is highly unlikely.

I'd suggest reading through this troubleshooting manual before you do anything.....

http://www.72hours.net/~zcar/

[sblake01]

I don't know if the 280ZX injector fan will solve this problem. I have both a 78 280 and a 79 810. Neither have the injector fan. The only time I experienced the problem you describe was on my 810 when the thermotime switch was bad causing the car to inject fuel from the cold start valve everytime I started it regardless of temperature. It would sputter and stumble until this extra fuel was cleared out. That's where I would look first. Test the two terminals on the thermotime switch for continuity. It shouldn't have continuity if the engine is warm. I'm have my doubts about that ZX injector cooler anyway. 75-78 280Zs didn't have it neither did any of the 810s or rwd Maximas. I wonder why Datsun/Nissan thought it was necessary on the ZX. Remember that the ZX was laden with all kinds of gadgets that IMHO were unneccesary.

[mdbrandy]

Originally posted by sblake01

The only time I experienced the problem you describe was on my 810 when the thermotime switch was bad causing the car to inject fuel from the cold start valve everytime I started it regardless of temperature. It would sputter and stumble until this extra fuel was cleared out. That's where I would look first. Test the two terminals on the thermotime switch for continuity. It shouldn't have continuity if the engine is warm.

Now THAT sounds promising. I had forgotten that I had downloaded that FI manual that 2manyZ's referenced. Just looked at it, and the thermotime would certainly seem to cause exactly this problem. I'll try to test it tomorrow. It's been idling high too, so I guess it's time to read that manual more thoroughly. The air regulator appears to be a possible problem for that...

Thanks!

[280z]

MDBRANDY,

i found the problem!! all it was was a bad fuel regulator, you can test it by removiving the hose and starting your car when hot.

[mdbrandy]

Originally posted by 280z

MDBRANDY,

i found the problem!! all it was was a bad fuel regulator, you can test it by removiving the hose and starting your car when hot.

Fuel regulator? So the flow is too low when hot, too high, or what? That's one I wouldn't have thought about. I haven't had time to look into it yet, but several days this week, it has started fine when hot, but today it ran like a tank when started hot again. What lead you to look at the fuel regulator?

Thanks for the tip!

[sblake01]

Originally posted by 280z i found the problem!! all it was was a bad fuel regulator, you can test it by removiving the hose and starting your car when hot.

Somehow, I think you are talking about the AIR regulator. If you disconnect the hose from the FUEL pressure regulator, the car shouldn't run at all. If you suspect a fuel pressure problem, perform the fuel pressure test outlined in the FSM. The problem you described at the beginning of this thread leads me to the thermotime switch/cold start valve combination. Have you checked those yet?

[mdbrandy]

Originally posted by sblake01

Somehow, I think you are talking about the AIR regulator. If you disconnect the hose from the FUEL pressure regulator, the car shouldn't run at all. If you suspect a fuel pressure problem, perform the fuel pressure test outlined in the FSM. The problem you described at the beginning of this thread leads me to the thermotime switch/cold start valve combination. Have you checked those yet?

The thermotime/cold start combo is what I'm planning on checking when I get the time. The air regulator really shouldn't cause this problem - it just raises the idle speed by supplying more air if I understand right. I am having a bit of trouble understanding what the fuel pressure would have to do with it.

[sblake01]

I don't know. In twenty years of working off and on with L24E and L28E engines, I've never known a fuel pressure regulator to go bad. But the electronic bits; thermotime switch, air pressure reg., water temp sensor, thermal transmitter, cold start valve, etc., I alwas keep at least a couple of each on hand tested and ready to go.