1976 280z Fuel Injection Issues

[wright_280z]

FastWoman's suggestion in Post #7 is most likely the cause. Commonly called the "hot start" or "heat soak" problem (NOT vapor lock). Very aggravating and common. Many people just learn to live with it.

I've done some experimenting and have found that directing cooling air at the injector bodies will stop the problem. Nissan did the same with the 280ZX, but installed a fan and ductwork to cool all of the components of the fuel supply system on top of the manifold. Heat from the engine block and manifolds heats up the injectors and fuel rail causing the problem. Many theories have been proposed but they all boil down to too much heat. 20 minutes seems like the point where things are the hottest. Earlier than 20 and the heat has not transferred to the fuel system, later and everything has cooled down.

I am looking into purchasing a fuel rail cooling fan. Any suggestions on attempting to insulate?

[EuroDat]

You could try a cooling fan from the 280ZX models. I have seen a 280Z fitted with one and the owner was happy with the results.

Where I work we use Armaflex on all our steam lines. Its nine mm thick which could be a little thick and bulky looking. If you grab the insulation, it is marginally warmer than room temp and that is with steam at 150degC. Its easy to cut with a stanely knife and you can glue it with contact cement.

http://www.armacell.com/www/armacell/ACwwwAttach.nsf/ansFiles/Armacell_UK&ROI_Product_Catalogue_Digital2013.pdf/$file/Armacell_UK&ROI_Product_Catalogue_Digital2013.pdf#page51

Maybe your local heating and AC specialist has something similar.

Chas

[superlen]

If you go the insulation route, you will have to test as I think it's possible that you might make the problem worse. If the heat is being from convected around the fuel lines them selves, this will help. If it's leeching (conducted) up through the injector body, this insulation will just make it harder for the heat to conduct *out* of the fuel itself.

One of the feature of the ECU I'm working on is a fuel pump prime feature to see if topping off the pressure with a few secs of fuel pump activity at key switch "ON" will help this common Z ailment. I believe that people have ran a hotwire switch bypass for just this function and reported back good results. That might be a good thing for you to try as well. You could do a quick test, just: start, warmup, stop, wait..... unplug starter solenoid wire and try to start (which will ONLY run fuel pump because of disconnected solenoid). This should ensure the line is primed as good as it's going to be..Then reconnect starter and give it a try. I personally would be interested in the results of this test on your car since it's behaving like a nice test subject currently.

Lenny

[superlen]

BTW. I would shoot for 2-3sec total of fuel pump priming. More won't hurt anything.

[Zed Head]

I have a switch for my fuel pump and I also had the hot start problem. I've primed the fuel rail for about a minute, until I couldn't hear the bubbles from the gas tank anymore, and still had the problem. The rail was full of cool fuel and the engine still choked and sputtered for an eternity of 10-20 seconds. The only fix I've found is cooling the injectors directly.

The theories that I like are that the injectors get hot enough that the fuel vaporizes as it passes through giving incorrect, lean, metering, or that the pintle inside the injector gets stuck, doing the same, rich or lean, or both.

[FastWoman]

My experience is the same as Zed's. I think priming helps a little, but it doesn't cure the problem -- not by a long shot. I think that's because the injectors and their hoses are still full of gasoline vapor even after priming. I've not tried the ZX cooling fan, but I bet that's the ticket.

[EuroDat]

I think a combination of the 280ZX cooling fan and a sort of heat sink on the fuel manifold piping. Something like you see on nitrogen or oxygen tank heat exchangers, but then to disipate heat rather than absorb it. Anything that will disipate heat and increase the efficiency of the fan will help.

That and the auto priming function on superlen's digital ECU;)

Chas.

[superlen]

Zed,

What about the fuel that is trapped in the short hose from the rail to the tip? (lets call it the stub) Could that amount of fuel represent the 10-20sec of run time? This fuel would not be flushed back to the tank during the prime function as it would simple be bypassed. We could do the math on the flow rate and the ID/Length of that stub to see if the numbers are half way close.

Also, if the entire car, or more importantly the injectors themselves, cool back down, I'm assuming the 20sec stumble is gone or at least greatly reduced. Does this agree with what you have seen?

Len

[Zed Head]

The "stub" is below the main rail and there is no surface tension to speak of to keep it from filling if fuel is in the rail. No reason for it to stay empty once the rail is primed. I think that the fuel probably sits and percolates in the injector body until enough vapor passes through it to cool it back down to where it's full of liquid. That's my theory and I have one experiment to support it, but not prove it.

But your comment shows the difficulty in figuring out what's really happening. You can come up with a variety of possible causes. As far as cooling everything, yes that seems to work. The problem doesn't happen unless the engine is nice and hot. It's also been proposed that using a lower temperature thermostat to keep the engine block and components cooler will also mitigate the problem. But that has other effects that may or may not be desirable. It's a dilemma.

[superlen]

Zed,

Yes, I agree with you. I kept thinking about the stub (it's just a short amount) & I think I was off the deep end. With some experimentation one may be able to find where the heat is getting in and causing the issue, but I think with a little characterization of how much additional fuel needed in a hot start/soak it would be a moot point. Just have the ecu dump additional fuel for x amount of revolutions/x time. It may take a bit to figure out the table, but makes for a simple solution when the work is done.

Len

[Zed Head]

I have a potentiometer on my coolant temperature circuit (installed specifically for the hot start issue, but later used for general tuning) and I've found that it helps to crank up the fuel when hot start happens. But based on the results, I've also come to the conclusion that not all of the injectors are affected. To add more detail to my theory, 1 - 3 injectors, probably 1, 5 and 6, are getting too hot and lose metering ability. That's why the engine will start and run, because these engines run okay on three cylinders. So, any solution that adds fuel to all six cylinders will make three rich and three right, as opposed to three lean and three right. I found that adding fuel to all six would make things better but never get all six running right.

Not trying to kill your motivation, I may be wrong and you might have a solution. But it's a rabbit hole.

[Zed Head]

Can't edit for some reason...

Here's something that I've wanted to try by finding a different set of injectors, but maybe you can do it with tuning - run at a higher fuel pressure. The 36 psi standard didn't last long before everyone went to 45 psi or higher. I've wondered if there was another reason besides better atomization to do that.