HellFire Status & release date annouced

[FastWoman]

Lenny, I strongly suspect fuel pressure is the culprit with the hot restart issue. Engine compartments have been getting smaller and more cramped, and they've even become more insulated, so I can't imagine that heat soaking has become any less of an issue over the years. But with the higher fuel pressures of modern cars, the fuel is less likely to boil. I would think the solution is to change out both the pump and the FPR for a higher fuel pressure and to make adjustments as required with the Hellfire tuning.

Anyway, I might be one of those people who discussed fuel rail priming with you. I know a few of us have tried it. Trust me that no amount of priming seems to eliminate the hot restart issue. I think that's because the injectors themselves are hot enough to flash any fuel that flows into them, so they are initially injecting vapor.

[superlen]

Sarah,

It was you that primed forever.... I knew it was one of us on here I was just too lazy to go look it up. I think Captain O may have done this as well. He's probably lurking but he's been busy with suspension duties.

I agree on the higher FP. I have an adjustable and will test hot soak at different pressures when I get to that point. I still would like to have a solution for the stock system of course as many won't want to change out the FPR.

I'm not convinced it's just the injectors as there isn't much fuel from the rail through the stub and injector body. It seems that after a few shots the vapor would be purged by the liquid fuel pushing it out...like burping the brake line when bleeding the valves. The first crack and the vapor(air in this case) pops out fast. I'm still a believer that a large portion of one of the fuel rails (specifically the rear one) has flashed over and the rear three cylinders are running lean.

All conjecture of course. This phenomena really bothers me and piques my interest. Given enough measurements and studying and we will figure it out. To my knowledge, no one has done a qualitative study of all the variables to properly understand the physics going on under the hood of a soaking Z.

Soon we will have have the perfect test bed with the HellFire. It will be easy to adjust for different FPRs, prime times, log temperatures on all the injectors, etc.

Len

[Zed Head]

Actually, I've also primed "forever". Until the bubbles in the tank stopped plus more. No effect. I've also rigged up a perforated tube and a fan to blow air directly on the injectors. Which works for me. That's why I think that it's the injector's getting hot that is the problem. What exactly happens when they get hot is unclear. It could be electrical; the solenoid gets weak when it gets hot, or mechanical; something expands or warps and the solenoid can't move it, or physical (like physics); gasoline flashing to vapor before it can get through the orifice.

Somebody check my punctuation, I've never used that many semicolons at one time. Not an English Major.

[FastWoman]

"gasoline flashing to vapor before it can get through the orifice."

^^^ I think this is the most likely explanation. If the injector is well above the boiling point of the gasoline at the operating pressure, it will take a certain amount of gasoline vaporization to cool it down to the boiling point, and all of that amount will have to pass as a vapor through the jet. When the hot restart stumbling resolves, it resolves SUDDENLY, as though there is a quick transition from injecting vapor to injecting liquid fuel. The cylinders catch, and then all is well. It is for this reason I am skeptical a fix is possible through intelligent control by the ECU. I would think the ECU would have to detect stumbling, apply its corrections, and then abruptly cut off those corrections when stumbling disappears. But some cylinders run rich, while others run lean. An O2 sensor is unlikely to distinguish between rich stumbling and lean stumbling. Maybe you can do it, but I have my doubts.

Sadly, I think the only "real" solution is to find a way to cool the injectors, as with the ugly ZX blower.

[Zed Head]

I've wondered if the heat isn't coming from the bulk of the exhaust manifold. I also bought some used headers to experiment with but they need work. A survey of behavior with headers and EFI versus stock manifolds and EFI might tell something.

We're filling superlen's announcement thread with hot-start discussion.

[superlen]

Zed,

That's fine, I can start another thread. I think you are right on the money with the manifold mass & I believe someone said that when they switched to headers their hot soak improved.

It sounds like Sarah and you both have good arguments/evidence pointing to the injectors themselves being the culprit. That makes my fuel rail argument less convincing...even to myself. I also believe I understood that plastic injector bosses were better than the aluminum ones & that would also support injector vs rail.

Hopefully, I can still correct it just by metering more fuel than calculated, for x amount of time. It would be open loop in this case unless someone is running on O2 with the system which is easy to do, but they will have to weld the sensor bung into the exhaust.

Lenny

[siteunseen]

I didn't mention that I have the ceramic coated header from MSA. That could be important as I've never had the hot start problem since the header and new injectors on a open fuel supply with the aluminium rail and the FPR out the rear barb to the return line to the tank. It recirculates but not on top of the intake manifold.

[superlen]

The ceramic header I'm sure keeps a lot of heat from pooling under the intake during a soak. That combined with your rail and FPR is a great solution....and it looks cool.

Len

[Zed Head]

Started a new hot-start survey thread - http://www.classiczcars.com/forums/fuel-injection-s30/51835-hot-start-issue-efi-who-has-who-doesnt.html#post452360

I'll let people post their own details so I don't screw them up. SU, I remembered that you had not had issues. Referred back to your post in this thread from the new thread. If you could re-post over there that would be most excellent.

[Captain Obvious]

Haha, yeah, I'm lurking. Visions of spindle pin bushings dancing in my head.

I tried the forever priming as well, and it didn't help me much, but my (first order) problem was a non-existent check valve back at the pump. So I could have great fuel pressure as long as I was cranking, but as soon as I let off the key, it went to zero. I found the check valve issue at the trailing end of last summer so I haven't spent a full blown hot summer afternoon parked on the blacktop yet after getting that part sorted. This year will be the test for me.

My artificial enrichment circuit got me through last summer. That was when I hooked up a relay in series with the temp sensor and fooled the ECU into thinking my engine was very very cold while it was cranking. This summer I'm hoping that a new check valve will make the electronic choke unnecessary.

But I gotta get wheels under it first!! :classic:

[CG240Z]

I didn't realize we started a new thread. I was wondering why the old thread got so quiet. None the less, sign me up for an ECU!

[superlen]

I just wanted to post a quick update. As some of you know, my wife and I were expecting & we now have a new daughter. All is well, but progress on HellFire has been slowed of course. I do have my flowbench operational so the next step is to begin testing all the AFMS I have accumulated and establish a baseline curve, then plug that data back into the Hellfire and see how it behaves.

Len