Fuel Starvation Problem

[Marty Rogan]

I felt the fuel rail and it was HOT. I felt the mechanical pump and it too was HOT. I decided to remove those things from the system, and I ran a rubber hose from the filter across the rad support back to the SU's and installed an electric pump. My theory is that even if you wrap the fuel line with some heat shielding you haven't disconnected it from the head, which is damn hot. So the heat just travels up the mounting tabs right under the heat shield. Anyway, I never had a problem with vapor lock again. I just dead-headed the fuel system, no return line. Since it worked great with no return, I can surmise that the fuel wasn't boiling in the float bowls, but was boiling as it went through the mechanical pump and fuel rail.

Do you have any pictures of this set up? What is holding up the fuel line going into the SU's?

thanks,

Marty

[jmortensen]

I don't have any pictures anymore sorry, been running Mikunis for years now. I had the rubber line zip tied to the radiator core support and had it zip tied to some wiring on the distributor side of the engine. I can't remember how I had it split, I think I just got a little T from the auto parts store and hooked it up. It was easy, that much I do remember.

One more thing, you need to plug the return line if you dead-head the fuel system. Otherwise fuel will come from the tank and spill out the front under hard braking.

[Marty Rogan]

Ok, Thanks for the response.

Marty

[Chino 240Z]

Marty, I had simular issue, and found fuel pump was starting to go bad. Had a Holley Red, always looked dry, but you know how fast that race gas evaporates, finally had my head under the tank & pump area while Cindy was running the car and noticed fuel residue out the weep-hole! Fuel pump worked 95% of the time but started to give me starvation issues more & more often.

Your car ran fine with your set up (so you know it works, right), and now more and more it is happening. You haven't changed fuel line diameter, pressure regulator settings, fuel bowl float levels, needles or jets, clean fuel filter, no kinks in fuel line, clear fuel pick-up line in tank, clear vent line in tank, clear vent lines on both carb fuel bowls... so how is your fuel pump? it's the only mechanical piece of the puzzle that is most likely to change or wear out?

Just me remembering my fuel issue...

Craig

[jmortensen]

That's a good point Craig. I probably should have mentioned that the one time I had the vapor lock it was 105º and I normally didn't run in those kinds of temps.

[Sean240Z]

Marty,

I've had fuel vaporization problems twice since I've owned my car. The first Fuel vap. problem was due to a leaking exhaust manifold. While learning how to maintain my car, I ran the carbs overly rich for an extended period and warped my exhaust manifold and ended up shearing off the manifold studs at both ends.

Prior to fixing the actual problem, I ended up installing a later Z hood with louvers. This had two benefits: underhood temperatures where reduced and the front end lift at high speed was also reduced (evidenced by the fact that the rear corners of the hood no longer lifted at high speed). To further alleviate the fuel vapourization problem I re-wrapped the fuel rail with insulation and covered with aluminum tape.

More recently, I installed a MSA 6-into-1 header (coated) and ended up having a fuel vap. problem again (especially if I went through a drive-thru). The problem was that one of the header tubes passed directly underneath the rear carb float bowl. After a few minutes at idle, the float bowl would get very hot and fuel vap. symptoms would occur. To rectify the problem, I got some scrap metal and manufactured an extension to the stock heat shield which extended under the rear carb bowl. I covered the under side of the new heat shield with insulation and aluminum tape and have since had no problems. This solution is not the most aesthetically pleasing as the view of my coated header is obscured but it works.

Cheers,

[Chino 240Z]

I don't have alot of experience with vapor lock, but did run in 105-110* temps then pitted and turned off car, thought I boiled fuel in fuel bowls from the sitting heat. Little later wouldn't start? Went home early that day thinking floats out of whack, possible vapor lock. But silly me, while I was working under the hood I accidently pinched the fuel bowl vent closed and learned that it no likey! With out a good vent to the bowls the fuel comes pouring out the carbs down all over the hot exhaust. (no flames, I was lucky)

Guess getting too hot could possibly be a fuel problem but while moving fast should be enough air flow under the hood and fuel consumption to not let new fuel get too hot to vapor lock it? It can get pretty hot, July 2nd at WSIR we measured 112 to 115* in car temp on the Ultra lap timer. :eek:

We started and stopped the car a dozen times making driver changes then right back out again, and again... no problem with the heat.

Craig

Just to be sure no heat issues, I have installed a heat shield the entire headed area, a 3" air duct directed into header area under the shield, and the original NISMO header is getting JetHot coated right now, better safe than sorry.

[Marty Rogan]

Marty, I had simular issue, and found fuel pump was starting to go bad. Had a Holley Red, always looked dry, but you know how fast that race gas evaporates, finally had my head under the tank & pump area while Cindy was running the car and noticed fuel residue out the weep-hole! Fuel pump worked 95% of the time but started to give me starvation issues more & more often.

Your car ran fine with your set up (so you know it works, right), and now more and more it is happening. You haven't changed fuel line diameter, pressure regulator settings, fuel bowl float levels, needles or jets, clean fuel filter, no kinks in fuel line, clear fuel pick-up line in tank, clear vent line in tank, clear vent lines on both carb fuel bowls... so how is your fuel pump? it's the only mechanical piece of the puzzle that is most likely to change or wear out?

Just me remembering my fuel issue...

Craig

I have recently replaced all of the fuel hoses, so there should be no clogging or kinking. I did not change any of the metal fuel lines. Have not touched the fuel bowls, needles or jets yet. I am only running the stock mech fuel pump, which I replaced with a new one when I bought the car just shy of 2 years and 5,000 miles ago.

The tank looked clean when I replaced all of the fuel hoses back there. Is it possible that removing some of the vent lines have caused this? The Z was set up without all the extra vent likes for other countries, and I have not heard of them having problems like this.

The track days I have been running this year have been pretty hot, around 90+. Track conditions do raise the under hood temps. But I also had this problem in October.

On the other hand, driving it to work should not have raised the temps that much, and it was maybe in the 80's in the morning.

ARRRRRRGH!!! This is really getting on my nerves!

Anybody have other ideas?

Marty

[Marty Rogan]

Marty,

I've had fuel vaporization problems twice since I've owned my car. The first Fuel vap. problem was due to a leaking exhaust manifold. While learning how to maintain my car, I ran the carbs overly rich for an extended period and warped my exhaust manifold and ended up shearing off the manifold studs at both ends.

Prior to fixing the actual problem, I ended up installing a later Z hood with louvers. This had two benefits: underhood temperatures where reduced and the front end lift at high speed was also reduced (evidenced by the fact that the rear corners of the hood no longer lifted at high speed). To further alleviate the fuel vapourization problem I re-wrapped the fuel rail with insulation and covered with aluminum tape.

More recently, I installed a MSA 6-into-1 header (coated) and ended up having a fuel vap. problem again (especially if I went through a drive-thru). The problem was that one of the header tubes passed directly underneath the rear carb float bowl. After a few minutes at idle, the float bowl would get very hot and fuel vap. symptoms would occur. To rectify the problem, I got some scrap metal and manufactured an extension to the stock heat shield which extended under the rear carb bowl. I covered the under side of the new heat shield with insulation and aluminum tape and have since had no problems. This solution is not the most aesthetically pleasing as the view of my coated header is obscured but it works.

Cheers,

Sean,

That's another good thought. I'll have to see if the exhaust manifold is leaking. It's all stock, so no headers to add extra heat. But if there is a leak, it could cause a hot spot near the carbs.

Marty

[Marty Rogan]

Well, I had a chance to look into this a little further on the weekend. I cut open the second fuel filter. I did find some chunks of rust. So, I guess there is rust in there that I couldn't see when I had the tank down last summer. I also took out and cleaned the bango fitting screens. Those did not look too bad at all. I think Ihave heard of people removing those. Not sure if that's a good idea. Afterall, is Nissan put them there, they must have had a good reason.

I got a quote from a local radiator shop. They will boil it out, acid etch the interior and seal it with a product called "Red Coat Liner". Comes with a 3 year warranty on the liner. Cost $100-175 depending how bad the rust is. Another $70-80 for the installation labor. About $250 total, which doesn't seem too bad. I may do the install myself if I can find the time.

Anybody ever hear of this product?

I sure hope this ends this long and annoying problem!

Thanks for all your input. I'll let you know if this does the trick.

Marty

[Marty Rogan]

Here's an update on my continuing fuel problems.

I got the car back from the shop Friday after the tank was boiled out and sealed. Just in time for a track day on Saturday.

The problem has not totally gone away. The car still starts bogging and losing power at about 5500 RPM. I car shift and keep it going, but it certainly does not give its full potential. A couple of times it revved to 6K without problems, but not consistently. Even with the car running crappy, I came within 1/10 of a second of my best time ever at that track. So, it wasn't a total loss and I still had fun.

It seems that I have solved part of the problem. Looks like I will need to tear into the carbs next.

Marty

[ChrisA]

Hi Marty, sorry to hear the fuel tank repair didnt fix the issue. Could you be having an ignition related problem? I know you're running a Pertronix, could it be dying at higher RPM? I dont really know about that type of ignition. Could there be a saturation time problem at 5500 to not load the coil?

I'm really at a loss to help you. Any wobble in the dizzy shaft?

I'm assuming your getting good clean fuel at good pressure at the carbies now?

Chris