Hot-start issue with EFI - who has it, who doesn't

[rcb280z]

rossiz, that only works on carbureted cars.

[FastWoman]

ok, so i'm a little confusticated here...

if the accumulator "stores pressure" between the fuel pump and the rail, wouldn't a drop in pressure due to throttle opening/closing simply pull a little of the "stored" pressure, which would be continually replaced by the pump? i'm not sure how the accumulator would make the system any less responsive, as the pressure would always be there - either from the accumulator or the pump itself, right?

Well, when the pressure NEEDS to drop, this is achieved by the FPR spilling fuel into the return line. If there's no accumulator, relatively little fuel needs to be spilled to drop the pressure. On the other hand, if you have an accumulator full of pressurized fuel, much of the volume of that accumulator must be spilled before the pressure can drop. More spillage into the return line takes more time, hence less responsiveness. And then when pressure must again be built, that's accomplished by the FPR blocking fuel flow to the return line. However, the pump no longer simply has to repressurize the line. Instead, it has to fill up an accumulator, which again takes time and decreases responsiveness. Not mentioned in all of this, an accumulator could increase the work load of the pump considerably, as the pump would have to keep re-filling it after every dumping.

As I said, I can see an accumulator being useful in a constant pressure system, but the L-Jet system is NOT constant pressure.

[hr369]

the volume of these bosch accumulators is pretty small. what would it take to fill one? 3 -5 seconds? if that indeed is a

problem, they do make a very small accumulator. see the size

difference. Big one is first picture.

if you search on volvo hot start problem there are lots of forums with the same problem. i do believe many of the 70's and 80's volvo's use the same fuel pump check valve. I do

believe thats part of the problem. Its too weak and allows

bleed back when pressure rises after shutdown. Perhaps a

different-better higher pressure check valve?

The jeep cherokee guys have the same issues and they have resorted to the hood vents and injector fans too. they even

had a recall because injector #3 was throwing a code. the

band aid was a injector shield for #3 but didn't work very

well. they use this fan timer if anyone is interested

ELK-960 - ELK Delay Timer

im wondering if there is any difference between the z and zx exhaust manifolds other than the additional o2 sensor. perhaps nissan made a beefier exhaust manifold that reduces

heat soak?

Edited March 30, 2014 by hr369

[FastWoman]

HR369, those are interesting looking accumulators -- different from what I was understanding! They appear to have a vacuum fitting on the back end. Mentally reverse-engineering the things, if I'm understanding right, the pressure of the accumulator would rapidly track changes in the manifold vacuum, essentially maintaining constant pressure across the injector. In that case (with the vacuum fitting), that WOULD work. Furthermore, this accumulator design would raise the fuel rail pressure to 36 psi immediately upon engine shut-down, rather than the typical running pressure of 29 psi. This would help prevent vaporization.

Are these mounted on the fuel pump assembly, with vacuum lines running that entire distance from the intake manifold, or are they mounted in a cooler part of the engine compartment, where a shorter run of vacuum line would be required? I'm wondering whether a part like this could be mounted in the vicinity of the fuel filter.

If we're right about hot injectors flashing fuel that flows into them, this accumulator idea would not be a panacea, but it might help. It's interesting the Jeep folks have the same problem. Do the Jeeps have these accumulators?

There shouldn't be any issue with weakness in the check valve. These valves fail through wear. If you've replaced yours and are still having your pressure leak down, you probably have an issue elsewhere -- FPR, injector, cold start injector. FAIW, I'm running the same Volvo check valve, along with new injectors, and my system holds pressure almost indefinitely. So there's no design issue with the check valve not being strong enough.

Others have suggested the more massive exhaust manifold, relative to headers, would hold and radiate more heat, and I suspect there's merit to that thought. I suspect beefier manifolds would only aggravate the problem. Perhaps someone will will pipe in and say, "I had hot restart issues until I replaced my manifold with headers."

FAIW, there was an evolution of manifold design over the years. In '75/'76, the intake manifold had no webbing and would allow free airflow from the exhaust manifold, up around the intake runners. In '77/'78, there was webbing between the intake runners and heat shielding around the exhaust manifold -- both between the exhaust and intake, and surrounding the exhaust. Then of course they went with the fan in '79.

[mjr45]

My 75 had the problem last summer time(when it was running right) it has a 79 block with a N42 head, new injectors, 79 fuel rail, new FPR, stock 75 exhaust manifold and heat shields. Did not have it this winter before I started having some issues with timing and fuel. My check valve is toast, holds pressure for maybe a minute. Don't know if this adds to the discussion, but there it is, I am convinced it only occurs with higher atmospheric temps, at least at it seems that way up here in the thin air.

[Zed Head]

Thanks for posting, they all count. So far, only one person with headers and EFI has posted, siteunseen, and he is the only one who doesn't have the problem. Wish we had more EFI and header people who would chime in.

On the accumulator patent - That's a good find. The same reasoning was probably used by Nissan to develop the five second timer relay on the fuel pump circuit. It night be a typical covering-the-bases patent though. Filed in 2005, published in 2007, but no data on actual engine restart effects, just some pressure-time curves. With an FPR on one side and a big reservoir of fuel on the other, the net effect is to use fuel as a cooling agent. Hot fuel gets pushed through the FPR, back to the tank, and is replaced by new cool fuel from the accumulator. Interesting that they called out the 10% ethanol blend.

Thanks for the link, the other citations in the patent will be some good rainy day reading.

[EuroDat]

My 75 had the problem last summer time(when it was running right) it has a 79 block with a N42 head, new injectors, 79 fuel rail, new FPR, stock 75 exhaust manifold and heat shields. Did not have it this winter before I started having some issues with timing and fuel. My check valve is toast, holds pressure for maybe a minute.

Captain Obvious started a thread on alternative check vavles to solve the same problem you are having.

http://www.classiczcars.com/forums/fuel-injection-s30/51862-fuel-pump-check-valve-alternatives.html

Which inlet manifold are you running; webbed or open?

[darom]

1976, stock exhaust manifold, catalytic converter, heat shield, 1979 fuel rail and injector fan (with eBay time relay circuit set for 12 minutes), new fuel pump, FPR, ignition timing 15*, inline resistance pod.

Previously I had heat soak issues, esp here in Bako town in the summer. After installing the 79 injector rail fan with a timer relay, there are no more incidents.

[mjr45]

Euro, I just read the thread on the check valve and will definitely be doing that, my intake is the open type. I'll also be changing the fuel rail to a single aluminum rail as I believe there is a very tiny almost microscopic leak right at where my #1 line comes off, I always find some burnt brown looking crud on the heat shield after running, but never see any fuel leaking. Oh well, another mystery for me to follow.

[ZCurves]

They use this fan timer if anyone is interested ELK-960 - ELK Delay Timer

Thanks for posting the link to the timer! This is great! I am planning to install with a ZX Fan.

[hr369]

fastwoman - vw's that i've looked at have the accumulator located next to the fuel pump. Thats not to say other cars

might have them far away from the fuel pump. But this is how

bosch designed the system for vw's.

good question on the cherokee using accumulators. i googled

cherokee fuel accumulator and did not turn up any and the

cherokee forums don't mention them.

a vacuum line going from front to back would a pain and you might need a vacuum tank reservoir to keep a stable vacuum.

i guess you could use the a/c hvac bottle in the engine compartment.

the deloreans have the accumulator about 7' from the rear and it appears to be located away from the fuel pump.

but... everyone knows how crappy that car performed.

Fuel Accumulator Removal and Installation

check valve alternatives -

I ran across this high flow bosch check valve for the Bosch

044 fuel pump it threads right into the fuel pump and uses AN style fittings. Bosch 044's are around 100 bucks new.

Bosch 044 High Flow Check Valve, -6AN outlet [bFP-CV6] - $50.00 : Jay Racing

Edited April 3, 2014 by hr369

[siteunseen]

The people at Nissan realized the big 'ole exhaust manifold wasn't too good an idea. This is stock on a '82 ZX. A whole lot less heat holding cast iron!

Oh yeah. I've got a ZX fuel injector fan somebody can have. The twice pipes that go over the valve cover is missing, fan and bracket only. Free, if someone wants to pay shipping. It works off my battery charger.

Edited April 9, 2014 by siteunseen