So where does the fuel pressure go?

[Willoughby Z]

I had the "hot start" problem return this fall. The fuel pressure also dissipates over a few hours after shut-off.

The PO removed the stock fuel rail "trombone" pipes and replaced them with a single billet rail. There is a Summit Racing pressure guage on the rail. With engine started and running, there is approx. 29-30 psi. Without vacuum to the FPR- approx 36 psi.

I knew that the loss of fuel pressure, over time, can be (a) leaking:

- check valve in the fuel pump

- cold start valve

- FPR

- injectors

The pump is fairly new, so I added a Parker check valve down stream, between filter outlet and rail inlet.

No change.

The CSV was original, so I replaced it.

No change.

The FPR was for a '77-without the stock fuel pipes, there was a blocked off inlet. I found one online for $45, so I replaced it with a'78 model FPR. Tidier.

No change.

The injectors were original, though they had been refurbed/ flow-tested by an injector service in the last few years, the connectors were getting brittle, so I replaced with new- not reman- Bosch units.

No change.

The pressure still drops to zero after a few hours. So where else can the fuel go if it can't flow backwards to the tank/pump (check valve) and it can't return through the FPR and the CSV/ injectors are brand new?

I'm stumped.

The "hot start" issue- engine stumbles and seems to run rich for a few minutes after a 10-20 shutdown having come to full temperature (180) is slightly improved. Before the work done, it was a b*tch to start and keep running for several minutes. Now it starts okay ,just runs a little rough.

So there is some improvement.

The CTS/ thermotime switches are new, all connectors and grounds have been cleaned and protected ( Caig De-oxit/ dielectric grease, where appropriate).

The AAR was removed by PO.

Vacuum is within specs, AFM checks out (Atlantic Z bulletins).

I welcome any suggestions/ thoughts.

Thanks for reading.

[Zed Head]

I've been having the "hot start" problem recently.

Here is an interesting thread on the topic - http://forums.hybridz.org/index.php/topic/76282-heat-soak-vapor-lock-problem/page__p__746890__hl__%2Bvapor+%2Block+%2Binjectors__fromsearch__1#entry746890

The thread above spends a lot of time on defining the words and terms, but hot injectors seem to be the common point.

I am leaning toward cygnusx1's theory that the problem is actually caused by overheated injector bodies causing the fuel to vaporize as it passes through, or percolate while it's in the channel. This actually results in a lean condition. You might have noticed that you don't really smell any excess gas, it just runs like it's flooded. A miss is a miss, lean or rich. I've had a pretty good lean backfire through the intake when this happened. I used to think the problem was leaking injectors, but the lack of fumes fits a lean running situation better.

One change that I made recently was to the holders around the injector bodies. In the past, I had the one piece holder, recently I went to the two piece holder, with the aluminum upper ring and phenolic lower. I almost never had the problem before, I'm having it more often now. Maybe the two piece holder lets the injector body get hotter while sitting. It is also a different engine, but the EFI, intake and exhaust system are the same.

I think that the Nissan engineers attempt to fix the problem was the injector fans, controlled by coolant temperature. I've insulated my injectors within the last week to keep them from absorbing radiant heat when the car is sitting, but still have had the problem. Still looking for ways to avoid an injector cooling fan retrofit, but I have one and might try it.

Edit - I also have an Aeromotive FPR that leaks down rapidly. So maybe the dry injectors do not get the cooling that a "filled" injector would.

Edited December 16, 2011 by Zed Head

[Willoughby Z]

I understand your description of lean vs. rich. characteristics. It seemed to be more descriptive to compare it to a rich condition.

The injector holder was a two-piece- phenolic lower/ metal upper assembly. With the injector swap, I installed the new one piece holder. The hot start was worse before and now it's marginally better. That is to say, it would require multiple starter cranking to start and then very rough running, front fire, stalling,etc. Now, it starts with one key turn but runs rough, stumbling, chugging- clears up much quicker. I assumed that the minor improvement was that the new injectors weren't leaking and that the hot start problem was still there because of something else. The hot injector bodies would explain the "something else".

I have also read comments regarding winter/ summer fuel blends that are interesting and when I commented earlier about the problem re-occuring, I had not had the problem for a long time- in the late spring I had replaced the thermostat, CTS/thermo time switches, AFM calibration, etc.- assuming that these changes had solved it. No problems all summer. Then in late October, I filled the tank and then just like the snap of my fingers, the hot start issue was back. I've always had the pressure leakdown issue, so my thought was that if I have leaky components and I replace them one at a time, I may solve two problems.

I know that opinion is divided (strongly) about fuel pressure leakdown. Many have experienced it and fixed it with some of the changes that I made, others have never had any loss and express the opinion that the rail should never lose pressure- definitely not within a few hours. Some say that they lose pressure, it doesn't matter, " who cares?"...

When the pressure reads zero and the engine is cold, or has "rested" from a hot run, the fuel pump pressures the system instantly with a turn of the key and starts easily, so I'm not confident that a momentary switch for priming is the answer- unless the argument that pre-priming cools the rail/injectors is valid.

It's just frustrating to not be able to solve the two issues and particularly to be unable to prove/disprove whether they are interrelated.

[Z train]

I installed a in-line check valve(to replace the one at the fuel pump),had all injectors cleaned & flowed(including the CSI)and my hot start issue went away.If the fuel system(from check valve to reg is "tight",then you CAN NOT have "vapor lock".

[Willoughby Z]

Z train- so having done all of those (check valve, new injectors, CSI) plus the FPR, where wouldyou think the pressure dissipates?

[Z train]

You're losing it with ALL those things done?Injectors flowed & tested?

[Zed Head]

I had a check valve on my return line, but found that it quit working after a short while. It was only designed to hold to about one psi anyway. I know my lines were emptying because I could hear a large quantity of bubbles in the tank for about 20 seconds after I turned on the pump (I have a switch for priming).

The best check valve is a fuel pressure regulator. So I dug through a few parts that I've accumulated and found an FPR that regulated and held full pressure with the pump off. Replaced my Aeromotive FPR with it last night and will see how things work now. Hopefully, a good 30+ psi on a full rail of fuel will keep the injector bodies cool enough (that's my hypothesis of the moment that seems to fit the facts) to avoid the hot start problem. Basically using the fuel as a conductive cooling path.

Since I first installed the Aeromotive adjustable FPR (I bought it 'cause I knew my stock FPR was bad and the Aeromotive was on CL for cheap at the time, plus I thought adjustability would be nice in the future) I've made quite a few changes and coincidentally ended up with a fuel rail that fits the good regulator, which is the two port, 1978 style. So I am essentially back to a stock 1978 fuel setup, in my 1976 car.

I also tested another 78 FPR that I had and it regulated fine but leaked down to 10-15 psi as soon as the pump turned off. With all of the sitting that these old cars tend to do, I suspect that there a lot of marginal FPRs out there, with seats that don't quite seat.

[Willoughby Z]

You're losing it with ALL those things done?Injectors flowed & tested?

Yup.

Brand new Bosch injectors

Brand new CSI (NAPA-mfg unknown)

Brand new FPR (Beck-Arnley) '78 single port- installed exit of rail

Parker Fittings industrial liquid/gas check valve- 2600psi max pressure/ 3 psi crack pressure- installed exit of fuel filter.

I'm thinking that I can open the line between check valve and fuel filter after shut-down to verify that fuel is, in fact held by the check valve. second test, assuming check valve is good, removing hose from exit port of the FPR to verify that the diaphragm is seating and not allowing fuel to go back to the tank. Or just remove the check valve and FPR and pressure test them (in the direction of flow for the FPR and back-flow direction for the check valve), ie. Mighty-Vac, and see if they hold pressure. If those two items work properly then fuel has to be leaking through new injectors/ new CSI.

[FastWoman]

Even a tight system leaks down after a while. I have a fuel pressure gauge on my system and have observed that pressure will be held anywhere between a few hours and just about as long as I would care to wait (at least a week or two?), so there's some variability in how the parts seat.

Some of your pressure loss might occur when fuel vaporization in the injectors and/or fuel rail causes pressure to rise (which you can observe after engine shut-down), forcing fuel through the FPR into the return line. Then when the injectors cool, the pressure drops markedly, possibly even pulling a vacuum that draws in new fuel through the check valve. This is of course heresy, because fuel under pressure "never" boils. However, in practice, I think it does, often because the formulation sold to you is wrong for the season or climate in which you're using it.

[Z train]

O.K.,back to the basics:THe fuel system from the check valve to the FPR and the seven injectors is a"pressure vessel"of sorts.Assuming no visable line leaks,the pressure is escaping SOMEWHERE(either the check valve,injectors or FPR).If the rail is charged at 30psi,fuel CAN NOT boil/expand/vaporize(chemical content of the fuel be damned).But when the psi drops(fuel is escaping somewhere)THEN vaporization(expanding vapor) can occur.

So now we are back to chasing a leak.You can plug the csi and rule that out.One idea,is to remove the entire fuel rail WITH injectors & FPR attached,charge the line and look for leaks.

[Zed Head]

The pump is fairly new, so I added a Parker check valve down stream, between filter outlet and rail inlet.

The FPR was for a '77-without the stock fuel pipes, there was a blocked off inlet. I found one online for $45, so I replaced it with a'78 model FPR. Tidier.

The pressure still drops to zero after a few hours.

The "hot start" issue- engine stumbles and seems to run rich for a few minutes after a 10-20 shutdown having come to full temperature (180) is slightly improved. Before the work done, it was a b*tch to start and keep running for several minutes. Now it starts okay ,just runs a little rough.

So there is some improvement.

WZ, I edited your original post for brevity. I have a couple of questions and some comments, if you don't mind.

What brand of pump are you using? I'm no expert on brands, just wondering for the record.

If you look back at my post, I mentioned that I tried two different 1978 FPRs. One leaked down to about 15 psi right away, the other held pressure at >30 psi, consistently. So there's no guarantee that a used factory FPR will hold pressure. Also, who knows about Beck Arnley quality? They're mid-level quality, I believe.

I mentioned a check valve that I was using. I had my check valve on the return line, because it was the FPR that was leaking down. Sounds like you had yours placed on the assumption that the fuel pump was leaking down, stopping fuel from flowing backward. Mine was spring-loaded and designed to hold one psi ( I was just trying to keep the line full). I'm not familiar with yours, if it's an inertia or gravity type valve, it might not work on the return side.

As far as results for me, after going back to the 1978 FPR that actually holds pressure, I've only had one very small hot start symptom after a long drive and short visit to a supermarket, a typical scenario for the problem in the recent past. It only stumbled for about 3 seconds, as opposed to a few times before going to the stock FPR, where it felt like 30-40 seconds of waiting (praying) for it to stop. I've also made several trips where it normally would have exhibited some hot start symptoms but got nothing.

And the FPR I'm using holds pressure overnight. The engine actually started on one revolution this morning, No more fuel pump priming, which is great, because the novelty had worn off completely.

Edited December 22, 2011 by Zed Head

[Zed Head]

Here's a followup to my last post.

I still have the hot start problem, even with an FPR that holds pressure over time. It seems to be holding close to full pressure also, because the engine starts right up over night and after sitting a few days, just like a modern car. But even with a rail full of fuel, if I get the engine nice and warn, then let it sit for about 20 - 30 minutes, I get a nasty thirty seconds to one minute of stumbling, low idle poor running. I'm wondering now if the problem might be an overheated solenoid, sticking and causing the injector to stay closed. Just another way to get to a lean condition, which is what my hot start problem is like. It seems to be isolated to one or two cylinders, based on the way it runs.

Just wanted to followup. On the plus side, a system that holds pressure is nice for restarting in general.