Fuel pressure issue
 

So I have been noticing fuel pressure issue ...start the car from cold start fuel pressure is set at 48psi...volt gauge reading just past 14 .....drive for around 20 mins notice the volt gauge comes down to 14 maybe just under .....check pressure again it has dropped to 40 psi.....this has been a consistent drop of around 5 psi from start up to driving for at least 20mins.......I am running 340aerometer pump with 750cc injectors I also have the voltage stabilizer kit installed not sure where to look now

I can't comment on your fuel pressure issue, but I have noticed the variance in the voltmeter from cold engine to hot...probably more than a volt difference.

The voltage seems normal. As my car warms up, I notice manifold pressure drop (greater vacuum), which would drive fuel pressure down with it. I have the same pump and wiring kit but with larger injectors (1050x). Set reference (@ atmospheric pressure) to ~52 psi and then reconnected the manifold air line to the regulator. Hven't touched it since, no issues.

With my other nismo I had a supercharger on I never noticed any pressure drop and it was set around 52 as well ... I’m wondering if maybe it’s a regulator issue

Mine was a regulator issue, you could try another regulator.

Could be blocked filter?

The pressure should be set with the reference line (air line from the manifold to the regulator) disconnected. Manifold pressure with the car warm should be about 22in vacuum (-22 on my pressure meter). I notice that when I start my car in the mornings, vacuum starts out around 12-14in vacuum. This leads to pressure being lower than the set 52psi with the line open to atmosphere.

Not trying to be facetious or anything, setting of the reference at atmospheric pressure and the reason for the manifold tap are to ensure that fuel pressure is always the same relative to manifold pressure. Again, I'm not saying that you don't know this, just for anyone who might read this and not know why that line is there.

With your Nismo, the reference could have been set up incorrectly or differently at the recommendation of a different tuner. Seb told me 52psi at idle with the line disconnected. Then reconnect the line and all was good. Perhaps someone either wanted a higher pressure or set the reference at idle with the line connected to the manifold.

I hope this helps, I will look at my regulator pressure when I get home to see what it reads.

I know how to set the pressure ...but once the pressure is set ,shouldn’t it remain consistent ..the manifold does have a brief period when starting that the vacuum is less but once it idles down the vacuum should be at it max ( mine is around 18) and at this time fuel pressure should be stable ....which mine is ,what I’m seeing is if I go for a drive after this and come back and check it the pressure is about 5 lbs lower even with the same vacuum (18) ....I assume the voltage was the issue because at the time I was still on the stock wiring...and I noticed that fuel pressure was higher when my voltage gauge was over 14 and lower when voltage was under 14 ...thinking that was the issue added the voltage wiring kit ..but still same thing ....to be fair with my other car the fuel pump was the 255 walbro and it held pressure consistent at 52 ....when this car was tuned it was around 45 it drops to 40 ....not sure what could be changing to cause the drop fuel filter is new ...I guese it leaves the regulator or pump

Also the vacuum reference would be for increase fuel pressure as boost increases in a idle state the pressure shouldn’t change ..yes it is different with the vacuum line off and it is open atmosphere but set point should remain the same after setting

Well I been through it with a fine tooth comb ...can’t find anything wrong so I guese it’s normal

Well I thought it was normal ...butter fuel pressure is too inconsistent ...set at 45 it will fall anywhere between 35-40 after driving ..I’m thinking maybe regulator could be the issue but I’m not sure ...are they serviceable or anyway to verify before buying a new one....maybe a faulty gauge ?

After the fuel is warm, the pressure can drop a few psi. But it shouldn't drop more than a few psi

I’m sure you have a good return fuel system & wouldn’t hurt to have CJM’s hat.
You should be able to maintain set fuel pressure,,,may be a faulty gauge or too much resistance in the wiring/relay for pump,,,causing extra heat & loss of efficiency.
Either way; you don’t want to lean out on a hard wot pull.
:)

Are you reading off a gauge in the engine bay? i had a similar issue and when i asked Charles he said that heat in the engine bay effects the reading of the gauge. i check mine when the engine is cold it always reads my set pressure (51psi)

I have the cjm return kit and the voltage stabilized wiring kit already... it never runs lean ... I’m thinking engine heat affecting the gauge or gauge isn’t right

I am reading off the gauge in the engine compartment... and it does seem to be heat related ... also I never see a lean condition when pressure is low .. if I adjust the pressure when it’s reading low I start running rich ... so I’m thinking gauge isn’t right

Its most likely engine bay heat effecting the gauge, or engine temperature effecting the intake manifold vacuum, or some of both. Do you have a vacuum gauge to compare manifold vacuum cold versus hot?

These inexpensive little gauges I really only rely on for basic diagnostics and setting the regulator for initial installation. For a high degree of accuracy I would recommend swapping it out for a more serious gauge.

But do at least compare engine vacuum. Typically an engine will have less vacuum during high idle cold start. Then more vacuum and lower idle once warmed up some. This would cause fuel rail pressure to be lower when hot, due to the vacuum reference to the regulator. I havent paid a lot of attention to VHR vacuum readings in a long time, but I suspect you will find similar habits in connection to cold start and full temp idle.

Or if you dont have a vacuum gauge, you could always pull the vacuum line off the regulator while its idling cold to confirm your base pressure. Then while idling hot, pull the line off again and see what the gauge claims your base pressure is when hot. This would subtract the variable of a difference in engine vacuum when cold/hot.

Pressure is set to 50 without vacuum source with vacuum source it’s is around 44-45 .... and while normal operating temps without driving vacuum is around 18 after driving it’s more around 16 and when hot fuel pressure is around 40 ..... but that’s going from the gauge gauge with the return kit .... I think it’s giving a inaccurate reading due to the AFR doesn’t lean out and if I try to increase pressure I start going rich

How fast should fuel pressure drop to zero with the return setup after the car is turned off

There are no intentions for fuel pressure once the pump is not running... so anywhere between immediately and a couple hours would sound about right to me.

Ok just checking all angles ..it drops to zero fast after engine is off... I still think pressure is dropping ...I never noticed my supercharged 370 changing this much ...I went to a cruise in today ...popped the hood pressure was around 37 ..was very hot ...I let it just sit and idle for about 5 minutes ...pressure came back up to 40....cold idle is 45 ....still need to verify pressure gauge but I feel it’s is fluctuation

So I have been looking at my fuel pressure as well...

Full blown fuel return kit, with a fuel pressure regulator, and pressure gauge. No vacuum line connected to the regulator from the manifold. The car is still stock other than the fuel return kit, which also has a 14 ga wire from the batter all the way to the fuel pump.

I had set the fuel pressure at 51psi, as per FSM, with the engine all warmed up. I did notice that the fuel pressure will go up as high as 56psi when the engine is cold, and it does take 10-15 minutes of driving for it to come back down to what I set it to. This is about a 9% fluctuation.

I also noticed that the OEM volt gauge will go from about 14.4, to below 13.6...ish cold vs warm, which is about 6%.

I think it is safe to notch the last 3% of fluctuation on the heat, and the influence it has on the spring in the FPR.

That’s strange ...mine is the opposite ...it’s dropping pressure from set point ....I had it set around 50 without vacuum line on it ...with vacuum line it dropped to around 45 ....after twenty mins of driving it would drop to 38 ish ...I watched it while idling and it would start to bounce from 35-38 .....sooooo i lowered the pressure down to 28 then increased it to 65 just to see if the regulator was hanging up ....I set it back to 52 without the vacuum it dropped to 50 with vacuum line on ....took it out for a drive for awhile got back home and checked it ..still holding 50 .....I’m wondering if there is a issue with the regulator that is magnified at lower pressure , now I’m just guessing ...there are really only a few things that can affect pressure ...vacuum source ..regulator ...pump ...voltage.....I’m using the vacuum lines off the throttle body for regulator source ..I’m gonna ty to switch it to another one to see if there is a change

I did find this info which almost fits my issues to a T .... Aeromotive AFPR problems | DSMtuners

and it came down to the gauge being liquid filled not being accurate and that aeromotive states not to use liquid filled gauges on their web site. And now that I think about it I had a fuel lab regulator on my other z with no fluctuations with a liquid filled gauge .....I’ll be adding a T in and add a regular gauge and compare the two

This may help as well. https://www.verociousmotorsports.com...-Filled-vs-Dry

That what the first thing I was saying... temperature effects the gauges.

You also need to always compare engine vacuum to your fuel pressure. Monitoring your idle fuel pressure without comparing manifold vacuum is sort of like expressing wheel offset without knowing the wheel width. Its mostly erroneous information when one of the major contributing factors is unknown.

Between temperature and engine vacuum fluctuation, you're probably fine. A loss of even a few psi will show a clear change in the A/F. So if you are already closely monitoring your A/F, you are already seeing the aftermath of the fuel pressure. I wouldnt dwell on the fuel pressure much at all unless I was first seeing a problem in the A/F that needed diagnostics. But if you really want to know for sure that everything is OK, you can get nice fuel pressure gauges for less than $200.

I was comparing the pressure with the vacuum and even with same amount of vacuum pressure I was seeing differences in fuel pressure ...reading from the gauge ...and I was never getting a lean condition ...I guese I was just paranoid ....better to double check something before hand than after a blown motor lol
What got me looking was a very light surging stumble during light acceleration ...checked everything couldn’t find anything ..changed plugs no change I thought the fuel pressure might have been bouncing causing it ....it is only under light throttle and very minor ..it’s around when the vacuum gauge is from 5-0 when I’m cruising at a steady speed .....no issue during boost

Ever find anything else about this or was it just the engine bay heat?

I chalked it it up to heat affecting the fluid in the gauge causing a inaccurate reading ....but one thing to note ...once you increase from vacuum to boost (0+ boost ) the gauge will go to original set point and increase as boost increases...doesn’t seem to affect the tune or set points.

UPDATE ..the surging I was feeling was in the tune ...I took John from Z1 for a ride and was able to show him what I was feeling while he had it connected to the laptop and he adjusted the fuel to eliminate it