Fuel pressure issue

Jinxx

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

bcfromfl

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.

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turtle64b

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.

Jinxx

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

shadow85

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

Could be blocked filter?

turtle64b

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.

Jinxx

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

Jinxx

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

Jinxx

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 ?

shadow85

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

jchammond

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.

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7ssain

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)

Jinxx

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

Jinxx

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

phunk

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.

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