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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

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Old 08-02-2018, 01:33 PM   #1 (permalink)
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Default 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
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Old 08-02-2018, 06:31 PM   #2 (permalink)
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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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Old 08-02-2018, 09:11 PM   #3 (permalink)
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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.
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Old 08-03-2018, 07:05 AM   #4 (permalink)
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Originally Posted by turtle64b View Post
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 ... Iím wondering if maybe itís a regulator issue
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Old 08-03-2018, 07:52 AM   #5 (permalink)
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Mine was a regulator issue, you could try another regulator.

Could be blocked filter?
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Old 08-03-2018, 11:15 AM   #6 (permalink)
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Quote:
Originally Posted by Jinxx View Post
With my other nismo I had a supercharger on I never noticed any pressure drop and it was set around 52 as well ... Iím wondering if maybe itís a regulator issue
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.
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Old 08-03-2018, 02:16 PM   #7 (permalink)
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Originally Posted by turtle64b View Post
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

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Old 08-03-2018, 08:47 PM   #8 (permalink)
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Well I been through it with a fine tooth comb ...can’t find anything wrong so I guese it’s normal
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Old 08-04-2018, 02:31 PM   #9 (permalink)
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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 ?

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Old 08-05-2018, 03:43 AM   #10 (permalink)
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After the fuel is warm, the pressure can drop a few psi. But it shouldn't drop more than a few psi
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Old 08-05-2018, 03:56 AM   #11 (permalink)
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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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Old 08-05-2018, 09:41 AM   #12 (permalink)
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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)
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Old 08-05-2018, 02:19 PM   #13 (permalink)
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Iím sure you have a good return fuel system & wouldnít hurt to have CJMís 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 donít want to lean out on a hard wot pull.
I have the cjm return kit and the voltage stabilized wiring kit already... it never runs lean ... Iím thinking engine heat affecting the gauge or gauge isnít right
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Old 08-05-2018, 02:22 PM   #14 (permalink)
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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 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 itís reading low I start running rich ... so Iím thinking gauge isnít right
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Old 08-06-2018, 05:19 PM   #15 (permalink)
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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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