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Discussion Starter · #1 ·
Looks I will need new fuel pump internals! I have experienced the worst knock that I have ever had with this car. Anywhere from 2.1~5.25 at wot. I have experienced knock in 3rd and 5th gear. I have logged for the last two days to figure the problem out and here it is.

Mods:
Stage1 +SF 93 v103, Cobb SF Short Ram Intake, Synapse Synchronic BOV(100% recirc.), RPMC Motorsports Turbo Inlet, StreetUnit Silicone Boost Tubes, Redline MT-90 Gear Oil, Royal Purple 5w30, Denso Iridium Spark Plugs(ITV22).

Problem:
The stock fuel pump can not supply the proper fueling on the “fly” that the map requires. Every single instance where I had hardcore knock, I lifted of the throttle, then shifted, then went back wot.

Hypothesis:
Stock fuel pump is fine when it is flowing at its max, continuously, during wot. No lifting of the throttle when shifting keeps the fuel pump running max flow. I have several logs showing little to no knock when I do not lift off the throttle to shift. I have other logs showing that when I lift off the throttle to shift, the fuel pressure drops and stock fuel pump struggles to deliver the required fuel when going back up to wot.

Evidence:

No lift, 3rd & 4th gear pull, kr=0



No lift, 3rd & 4th gear, kr<1.05, lift/shift 5th gear pull, kr>4.9



Cruising-WOT, 3rd gear pull, kr>2.1



I will not be going wot until I get my fuel pump internals from pg. Normally I do lift off the gas to shift when I am going wot. It was a just a fluke that it happen on one of these nights, that revealed a huge problem. So, until I get my internals, I will be babying the car and watching for any more hardcore knock.

Anyone else seeing this problem? Please post your concerns and opinions.
 

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The lift/no-lift has nothing to do with, but you are right you do need new fp internals, your last graph shows fuel pressure around 1100, well below the allowable specs Cobb put out.
 

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Discussion Starter · #3 ·
[quote author=pwrful4 link=topic=137644.msg2967540#msg2967540 date=1235862647]
The lift/no-lift has nothing to do with, but you are right you do need new fp internals, your last graph shows fuel pressure around 1100, well below the allowable specs Cobb put out.
[/quote]

The problem only occurs when I lift. Lifting off the throttle, even for a second, cause the fuel pump to reduce its flow. Then, in an instance, getting back on the throttle cause the fuel pump to sputter due to it can handle the dramatic change in fuel demand that the map requires. So it sputters untill it can keep up with the fuel demand causing knock. Or am I thinking to hard about this and just need new internals? :)
 

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this may not be the case... too bad you can REALLY tune the car to see what the capacity of the pump is. Timing is really important in the tuning mix... so even timing that is a little off can lean you out and possibly cause that knock.

Have you thought about finding a Cobb dealer to dial you in? Just changing internals on the pump will not help if your car is not asking for the right amounts of fuel.

I have a ton more requested from my stock pump with no issues... but my timing is tuned specifically for my mods. I would bet you are just hitting a boost limiter, then the car is purposely pulling back fuel.. at least in the one log.

Last request... Can you borrow a dashhawk so the display is easier to read?

It is tough as balls to get this car to act properly once you have some mods on.
 

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It would be interesting if that's the case.

I've noticed on my car when I do a pull if I start the pull ~3000rpm I get KR higher than if I start the pull around 3500 + rpm. I'm not sure if this would tie in with your problem. Also I've never seen my FP anything lower than 16XXpsi while going WOT, I've seen my FP go as high as 1800 before... but it's typically between 1600-1700psi while at WOT.
 

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Fuel pressure has to drop when you lift off the throttle.....if the injectors stop opening, where do you expect that 1600 psi of fuel to go? It gets purged so you don't blow your lines up. Looks like on that graph the pump can't handle the boost spike. If that could be tuned out you might be ok.
 

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[quote author=smakdown61 link=topic=137644.msg2967734#msg2967734 date=1235871920]
Fuel pressure has to drop when you lift off the throttle.....if the injectors stop opening, where do you expect that 1600 psi of fuel to go? It gets purged so you don't blow your lines up. Looks like on that graph the pump can't handle the boost spike. If that could be tuned out you might be ok.
[/quote]

I agree. When you don't lift you can maintain fuel pressure and your pump is crapping out with the boost spike then recovers when boost comes down. I disagree with diablo you are not hitting the boost limiter. Every stock pump is different, some can handle it and other can't
 

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[quote author=smakdown61 link=topic=137644.msg2967734#msg2967734 date=1235871920]
Fuel pressure has to drop when you lift off the throttle.....if the injectors stop opening, where do you expect that 1600 psi of fuel to go? It gets purged so you don't blow your lines up. Looks like on that graph the pump can't handle the boost spike. If that could be tuned out you might be ok.
[/quote]

From observing my FP when driving normal I noticed that when I lift off the gas my FP stays up for a few seconds then begins to drop slowly. I havent observed this at WOT though, just normal cruising.
 
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