Fuel pressure too high - what the???

PL · Post by PL » Tue Dec 11, 2007 3:05 pm

Hey Guys,

This should probably be in the SR20 section but I thought I'd post it here so it might get more people to read it.

I was tuning an(other) S13 SR20DET on the weekend. T28 turbo, 480cc injectors, Z32 AFM. I had a map that has worked in other engine with the same setup but this one was running like a hairy goat with the same map. Had to lower the K value to about 60% of where it started!!!

Long story short, the fuel pressure was running about 10psi higher than it should be (52psi at 0psi boost). I tried 2 different FPR's to no avail.

All I can think of is that the fuel pump was flowing more than the FPR was capable of controlling. It was a new Bosch 040 pump. Do these things really flow that much fuel? Fuel flow was a bit over 3 lpm at idle.

Anyone seen this? Ed?

Thanks
Pete L

kenbam · Post by **kenbam** » Tue Dec 11, 2007 6:31 pm

Id take the return line off and see if pressure drops, your return line might be blocked/kinked/damaged. I have an 040 in my rb20, pressure is fine.

PL · Post by PL » Tue Dec 11, 2007 6:36 pm

Yeah, that was the 2nd thing I did (after trying another FPR). No change.

It's really gotta be the pump. I guess we could bleed a bit of the pressure back to the tank but it just seems a bit wrong!

Thanks
PL

kenbam · Post by **kenbam** » Tue Dec 11, 2007 7:04 pm

Does the the vacuum line for the regulator have a solenoid on it? it might be faulty and cutting off manifold vacuum to the regulator. Most nissans do i think, raises fuel pressure on engine shutdown to stop fuel vapourisation in the rail from heat soak.

PL · Post by PL » Tue Dec 11, 2007 7:13 pm

No, it goes straight into the manifold. Even with the vacuum line removed the fuel pressure was 52psi when it should have been around 43.

PL

kenbam · Post by **kenbam** » Tue Dec 11, 2007 7:19 pm

with vacuum line removed the pressure should be higher (no vacuum=atmospheric pressure=full load). Do you have access to a mighty vac, hand held vaccum pump, something to apply vacuum to the regulator and see if it decreases pressure. In order to have pressure you need a restriction to flow, need to find out where its coming from. Could be too much from pump as you suggested but i think its unlikely.

PL · Post by PL » Tue Dec 11, 2007 7:28 pm

Yeah, I agree. I've tuned bucketloads of SR's now - and never had this problem! From checks on my own SR I know that fuel pressure should be around 36psi at idle, 43psi (3 bar) at atmospheric and then it should increase with boost from there on. This one was around 10psi above where it should be all through the range. Hmmmm...

PL

Hendrick301 · Post by **Hendrick301** » Thu Dec 13, 2007 7:32 am

I know on Z31's the ECU has a bit of control on the powerflow to the fuel pump.

I too have this problem having the injection multiplier reduced from 365 to about 300. I have a walbro 255 fuel pump. I need to recheck my fuel pressure because i believe it too was a bit higher then normal. I have a new nissan FPR and the vac line straight to the intake.

here is a little bit more info on how it is set up.

http://www.redz31.com/images/fuel/pumpvolt.JPG

PL · Post by PL » Thu Dec 13, 2007 9:08 am

SR20 also has a form of fuel pump voltage control (they just switch a series load resistor in/out of circuit). I always believed that it was just to decrease fuel pump noise at low load. Once boost comes up the pump gets full voltage.

Most (all) of the engine conversions I've seen run the pump at full voltage continuously - without a problem.

I have a Walbro 255 to go on my SR so it'll be interesting to see how fuel pressure goes!

PL

Hendrick301 · Post by **Hendrick301** » Thu Dec 13, 2007 10:38 am

well im going to try and figure out how to hard wire the pump this weekend and update with results

PL · Post by PL » Thu Dec 13, 2007 11:34 am

The problem with fuel pumps is usually getting enough voltage to them to get the full flow from them. That's why this "too much fuel pressure" problem is such a weird one for me.

Those Walbro pumps use A LOT of current (10A) so make sure you have decent wire feeding it. If you wanna take advantage of the full flow of course. If it's causing excess fuel pressure then this will be the last thing you want!

PL

knightrous · Post by **knightrous** » Thu Dec 13, 2007 12:53 pm

Hi Pete,

I know this doesn't look at solving the real problem, but you could pick up one of these cheap little PWM speed controller kits from Dick Smith, solder in a bigger transistor capable of 20+amps and bolt a block of aluminium to it for a heat sink, wire it up with the pump, then you can at least turn the pump down a bit on command... You could even be tricky and remove the pot, and feed a signal into the little controller from the TPS and have it increase the pump pressure with throttle increase.

It's a bandaid solution, but it might help you out.

PL · Post by PL » Thu Dec 13, 2007 1:12 pm

Yeah, true. There are plenty of FETs around that would do the job. The other way would be to just wire in a big power resistor to drop some voltage. It goes against the grain though!

Probably the easiest solution would be to just use a brass tee in the delivery line so he could bypass some of the pressure back to the tank. Just need to braze up the fitting and drill a small hole in it to adjust the amount being bled off. Quick 'n' dirty!

PL

LMQT · Post by **LMQT** » Fri Dec 14, 2007 6:16 am

PL wrote:Hey Guys,

This should probably be in the SR20 section but I thought I'd post it here so it might get more people to read it.

I was tuning an(other) S13 SR20DET on the weekend. T28 turbo, 480cc injectors, Z32 AFM. I had a map that has worked in other engine with the same setup but this one was running like a hairy goat with the same map. Had to lower the K value to about 60% of where it started!!!

Long story short, the fuel pressure was running about 10psi higher than it should be (52psi at 0psi boost). I tried 2 different FPR's to no avail.

All I can think of is that the fuel pump was flowing more than the FPR was capable of controlling. It was a new Bosch 040 pump. Do these things really flow that much fuel? Fuel flow was a bit over 3 lpm at idle.

Anyone seen this? Ed?

Thanks
Pete L

Exactly the same problem with a new SARD 235L/h on my CA18DET

Approximatly 8PSI more than my old JECS (Nissan) pump !!!!

I've got a Nismo adjustable fuel pressure regulator, but it can't manage pressure correctly

I also think that pump send to much pressure for him

Pingu · Post by **Pingu** » Tue Jan 01, 2008 4:22 pm

Use a larger diameter / high flow capacity regulator - they are able to send large flows back to the tank which suits eager pumps

Most RB20's have two factory regulators - which seems to handle the jandle.

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Fuel pressure too high - what the???

Fuel pressure too high - what the???

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