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Author: Subject: Fuel Pressure Overrun
rgb247

posted on 13/7/20 at 10:11 PM Reply With Quote
Fuel Pressure Overrun

Just wanted to get some second opinions before i investigate more.

I have a 1600 xflow running with fuel injection. Since converting to injection i've been chasing out various issues and now drives okay if i'm very gentle with it. If i've been running at medium/high revs and i lift off i get lots of overrun (popping in the exhaust). I've tried richer and leaner on the fuel table but can't get rid of it.

I had a bit of an epiphany when out for a walk today. I have a temporary fuel pressure gauge rigged up and have noticed it at cruise the pressure has been higher than it should be, but it's on target when idling. A little research tonight tells me fuel pressure regulators should be set at zero vacuum, so either WOT or with the vacuum hose unplugged.

So my uneducated thought is that i've set the pressure too high at idle and then get's higher as i reach zero vacuum (fast cruising speed). The injectors can cope with this high pressure and work ok, compensated by my fuel table. But as soon as i lift off the vacuum in the manifold increases faster than the fuel pressure reduces so the added 'suck' of the high vacuum at closed throttle means far too much fuel is sucked in making the mixture rich and hence the overrun. I have an AFR gauge but the overrun pops make it unreliable when it's happening.

So my question is does that sound plausible?

Plan of action is to set fuel pressure conservatively at idle with vacuum hose unattached. Reattach hose and the pressure should drop, if not i've got a problem with my vacuum system. If it does go up then test drive keeping a close eye on a fuel pressure gauge.

I had assumed that fuel pressure should be constant but now i don't think it does. It needs to be relative to manifold pressure not atmospheric pressure so will change with vacuum.

Thoughts welcome.
Thanks

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rusty nuts

posted on 14/7/20 at 06:35 AM Reply With Quote
My Xflow set up doesn’t have the pressure regulator vacuum connected so has no variable fuel pressure, it was built and set up like that? Initially it would bang and pop on over run which was gradually improving as I was making adjustments but it was totally stopped after being set up on a rolling road by someone who knew what he was doing.I believe it is now set to cut the fuel on over run?
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CosKev3

posted on 14/7/20 at 09:41 AM Reply With Quote
My V6 does not run a vac line onto the reg either, my mapper said it's only required for boosted cars when you are running a reg that increases pressure as the boost rises.

Are you sure you aren't running over run fuel cut in your ECU?

That will make it burble and bang on the over run.

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ttalps2000

posted on 14/7/20 at 10:15 AM Reply With Quote
just fitted one to mine whilst at rolling road session. No vacuum hose connected to the reg as not required. Pressure is rock solid throughout.

Get it on the rollers and sorted properlly, money well spent if you ask me!

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rgb247

posted on 14/7/20 at 10:51 AM Reply With Quote
I see what you're saying. That you can run without the vacuum connected because the fuel table takes it into account based on the MAP sensor. I'll make sure pressure is set correctly and maybe also try a drive without any connection. I might have too many inputs,vacuum to regulator and MAP to fuel table confusing things. I had always planned to take it to a rolling road for final tuning but wanted to get it 'close' on the road first.

Is anyone running megasquirt/microsquirt ECU with their Xflow? If so it would be great if i could have a copy of your tune file.

Thanks

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ttalps2000

posted on 14/7/20 at 10:56 AM Reply With Quote
disconnect the vacuum line, set the pressure at idle to 3.5 bar using a gauge.

No point in trying to get it near as they will just re-do the table again mostly likely anyway. From my recent experience you will be constantly chasing your tail

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rusty nuts

posted on 14/7/20 at 11:57 AM Reply With Quote
quote:
Originally posted by rgb247

Is anyone running megasquirt/microsquirt ECU with their Xflow? If so it would be great if i could have a copy of your tune file.

Thanks


Mike R might be able to help ?

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CosKev3

posted on 14/7/20 at 03:40 PM Reply With Quote
quote:
Originally posted by rgb247
I see what you're saying. That you can run without the vacuum connected because the fuel table takes it into account based on the MAP sensor. I'll make sure pressure is set correctly and maybe also try a drive without any connection. I might have too many inputs,vacuum to regulator and MAP to fuel table confusing things. I had always planned to take it to a rolling road for final tuning but wanted to get it 'close' on the road first.

Is anyone running megasquirt/microsquirt ECU with their Xflow? If so it would be great if i could have a copy of your tune file.

Thanks


Is over run fuel cut activated in the ECU?

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rgb247

posted on 14/7/20 at 05:37 PM Reply With Quote
Currently fuel cut is turned off. I didn't like the way it was cutting back in to restart. But i was trying to mask the sympyton, if i can fix the problem with the fuel pressure sugfestions it might be worth another look.

Thanks

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Oddified

posted on 14/7/20 at 08:40 PM Reply With Quote
In general popping in the exhaust on overrun can be stopped/reduced with more fuel and ignition advance in the effected areas on the maps.
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