390 Fuel pressure
Discussion
Wedg1e, help me !
I have my new toy in bits already, as I wanted to play with my new digital fuel pressure guage. Also the 390 didn't seem to have a scarey enough 1/4 mile time compared with my old 350. I tested the three fuel pressure regulators I have on the car, at idle with no vacuum:
Original with the car 2.30 bar
Weber Alpha brand new 250kPa 2.50 bar
Used range rover regulator 2.55 bar
So I swapped the used regulator I have to get me out of the low fuel pressure melted pistons region, and the car is significantly quicker now. Trouble is that whenever I take me foot off the 'go' pedal the revs drop very low for a couple of seconds, and sometimes stall the car. Now, in theory the flapper system should work fine at 2.5 bar and match the air mass properly at cruise. The ECU doesn't look to have been tampered with and I wanted to know if all 390SE cars used the standard 4CU, or if they had some bits tweaked inside to help with the extra fueling needed under acceleration.
I also have a wide band lamda sensor/display thingy from NGK, but I need to weld the boss in the exhaust - any volunteers?
-Jim
I have my new toy in bits already, as I wanted to play with my new digital fuel pressure guage. Also the 390 didn't seem to have a scarey enough 1/4 mile time compared with my old 350. I tested the three fuel pressure regulators I have on the car, at idle with no vacuum:
Original with the car 2.30 bar
Weber Alpha brand new 250kPa 2.50 bar
Used range rover regulator 2.55 bar
So I swapped the used regulator I have to get me out of the low fuel pressure melted pistons region, and the car is significantly quicker now. Trouble is that whenever I take me foot off the 'go' pedal the revs drop very low for a couple of seconds, and sometimes stall the car. Now, in theory the flapper system should work fine at 2.5 bar and match the air mass properly at cruise. The ECU doesn't look to have been tampered with and I wanted to know if all 390SE cars used the standard 4CU, or if they had some bits tweaked inside to help with the extra fueling needed under acceleration.
I also have a wide band lamda sensor/display thingy from NGK, but I need to weld the boss in the exhaust - any volunteers?
-Jim
Just a quickie as I'm about to shut down the PC... bu99ering off to France for a few days 
... the 4CU in my car has a resistor set unlike any I've come across on any other V8, whether TVR, SD1 or Rangie.
However it's also had the AFM replaced, both in the past and later by me (with a Jag one) so the fuelling is all over the shop anyway.
I also have the adjustable FP regulator and run about 1.8Bar... probably due to the fatter injectors
There are so many variables that can be fiddled, not to mention the different combinations of pistons, cam, valvetrain, fuel, that it seems impossible to recommend a particular setting for the fuelling, other than could be determined empirically or by using a dyno (which would involve a degree of experimentation anyway, of course).
As an example, today I did a road test with the AFM flap spring at +2 and -2 'clicks' from the setting I started at.
In each case I could identify points in the rev range where the response improved or deteriorated relative to the other two settings. Now changing the ECU resistors could perhaps optimise any of the three to be 'better' than the other two, but I bet that any of them would be a compromise...
ETA: so to answer your question, no the 390 ECU is not 'standard'... not that there was a 'standard' anyway.
... the 4CU in my car has a resistor set unlike any I've come across on any other V8, whether TVR, SD1 or Rangie. However it's also had the AFM replaced, both in the past and later by me (with a Jag one) so the fuelling is all over the shop anyway.
I also have the adjustable FP regulator and run about 1.8Bar... probably due to the fatter injectors
There are so many variables that can be fiddled, not to mention the different combinations of pistons, cam, valvetrain, fuel, that it seems impossible to recommend a particular setting for the fuelling, other than could be determined empirically or by using a dyno (which would involve a degree of experimentation anyway, of course).
As an example, today I did a road test with the AFM flap spring at +2 and -2 'clicks' from the setting I started at.
In each case I could identify points in the rev range where the response improved or deteriorated relative to the other two settings. Now changing the ECU resistors could perhaps optimise any of the three to be 'better' than the other two, but I bet that any of them would be a compromise...
ETA: so to answer your question, no the 390 ECU is not 'standard'... not that there was a 'standard' anyway.
Edited by wedg1e on Saturday 16th September 23:42
As a temporary measure try adjusting the idle screw - throttle stop to bring the revs up to, say, 1100 when idle. It should show less tendency to bog down when you come off the gas.
It might be that it's a bit rich and when you come off the gas gets over-rich.
Another (opposite) possibility is that the manifold vacuum is causing the fuel pressure to drop dramatically, it might be that the new regulator is more affected, you could try restricting the pipe from the inlet manifold to the regulator.
It might be that it's a bit rich and when you come off the gas gets over-rich.
Another (opposite) possibility is that the manifold vacuum is causing the fuel pressure to drop dramatically, it might be that the new regulator is more affected, you could try restricting the pipe from the inlet manifold to the regulator.
Definately too rich at idle - but the AFM screw was/is backed out 13 turns which is the limit on these flappers. I do have a newish AFM, which even has the plastic plug still installed - I may try this. I was looking into the history, and just before the engine was rebuilt the fuel pressure was changed from 2.8 bar to 3.1 bar so maybe I will get me lambda sensor installed before I start messing things up.
Incidentally, the accelerator circuit in the 4CU simply turns the injectors both on constantly, as opposed to a more civilised modification of injector timing - seems a bit odd, but about 40% of the circuitry is devoted to this function. It looks like a rat's nest of op amps and transistors - the rest of the gubbins is straight forward.
-Jim
Incidentally, the accelerator circuit in the 4CU simply turns the injectors both on constantly, as opposed to a more civilised modification of injector timing - seems a bit odd, but about 40% of the circuitry is devoted to this function. It looks like a rat's nest of op amps and transistors - the rest of the gubbins is straight forward.
-Jim
That may explain it. I saw some stuff on other threads about tweaking the AFM by changing the flapper internals around. Incidentally, eggs sucking etc, but have you verified the throttle pot is linear and has no unconnected spots? And - you've checked the ENGINE TEMPERATURE SENSOR!!!
P.S. I have a nice MiG welder if you wanna come by and weld in that Lambda sensor...
;^)
P.S. I have a nice MiG welder if you wanna come by and weld in that Lambda sensor...
;^)
Edited by adam quantrill on Sunday 17th September 22:09
I need to get me a M18 x 1.5 tap, as according to the install manual the thread will be warped after welding in. What you doing monday night?
No, I haven't checked the throttle pot, but I have discovered that if you disconnect the throttle pot, all the acceleration enrichment circuitry is cut out, and you should be able to get the basic Air fuel metering set up pretty accurately - assuming the flapper hasn't been diddled with.
-Jim
No, I haven't checked the throttle pot, but I have discovered that if you disconnect the throttle pot, all the acceleration enrichment circuitry is cut out, and you should be able to get the basic Air fuel metering set up pretty accurately - assuming the flapper hasn't been diddled with.
-Jim
I need to get me a M18 x 1.5 tap, as according to the install manual the thread will be warped after welding in. What you doing monday night?
No, I haven't checked the throttle pot, but I have discovered that if you disconnect the throttle pot, all the acceleration enrichment circuitry is cut out, and you should be able to get the basic Air fuel metering set up pretty accurately - assuming the flapper hasn't been diddled with.
-Jim
No, I haven't checked the throttle pot, but I have discovered that if you disconnect the throttle pot, all the acceleration enrichment circuitry is cut out, and you should be able to get the basic Air fuel metering set up pretty accurately - assuming the flapper hasn't been diddled with.
-Jim
jchase said:
I need to get me a M18 x 1.5 tap, as according to the install manual the thread will be warped after welding in.
I haven't had any problem with that. The boss should be quite substantial, and you won't be putting much heat into it since the other side of the weld is just thin steel.
jchase said:
Well, took the manifold off, and welded the boss in, with the plug in, just incase - and it is well and truely stuck in there. So I have ordered me a M18x1.5 tap. I guess I put a bit too much weld on.
Heat the area around the boss with a blowtorch or an oxy acetlylene welding flame and then try to take the plug out.
I used my erm, 180 amp Mig welder, on setting 3 - might have been too much power - but it didn't hole the tubes and I got penetration (ooo-er). I think because the amount of weld was more on the curved sides, it left a lot of risidual stress in the boss - should have banged the tube flat before I drilled the hole in - d'oh . They didn't tach us that when I took my C&G introductory welding course!
I got a sales brochure off flea bay for the 390, and it says the passenger door has a stale air extractor in it - so it isn't a speaker grill! It also says " A higher capacity air flow meter , revised injector system and ECU are utilised.." the AFM has the NCK number written on it, and the ECU has some other number written on it, both in thick black pen. If only I knew the fuel pressure it came out the factory with...
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