[urq] Re: URQ fuel pressure numbers

[Ameer Antar]

ya carbs are easy to troubleshoot, just replace the brass/rubber parts. But 
they can be a pain to set up and tune, especially w/ those DCOE webers on 
little British cars. Setting 2 SU's was hard enough for me. The other thing 
about carbs, is the auto-choke always goes. Manual is so much better, b/c 
it gives you the control. I've heard good things about the new electric 
chokes, though I'm sure that too has a lifetime. FI is great when it's 
working cuz the mixture is precise and there's less maintenance. But 
troubleshooting is complex and requires special tools.

Anyway, if the d.c. is low, then the mixture is too rich, ie. the ecu is 
trying to lean the mixture. The d.c you measure is the output of the ecu to 
the frequency valve. This means this valve is almost opening very little, 
almost 0%. If the valve is closed, almost no fuel is returned to the tank. 
This creates higher pressure on the other side of the valve. The other side 
of the valve is connected to the fuel dist. chambers, and the increased 
pressure in the chambers mean a leaner mixture, b/c the airflow sensor 
plate meets more resistance to going up. This is how the ecu leans the 
mixture by o2 sensor command or when the warm-up regulator warms up. The 
warm-up regulator also increases the pressure in the chambers when it is warm.

When d.c is high, the opposite will happen, or should happen. At 90%+ the 
valve is almost always open, so all the fuel pressure is relieved back into 
the tank, allowing the airflow plate to rise, increasing fuel metered to 
the injectors. This is the response to a lean mixture. You'd know all this 
from the Bosch FI book...

Sounds like the problem is intermittent and/or only occurs during certain 
conditions. A good Air-fuel ratio gauge can at least tell you what's going 
on when. It will tell you what the o2 sensor sees and what the ecu sees. I 
had similar issues. It helps to at least know if the problem is rich or 
lean running, b/c either will cause poor performance. I could see in the 
morning my mixture not able to get anywhere near the middle, so I knew the 
warm-up regulator had issues. I could also see, after certain adjustments, 
the WOT enrichment was too high, causing the car to actually lose power at 
WOT. It can be a helpful tool, but it won't tell you everything, and 
shouldn't be used to tune your 350HP engine. Those guys need analyzers and 
dynos.

sounds like you are on yer way to finding something out....

-ameer


At 12:25 PM 10/27/00, you wrote:
>Thanks for the reply.
>The gremlin I'm chasing is low end power fluctuations. At times it pulls what
>feels normal, and is a pleasure to drive. Other times it is very slow and
>sluggish. Full throttle is good. The crackle is usually in direct correlation
>to the other behavior. At times it has even had slight hesitations (not a
>misfire) at 1/2 throttle acceleration.
>If there is a constant, it is that as I turn the mixture screw CCW (giving
>higher DC readings) the low end seems better but the crackle is obnoxious. If
>I turn it CW (lower DC readings) the crackle disappears, but it is a dog to
>get going. Is the latter an indication of lean running, or is the higher DC
>number showing lean?
>It is confusing to me from the stand point of discussions stating that a
>reading of 4.6 indicates a rich condition, AND that the ECU is trying to
>richen the mixture. Those are opposite situations. In other words, what ever
>the DC is trying to do means the engine is experiencing the opposite. Right?
>Carburetors were soooo easy. I guess it must be a labor of love.
>Thanks for all the help.
>
>Dennis