Fuel Level related Ineffeciency

[Jared]

Really stumped:

My 1995 RRC LWB is acting up. When the fuel level is below 1/2 tank, it will not idle correctly- as erratic as several hundred RPM. It even died on my the other day. Of course, it only happens occasionally. I had a similar problem a year back- one of the O2 sensors went out. At that time, however, the check engine light came on- no such light this time around. Again, it only happens when the fuel is below 1/2 tank- and then it gets worse as the level drops.

A buddy suggested that the fuel cap may be defective. I am at a loss.

[Mr Tj]

The fuel cap idea is actually probably right.

If the RRC fuel system is anything as finickety as the P38A/L422 ones, then I've seen just that kind of behaviour when there is too much of a vacuum for the fuel-pump to work against as it draws more fuel out of the tank. This would probably lead to an air-rich mixture, but maybe not enough for the O2 sensors to signal lean burn (if indeed they do, they're imo more there to catch rich burn with the issues that holds for the cat etc).

Basically, too tight a seal around the fuel tank... who'd have thunk it.

---
Staffan Tj.
'95 RR 4.6HSE NAS
115k
Rioja Red / Walnut

[Landy-Andy]

Check the fuel tank breather tube, it may be blocked or kinked.
On earlier models this vented out by the filler spout.
On later models the breather tube is connected to the Fuel expansion tank & then up to the charcoal cannister which it breathes through (maybe your charcoal cannister is plugged & needs replacing), this would explain why it works ok when the fuel tank is full but as the fuel tank empties a vacuum is created as new air cant get into the tank to replace the burnt fuel.
To complicate things more there is a "anti-surge" valve on the hose between the fuel tank & the fuel expansion tank that could be plugged or failed.
Ahh..the price we pay for clean air !! (complicated emissions systems)
good luck,
Andy