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my 1999 s80 hesitates, sputters and eventually stalls only when the gas tank is between full and 3/4. After pulling over the engine restarts, and it may run fine for a while. It seems to stall predominately when I am decellerating when coming to a stop or slow down.
the car runs fine with less than 3/4 of a tank. I have tried different gas but noything seems to work. any thoughts?
 

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If engine tickover is normally completely smooth & steady, then wonder whether fuel filler cap/cap seal & fuel tank breather pipes are OK. If original still fitted, it's well worth replacing the fuel filter.

If tickover is lumpy or engine hunts or surges then since problem occurs when decelerating, cleaning the throttle body may assist. Do you know whether your ECU has most up-to-date software loaded?

Other than this it might be a process of elimination e.g. fuel pump, plugs, coil(s), leads or just possibly an air leak.

Good luck.
 

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If your car is a bi-fuel one and if you mean Compressed Natural Gas (methane) with "gas", we may have the same problem:

In the second of my family's (two) V70 bi-fuel cars (year model 2000) the engine frequently stalls and requires the driver to manually switch from CNG to petrol.

The CNG tank has been filled up only once and we have not been able to consume the methane contents during the three weeks we have owned the car. Therefore, I do not know if the stalling problem is reduced or otherwise affected by the tank pressure.

Sometimes (unfortunately seldom) the CNG tank valves are open, feeding the engine with methane. However, the engine soon tends to stall again, particularly when the accelerator pedal is released to decelerate the car. Before stalling I have sometimes heard a 'clonk' from the tank, which is louder than the valve sounds from our other V70 bi-fuel Volvo (the first one was bought one month earlier in August).

This, our 'August', V70 works better and its engine does not stall until the CNG has been consumed. Nevertheless, it then requires manual switching to petrol and the CNG tank pressure indicator needle has got stuck at 100% (the trip odometer is needed to estimate when CNG must be filled up).

I am now trying to find technical/mechanical descriptions of the CNG sensors and actuators to explain the symptoms. I want to repair or replace the *failing* components, not anything else.

This may be posted also under some other forum with emphasis on problems with bi-fuel and CNG tank components.
Anyway, I wish you good luck!
 
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