93 Altima stalling at random

Forgot to say, when it stalls it stumbles and sputters like it is fuel starved and when I try to restart it turns over good but won't catch at all, almost like a vapour lock but weather is mild.

How old is the CAT? -----

- gpsman

Sounds like a possibly failing fuel pump, I'd do a pressure/voume test.

any stored codes?

Check the oil pressure sender. If it is failing it can do this by shutting the fuel pump off. The oil change may have given it some help with increased viscosity in the new oil giving a slightly higher pressure.

Any codes?

When it doesn't start, is there a spark when you pull a plug out? Is there fuel pressure at the rail when you put a gauge on?

--scott

now /that/ is exactly the kind of knowledge/experience that makes this forum excellent.

The pressure/volume being adequate obviates the need for checking the oil presure sender.

If the sender itself is damaged or has a frayed wire or connector (intermittent signal), wouldn't that cause his symptom regardless of the correct amount of clean pressurized oil available?

I'm not sure as to your meaning, but I thought it obvious from my earlier post that I was referencing FUEL supply.

Nope. Just means that the pump is OK. The sender being bad allows the pump to run until the engine warms up a bit and the oil thins out fully. Then the switch shuts off the pump and the engine sputters to a stop on the residual pressure in the system. Once the engine cools down again the oil creates just enough pressure that the switch turns the pump on and it starts the cycle over.

The usual switch failure is due to the diaphragm in the switch failing and the oil manages to get into the mechanical side of the switch. This equalizes the pressure on both sides of the switch actuator and the switch responds to very small pressure changes. Instead of on/off at

Sorry but if there's adequate pressure/volume (with the engine running, of course), the pump is not only OK, it is energized, which obviates the need for checking the oil pressure sender. Perhaps you didn't notice my usage of the term adequate. The scenario you describe temporarily disables the pump and inarguably decreases the system pressure and volume.

The switch does NOT "temporarily disable" the fuel pump it will shut it off until the oil pressure is restored. Just like it is supposed to do if the oil pressure drops. The pump will stay off until you try to restart the engine. At that time the FP Relay kicks on, pressurizes the system and you start the engine. The relay only stays on for a few seconds. Once the oil pressure rises (which it should do almost instantly) and crosses the switches threshold it turns on and sends power to the fuel pump, either directly or by telling the ECM that there is oil pressure and turning the relay back on.

In the OPs case the pump turns on for the initial prime. The engine starts and the cold oil trips the switch. Once the oil heats up and thins out some the O/P sender trips off thinking the engine has no oil pressure. That in turn shuts the pump off. The engine the "acts like it is vapor locked" because the pump is now off.

Testing the fuel pump will show absolutely nothing if it's the switch. The proper testing procedure bypasses all the controls on the pump and only tests the pump. Low pressure or volume will not show this symptom. The engine will run fine until the fuel requirement exceeds the ability of the pump to supply fuel, then it will lose power. The common item you see with this is a vehicle that starts and runs fine, until you go over a certain engine % of load, then it falls off until it drops below that load %. The common complaint from the owner is " Well it runs ok but you can't go over XX mph" The same symptoms you get with a partly plugged fuel filter, and for the same reason, low fuel flow.

The clincher to the problem is the stated " Since changing the oil, which was very dirty. It has not stalled" The fresh clean oil can only effect ONE item that has anything to do with the fuel. That would be the oil pressure sending unit.

Of course it's "temporarily disabled" by a lack of electrical power does, your statement is an attempted obfuscation and a semantics difference without distinction.

If that doesn't fit the definiton of disabled, I don't know what could.

No, testing the fuel delivery system at the fuel rail, not simply a bench test of the fuel pump, will clearly indicate a lack of adequate pressure.

Sorry, but still not true irrespective of the number of times you choose to reiterate the same nonsense.

Not the same at all, the latter is a restriction but in both cases they result in and can be diagnosed as a non adequate delivery of pressure or volume at the fuel rail.

Sans any data acquisition whatsoever, that's a guess and no difinitive diagnosis exists. It is still POSSIBLY a failing fuel pump.

Cat was replaced a yr ago as was the fuel pump.

Will chk codes and spark if it stalls again.

Anyone know where the oil pressure sending unit is? - no mention of it in the Hayes manual.

You might indicate the engine type, but a common location is in the near vicinity of the oil filter.

Under the vehicle to the right of the oil filter just above the axle shaft.

i think you're being more than a little harsh. if, as steve indicates, and he's usually pretty reliable, the engine computer has a "fail safe" shutting down the engine on low oil pressure, then everything he says makes sense. if of course the engine doesn't have this fail safe mode, and many older vehicles do not, then your conclusion about the pump could be one that fits.

so, given that there's a question about the input control logic on this vehicle that needs to be understood before jumping to conclusions on its output, why not go check up on the fail safe functionality of this oil pressure switch, then let us know what you find?

but if the fuel is being switched by the ecu, and they usually are, then his point is perfectly valid.