I have a 2008 Sentra SE-R Spec V-2.5 Liter, 6speed manual, 68,500 miles which recently has had a significant drop in the gas mileage (from 30mpg to 24) and intermittently loses power (pedal to the boards and nothing!) when traveling at high speeds. Changed the plugs (2 times-replaced with OEM plugs), air filter (every 10K). Dealer replaced a cracked exhaust manifold (under warrenty) but problem continues. It feels like it is running lean or lacking in fuel. Dealer says the fuel pressure is fine. Help!! any suggestions???
A restriction in the exhaust system could cause this problem. The dealer can check for this condition with a vacuum gauge.
Have the dealer check the coolant temperature sensor reading for the computer.
If this sensor has failed where it’s telling the computer that the coolant never gets up to operating temperature when it actually does, the computer will think the engine is still cold. The computer will then introduce too much gas into the engine. This then causes a drop in fuel mileage and lack of performance from the engine.
Tester
Will check both-Thank you!!
When you have no power(pedal to floor) have you tried pushing in clutch and seeing it revs up?
If not potentially the drive by wire in pedal is failing if so equipped. Eg a wire to engine not cable from accelarator.
It is drive by wire, but that may not be the problem, only the symptom. I’d be looking at the coolant sensor first.
Could be a plugged fuel filter or faulty pump. Might not show up w/a standard fuel pressure test. The fuel pressure would have to be tested during driving to prove/disprove. Is the CEL on, or has it been?
George,
I don’t know where you come up with these theories, but when you test the fuel pressure it doesn’t matter if the engine is running or not. The fuel pressure is a constant pressure. The fuel pressure regulator determines how much fuel is delivered thru the injectors, and how much fuel is returned to the tank.
Tester
@Tester: I’m going to disagree with you. If the fuel filter is partially clogged, the VOLUME of fuel the engine needs at high speeds will not be met, and the PRESSURE will be observed to drop. With a partially clogged filter, the engine can run fine at low and moderate speeds, but upon high demand situations, such as high speed cruising or hard acceleration, it will be starved for fuel.
In a situation where the engine is starving for fuel, it should set a malfunction code and turn on the MIL.
Also, the logic on a 2008 is sophisticated enough that if the coolant temp sensor is bad such that the engine is being perceived as constantly below operating temp, it will eventually throw a code “engine stays cold too long” Too much fuel being delivered will also set a code if it’s running rich.
So the next question I have that the OP didn’t mention: Is the check engine light on? Have the codes been read for existing or pending codes?
Volume and pressure are two different things. You can have the proper fuel pressure and not enough volume. But a fuel pressure test won’t show that. You have to do a fuel volume test.
But look at the symtom the OP describes. The mileage dropped from 30 to 24 MPG. Does that sound like a fuel delivery problem?
Tester
@Tester: Yes, that could be a fuel delivery problem. If the engine is starved for fuel, it seems logical at first glance that you’d actually get better mileage, but really the engine’s efficiency is suffering overall. The engine is probably not remaining in closed-loop operation for very long, if at all, plus long-term fuel trim is likely all out of whack. I’d still like to know what malfunction codes are present.
But I’ll admit it’s all heresy without more information, and you could be right.
@Tester, I don’t agree. At least not completely. If the fuel filter was partially clogged and limited the flow rate through it to a fixed maximum rate, that could indeed show up as an unexpected drop in fuel pressure at the rail during conditions where a large amount of fuel was being used by the engine, such as rapid acceleration. The rail pressure would measure ok with the engine stopped and at idle, but as more fuel was being used, at some point not enough flow rate could get past the clogged fuel filter, and the only possibility at that point is that the fuel pressure after the filter (i.e. at the rail) would drop. Someone maybe can think of a better analogy, but in electronics there’s a thing called a voltage regulator. If too much series resistance is put in front of a voltage regulator, the symptom is that it will regulate the output voltage ok at low currents, but the voltage output will drop if the load current demand gets too high.
You could argue that if the fuel filter were clogged, the fuel pump would compensate by increasing the pressure upstream of the fuel filter. That indeed may be the case in newer cars. But if that were the case, then the fuel filter wouldn’t be clogged enough to cause a symptom.
Also, I don’t agree that the fuel pressure is constant. It’s true, it is at least supposed to be constant with the engine stopped. But I’ve tested my Corolla’s fuel pressure regulator by measuring the fuel pressure at the rail with the engine running, in order to verify that the regulator is properly compensating its output pressure according to intake manifold vacuum. With a properly operating fuel pressure regulator, the rail pressure is supposed to drop as manifold vacuum increases.
George,
Gasoline is a liquid. And if you remember from your physics class a liquid can’t be compressed. So a restricted fuel filter can restrict the flow of the gasoline but it can’t reduce the pressure applied to that liquid.
I just replaced the fuel pump on my daughters Honda. The fuel pressure spec on her car is 28-36 PSI. The bad fuel pump provided 22 PSI at idle and while reving the engine. And while reving the engine it would stall out. When the new fuel pump was installed is provided 32 PSI at idle and while reving the engine.
The fuel pressure regulator keeps the fuel pressure constant. When stepping on the gas, the fuel pressure regulator restricts the flow to the return line to the gas tank and forces more fuel thru the injectors. When at idle, the fuel pressure regulator allows more fuel to return to the gas tank and less fuel to be forced thru the injectors. But the fuel pressure regulator keeps the fuel pressure constant.
Tester
hmmm … are you saying there’s something about fuel filters that prevent a drop in fuel pressure, input to output, when there is a flow through them? I’ve never heard of that, but maybe something about their construction I don’t understand, so could be I guess.
George.
If we were talking about an air filter then yes, a restrictive filter can effect the pressure. But air is compressible. You would learn this if you took a Industrial Ventilation Course.
Tester
Actually 24 mpg is normal for this car. 30 mpg would be unusual.
I think everyone is arguing about the same thing, from a different perspective. If you check the static fuel pressure (key on, engine not running) with a partially clogged filter, the pressure will be observed to be the same as with a new filter, or no filter at all. But as soon as you start driving, the pressure will drop at the fuel rail because the clogged filter is limiting the amount of fuel that can be delivered. You can do a simple experiment with a garden hose to see how this works, as water is not compressible: Kink the hose so that it’s partially restricted. Now open the nozzle and see what happens—you’ll get a brief surge of high pressure water, followed by a thin dribble as you lose pressure after the restriction. Close the nozzle and the pressure will go back up. If you think of electrical voltage as pressure, you can have the same situation with a bad car battery that reads perfect voltage with no demand on it, then drops very low when you try to start the vehicle.
Probably the best test to rule out a fuel system problem would be to attach a pressure gauge with a flexible hose to the test port on the fuel rail, then duct-tape it somewhere so that it can be viewed through the windshield. Now go for a drive and observe what the gauge reads when the engine acts like it’s starving. If it’s lower than the minimum spec, either the fuel filter is clogged or the fuel pump is weak. If the pressure reads nice n’ high, then you have another problem.
Not sure why I’m bothering because it seems that the OP is long gone by now…
Good post @oblivion, you clarified the discussion, I agree w/your analysis completely.
Sorry for the absence-I’ve been away. There hasn’t been any SES codes only the marked decrease in mileage and loss of power. I keep track of the mileage in a notebook every time I fill up so I noticed the drop which was sudden. I was told by the dealership that there wasn’t a fuel filter which is able to be changed-is that right?
My husband (who is fairly good at car repairs)rented a fuel pressure tester but couldn’t find the test ports to connect it to. Where is this in the Sentra? And how do you check the cooling sensor and where is it located.
Thank you all for your posts!
Most japanese car manufacturers DON’T have a fuel pressure test port.
Hubby will have to tee in . . . he’ll need to rent the master kit