Strange car problem (fuel injection)

[Tsal]

Hey all you gearheads, can anyone give me a bit of input?

I drive a 1998 Nissan Almera (Sentra in the States, me thinks) 1.6i gasoline engine. I got this strange problem with fuel/air: whenever I press the accelerator quickly, the engine coughs a bit, and only after that the revs rise in normal pace. No matter what the revs are, is it hot or cold engine, a quick press of pedal results in a single cough, then back to normal.

It feels like it gets a more gas immediately, but it takes a second for air intake to reach the right position, or vice versa. Fuel filter has been replaced, fuel pump starts and stops by ear. All the control sensors are reading correctly.

I'm thinking it could be the aux. air intake needle valve is stuck, or a problem with fuel pressure?

[1958Bassman]

Originally Posted by Tsal

Hey all you gearheads, can anyone give me a bit of input?

I drive a 1998 Nissan Almera (Sentra in the States, me thinks) 1.6i gasoline engine. I got this strange problem with fuel/air: whenever I press the accelerator quickly, the engine coughs a bit, and only after that the revs rise in normal pace. No matter what the revs are, is it hot or cold engine, a quick press of pedal results in a single cough, then back to normal.

It feels like it gets a more gas immediately, but it takes a second for air intake to reach the right position, or vice versa. Fuel filter has been replaced, fuel pump starts and stops by ear. All the control sensors are reading correctly.

I'm thinking it could be the aux. air intake needle valve is stuck, or a problem with fuel pressure?

That seems like it may be a TPS (Throttle Polition Sensor) issue but it could be a problem with sensing how much vacuum you have. I'm not sure the aux air intake would cause this because the throttle plate is opening and you should have plenty of air getting into the intake. It could definitely be a fuel pressure or regulator issue. Check the pressure, preferably while it's on the road so you can see how the pressure changes while you drive. If it drops, you may have a badly clogged fuel filter.

[hbarcat]

When you goose the gas, you should get a short squirt of auxiliary fuel. This is dependent on the TPS telling you the throttle valve is rapidly opening. If the TPS is defective then there will be no aux fuel and the mixture will be lean for a second or two and the engine will cough. Check the TPS first.

I doubt it's low fuel pressure due to a clogged fuel filter or faulty fuel pump or regulator. This would usually cause a gross lean condition whenever the throttle valve is even close to wide open and would be progressively worse until full WOT.

A possibility is a vacuum leak. These are notoriously difficult to diagnose if you can't find it by poking around by hand with the engine running.

[Tsal]

Thanks, guys. TPS values are within normal limit, and seem to react to throttle position normally. My dad and I thought it might be that the aux air valve needle is stuck under the TPS, though - it's located in very difficult place to check.

There seems to be a minor sound from the intake manifold, it might be there's a small leak there. Will check the fuel pressure while running next, and if it doesn't help, I'll take it to a local shop that's good with manifold stuff.

[elgecko]

Have you visually inspected the TPS element? I know on Toyotas, a dirty one can cause the symptoms you describe.

[T-Bird]

Moi.

Vaikkakin omat kokemukset ruiskuista ovat vähäisempiä kuin kaasareista, veikkaisin 70% varmuudella imuvuotoa jos diagnostiikka ei herjaa viallista anturia tai boksia.

Ei tainnut Nissanilla olla tuossa vaiheessa erillistä ruiskusuutinta hoitamassa kiihdytyspumpun virkaa?
Jos oli, sen viallisuus aiheuttaa toki samanlaisen käyttäytymisen: menee laihalle, yskäisee ja kun pääpiiri tulee mukaan, antaa pääsuuttimet tarvittavan bensan ja kiihdytys alkaa hiljalleen.

T:Samuli

[Jay2U]

A bad lambda probe in he exhoust could be the cause.

[Bob Lee (QSC)]

I'm thinking vacuum leak, too.

[Mike N]

Does that car have a MAP or a MAF sensor?

If you have a MAF sensor, clean the element with a good aerosol cleaner and check the duct work between the MAF and the throttle body for any cracks or leaks.

If you have a MAP sensor, check the vacuum hose for any leaks, kinks or carbon buildup that could affect the signal the sensor sees.

[Mike N]

Originally Posted by Jay2U

A bad lambda probe in he exhoust could be the cause.

Doubt it, but that's the first thing Auto Zone would try and sell you.

[Tsal]

The local version of AutoZone actually DID sell a lambda probe to my dad, before we got access to a proper ECU reading device

I have to say the local Nissan dealerships aren't very good shops, the first shop didn't get any codes for my faulty MAF or even check the sensor voltages. The second located the problem to the MAF (how, they didn't tell me) but wanted to order a $1200 OEM part for the '98 model.. A new non-OEM part was located from eBay for $90.

But, FWIW, it's been a bonding moment for me and my step-dad, who's a retired bus mechanic. He's right at home fixing old junkers and small fuel engines, but ECU stuff goes over his head. I know zero about mechanics, but a little about electronics, computers and how to acquire shop manuals written in English. We come together pretty well on a late-90's Nissan

[Mike N]

I'd try cleaning the existing MAF before replacing it. A faulty MAF will kick a trouble code and turn on the check engine light, a dirty one won't. At this point you've got nothing to lose but $6 worth of cleaner and a little time. If you opt to go this route, use an aerosol cleaner ONLY, don't touch the MAF internals with a Q-tip or anything else. I've pulled MAF's out that looked like Chia pets, they were so dirty.

A quick overview of how a MAF works;

The computer (PCM) heats an element in the MAF to a pre-determined temperature. Incoming air cools this element. Depending how quickly or slowly the element is cooled "tells" the PCM how much air is entering the engine, and the PCM calculates how much fuel to add through the injectors. At the same time, the PCM is looking at other inputs, such as throttle position, vehicle speed, O2 sensor readings etc.....

Now, if the element in the MAF was dirty, the incoming air would have a harder time cooling the element, since the dirt would act as insulation. When this happens, the engine winds up running lean and can stumble upon acceleration, since the MAF is going to be somewhat slower at communicating correct information. The reason the check engine light doesn't come on is because the MAF is still operating within a normal range, and the computer isn't trained to look for a slow responding sensor.

If you have access to a scanner that can give you a data stream, see how fast the MAF responds to you snapping the throttle. Also, look at your fuel trims. The scanner should show short term and long term trims, possibly abbreviated as STFT and LTFT. Short term is kinda what's happening now, and long term is measured over a period of time, like the name implies. The numbers should be close to zero. Negative numbers mean the PCM is taking fuel away (the car is running rich and the PCM is trying to compensate), while positive numbers mean the car is running lean and the PCM is adding fuel.

[Bob Lee (QSC)]

Great info, Mike. Thanks!

[runmikeyrun]

If it doesn't have a carburetor I can't help you, sorry

good luck.