2001 Altima GXE & p0402

[msl3785]

I've got a 2001 Altima, 231k miles, runs great but the P0402 is pissing me off. I had the PCV valve changed for the first time and had a diagnosis completed for the P0402.

I had a tech tell me my EGR ports could be partially blocked which results in the exessive flow to the others.

Car runs great out of the garage when motor is cold.
After it warms up, i get a slight hesitation when i accelerate slowly from a stop.
1100 RPM @ 15-20 MPH and i can feel a small shake thru the gas pedal. Give it more gas and it goes away.

Does this sound like P0402?
Sometimes I get P0303 along with it. CEL comes and goes. Gas mileage was crappy all winter long too.

Is it possible to clean this out without pulling the intake manifold off?

I replaced the BPT valve back in the fall.
I checked the EGR solenoid. works the way it should with battery voltage applied / removed.
All vacuum hoses (that i can see from above) look good. I've replaced a few over the past year or two.
Trying to knock out the simple stuff first.
FSM mentions camshaft position sensor and MAF sensor. Is there any other way to check these components other than thru the ECM? I'm not crazy about back-probing into the car's brain and having something go wrong.

Any help would be appreciated.

Thanks

[msl3785]

Question about the hose that goes from the bottom of the BPT valve to the small pipe.

Inspected the hose today at lunch time. Hose is good.
I removed the BPT valve from the car.
I applied vacuum to one of the top connections and plugged the other. Bottom port remained open. Vacuum does not hold. If I understand the FSM correctly, this is the correct way to test this component and it is working as it should.

Now the real question:

Does the small metal tube that the bottom hose from the BPT valve connects to supply vacuum or exhaust pressure to the BPT valve?? is there an orifice built into this tube??

I hope someone can help.

[msl3785]

Can somebody please explain the proper operation of the BPT valve? Does the bottom port receive exhaust pressure or vacuum?
what happens when it receives exhaust pressure of vacuum. FSM is very confusing to me.

Thanks

[Timothy S]

According to the Hanes manual, the Altima's Backpressure transducer is connected at the top to manifold vaccum off of the egr valve control line. the bottom is connected to the Exhaust gas line in the manifold. When the engine is accellerating, exhaust backpressure closes the transducer's normally open leak to the atmosphere, allowing the vacum to open the egr valve.
In your test, the EG transducer should have held no vacum at the top port. The lower port should receive pressure when the engine is running. The pressure at higher RPM should become high enough to make the top port hold vaccum. This is a safety to prevent a vacum leak to the intake manifold caused by a egr valve open at the wrong time.
Have you tried moving the egr valve by hand- touching the diaphram? you can feel it move when you rev the engine if it is working properly.
If you clean anything, it should be the seat for the egr valve itself. This is a last resort, as the bolts are hard to remove and you'll need a new egr valve gasket.

[msl3785]

Saturday morning i cleaned out the exhaust tube with the orifice the connects to the bottom of the BPT valve. That thing was gunked up real good. Got some bottle brushes and tried to clean out the EGR pipe at the EGR valve connection and EGR temp sensor location as best as i could. Used some seafoam and throttle body cleaner. Now I'm thinking maybe the problem was blockage on the exhaust side. No exhaust pressure to the bottom port of the BPT valve would trip the light. Correct? Anyhow, put everything back together, drove to auto parts store and got the light turned off. Still off as of this morning. Still have that little hesitation stutter going on every now and then.

Any thoughts?

[msl3785]

Monday afternoon check engine light returned.
Autozone verified P0402 again by itself.
Checked BPT valve operation on car.
At idle, applied vacuum to top port while plugging the other, vacuum does not hold.
Had a buddy hold the motor to 2200 RPM, applied vacuum to top port while plugging the other, vacuum holds. BPT valve functions as it should.
I'm guessing the EGR pipe from the exhaust manifold is flowing the way it should as well. Producing enough exhaust pressure at 2200 RPM to activate the BPT.

Nissan FSM mentions testing the camshaft position sensor for the error code.
I'm wondering if this is the issue because of the hesitation the motor gets at low RPM.

BTW, i feel like i'm having a conversation with myself due to the lack of interest in this topic. Just sayin'.....
..

[Timothy S]

msl3785,
Sounds like your BPT is fine and exhaust egr passages are ok too. 2.4 liter Altimas (pre 2003) have the camshaft position sensor in the distributor. You might try unplugging the electrical connectors to the distributor a few times, or using electrical contact cleaner sold in home improvement and auto parts stores. Often in older cars like ours, contact cleaner goes a long way!

[msl3785]

Electrical contact cleaner. I never thought of that. Good tip. Can I use Mass Air Flow Sensor Cleaner? I have a can of that.

[msl3785]

Now i'm thinking camshaft position sensor...

I just noticed today, (Wednesday), that the tachometer seems to be behaving strangely. I acts like the "seconds" hand on a watch when accelerating with the O/D off. "ticks" its way up to 3,000 RPM. Put the car in neutral and rev it and the needle moves smooth and quick. I expect that in neutral. Does the camshaft position sensor trigger the tachometer on the dash?

[Timothy S]

I cannot find the information by looking at the wiring diagrams, but the tachomer must be controlled by the ECU reading either the crank position sensor in the transmission belhousing, the cam position sensor in the distributor, or both.
Also, you can use any alcohol based cleaner on electrical connections- I've seen people using drugstore rubbing alcohol!