QuestionI have a 2001 Hyundai Santa Fe 2.4L 4cyl that keeps hesitating when taking off and sometimes stalls or feels like it will while idling. I was told to replace (which I did) the EGR Valve and the EGR solenoid. I test drove it and not very long after test driving it feels like it may stall again and still hesitating. Check Engine light comes back on and I get the same error codes PO401 & PO172. HELP what is my vehicles problem???
AnswerYour trouble codes are likely unrelated to one another.
Let's start with P0401. Hyundai uses this to indicate any type of abnormal EGR flow. If the EGR valve opens at idle, the engine should run rough and nearly stall. Since you've replaced the EGR valve, we'll presume it's working properly (and not stuck open).
Here's how your vehicle operates the EGR valve: There's a vacuum hose coming off the throttle body (green stripe) that leads to a tee. From the tee, one hose goes to a vacuum control valve, and then from there to the EGR valve. The other hose goes to the EGR solenoid. From the EGR solenoid, there is a yellow striped hose that goes back to the throttle body. When the engine control module (ECM) activates the EGR solenoid, the solenoid closes, blocking the vacuum from venting back to the throttle body, and causing the vacuum to be applied to the EGR valve, as limited by the vacuum control valve, which bleeds off any excess vacuum.
Since it sounds like your EGR valve may be open (hesitation/feels like engine will stall), remove the green hose from the throttle body. If the engine begins to run normally, we can conclude that there is vacuum applied to the EGR valve. This implies there's a blockage in the circuit that vents vacuum back to the throttle body. You'll need to check to see whether the blockage is in the solenoid or the vacuum line that runs from the tee to the solenoid and back to the throttle body.
If the engine continues running exactly the same, reconnect the hose to the throttle body and pull the hose off the EGR valve. If the engine now runs smoother, we can conclude there's a one-way blockage in the circuit between the tee and the EGR valve that allows vacuum to reach the EGR valve but not be bled off.
If the engine still continues running the same, reconnect the hose to the EGR valve and pinch the yellow hose at the throttle body. This should cause the engine to run poorly. If it doesn't you'll need to find the vacuum leak in the circuit that runs from the tee through the solenoid and back to the throttle body through the yellow hose.
If the engine begins to run poorly when you pinch the yellow hose, we've now verified that the EGR passage in the intake is clear, the EGR valve itself is not defective, and the vacuum hoses are in good condition. In this case, check to be sure you replaced the proper solenoid. The EGR solenoid should have one green-striped hose going to it and one hose with a yellow stripe or no stripe. If the hose has a red stripe, you've replaced the wrong solenoid (purge solenoid). If you indeed replaced the correct solenoid, then you'll want to replace the vacuum control valve. Hyundai calls it something like a vacuum switch in the parts catalog. It's a plastic can about 1" in diameter and 1/2" long, with two green-striped vacuum hoses attached to one side.
Next is P0172. This code indicates that the ECM believes the engine to be running so rich that it is unable to properly compensate the fuel mixture. It detemines this by monitoring the data from the air flow sensor and throttle position sensor (TPS) and then adjusting the amount of time the injectors are held open in order to achieve the oxygen sensor reading it desires. There are three basic scenarios that can occur occur:
1. The ECM receives erroneous signals from a sensor causing it to think the engine is running rich when in fact it is not.
2. The engine is receiving too much fuel.
3. The intake airflow is restricted.
Scenario #2 is infrequent, but I have seen it occur. The most frequent problem is a stuck open purge control valve, allowing fuel vapors to be sucked into the engine when not desired. This is the solenoid valve with the red striped hose running to it that I mentioned above. To check it, with the engine off, remove the red striped hose from the throttle body and attempt to suck through it (very carefully-- there will be fuel vapors in the hose if the valve is stuck open). If you can suck through it, replace the purge control valve.
The other ways you can get excessive fuel are very rare. They include excessive fuel pressure and sticking open fuel injectors. Unless you've got the equipment to check this, it's probably not worth your time.
Scenario #3 is also very rare. But it's fairly simple to check. You'll want to make sure your air filter is in good condition and that there's nothing blocking the passage from the air intake through the air filter box and to the throttle body.
Scenario #1 is most likely. If you've found nothing yet, your issue will probably be with the air flow sensor. Although a faulty oxygen sensor can also cause this trouble code, it would typically also cause a trouble code indicating a problem with the oxygen sensor readings. Additionally, Hyundai oxygen sensors almost never fail in a manner that would lead to this code (normal oscillating waveform with values skewed to a higher than normal value). Unfortunately, the air flow sensor is difficult to diagnose without a scan tool and a known good sensor to use to compare values. Hyundai doesn't publish standard values for the sensor at a variety of engine RPM to be able to determine whether the readings are normal.
