QuestionQUESTION: Hello,
My Hyundai Sonata 4 cylinder 2.4 L, developed an error code of P0401. I got the EGR valve replaced but the check engine light came back on after 30 Km of driving. Then the throttle body was cleaned the light turned off. But after about 3000 Km of driving, it came back on. Do I need to periodically clean the throttle body or there is another problem that causes carbon build up in it.
I read your helpful comments for checking the EGR system under "EGR vacuum" but I appreciate your comments about my car as well.
Thank you very much,
Mehdi
ANSWER: Hi, Mehdi. Although carbon will accrue in the throttle body over time, I expect the cleaning of the throttle body is unrelated to your EGR issue. Normally, it's a good idea to clean the throttle body every 30,000 to 60,000 miles, but I've seen new cars go even longer than that without significant carbon buildup in the throttle.
Hyundai uses P0401 to indicate any sort of EGR flow-related issue, not just low flow.
If you look at your throttle body, you'll see three vacuum hoses coming off the top of it: one with green striping, one with red, and one with yellow. For simplicity, I'll refer to them as the green, red, and yellow hoses.
The green and yellow hoses are used for your EGR. Basically, these hoses are a loop, with a tee in the green hose running to the EGR valve. In addition, there's an electrically activated EGR solenoid at the end of the green and yellow hoses. Vacuum is applied to the green hose, and is returned to the throttle body through the yellow hose. When the engine control module (ECM) activates the EGR solenoid, it closes, blocking the vacuum from returning through the yellow hose and causing it to be applied to the EGR valve, which in turn causes the EGR valve to open.
The first step I always recommend in checking the system is to, with the engine idling, take a pair of needlenose pliers and pinch the yellow hose shut. If the engine starts running poorly, this indicates that the EGR valve is working, the EGR passages are clear, and that the vacuum lines are correctly run. At this point, the most likely culprit is the in-line valve (which I've previously unmentioned) that regulates the amount of vacuum which can be applied to the EGR valve. Hyundai calls this valve a vacuum switch in their parts catalog. It's often difficult to find in the catalog, so I'll tell you the part number is 28378-38050. In addition, the EGR solenoid valve is prone to intermittent failures. Although the near-immediate return of your check engine light makes it likely that the vacuum regulating valve is the issue, I'd still recommend replacing the solenoid valve. It's difficult to tell for certain whether the vacuum regulating valve is working properly, and as I've said, the solenoid valve tends to work okay sometimes and then not work others. So it's usually difficult to pinpoint for certain which of these valves is the culprit. The part number for the EGR solenoid valve is 39460-38250.
(Keep in mind with reference to the part numbers that they may have been superseded by others. Your dealer will be able to tell you this if you give them the number. I've obtained the numbers by looking in the parts catalog, which lists the number originally installed on the vehicle.)
---------- FOLLOW-UP ----------
QUESTION: Thanks for your helpful comments. I don't understand if the EGR system problem is the solenoid valve why the throttle body cleanup should resolve the problem for 3000 Km. If the EGR solenoid was faulty, the MIL should have come back on much earlier. Since the throttle body cleanup resolved the problem temporarily, isn't that likely that some sensors on the throttle plate are dirty and do not send the correct signal to the ECM to operate the solenoid valves properly?
By the way, shutting off the yellow hose at idle caused the engine to nearly stall which indicates that there is no clogging in the EGR system. Is the EGR solenoid valve repairable?
I appreciate your time answering my questions.
Regards,
Mehdi
AnswerI think the cleaning of the throttle body is nothing more than coincidence. Most likely, the failure is intermittent as I alluded in my initial comments. Not only are there are no sensors on the throttle body that control the EGR, the ECM controls the EGR by electrically activating the solenoid. In addition, the fact that you've caused the EGR to operate by blocking the yellow hose indicates that the throttle body is properly applying vacuum to the green EGR hose.
At this point, I think the best avenue is to replace the solenoid and vacuum bleed-off valve as mentioned previously. You won't be able to repair either one of these items without replacing them.
