QuestionQUESTION: I have a 2004 Hyundai Elantra GLS and am having issues stalling out while coasting in neutral. To give you some history I recently had to have some repairs done. I blew a head gasket a few months back then had to replace the thermostat a few weeks later. My car also has had some hydrolocking problems due to a low cold air intake. In the last week my car has been stalling out at least once a day while coasting downhill in neutral. It starts back up fine while coasting but this is a big concern since stalling out causes me to lose power braking and steering. I recently took it to get inspected and it failed because the CEL was on. The mechanic said it was a bad MAF sensor. After having that replaced I did notice impressive power gains (the MAF must have been terrible) but the stalling problem continues as well as the CEL remaining on. It is only while coasting. I have not had any idling problems while parked. My mechanic is baffled at what could be bad. I have read it could be a bad fuel pump or leaks in the vacuum system but I have no idea. Any thoughts?
ANSWER: I must say that my primary interest is why your check engine lamp is still on. If you can, retrieve and report the trouble code-- I'm looking for the code number, not what someone thinks that code number means.
It's possible you have a large vacuum leak or a leak in your air intake between the air flow sensor and the throttle body. I'll note that this directly relates to the air flow sensor readings because such a problem would introduce air into the engine without the sensor measuring it. Additionally, if you're using an oiled filter in your intake, the oil can contaminate the air flow sensor so it cannot read properly.
It's also possible that you need to replace your cylinder head. I have yet to see a Hyundai, where the head gasket was damaged from overheating, that the cylinder head didn't need to be replaced.
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QUESTION: Thank you for the quick reply!
I will be going back to my mechanic in a few days to have it hooked back up to the scanner and looked at again. Anyway, I noticed that my car does indeed idle rough. Whenever I put the car in neutral while driving the RPMs jump down and overshoot the correct idling level. What I mean is that my car will usually idle at around 700 RPMs when warmed up. The RPMs will jump down to around 400 and then up to 900 then finally settle at 700. Sometimes it will overshoot 700 and go all the way to 0, thus shutting off my car. I have talked to a few people that suggested it is most likely a leak in the vacuum system or a problem with the IAC.
Also, you mentioned my cylinder head could be bad. Would this cause the CEL to come on?
I do not have an oiled filter, so that rules that out. Also, I'm am pretty sure my head gasket blew as a result of intense hydro-locking, not overheating. I didn't recognize what was happening and drove about 10 miles with the hydro-locking getting progressively worse. I finally pulled over and noticed coolant in my oil. However, my engine never overheated until a few weeks later when I had a bad thermostat.
Anyway, after I get it looked at again I will let you know the problem (assuming my mechanic can figure it out). Thank you so much for your time!
AnswerIf it's idling roughly, then I'd suspect you have a misfire. The most common causes if misfires are in the ignition system, but if you normal power on acceleration, it's more likely that your intake manifold gasket may have failed. Also, based on your issue with the hydrolock, you could have serious internal engine problems.
A warped or damaged cylinder head can cause a check engine lamp. The ECM won't have any way of detecting the warpage, but it may come on because of the symptoms, typically overheating or misfire. This damage could have occurred during the overheating due to the thermostat.
Additionally, if the engine has been hydrolocked, it's possible one or more of the connecting rods is bent.
