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New Member Virginia Beach Help please

1K views 10 replies 6 participants last post by  James21 
#1 ·
Hi, I have never joined a forum before so I'm not sure if Posting my first thread is where I need to be. I'm just a Dad trying to help his daughter with her 2005 Hyundai Elantra GLS with about 97,000 miles. I am usually the first stop with all my kids with working on their cars but I could use some technical help on a problem.

My daughters 2005 Hyundai Elantra GLS with automatic transmission has intermittent drop of RPM at idle at both cold and hot engine temperature while in Park, Drive and Reverse. RPM drops with slight engine shake then returns to normal idle within 2-3 minutes at a stop. There is not a drop in RPM in Neutral. Spark plugs and Idle Air Control valve (IAC) have been replace with no change.

Any suggestions will be greatly appreciated.

Thanking all of you in advance for any support and assistance!
 
#6 ·
Previously during initial troubleshooting with transmission in Park, Drive and Reverse the rad fans were already running when there was a drop of RPM. A/C was off and compressor clutch was not engaged. No other load to the engine besides transmission. There is no engine check light. I will have code scanner check any other possible computer codes. To clarify the RPM issue: it takes approximately 2-3 minutes after vehicle is stopped before RPM drops and reaction time to return to normal is instantly. RPM Drop can occur 2-3 times during one single stop.
 
#7 ·
I don't really have any help to offer at this point, but it is interesting that this drop in RPM can happen in Park but not Neutral. As far as the engine PCM is concerned, there is no difference between the two. IE, there is no input to the engine PCM when the selector is moved to either P or N, only when D/2/L or R are selected.

If your scan tool allows you to view live engine data, see if there is a PID for idle control duty. Maybe monitor the duty of the ICV and see if the engine PCM is reacting to the drop in RPM by increasing the ICV duty. It might be worth disconnecting the alternator main supply cable to see if that stops the RPM drop. At least you could rule out an electrical load/charging system issue as the cause.

Scott.
 
#8 ·
I have two ideas
1. Could the problem be in the transmission? That might explain the problem, especially as there are no error codes. Has the transmission been serviced? Might be a good time to try a drain and refill and see if that helps.
2. (this is a long shot but worth mentioning)
I had a similar problem in a 1999 Saturn SW1. It turned out to be a leak in the intake manifold gasket. But that car was producing error codes for a cylinder misfire and low idle RPM. Maybe when you check for error codes you should also check for pending error codes.
Good luck!
 
#9 ·
given what we have here, i'm leaning towards some odd transmission problem but i can't imagine what at this point. the ICV is worth a look too.

i'll be surprised if this is the problem, but what about the purge valve? if there was a leak downstream of the valve and it opens it could cause a significant vacuum leak. very much a long shot, but easy to check. just disconnect the purge valve and see what happens. it'll throw a code, but if it solves the idle issue then you've got it.

intake manifold gasket is out. looking for a leak leaning out the mixture is a great thought, but a broken gasket would cause problems at all times during idle not intermittently like this. this idea did lead me to think of the purge valve though.
 
#10 · (Edited)
but what about the purge valve? if there was a leak downstream of the valve and it opens it could cause a significant vacuum leak.
Is the purge control strategy different in Park than it is in Neutral? I wouldn't have thought so.

Also, a vacuum leak on an Elantra engine would normally cause an increase in idle RPM, not a reduction. (Assuming it has the same engine as a UK model would have. The OP didn't mention what engine was in the car.)

Scott.
 
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