Hello all!

First and foremost thank you so much for taking the time to help me with this, I'm truly out of ideas on how to solve this issue. I will do my best to give you a very detailed picture of what has happened to this date. I consider myself a novice when it comes to car repair as I've always DIY'd my own repairs as well as my parents, friends, etc.

Car:
2005 Hyundai Elantra
5-Spd MANUAL
170k miles (engine replacement at 120k)

Recently replaced/fixed/new:
radiator
clutch
serpentine belts (not sure if others were but three belts were replaced at the same time)
spark plugs (Bosch Platinum +4)
major sensors (inus the speed sensors, I'll cover this below)
fuel filter
regular oil/air filter changes

What the car is doing:
I'm pulling a P0501 and P0301 error from it.
What I can gather is P0501 is related to the speed sensors, which I've read is a notorious issues for this car, and the P0301 is related to a misfire.

In a nutshell it feels like it doesn't 'catch' into gear quickly. I'll be driving and until about 3k RPMS it feels extremely sluggish then all of a sudden it will 'catch' into gear and be more responsive to the gas pedal. When the AC is turned in this is much, much more dramatic.

The engine symbol is always on and will sometimes flash at me whenever it 'catches'.

What I've done recently
After I pulled the P0501 and P0301 the first thing I did was change the fuel filter. After I did this and turned the car on the check engine light was off...but it came back on within 15 minutes and the car was behaving the same.

I then pulled the new plugs out to see if maybe I got a faulty plug. Nope, all the plugs looked great.

I then reset the P0501 and P0301 after I pulled the plugs and the engine light came on again after the first time it was choppy.

What I'm inclined to think:

After eliminating the plugs and the fuel filter I'm thinking it may be the spark plug wires but I really don't think that's the case.

I'm also inclined to think it's a vacuum leak somewhere? Since there is more air intake in the low rpms than high rpms maybe that's why it's kicking in at around 3k rpms?

I'm also a grad student so I'd like to exhaust all my options before I have to take it into a shop to get the work done. Grad School is expensive enough :P

Any and all help is GREATLY appreciated as I would love to finally find out what the deal is with this issue.

Thank you all so much

 

Your miss fire problem is your spark plugs. Hyundais don't like the Bosch plat. Plugs. Install NGK or champion plugs only. No platinum of any type. As far as your speed sensor problem the vehicle speed sensor is located on the front right hand wheel. Inspect wiring and reluctor wheel on axle for damage. May have a bad sensor

 

Based on your advice I did some searching on NGK/Bosch Plugs.

Long story short I definitely things that's the problem. I'm going out to get a set of NGK Iridium IX plugs and I'm confident that will fix the issue at hand.

Thank you!

 

*Update*

I swapped out the Bosch Platinum +4 plugs with NGK Iridium IX and everything was fixed. Although the true OEM plugs for the car was NGK Laser Platinum, the Iridium IX was $6 cheaper per plug and users with similar experiences said it the Iridium IX plugs would work fine.

Swapped out the plugs, started it up, no more misfires, steady acceleration through all ranges of RPMs and Gears, and seems to have fixed the problem.

In the end it really was an issue of NGK single-prong vs Bosch four-prong plugs. The rep at NAPA originally sold me the Bosch+4 plugs saying they were a match for the car. I used them because that's what I've used in the past for my other car (BMW uses OEM Bosch) so I figured it would be fine.

I consulted an employee at O'Reilly and he said that +4 wasn't even listed in their system as compatible.

So yeah, thanks for the suggestion! Hopefully others can use this knowledge if they have similar issues.

 

Most of the time, if possible, stay with OEM parts same as removed . It prevents a lot of problems.