Engine Drops 50 RPMs at idle?

Tiny
MIKELINE
  • MEMBER
  • 2016 HYUNDAI TUCSON
  • 2.0L
  • 4 CYL
  • 4WD
  • AUTOMATIC
  • 106,800 MILES
Engine replaced at 87533 miles (long block). At 106,812 miles the engine started to stumble at idle dropping about 50 rpm intermittently. Sometimes it won't do it for 10-15 minutes. Once in a while it will also surge up to 1100rpm and immediately drops back down. There is no check engine light. Using AI I methodically replaced the following parts.

plugs
coils
battery
high pressure fuel pump
throttle body (imbedded TPS/MAF)
Acceleration Position Sensor
temp sensor
fuel rail
fuel injectors
air filter
PCV

I disconnected O2 sensor (primary and secondar) one at a time and the engine check light came on immediately in both instances. I assume they are working.

I used an OBD II tester to check live settings (I do not know all of what they mean. But was checking for catalytic temperature since AI suggested that if the temp was too high it might indicate a blockage [temp was 890 and Hyundai specs say between 800-1100 is normal]).

Dealer couldn't find a problem/source and said since some of the parts I used were not Hyundai OEM (E3 plugs) they wouldn't go any further with diagnostics. What rot.
Saturday, June 14th, 2025 AT 3:27 PM

19 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
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Yep, the dealer will bow out without OEM parts. But I can tell you I have seen the fuel pressure control valve cause this issue, but you said you replaced it, correct? Also, if you have a vacuum leak, it will do this as well, ie: lose intake manifold, broken vacuum hose. Please go over this video to help you find it, the machine costs about $65.00 on Amazon.

https://amzn.to/408Y25d

Can you tell me what the short- and long-term trim is on the scanner? Here is a guide that can help as well:

https://www.2carpros.com/articles/low-engine-idle

Please go over this guide and get back to us.
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Sunday, June 15th, 2025 AT 11:10 AM
Tiny
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Thanks for your reply. I did not replace the fuel pressure control valve. I will get one and see what happens. Thanks I will post again to let you know.
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Wednesday, June 18th, 2025 AT 1:06 PM
Tiny
KEN L
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Did the high pressure fuel pump come with the valve? All of the pictures I have seen have the valve on it?
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Thursday, June 19th, 2025 AT 9:34 AM
Tiny
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Looking at OEM part for a picture of the valve, I realized that it did not, and I ended up using the original one. I should get the valve this coming Thursday and will let you know if it is a cure. Thanks again.
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Saturday, June 21st, 2025 AT 6:37 AM
Tiny
KEN L
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Sounds good have a great Sunday!
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Sunday, June 22nd, 2025 AT 12:16 PM
Tiny
MIKELINE
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  • 11 POSTS
I got another reader because my old one would not exit. Anyway. Here are the pics off the new reader
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Sunday, July 6th, 2025 AT 2:15 PM
Tiny
KEN L
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It still sounds like you have a vacuum leak, did you get the smoke machine and try it? If you can get the long and short trim values I can tell you if the engine is lean or not. It seems like it can be the intake manifold gasket is leaking. Please let me know.
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Monday, July 7th, 2025 AT 10:05 AM
Tiny
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Replaced the fuel pressure control valve - no change. Will get the smoke machine. My friend has a better reader. Going there on 7-14 and will upload pic of reader findings.
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Saturday, July 12th, 2025 AT 5:46 AM
Tiny
KEN L
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Sounds good, we are getting closer.
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Saturday, July 12th, 2025 AT 9:49 AM
Tiny
MIKELINE
  • MEMBER
  • 11 POSTS
Local shop pressure tested fuel and smoke tested vacuum. Both good. Ended up replacing the fuel rail pressure sensor recommended by AI bot. It worked perfectly until the next morning when I started out and within two minutes it went right back to cutting out, but no worse. It really bugs me with less than 25k on the engine. ECU?
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Monday, August 4th, 2025 AT 12:36 PM
Tiny
KEN L
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Can you get the long and short fuel trims so we can see what the computer is doing? It sounds like the MAP sensor is weak, also can you pull the spark plugs to see what color they are? Here is the location of the sensor so you can check it out.
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+1
Monday, August 4th, 2025 AT 1:53 PM
Tiny
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Plugs are brown. MAP was replaced with the throttlebody. I will ask him if his reader will do trims and bring to me on Wed. Thanks
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Monday, August 4th, 2025 AT 5:52 PM
Tiny
KEN L
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When installing the sensor did the O ring seal get replaced/installed? This is what seals the sensor to the vacuum. Also, be sure there is no exhaust leak as well, this can allow fresh air into the exhaust system and fool the computer which can lower the idle. Please go over this guide as well

https://www.2carpros.com/articles/low-engine-idle

Please go over this guide and get back to us.
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Tuesday, August 5th, 2025 AT 8:23 AM
Tiny
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Yes new o-ring used. No sign of exhaust leakage. Here are pics of the reader. Note the two which show the TRIMs LT ST reflect the ST changes but not the LT.
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Wednesday, August 6th, 2025 AT 1:54 PM
Tiny
KEN L
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I am sorry I cant read any of these they are two small. Can you break them up please?
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Thursday, August 7th, 2025 AT 5:55 PM
Tiny
MIKELINE
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I put four in a row. I hope that helps
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Thursday, August 7th, 2025 AT 6:31 PM
Tiny
KEN L
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Thanks you, okay it was like I thought, the PCM is thinking the system is lean, here are choices for the cause and a guide as well.

1. Small vacuum leaks (hoses, intake gasket)
2. Slight MAF sensor contamination
3. Weak fuel injector spray pattern
4. Dirty or restricted fuel filter
5. Low fuel pressure from fuel pump beginning to fail

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

I would recheck the vacuum leak, did you use a smoke machine? Please go over this video and get back to us.

https://www.youtube.com/watch?v=LRKWV4Are5c&t=3s&pp=0gcJCa0JAYcqIYzv

I am curious to what you find.
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Thursday, August 7th, 2025 AT 7:00 PM
Tiny
MIKELINE
  • MEMBER
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MAF is new. Injectors are new. Used carburetor cleaner around every connection I could see without change in idle pattern. Local shop paid to check it but tried an old manual way after your comment. I ordered a new fuel pump assembly which incls the filter. Will change it out when it arrives. If no change I will buy a smoke machine. Will get back to you.
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Monday, August 11th, 2025 AT 7:47 PM
Tiny
KEN L
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Okay, it looks like you are on the right track, thanks for getting back to me.
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Tuesday, August 12th, 2025 AT 8:35 AM

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