i30N cold start issue: Severe rev hang and jerking/bucking – Throttle body?

[P4TR1C3]

Hi everyone,

I’m looking for some advice regarding an issue I’m having with my i30N.

Car details:

• Model: Hyundai i30N
• Current mileage: 87,000 km
• Maintenance: Spark plugs were replaced recently at 80,000 km.

The Problem: When the engine is completely cold, it behaves strangely when letting off the gas. If I accelerate and then take my foot off the pedal, the engine holds the RPMs for about 2 to 3 seconds (severe rev hang). Furthermore, if I lift off the throttle quickly, the car jerks and bucks quite violently.

Interestingly, once the engine reaches its normal operating temperature, the issue becomes much less noticeable or goes away entirely.

Another Symptom (Coolant Temp): I also noticed that my coolant temperature gauge only goes up to 85°C and stays there, never quite reaching the usual 90°C.

My Suspects:

• My main suspicion at first was a dirty or sticking throttle body.
• However, given the temperature reading, could this be a faulty engine coolant temperature (ECT) sensor or a stuck thermostat? If the ECU is getting incorrect temperature data during the warmup phase, I assume it could mess up the air/fuel mixture and cause this weird cold start behavior.

Has anyone else experienced similar symptoms with their i30N? If so, what was the fix? Should I start by cleaning the throttle body, replacing the temp sensor, or looking for something else like a vacuum leak?

Thanks in advance for your help!

 

[Bu11eT]

Regarding the main issue: Check the injectors.
Regarding the temp issue: Thats normal. Usually, cars dont reach the 90C coolant temp, but the gauge still shows it, so people won't ask questions. On the i30N the gauge is more accurate. So consider it normal and nothing to worry about.

 

[Jajodu]

For a quick check of the thermostat start the engine from cold and keep touching the bottom hose, it shoul stay cold until the engine reached operating temp, if it warms quickly then suspect a stuck open stat, if you've got access to a diagnostic tool you can easily check the real temps.