I have a lemon car Hyundai sonata 1999, 180’000 km, well maintaned tough I spent 9000$ in 4 years in maintenance.


My symptoms:



1) I have difficulties to accelerate few seconds after starting the car and getting out from the garage in the morning, and during the first 5 minutes.
2) I have LTFT (long term fuel trim) up to 16% in a regular basis with O2 sensors indicating high voltage (more than 0.45V – up to 0.8V)
3) I have alternatively the codes P0171 with P0300, with a majority of P0171 code
4) I have an exhaust leak in the extension pipe between the catalytic converter and the muffler (during the installation of a new muffler) but a muffler specialist checked it one day and told me that it is OK, even if it was bad few month before…Could a small leak get repaired my itself with carbon deposits ?
5) My instant fuel consumption drivin town is up to 28l/100km from a light stop aceeleration (with an average overall of 12l/100km in town)


What I’ve already done:
1) For P0300 -> changed my spark plugs, sprayed some water onto spark plug wires and wires seem OK.
But I’m suspecting a valve cover gasket oil seep (found oil in one spark plug), but I don’t think it is an oil “leak” in the valve gasket, but I might be wrong…
2) For P0171-> cleaned MAF sensor, changed air filter, tightened intake tube (no holes, in top shape), cleaned throttle body
3) Ordered a PCV valve (14$) (without even checking for excess pressure and vacuum in the valve compartment)


What I’m planning to do:
1) Change PCV valve
2) Repair exhaust leak
3) Change valve cover gasket
4) Change injectors
5) Check fuel pressure in the system (recently changed my fuel tank and whole fuel lines, but fuel pump was not changed)


What do you think? Any suggestions ?



Thanks!

 

Update:



I reset my ECU all night long, and found that:
At idle, o2 sensors are around 500mV and STFT remains low,
BUT at acceleration, all four O2 sensors voltages become NULL and STFT start increasing and move up to 16% and remains, which increases gradually my LTFT
I think it is most likely my exhaust leak, I just bought an exhaust repair paste that I am going to try

Am I in the right direction ?