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Discussion Starter · #1 ·
Our 2002 Sonata (4 cyl) started losing power over the past couple weeks. Yesterday it would only go 40mph on the highway. I pulled codes (the CEL was on briefly) and got P0300, P0301, and P0304. I figured the coil that feeds #1 and #4 cylinder was bad so I replaced it, but it did not solve the problem.

I can sit in the garage and step on the gas. The engine revs up, the slowly starts dying. The more gas you give it the more it wants to die. Eventually it'll stall and be difficult to restart.

Any ideas?
 

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for starters, stop replacing parts randomly. as per your replacement of coils, it was a waste of time and money. not disagreeing with that being a very possible cause, but those coils could have been easily tested.

first thing i'd look at is a fueling issue. guessing the fuel pump is not putting out enough pressure to support the air flow into the engine. result is mixture leans out, misfires and eventually it stalls. get a test gauge on the fuel system and test. its not an expensive tool.
 

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While the codes suggest "misfire", that can be caused by multiple situations. Coils and wires could be involved, but spark plugs would be much higher on the list as a cause (and cheaper too). A vacuum leak to the fuel pressure regulator, a dirty/gummed-up IAC valve, dirty MAF -- I can think of many more potential causes of misfires. If it has not been done recently, I would think that a good tune-up may be in order, which should include a good cleaning of all the items in the intake system.
 

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the key here is that it revs up and then slowly starts to die. that puts sensors and plugs out. either they work or they don't. the fuel pressure regulator is another good suggestion that should be checked.

regardless, if the plugs have not been done in a long time they are worth replacing as a maintenance item. i don't believe they will solve your problem though.
 

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Discussion Starter · #5 ·
The plugs were done relatively recently. I pulled one to verify and it looks fine.

I have a fuel pressure tester and have exposed the rail but I don't see the valve to connect the tester to. Is there one?
 

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Sound like he has a 4 popper... they horrible for by-products of on-going misfire..

Primary cat has melted down from the misfires that left to go on too long.

Unscrew the "pre-cat ox sensor, and fire up the engine, and flip the throttle and see if rev better.. Vacuum gauge to intake manifold be another test.. drop front pipe off bottom of primary cat and see if "brick" all melted/broken and chunks missing.
 

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brilliant as usual sbr :) very good test and worthwhile performing.

the fuel pressure tester can be plumed in using a simple T fitting and a small piece of hose. easy to get a multi-sized T fitting at any auto parts store.
 

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Discussion Starter · #8 ·
I just went out to work on it and now the car won't start. It started once, ran for a bit then stalled. Now it just cranks and sputters but won't catch.

I disconnected the vacuum line from the fuel pressure regulator to see if there's fuel in it -- there is not.

After it warms up (it's about 0 degrees right now) tomorrow I will see if we're getting fuel pressure. It's pretty cold out there right now.
 

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Discussion Starter · #9 ·
the fuel pressure tester can be plumed in using a simple T fitting and a small piece of hose. easy to get a multi-sized T fitting at any auto parts store.
There's two lines connected to the rail - one small and one big. Looks like the smaller one has a metal end on it and is bolted to the rail. Are you saying to cut that line to test fuel pressure?

I looked at the fuel line connection near the fuel pump as well (under the access hatch in the trunk) and it's metal as well. It's hard to believe that Hyundai didn't see fit to install a shrader valve somewhere in the system.
 

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We have an adapter that fit between fuel line and fuel rail flanges.. DO NO CUT any of your factory fuel lines !!

Pull the pre-cat ox sensor and fire it up, or drop the front pipe off and look at honeycomb.

We had a 2.7 come in that would not run.. tech pulled 2 front spark plug, the electrode erroded away all way down to porcelin.. a few too many mile on them, the wires turned all white haze, customer stated oil light came on, added a qt, still not on dipstick,, they ended up adding 5qt (what I heard) before oil level correct..

Customer declined plugs and wires based on recent experiences with car..

275,000+ mile..
 

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Discussion Starter · #11 · (Edited)
I'll pull the O2 sensor and test. In the meantime, the previous owner (a friend) gave us all the maintenance records from the car:

Car currently has 165,000 miles.
On 05/30/13 at 157,000 miles, fuel pump/strainer/Crankshaft Position Sensor/Timing belt replaced due to a no-start.
06/30/12 at 147,000 miles, spark plugs and ignition wires replaced due to "has no power up hills or from stops".

Interestingly enough, even though the CPS was replaced, soon after we got the car it died and wouldn't restart. The balancer belt had shredded itself and destroyed the CPS. Apparently the shop that replaced the CPS & timing belt didn't spend the extra few minutes to replace the balancer belt... the pieces of balancer belt I fished out of there were old and brittle.

tech pulled 2 front spark plug, the electrode erroded away all way down to porcelin
Good point. I'll pull all the plugs and check their condition, though I don't think that's causing this problem.

Is there no convenient place to check fuel pressure on this car??
 

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Discussion Starter · #12 ·
Primary cat has melted down from the misfires that left to go on too long.

Unscrew the "pre-cat ox sensor, and fire up the engine, and flip the throttle and see if rev better.. Vacuum gauge to intake manifold be another test.. drop front pipe off bottom of primary cat and see if "brick" all melted/broken and chunks missing.
Bingo! Pulled the upstream O2 sensor and the car runs great. Looks like one or both of the catalytic converters are plugged.
 

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Discussion Starter · #13 · (Edited)
It was the second catalytic converter. Replaced it with a unit from RockAuto and all is well. Credit to sbr711 for proper diagnosis even though I didn't believe it. :)
 
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