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Discussion Starter #1
This is 2.5L V6 engine. The car started to miss and the CEL came on with the code for Cylinder 6 misfire.

I have replaced all the spark plugs, just because it wasn't too expensive. The code was still there.
I have bought one injector to replace #6 and while replacing it I noticed the that connector wasn't clicking in place, like wasn't going in all the way. So I have bought also a pigtail connector and soldered it in place of the old one (cut wires).
Now the engine it is idling fin until I put it in "D" or "R". Then it stumbles, almost like is missing again. But no CEL. When I accelerate it hesitates at low RPM, but, once I get up on RPM's, it feels fine.
I have replaced the air filter and checked all the connectors and vacuum hoses that I can see.
I have cleaned the IAV valve and verified the PCV valve - both looked clean.
Pulled the MAF and spray it with MAF cleaner. It wasn't looking dirty but whatever...

After each iteration, tried the engine and it has the same symptoms. No CEL, but the Emissions Cycle is incomplete because I had cleared the early missfire code.

Any ideas?
 

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Discussion Starter #2 (Edited)
I just noticed that the reader shows the temperature for intake air at 107-125C. I am not sure if it has anything to do or even if is a correct reading.

LE: I have checked the connection it seems OK. It reads the temperature inside the intake manifold, so it might be right, that thing is hot.
 

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Usually a cylinder missfire is caused by a coil or lead. Your high reading on the air temp may be an open circuit. Some codes don't come up straight away so I suggest you take it for a drive and see what happens.
 

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Discussion Starter #4
UPDATE: I have replaced all the spark plugs, all injectors and the MAF sensor and, sooner or later, I still have the miss on cylinder #6. The car feels like missing, but only at low RPM.
At this point I have suspected compression, but I don't have a tool to check that. However watching a video about the sound of a compression loss engine starting (with no fuel added), I deactivated the injectors (pulled the fuse) and I have recorded the sound of the engine starting. It very much sounds like I am missing compression on that cylinder.
I guess it's time to retire the trusty Sonata after 19 years... ?

Sound is here:
 

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Doesn't sound bad to me. The air temp will vary depending on it's location and if it's been thru a heat soak and you checked it without the engine running 10 minutes after it was turned off but that reading is still high.
 

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Most temperature sensors used on cars are NTC thermistors (Negative Temperature Coefficient), meaning their resistance decreases as the temperature rises. Since an open circuit causes high resistance, the scan tool temperature would be very low if the sensor signal wire was open. In this case the scan tool temp is way too high, indicating that the sensor signal wire has shorted to ground (short circuit = low resistance).
 

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Discussion Starter #7 (Edited)
I have read the temperature sensors with the engine off, cold, right now. Used a Autel MaxiDiag MD808Pro. Previously I used an Android phone with Torque and a Bluetooth OBDII adapter.

Coolant sensor: 64F
Air intake sensor: 62 F
My house thermostat outdoor sensor: 64 F
So the temp sensors are all right.

The engine sound to me is not "round" like it would miss compression on one cylinder. That would explain why it would be irregural at low rpm but will be fine at over 2000 rpm...

I see that HF has a compression tester for $35: https://www.harborfreight.com/quick-connect-compression-tester-62622.html

20191230_215630.jpg
 

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Well you can't question those readings. While watching the scantool wiggle all the wires you can see watching for any change in the air temp sensor reading. If there is an intermittent open circuit or poor electrical connection it should spike up. Thats a nice looking compression tester. Wish I could but the same for that price.
 

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Discussion Starter #9

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Just a thought...have you checked for a vacuum leak? Misfires at idle that go away when you open the throttle are often caused by vacuum leaks. The wider the throttle is open the less vacuum there is in the manifold which is why things improve when you open the throttle. Perhaps there is a leak in the intake plenum gasket or the manifold gasket.
 

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Discussion Starter #11 (Edited)
Thanks for suggestion. I don't hear anything whistling in the engine bay. And what baffles me is that it affects only one cylinder. I did mess with the intake manifold gasket tough when I removed it to change th erear bank plugs and injectors. It didn't look damaged...

The new spark plug wires are on the way, even if unplugging the wire I can hear sparks inside of the spark plug well. Maybe not strong enough for the idle mixture?
 

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When I suspect a lead I put another lead in series with the lead in question and the plug and run an earth wire around the suspect area (usually the hard plastic) looking for a spark. It's usually a tiny almost invisible pin hole. That's if you can get to it.
 

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Since the plugs have been replaced and the plug wires are also being replaced at this point, what about the ignition coils that was mentioned earlier (for cylinder 6)? You can move the #6 coil to another cylinder location and see if the misfire code also follows the coil. I am assuming that the 2.5L is the same setup as my 2.7L.
 

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Discussion Starter #15
The coil packs are hard to reach. The whole assy is bolted in four places, one bolt particularly is obstructed by coolant hose that goes to the top of radiator... I think I will need to drain that first to get it out of the way.
But since this is a wasted spark system, usually a faulted coil will affect two cylinders at once. I didn't see that yet.

I will try a new spark plug wire first, it's in the mail, easier to replace...

 

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Measure the primary and secondary resistance of the old coil and compare the results with the new coil. In theory waste spark should knock out 2 cylinders but the reality is they don't always.
 

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jumping in this thread late. any chance you have too much ethanol or any water in the gas? in my expierence the 2.5/2.7 don't like too much ethanol which attracts water. I had a similar issue on cylinder 2 total misfire and spent the whole day trying to figure it. turned out it was just not soo good gas and the tank was a bit low. put a couple gallons of ethanol free and problem went away
 

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Discussion Starter #18
Car is still not perfect. I have replaced the coils #4 and 6 spark plugs but I still get some misfires. Not as bad as before, but still...
Maybe I need to do a tank fill indeed.
 

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Sorry I hear that all the time. "it's not as bad as before". You still haven't nailed the problem. What about the leads? Forget fuel. You need to look at what can affect cyl 6 (i think is the continual offender) and go from there. My suspicion is the lead and I mentioned how I test them. You did say new leads are on the way but not said if you have fitted them yet. You may have a mechanical issue or air leak as has been mentioned. I had one once and it had great compression, spark and fuel and it turned out the be the exhaust cam lobe had completely worn down and it could not open the valve for the exhaust stroke but that had a big impact on the engine performance. You should either put a vacuum gauge or use a scantool and measure the manifold vacuum at idle.
 

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Discussion Starter #20
As I said above, I installed a new lead. That's what made it "better".
 
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