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Discussion Starter · #1 ·
I'm trying to help a friend out who has a 2003 Santa fe 2.7L. I am using Torque on my droid phone with a bluetooth ODB2 adapter to connect to the car and monitor it. With all sensors and components connected correctly it is pulling 30% of the fuel trim and runs horribly, hesitating when you try to accelerate from 0 bad enough that you can't pull out into traffic.

Diags tried -
If I disconnect a fuel injector, it starts running fine (with the exception of 1 cyl missing) and lets the fuel trim zero out. In this state it has almost no hesitation. It doesn't matter which injector I pull - I have tried this with each of the 3 on the front side of the engine and 1 on the back side closest to the belt (1 at a time of course) and I get the same result with each. I believe it is forcing the ecu into open loop due to failure, and whatever that does causes it to fix the fuel trims. What does it ignore in. If I pull an upstream o2 sensor that bank's fuel trim corrects itself.

Components replaced/swapped so far:
-Spark plugs,wires
-1 injector
-ecu
-MAF sensor - temporarily swapped for a known good one, no difference
-all 4 o2 sensors

Items I've noticed:
-seems to pull 30% fuel trim in closed loop, but clears up in open loop
-MAF seems to be reading AIT's right, but I don't know what normal airflow readings should be. Between 70-90% throttle its seeing around 120... whatever... in the MAF flow reading in Torque. What type of MAF do these cars use? What is a normal flow?

ask many whatever questions you can think of - this one has me stumped...
 

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Discussion Starter · #2 ·
Sorry to bump my own thread - especially as a noob on the forum - but I need to get this figured out so he can pass emissions when the time comes around. We've already taken it to the Hyundai dealership - they diagnosed for 4+ hours and couldn't figure it out. We've taken it to another mechanic before that, which is why we replaced the ECU.

My big questions at the moment are:
1. What changes between open loop and closed loop on these? What sensors does it use/stop using when it switches?
2. Is pulling 30% fuel trim a normal thing in closed loop on these cars?
 

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Sorry to bump my own thread - especially as a noob on the forum - but I need to get this figured out so he can pass emissions when the time comes around. We've already taken it to the Hyundai dealership - they diagnosed for 4+ hours and couldn't figure it out. We've taken it to another mechanic before that, which is why we replaced the ECU.

My big questions at the moment are:
1. What changes between open loop and closed loop on these? What sensors does it use/stop using when it switches?
2. Is pulling 30% fuel trim a normal thing in closed loop on these cars?
What does it report for engine temperature? A faulty coolant sensor will create havoc..make sure it's reporting correctly first.

However, what's going on here is it's compensating for a lean condition, meaning it's under the assumption that there is unmetered air getting into the engine after the MAF or before the upstream O2 sensors ( the sensors before the Cat ).

Positive numbers mean it's running lean, negative mean it's running rich. The computer is trimming the fuel injectors to have a longer pulse to compensate, but 30% is the maximum they can adjust for, meaning the air getting in is much worse than it can regulate.

Look for a crack in the PCV hose, or the connection between the MAF and the upper plenum...anything made of rubber that would allow air to pass through the engine needs to be checked for integrity.

Beyond that, it could be a gasket leak at the upper intake plenum/intake gasket or the intake/cylinder head gasket. You can watch the readings while using an unlit propane torch and passing it around the seams to see if the engine will suck in propane, which will correct the fuel trim temporarily.

Another test you can do is to watch the short term fuel trim numbers and see if they drop when the RPM's are raised to 2500. That's an easy way to pinpoint towards a bad hose/etc.

However, the fact that removing any injector lead corrects the fuel trim and how well it runs also makes me wonder about the quality of the fuel pressure.

To answer your 2 questions directly however...

1. What changes between open loop and closed loop on these? What sensors does it use/stop using when it switches?

Open loop uses preset parameters to control fuel trim and spark advance based on a table in the computer that uses that last set of values the car last ran on or the ECU is using from adaptive learning. The only sensors it really uses are the MAF and Temperature sensors, as well as throttle position.

Closed loop brings in the O2 sensors as they have now warmed up enough to properly read the amount of unburned fuel in the exhaust stream and begin to oscillate slightly above and below 0 ( or .450mv ) to allow combustion to make the two gasses the catalytic converters need to burn unburnt fuel and create a clean exhaust. It also brings the computers short term adaptive learn tables into the equation as it senses changes in load and RPM.

2. Is pulling 30% fuel trim a normal thing in closed loop on these cars?

In short, no. You ideally want to see single digits as close to zero as you can manage. This also goes for the Long term fuel trim. STFT and LTFT want to be at Zero. Watching live data on them will see them fluctuate above and below zero as the O2 sensors are actively switching from slightly lean to slightly rich.
 

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Discussion Starter · #4 ·
Thank you very much for taking the time to respond, Redfury. That was a great amount of data and I will dig into that and report back when I can.
 

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HEY, check your fuel pressure regulator...it's at the end of the fuel rail, and has a rubber vacuum line that goes to a nipple on the intake....
pull the line off with the engine running and see if fuel comes out of the nipple on the regulator side...if fuel comes out, regulator is bad....possibly could be intermittent so if it starts running rough do that real quick. It causes three problems, vacuum leak, low fuel pressure, unmetered fuel into engine AFTER the throttle body so it will run rich.
BTW it won't throw a code...I found it by trying to figure out what the car was doing and scouring the forums until I found that little nugget of truth....$35 on Amazon
 

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HEY, check your fuel pressure regulator...it's at the end of the fuel rail, and has a rubber vacuum line that goes to a nipple on the intake....
pull the line off with the engine running and see if fuel comes out of the nipple on the regulator side...if fuel comes out, regulator is bad....possibly could be intermittent so if it starts running rough do that real quick. It causes three problems, vacuum leak, low fuel pressure, unmetered fuel into engine AFTER the throttle body so it will run rich.
BTW it won't throw a code...I found it by trying to figure out what the car was doing and scouring the forums until I found that little nugget of truth....$35 on Amazon
I replaced mine after noticing my car took longer to start up... slapped a new one in place and she was good to go. Can't hurt as they tend to go out anyway.
 
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