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Discussion Starter · #1 ·
Symptoms:
Random Misfire on cylinders 1 and 4. (Says the computer, does not feel like it.)
Engine light on, sometimes flashing, sometimes 'dings'.
Car is getting almost 1/2 as many miles per tank of gas that it used to.

Checked and tried a lot of stuff myself. Finally gave up and took it to the dealer, after hours of testing and a bunch of parts, still don't know.

What it is not:
Plugs: tried different different plugs, dealer put in new plugs, swapped plugs to non-misfire cylinders
Coils: tried different coils (from parts car), swapped coils to non-misfire cylinders
Fuel injector: tried different injectors and fuel rail, swapped injectors to non-misfire cylinders
Compression: 150 to 175psi each on a couple of cranks
Pressure?: dealer did a pressure test on cylinders, said engine is tight, nothing wrong that way.
Vacuum Leak: computer says -10 at idle, sprayed ether around intake, no difference in idle.
Crank sensor: Car had also been going into limp mode. Crank sensor fixed that, but not this.
Cam sensor: tried a different cam sensor.
Carbon cleaning: ran a couple bottles of seafoam cleaner through it, no change.

My Bluetooth tool only says Random cylinder misfire, dealer tool was able to tell it is cylinders 1 and 4.
Visually checked wiring for any signs of damage, cleaned up grounds, etc.
They said if it was an O2 sensor, or any other sensor, there should be a code for that too.
They thought about flashing PCM but said it probably would not cause or fix this issue.


Anyone ever run into this before? Or have any idea what this could be?

Thanks for any help.
 

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Wiring harness issues? Maybe a short or poor contact on one of the injector signal wires is causing issues, causing that cylinder to go lean and misfire, causing the O2 sensor to read lean, causing the computer to pump in more fuel overall...leading to poor fuel economy. Grasping at straws? Yes, but you've eliminated everything else.
 

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Discussion Starter · #5 ·
Fuel Pressure is 55psi, which is about what it should be for R/T 2.4. Still back at square 1. ?
 

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Hook up a scan tool and look at your short/long term fuel trims...they should all be roughly zero. If any of them are excessively positive (like higher than 5-7) then the car is adding additional fuel because it thinks the fuel mixture is too lean. If the numbers are excessively negative (more than -5 to -7) then it's cutting back on fuel because it thinks the mixture is too rich. I believe the trims being off by enough in either direction could cause misfire issues.

There are lots of things that can cause the ECU to erroneously adjust the fuel trims, but find out if that's the case first and we can go from there.
 

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Discussion Starter · #7 ·
Hook up a scan tool and look at your short/long term fuel trims...they should all be roughly zero. If any of them are excessively positive (like higher than 5-7) then the car is adding additional fuel because it thinks the fuel mixture is too lean. If the numbers are excessively negative (more than -5 to -7) then it's cutting back on fuel because it thinks the mixture is too rich. I believe the trims being off by enough in either direction could cause misfire issues.

There are lots of things that can cause the ECU to erroneously adjust the fuel trims, but find out if that's the case first and we can go from there.
Finally got to do this with a scan tool and Torque Pro App.

Got Short term results from +2 to -7. Mostly at 0, but when it goes off 0 is usually in negative numbers.
Long Term always stayed above 0, up as high as +11.7

So what does this mean?

17018
17019
17020
17021



Thanks for all the help.
 

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It means it's probably not a fuel issue.

How many misfires are the counters displaying? Are we talking like dozens a minute or just one or two?
 

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Discussion Starter · #9 ·
Dozens. But just on cylinders 1 and 4. (And it still doesn't "feel" like there is any misfire happening.)
 

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Realistically the next step would be to hook up a scope to the coil control wires and see if 1 & 4 are not receiving the necessary signals to fire. The signals could be checked at the coil and could be checked again at the PCM to determine if the PCM is at fault or if it's a wiring fault between PCM and coils
 

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Discussion Starter · #13 ·
Realistically the next step would be to hook up a scope to the coil control wires and see if 1 & 4 are not receiving the necessary signals to fire. The signals could be checked at the coil and could be checked again at the PCM to determine if the PCM is at fault or if it's a wiring fault between PCM and coils
FIXED! It was the PCM.

It just seemed like it had to be a bad wire, or the ECM. I checked and checked wires, couldn't find anything. I wouldn't have bought an ECM, but since I had one from my parts car, I went back to the dealer to have it programmed for this car (even though they didn't think that was the issue), it only took about 30 minutes of labor to do. Engine light is off, and I'm getting my 'regular' mileage with it again (about 23mpg).

Thanks everyone for all the help.
 
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