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PCM or Injectors?

Inline.TJ

TJ Enthusiast
Joined
Jan 31, 2022
Messages
146
Location
Colorado
It appears the little burble in the idle and whatnot are signs of a misfire, the fuel pump was on it's way out and replacing it seems to have shined the light on the real problem.

Through diagnosis it appears cylinder 3 and 6 spark plugs are pretty sooty, all the others look normal. I replaced them all maybe 6 months ago.

Next was to check the injectors:
Brought motor to operating temperature. Measurements taken in ohms:
Cylinder 1: 13.2
Cylinder 2: 13.3
Cylinder 3: 13.3
Cylinder 4: 13.1
Cylinder 5: 13.4
Cylinder 6: 13.4

These are all within spec for operating resistance on injectors for the 4.0L, the Range being 10-13.5 IIRC. I did however swap injector 6 with injector 5 and the Code 27 moved with it, making it a misfire on 3,5,& 6.

I guess that means it's time to replace a couple injectors, however the hard start is still a problem and it's making me think the PCM might be on the way out.
 

Jerry Bransford

TJ Guru Moderator
Staff Member
Supporting Member
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Messages
25,424
Location
Fleming Island Florida
Don't replace injectors until you have moved them around to see if the misfires follow them or if they stay at the same cylinders.

And exactly what brand and model spark plug is in your engine? Specifically?
 
OP
Inline.TJ

Inline.TJ

TJ Enthusiast
Joined
Jan 31, 2022
Messages
146
Location
Colorado
Update: Pulled the Idle Air Controller... it was caked in soot.

Cleaned it up and blasted out the throttle body. Three shop towels completely blacked out.

Put everything back together and it fired right up. I'll let it sit overnight to see if that was really the issue.
 
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Inline.TJ

Inline.TJ

TJ Enthusiast
Joined
Jan 31, 2022
Messages
146
Location
Colorado
Nope. I turned the key on-off-on-off-on then it fired right up, which tells me the fuel line isn't getting primed properly. Considering I installed a new fuel pump literally days ago I'm puzzled. Is there some other pressure regulator in the system that I should be looking at?
 
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Jerry Bransford

TJ Guru Moderator
Staff Member
Supporting Member
Joined
Nov 9, 2015
Messages
25,424
Location
Fleming Island Florida
It does move.


Champion RC12LYC

OK they're not the problem.
Nope. I turned the key on-off-on-off-on then it fired right up, which tells me the fuel line isn't getting primed properly. Considering I installed a new fuel pump literally days ago I'm puzzled. Is there some other pressure regulator in the system that I should be looking at?
If it won't start right away after it has just sat a few minutes that's usually caused by the fuel pressure regulator. If it doesn't start right away after sitting a while like overnight that's usually caused by the fuel pump. Which is it for you?

Aftermarket fuel pumps and fuel pump assemblies are a real crapshoot, they can fail any time.
 
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Inline.TJ

Inline.TJ

TJ Enthusiast
Joined
Jan 31, 2022
Messages
146
Location
Colorado
If it won't start right away after it has just sat a few minutes that's usually caused by the fuel pressure regulator. If it doesn't start right away after sitting a while like overnight that's usually caused by the fuel pump. Which is it for you?

Aftermarket fuel pumps and fuel pump assemblies are a real crapshoot, they can fail any time.
Both. I basically have to "prime" it every time I start it. The fuel pump itself seems fine, once running it's fine even under full throttle. The regulator seems to be the problem.
 
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