Idle surging and rough idle on 1998 4.0

TheDukenasty

TheDukenasty

New Member
Joined
Mar 29, 2021
Messages
16
Location
Rhode Island
Happy Monday all!

So to make this brief, here is the symptom and then what has been done recently. A lot of these items were done as part of rebuilding effort. Jeep still runs and drives.

Random rough idle/surging idle, especially when cold start in the morning. Letting it warm up for at least 10 mins.

Items replaced in last 6 months with OEM only parts: IAC, TPS, MAF Sensor, heater core, condensor/evap.

There has been a ton of other work done in this restoration effort, but the rest is not likely to be related.
Vacuum lines have not all been thoroughly inspected, and I believe the ECU is an aftermarket replacement.

Help me @Jerry Bransford wan Kenobi, you're my only hope :)

p.s. side note, if anyone is interested in the progress pics of rebuild lemme know and I will post'em.

Thanks!!!
 
TheDukenasty said:
Happy Monday all!

So to make this brief, here is the symptom and then what has been done recently. A lot of these items were done as part of rebuilding effort. Jeep still runs and drives.

Random rough idle/surging idle, especially when cold start in the morning. Letting it warm up for at least 10 mins.

Items replaced in last 6 months with OEM only parts: IAC, TPS, MAF Sensor, heater core, condensor/evap.

There has been a ton of other work done in this restoration effort, but the rest is not likely to be related.
Vacuum lines have not all been thoroughly inspected, and I believe the ECU is an aftermarket replacement.

Help me @Jerry Bransford wan Kenobi, you're my only hope :)

p.s. side note, if anyone is interested in the progress pics of rebuild lemme know and I will post'em.

Thanks!!!
Click to expand...
Also new fuel sending unit this past weekend.
 
(New Jeep Owner)
I have the same issues with my 98. I just bought it about 2 weeks ago. I took apart and cleaned the throttle body, idle control valve, replaced all plugs, checked as many lines as I could and couldn't find any leaks or tears. when the AC or Heat is on the idle moves from the normal 800 up to the 1800 range. prior to cleaning the throttle body with out AC running it was at a crazy high idle from 12-1800. I have no idea what is causing this and I'm getting frustrated trying to chase problems...
 
@TheDukenasty I just saw this thread, did you ever get that problem figured out? That it only happens when cold kinda makes me think you might have a bad coolant temperature sensor or IAT sensor telling the computer the engine is warm when it's not. When the computer thinks the engine is warm it will use the upstream O2 sensor to set the air/fuel mixture. If the engine is actually cold it should not be using the O2 sensor to set its air fuel mixture.
 
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