Vacuum issue

[SandMountainJeep]

1988 4.0 automatic had a fast idle, I found a rotted rubber vacuum line and replaced it.  Now it will not idle with all vacuum lines on where they are supposed to be and no vacuum leaks.  Can remove a line and idle comes back but is too fast.  Thoughts?

[Ωhm]

If idle improves the more you START and RUN your vehicle, could be ECU is relearning itself.

If not, check TPS adjustment.

 

Renix TPS Adjustment | cruiser54

[cruiser54]

How long have you owned this Jeep?

 

Are you positive the hoses are where they're supposed to go or did you just replace the bad hoses?

 

Here's a diagram for verification if that's the case. 

 

 

[pizzaman09]

On 9/22/2023 at 3:21 AM, Ωhm said:

If idle improves the more you START and RUN your vehicle, could be ECU is relearning itself.

If not, check TPS adjustment.

 

Renix TPS Adjustment | cruiser54

I wasn't aware that the Renix ecu had any memory to learn over time.  I was under the impression all the memory was erased every time the engine is shut down.  However I am also not an expert in this topic.

[Ωhm]

14 minutes ago, pizzaman09 said:

I wasn't aware that the Renix ecu had any memory to learn over time.  I was under the impression all the memory was erased every time the engine is shut down.  However I am also not an expert in this topic.

 

TPS, Short Term Fuel Trim (STFT) & Long Term Fuel Trim (LTFT).

88 FSM Computer Relearn Procedures.pdf

[cruiser54]

Ohm, you are correct. 

[SandMountainJeep]

Cruiser, hoses are correct.  I've owned it for about three years.

 

[SandMountainJeep]

Just a little more info, switch throttle body from a running engine and still does the exact same thing. I left all electrical on new throttle body.   Will idle when cold but once warmed up won't idle

[cruiser54]

Have you completed any of the Tips on my website? 

[SandMountainJeep]

I have found that the IAC is closed when warmed up.  Is this correct?  As long as it's open it idles and it's open until it reaches operating temp.  If it's closed and butterfly is closed how could it get air?  Thanks for your help.

[cruiser54]

You should see it move.

[Gojira94]

GM ECMs from the same period have 4 primary parameters for IAC, where 0 IAC steps/ counts is pintle fully extended, closing off airflow and max IAC steps is fully retracted with max airflow possible:

 

IAC park steps (where the pintle moves to at key off); shouldn't change.

IAC initial steps (where the pintle moves to at key on) most likely CAN change/ "learnable" on Renix ECU.

IAC minimum steps (lowest number of steps open possible/ pintle fully extended); usually low but not quite a zero value. Probably learnable on Renix ECU?

IAC maximum steps (highest number of steps open possible/ pintle fully retracted); high, maybe 255 (256 byte words is values from 0-255). Probably learnable on Renix ECU?

 

There are also stall saver IAC functions as well as target idle, acceleration enrichment ("throttle cracker") and deceleration enleanment IAC trims that operate in tandem with fuel and timing advance/ retard trims for these conditions (topic for another time).

 

Also wondering if target idle has been learned artificially low (operates as a function of IAC vs TPS position) due to the vacuum leak it's been living with.

 

A large vacuum leak can 'learn' the IAC positions to maintain idle/ run pretty far from default values. Have you disconnected the battery since fixing the vacuum leak? You might try it so the ECU can 'learn' from default values for IAC initial, min/max parameters, and from there follow the relearn procedure Ohm provided as needed. And TPS check/ adjust also a good suggestion from Ohm & pizzaman09.