Low idle.

[feralyouth]

Ok so I’ve been battling a low idle for awhile. Couldn’t really pinpoint why it starts tell recent.

It seems to start after going to 55mph+. It runs fine after ECU reset, driving through town for a few days. Jump on the highway and when getting off and coming to a stop (foot off the brakes or rolling to a stop) rpms drop to below 450 rpms. Then the issue is sticks around and is intermittent. Until another ecu reset. Also sometimes if I turn the truck off and back on the problem will stop for a bit.

TPS is good, IAC is good, MAF is good, new O2 sensors, new spark plugs/wires, new air filter, new intake hose, new fuel filter, throttle body was pretty clean but I did a quick wipe down. Hoses seem fine with vacuum test. I just don’t know.

Maybe a new distributor cap, maybe a better cleaning, maybe a vacuum leak. Those are all that I can think of now. I’m even thinking about replacing spark plugs and wires again.

I need help!!

 

[voodu3]

Have you checked your battery cables? I had a similar problem when driving on the highway. It would be running fine and then start bogging down rpms would drop. The positive battery cable terminal was vibrating loose because it was broken. Might not be your problem but worth checking.

 

[jonheld]

Ok so I’ve been battling a low idle for awhile. Couldn’t really pinpoint why it starts tell recent.

What year?
What engine?

 

[feralyouth]

What year?
What engine?

95 with 4.5. Can’t believe I forgot to add this.

 

[feralyouth]

Have you checked your battery cables? I had a similar problem when driving on the highway. It would be running fine and then start bogging down rpms would drop. The positive battery cable terminal was vibrating loose because it was broken. Might not be your problem but worth checking.

I’ll check it out.

 

[ZackR]

I had a low idle for years on my 94, it would be around 400rpm and sometimes drop as low as 200rpm. Here's what solved mine:

- Remove and clean the IAC sensor, test for functionality per the FSM (stepper motor), run it all the way open and closed several times on the bench
- Clean the IAC air passage in the throttle body
- Deep clean of the throttle body, all passages, and the throttle plate so that it will fully close
- Adjust the throttle cables so that the throttle will fully close
- Adjust the TPS sensor per the FSM, using a multimeter make sure the signal output matches idle vs off idle
- Replace all vacuum lines (removal of the upper intake)
- Replace the oil cap (mine was old and didn't seal fully causing a small vacuum leak)
- Replace the PCV and hoses
- Reset the ECU (pull the fuse)

If I had to narrow down the most likely causes it would be vacuum leaks, a issue with the IAC, and or a misadjusted TPS.

Now upon starting the engine revs up to a high idle then drops back to a solid 650rpm (with the AC off) where it stays.

 

[OfficialGamings]

Simple Question:
Do you have a CEL or checked for codes stored in ECU? Seems like there should be if a ECU reset temporarily fixes the issue.

From what I remember some codes have a two trip detection system. First drive stores the code in the ECU, Second drive then checks for that same code again and then trips it.

Resetting the ECU will clear all codes so check when issue is present.

All this information should be obtainable within the FSM.

 

[feralyouth]

Simple Question:
Do you have a CEL or checked for codes stored in ECU? Seems like there should be if a ECU reset temporarily fixes the issue.

From what I remember some codes have a two trip detection system. First drive stores the code in the ECU, Second drive then checks for that same code again and then trips it.

Resetting the ECU will clear all codes so check when issue is present.

All this information should be obtainable within the FSM.

I have received no codes. If been fighting this for awhile too…