Funky Idle

[Porquer]

My 2.5 seems to have trouble idling. It will idle at what seems like a normal RPM (no tach) for a while, then the RPMs drop and it sounds like it’s struggling to run. Then, after a little bit, the RPMs rise back to normal. Also smells like it’s running rich. I can see the idle stop moving as the RPMs cycle from high to low. I let it run for a bit with the TPS unplugged, and it behaved exactly the same. I tightened all of the manifold fasteners and there was also no change. The rhythm from high to low RPM seems fairly consistent. What are some things I can check to troubleshoot?

 

I have a video, but it’s too large. What’s the easiest way to share it?

[eaglescout526]

How are your grounds?  Start with the basics. 

[Porquer]

13 minutes ago, eaglescout526 said:

How are your grounds?  Start with the basics. 

Where are the sensor grounds on the 2.5? I know about the ground strap from the engine to the fire wall, but isn’t there single place on the block where all of the sensor grounds attach?

[eaglescout526]

The ground for the sensors and ECU ground out at the dipstick mount stud or bolt. 
 

Hopefully we can eliminate electrical being an issue before we look to mechanical. Part of me thinks headgasket is failing as I had similar symptoms but let’s check the free and less labor intensive stuff first. 

[AnotherOldJeepGuy]

On 4/15/2026 at 7:19 PM, Porquer said:

I have a video, but it’s too large. What’s the easiest way to share it?

I recently successfully share some video by putting the files on a free Google Drive account.  See my post, that coincidentally is on an issue very similar to yours, here.

Good Luck!

[Porquer]

Here’s a link to the video: https://photos.app.goo.gl/ZFG6bUoPDHXdpSko6

[Porquer]

@eaglescout526 I read in your ISA adjustment write up that a faulty throttle closed switch can cause the engine to hunt up and down for proper idle speed. Can you explain to me why that would be? I can see my ISA constantly extending and retracting as the rpm’s alternate from high to low. 

[eaglescout526]

Interesting.

Hunting makes me thing the ISA screw is too low.  So when it idles the ECU adjusts it to make contact with the closed throttle switch to tell the ECU it’s at idle or coasting in gear. 
So if it’s idling and it gets too low on idle, the ECU will kick the ISA up and try again. If it fails to maintain an idle while in closed loop, it will default to open loop fun little fact. Learned that with a bad headgasket while watching the REM. 
 

These do have grease inside and can harden and or not make good contact anymore. 
 

I hope that explains your why. How? Could be a couple reasons IE headgasket or the ISA was played with for another issue. 

[Porquer]

A few finds so far. the ground path for the IAT sensor has about 2.1 ohms of resistance. I suspect but have not confirmed that a lot of that resistance may be from the C101.

 

I started unplugging, cleaning, and applying dielectric grease to every connector I can find under the hood. In doing so, I found that the ground pin for the ICM looks burnt. Are individual pins available for purchase?

 

I tested the throttle closed switch at the diagnostic port with a volt meter. 0 volts at closed throttle, 5 volts when the throttle is open.

[eaglescout526]

You can get new terminals for that connector. They’re just weather pack terminals. Common GM connector of the era. 
 

I have witnessed the C101 do some damn weird things. It would always screw with my O2 sensor. 

[Porquer]

I did some probing of the o2 sensor plug. I have a good ground on pin B, 0.1-0.3 ohms. Key on, not running, I have 11v at pin A and 0v at pin C. Shouldn’t I see 5 volts from the ECU at pin C with the key on?