Rough Idle | No Codes

[Scry0402]

Hey All-

Thanks for reading my post. I've read through several rough idle posts on the forum here, but nothing that quite touches on what I'm experiencing.

I've got a 2003 Trailblazer EXT, 4.2 I6 - it just developed a rough idle. No service engine light, no codes, no nothing.
Vehicle only idles rough when sitting. Does not matter if it is in Park, Drive, Neutral. Idles rough regardless.
When accelerating the vehicle operates smoothly. Driving around town and/or highway it runs as smooth as it can be.

I have read and saw all sorts of things people have suggested: Clean Throttle Body, bad ignition coils, Camshaft sensor, etc...before attacking any of that, I'd rather look at a couple of other things regarding vehicle to get thoughts from you if whether any of it could be related.

Recently I fixed heater control valve found on firewall to get heat to rear of vehicle. I have ensured vacuum line is attached to valve properly. Vehicle has rear heat - so valve is operational.

In conjunction with the rough idle - I have noticed that the vehicle is absolutely in need of a water pump. There is excessive play in the water pump pulley with obvious noise that the fan is coming into contact with the shroud.

The vehicle itself is not running hot. The temp needle doesn't quite reach 210, so I have to believe the pump itself and thermostat are still functional despite the bad pulley.

I know stranger things have happened, but just the sheer fact that the rough idle and noise coming from/caused by water pump have seemingly happened simultaneously has led me to believe these two items are possibly intertwined.

Has anyone ever seen a water pump causing a rough idle at low RPM's - or have any justification that a pump could create this condition?

The fact that there are no codes (been driving it daily with rough idle for a week) and it does not accelerate rough or have any loss of power when accelerating leads me to believe its not ignition related.

I came across a guy who was experiencing similar issues in his 06 TB (rough at low RPM smooth through acceleration & at speed), same motor - he replaced what he identified as a MAF sensor on his 06 and he also replaced his MAP sensor and he said this fixed his problem.

I don't have any tools to read live data unfortunately. I do know a guy in our area who owns a shop, but he's busy so I thought I'd turn here first for any suggestions before scheduling something with him.

Thanks for taking time to read my post. I look forward to any feedback you may have.

 

[TollKeeper]

If the bearings in the waterpump are seizing, yes it can cause a rough idle.

But to know if thats the issue, just take your waterpump belt off, and start the truck. No more rough idle, problem found..

But I doubt its the cause. I would start by doing the normal things for the 4.2.

Disconnect the battery
While battery is disconnected, clean the throttle body
After 30 mins, reconnect battery
Start engine
Check MAF, by tapping on it with the back end of a screwdriver

Whens the last time you had a tune up done? What spark plugs were used?

After that, you are likely going to have to drive it around for 30ish minutes, and see if any codes pop up. Did you run a code reader on it? Were there any pending codes?

 

[Mooseman]

What @TollKeeper said except yours doesn't have a MAF. Do check/clean the MAP on top of the intake manifold.

For ANY rough idle issue on this engine, cleaning the throttle body is always the first step.

After that, you absolutely need a tool to scan live data. A cheap Vpeak bluetooth adapter and Torque Pro on Android will allow you to see what your O2 sensor is doing and what your fuel trims are like.

Another thing to check are the engine mounts. They are infamous for making people think the engine is shaking when it's the mounts that are defective. If it's a regular ~50hz vibration only at idle, it's the #1 suspect.

 

[Scry0402]

A mere couple of hours later after making this post I can with 100% certainty say that the issue was being caused by the vibration in from the water pump.

Bearings got so bad earlier this afternoon and forced such a violent wobble on the pulley that the serpentine belt slipped off. This happened while sitting at a stop light. Immediately after the belt came off the water pump pulley, the idle smoothened right out.

You guys would lose your minds if you saw how much play was in the fan now that the belt is not on it. Crazy bad.

Thanks again for all the suggestions. Really enjoy this forum and what enthusiasts there are for these vehicles.

We bought Trailblazer EXT from from a guy who had it and was experiencing a no crank/no start. After doing all the basic checks, the culprit causing that issue was found on a corroded pin to the ignition one circuit on the electrical ribbon that connects to the fuse block under driver side middle seat. Incredible.

Gotta love being in the rust belt up here in Minnesota!

Take care!