Minor fluctuations to RPMS while idling. Is this normal? Tune up? please help

[jasindude1]

I have a 94 4Runner its new to me, 241K on the clock, the air filter has been replaced the oil just changed by PO. I noticed while in line for at a drive-thru that about every 45 seconds there is a slight fluctuation in the idle, by looking at the gauge, i have the idle set at 750 (per tach), it will drop to as low as 500rpms and then go back to 750. The AC was on so i thought it was that, but when the AC was off it would do the same thing but wound drop as much. is this a normal thing? or should i be looking at other things? I figured it was too like clockwork to be fuel, maybe spark? Timing? Or maybe clogged injectors? The truck runs fine, never feels like its gonna die, just a little shaky when it drops RPMs, but then evens out. I wanted to get suggestions before just throwing parts at it. My idle is set at 750, is that too low? any suggestions would be greatly appreciated, I am about 5300 feet above sea level and i only put the cheapest fuel as this is my around town vehicle. Thanks in advance!

 

[3VZE93]

This can be a few issues. I have two of these motors and both idling issues were solved by different solutions.

My first bet would be to check out the Throttle Position Sensor (TPS). They do go bad and are very easy to replace. It's the black/silver plastic piece on the firewall side of the throttle body. Try cleaning out the plug first with some electrical cleaner spray.

Also take a look at the air flow meter, it's the large silver housing behind your air filter. It has a plug attached to it. Spray out the inside of the meter with some Mass Air Flow sensor cleaner, do not use anything else other than that. Best practice to disconnect the negative terminal on the battery to avoid shorting anything out also.

With that many miles I would say go ahead and replace all tune up items unless you know when the last time they were done (Plugs, plug wires, distributor cap, rotor.)

 

[MTX018]

Had a similar issue on my ‘93 4Runner with a 3.0 V6. My symptoms were more like an engine stumble. When the RPMs were dropping, the engine would shudder slightly and when it recovered, the RPMs went back up. If you RPMs are “searching” the engine won’t stumble but the RPMs will constantly fluctuate up and down. Searching is usually a result of a vacuum leak. Stumbling is usually a result of low RPMs or misfires (plugs or injectors).

Here is how I resolved my issues.

1 - Replace all spark plugs. This resolved 85% of my symptoms and was a quick, cheap fix. Some plugs were obviously fouled.

2 - Inspect all engine vacuum lines for cracks or tears. I ended up replacing them all 1 by 1. Again, a very cheap fix and easy. Will take you 30min. This will cure the majority of your vacuum leaks if the lines are older.

3 - Inspect the cold air intake (rear of air box to throttle body). It’s that big black pipe on the driver’s side of the engine. Replace if it is cracked. They are not cheap (~$30 or so).

4 - Drive vehicle again and see if symptoms are improved. If symptoms didn’t improve, continue.

5 - Remove throttle body, clean, and reinstall with NEW throttle body gasket. The throttle body gasket can leak also, throwing things off.

6 - Add a can of fuel injection system cleaner with your next tank of gas.

7 - Drive again and see if symptoms improve. If not, continue on.

8 - Raise your idle RPM up to 800-850. Sometimes, if your idle is too low, the 3VZEs can stumble a bit and try to die. Set when the engine is warm.

9 - Drive again and see if symptoms improve. If not, continue on.

10 - consider replacing your upper plenum gasket. This will also give you a chance to look at the fuel injector rails and connectors. Make sure they are not leaking and the connectors have a positive connection with the injectors. Clean the upper plenum before reinstalling. You do not have to remove the throttle body from the upper plenum.

11 - Drive again and see if symptoms improve. If not, continue on.

12 - Watch a video on YouTube by Practical Enthusiast “high idle speed easy fix”. He shows how to clean the idle air control valve on the back of the throttle body.

13 - consider removing all fuel injectors and sending them in for cleaning and a new o-rings


My guess is you will be back in action by Step 5 or 6 at the worst. Good luck.