Argh! Help findng cause of rough low idle

[EvilMole]

Two things really irk me about this truck 1) ongoing alignment issues and 2) this darn rough low idle that I can't seem to pinpoint. The RPM when in gear is around 600 or slightly lower. If I take my foot off the gas then it kind of chugs along and doesn't move smoothly. The truck does appear to be running a little richer than it used to. Even when warm I notice a stronger exhaust smell than what it had when I first got it.

I currently have no CEL. The plugs and both o2 sensors have less than 5k on them, recently did the whole timing belt kit plus new pcv valve, grommet and plug wires. Cleaned the MAF (twice) and cleaned the IAC valve. Ran seafoam thru the tank.

Would a bad MAF or IAC throw a CEL? Anything else I should be looking into? Vacuum leak, temp sensor, anything...Thanks

 

[sean2202]

There could be many causes your truck is out of alignment. Inner and outer TR steering rack loose or LCA bushings worn. LCA bushings snuck up on me. I checked by jacking up front wheels seperatly and put a pry bar in between LCA and frame. Mine was cracked and had play. You can also check your tie rods when your truck is jacked up see if any play. Steering rack rubber bushings wear too. Did u use the proper plugs? Could also be an injector going bad. Listen next to each injector with long screw driver or stethescope for any differences. When was last fuel filter changed?

 

[nissanh]

When did you start noticing the change? Is it right after what repair?

Can the timing belt is off by one notch?

Also did you use all factory parts (PCV, spark plug wires, O2 sensors)?

Tell us more about the alignment issue.

 

[EvilMole]

There could be many causes your truck is out of alignment. Inner and outer TR steering rack loose or LCA bushings worn. LCA bushings snuck up on me. I checked by jacking up front wheels seperatly and put a pry bar in between LCA and frame. Mine was cracked and had play. You can also check your tie rods when your truck is jacked up see if any play. Steering rack rubber bushings wear too. Did u use the proper plugs? Could also be an injector going bad. Listen next to each injector with long screw driver or stethescope for any differences. When was last fuel filter changed?

The steering rack guide and bushings were replaced recently. Didn't notice any play when I had it jacked up. I believe the alignment issues stem from a shitty tire place doing the work. The truck has pulled to the right, with a slight right steering wheel, ever since I bought it. Now after new tires and two alignment attempts the truck still pulls slightly to the right but it now has a steering wheel slightly turned left.I'm planning on taking it to the Toy dealer and having them align it. I do plan on replacing the LBJ's but don't know if it'll happen before or after the Toy alignment.

As far as I know the fuel filter has never been changed.

 

[AZBub]

When you cleaned your MAF did you have it out of the vehicle and inserted MAF cleaner straw down into the MAF itself to clean it?

 

[gamefreakgc]

The steering rack guide and bushings were replaced recently. Didn't notice any play when I had it jacked up. I believe the alignment issues stem from a shitty tire place doing the work. The truck has pulled to the right, with a slight right steering wheel, ever since I bought it. Now after new tires and two alignment attempts the truck still pulls slightly to the right but it now has a steering wheel slightly turned left.I'm planning on taking it to the Toy dealer and having them align it. I do plan on replacing the LBJ's but don't know if it'll happen before or after the Toy alignment.

As far as I know the fuel filter has never been changed.

If the tie rods, inner or outer, have excessive play they will never be able to get the alignment right. That may be what's wrong. I second the Toyota dealer, mine does an alignment for $70 which isn't too bad.

The rough idle could be an out of spec valve. Have you ever checked the valve clearances? It's a little bit of work to get to the valve covers but if you've done all the easy stuff it might be time to check. You can get to the driver's side cover without removing much though. Plus a great opportunity to change out those leaky valve cover gaskets!

 

[EvilMole]

When did you start noticing the change? Is it right after what repair?

Can the timing belt is off by one notch?

Also did you use all factory parts (PCV, spark plug wires, O2 sensors)?

Tell us more about the alignment issue.

It wasn't noticeable after any one change. It's kind of gradually getting worse over time. It's not the timing belt as the truck runs good as soon as I give it some gas. It runs smooth If I give it just enough gas to raise the idle to 650-700 while in gear. Now it's 600 or less.

The PCV was the only off brand part I used. The plugs were NGK and the wires and sensors were Denso. Timing belt kit from aircabinman on ebay.

 

[gamefreakgc]

It wasn't noticeable after any one change. It's kind of gradually getting worse over time. It's not the timing belt as the truck runs good as soon as I give it some gas. It runs smooth If I give it just enough gas to raise the idle to 650-700 while in gear. Now it's 600 or less.

The PCV was the only off brand part I used. The plugs were NGK and the wires and sensors were Denso. Timing belt kit from aircabinman on ebay.

Sounding more and more like an out of spec valve, they don't fail suddenly just very slowly drift open over time. Probably an exhaust valve that's not seating correctly and causing the bad idle. Not very common for these engines since they are well made but still does happen.

A compression check would verify this for you. My old shop would do it for $25 if I did it along with another service. If there's a cylinder with low compression that's the source of your bad idle.

 

[sean2202]

Sounding more and more like an out of spec valve, they don't fail suddenly just very slowly drift open over time. Probably an exhaust valve that's not seating correctly and causing the bad idle. Not very common for these engines since they are well made but still does happen.

A compression check would verify this for you. My old shop would do it for $25 if I did it along with another service. If there's a cylinder with low compression that's the source of your bad idle.

Have you ever done a compression test on your engine and if so what did you get.

 

[BlazeRunner]

It's worth checking out your engine coolant temperature sensor.
Why am I mentioning the engine coolant temp sensor? The ECT tells the ECU what the internal temperature of the engine is which determines a few things. One thing that it affects is how much fuel is dumped into the cylinders. When an engine is cold, the ECU delivers more fuel and runs more timing advance because cold fuel does not atomize as well as warm fuel. If your sensor is bad, the ECU is seeing a cold engine even though it may have warmed up. It will keep running in open loop (default rich mixture) setting until it gets a signal it likes from the ECT sensor. Using the FSM, you can test your sensor to see if it is in spec using a multimeter and go from there.

 

[Bstrom]

Would it not be a good idea to clean the TB before all this other work is done? Poor idle is often corrected by doing this. Just my 2 cents...

 

[AntleredRuin]

Would it not be a good idea to clean the TB before all this other work is done? Poor idle is often corrected by doing this. Just my 2 cents...

+1, unless TB was cleaned along with the IAC and the OP just didn't mention it.
How did you clean the MAF? All you need to do is remove it and hose it down with MAF cleaner and let air dry for a few minutes. Touching it can break it or get oil or fibers from the cleaning utensil on it that throw off its reading.

Also, have you noticed any gas at all in the Air Intake Silencer at all? There is a hose that runs from it to the Fuel Pressure Regulator. My FPR started leaking about a week after I bought my '99 used. I fought with it for weeks; rough idle, random stalling (especially after a heavy acceleration), and hard starting. Did all that you did; new spark plugs, wires, fuel filter, PCV valve, cleaned MAF, TB, IAC, etc.

I finally noticed while doing another look through that when I removed the Air Intake Silencer to get to the passenger side spark plugs that it sounded a bit sloshy. Turned it this way and that and dumped a couple OZ of gas out. Followed the line back around and found it was the Fuel Pressure Regulator.
Got a used one off eBay for about $40, and a week later (and almost 10hrs of labor due to my breaking a fuel injector head off into one of the valves) it was installed and good as new. Started right up with no issues, even with the fuel line being disconnected for several hours for depressurization.

Make sure also to either pull the EFI relay or detach the negative battery cable for a bit to help reset the ECM after every repair you do (fluid changes not necessary). This helps the ECM remap fuel/air ratios based on the new repair.

 

[EvilMole]

Sounding more and more like an out of spec valve, they don't fail suddenly just very slowly drift open over time. Probably an exhaust valve that's not seating correctly and causing the bad idle. Not very common for these engines since they are well made but still does happen.

A compression check would verify this for you. My old shop would do it for $25 if I did it along with another service. If there's a cylinder with low compression that's the source of your bad idle.

Wouldn't an out of spec valve cause more than just a low idle issue? It really only runs low and rough when in gear. It idles around a smooth 700rpm if I slip it into neutral.

It's worth checking out your engine coolant temperature sensor.
Why am I mentioning the engine coolant temp sensor? The ECT tells the ECU what the internal temperature of the engine is which determines a few things. One thing that it affects is how much fuel is dumped into the cylinders. When an engine is cold, the ECU delivers more fuel and runs more timing advance because cold fuel does not atomize as well as warm fuel. If your sensor is bad, the ECU is seeing a cold engine even though it may have warmed up. It will keep running in open loop (default rich mixture) setting until it gets a signal it likes from the ECT sensor. Using the FSM, you can test your sensor to see if it is in spec using a multimeter and go from there.

This was one of the first things that crossed my mind, but when I had this problem on our van, it ran like total crap. May be worth investigating.

Would it not be a good idea to clean the TB before all this other work is done? Poor idle is often corrected by doing this. Just my 2 cents...

I did this when I cleaned the IAC valve. It's a quick thing to do so I may pull it off and try to clean it again.


One other thing. I used contact cleaner to clean the MAF sensor which I now know is different than MAF sensor cleaner. Looks like I'll be swinging by the store later to pick up some MAF cleaner. Don't expect to see a difference, but you never know.

 

[gamefreakgc]

Wouldn't an out of spec valve cause more than just a low idle issue? It really only runs low and rough when in gear. It idles around a smooth 700rpm if I slip it into neutral.

Not if it's slowly deteriorated or happened before you bought it. Valve issues are most noticable at idle, once you build up RPM it will run fine.

My last vehicle, 91 Toyota Pickup, had some more severe valve issues. At 1100 RPM it ran perfect but once you dropped to 900 it was very sloppy and the engine wobbled a ton at idle. If it dropped under 700 it would threaten to stall out. Different engine management system and ECU on that vehicle (idle is not controlled by ECU) but same sort of symptoms.

Also, the ECT is a good idea to check and quick with a multimeter. If you don't have one, pick one up for 10 bucks at Harbor freight or Amazon. It's a great tool for both home repairs and car repairs. However, with the 4Runners being ODB-II vehicles the ECT's don't mess up as often as the old OBD-I vehicles since they can actually throw a code for the ECT sensor. On old OBD-I vehicles the sensor would go bad but you had no way of knowing it.

 

[Jasonredwood]

Compression and leak down test would help narrow things down.
I have been searching, cleaning and replacing parts left and right in search of my low/irregular idle.

 

[nissanh]

Sounds more like a faulty IAC valve. Did you remove it from the TB to clean? Make sure to get new gaskets for IAC and TB. When you first remove the IAC, was it covered with lots of oil/grime in the air vents? dirty coolant lines?

 

[raty4R]

You guys may not believe this, but I had the low idle almost stall issue as well. Especially with the a/c on. Tried all the mentioned stuff, took it to the mechanics several times, still had the same problem. This was over about eight years time. Recently my power brake booster went out, so after I replaced it... no more rough idle. Not sure how you test a brake power booster, maybe pinch off the line and see if it idles better?