Rough Idle Help

[SanDiegoCruiser]

For the past 6 months I've been trying to trace down a rough idle. It's started around the same time I had my alternator replaced. There is a ton of information on here about various things that people have done to try and fix their idle issue. I wanted to start a new thread since I've read them all and still have not fixed my issue. No CEL and everything looks fine according to my Scangage.

Symptoms: Rough idle when sitting at a stop light with the truck in drive. Less noticeable with A/C on since RPMs are higher. Shifting to N appears to alleviate the issue. At HIH I noticed less of an issue when at over 10K+ feet. RPM at stop in Drive with AC off is around 600-620. My mechanic who's well versed in all things Land Cruiser is stumped & I am tired of throwing parts at this.

Here is what I have replaced / tried - all OEM parts
1. Fuel Filter
2. APPS
3. TPS
4. 8 new Denso coils/plugs
5. Clean MAF/Throttle Body
6. Had mechanic clean fuel injectors
7. Replaced air filter
8. Mechanic checked for vacuum leaks
9. O2 Sensors




Any suggestions on what I should try next?

Thanks

SDCruiser

 

[DirtDawg]

Just spitballing, but have you checked your engine/tranny mounts?

 

[Julian Stead]

Have you looked at your fuel trims, also disconnect the battery for 5 mins and then let the vehicle idle up to temp. I know you have changed the alternator but did you get the battery checked out. Also check your battery connections and grounds.

 

[Jeepster09]

I am not sure what Toyota uses but on other makes a carboned up idle air control [I.A.C.] valve will cause this and not set a code.

 

[SanDiegoCruiser]

Have you looked at your fuel trims, also disconnect the battery for 5 mins and then let the vehicle idle up to temp. I know you have changed the alternator but did you get the battery checked out. Also check your battery connections and grounds.

I have not looked at my fuel trims. I assume the scangage can tell me this. What would bad trims point to? Blocked injector?

 

[SanDiegoCruiser]

I am not sure what Toyota uses but on other makes a carboned up idle air control [I.A.C.] valve will cause this and not set a code.

Never heard of an IAC. I'll do some research. Thanks!

 

[SanDiegoCruiser]

I have not looked at my fuel trims. I assume the scangage can tell me this. What would bad trims point to? Blocked injector? I replaced my O2 sensors a few years ago.

Forgot to mention I replace the forward O2 sensors. I wonder if I should replace the other bank...

 

[Julian Stead]

I found this link as an explanation.

Fuel trim can be a valuable diagnostic tool - Eastern Manufacturing

 

[SanDiegoCruiser]

Just spitballing, but have you checked your engine/tranny mounts?

Just checked motor & trans mounts - all look solid with no movement.

 

[Julian Stead]

^^ to check the mounts properly you should remove them, then you might aswell replace them. I removed mine and was suprised how much the rubber was separating from the metal, it is hard to see this when they are in place. I didn't have your symptoms but was suprised with the improvement the new ones made.

 

[SanDiegoCruiser]

I am not sure what Toyota uses but on other makes a carboned up idle air control [I.A.C.] valve will cause this and not set a code.

I believe IAC is the same as toyota's TPS - throttle position sensor. I've replaced this.

 

[Jeepster09]

TPS and IAC are not the same. Throttle position senser and idle air control are different. Other cars such as Ford uses both.

 

[SanDiegoCruiser]

TPS and IAC are not the same. Throttle position senser and idle air control are different. Other cars such as Ford uses both.

As far as I know, the 100 series does not have an IAC. Anyone know otherwise?

 

[Julian Stead]

The 100 series has a motorised throttle body, without looking at it properly I cannot say for sure, but I think it just uses the butterfly to control the idle, that is why it is critical to clean it out thoroughly as we are only talking small movements to Control idle.

 

[AimCOtaco]

I think the fact that shifting to N smooths it out and that fast idle and normal driving are OK points more to a transmission condition. The idle can only be stable when no load or a consistent load is presented, otherwise it's hunting for stability.

I wouldn't change the other o2's those just verify the cats are good, the forward o2's are used for mixture control etc.

 

[jerryb]

there isn't an idle control valve. Not in the sense that they are talking about. If you differentiated a little more on high idle, low idle, or stumbling idle you might get closer to some info.
I agree with Mr. taco about pointing to the transmission. I would want to be 100% sure there's no air leak, before or after the butterfly, and fsm mass ariflow checks are good. Seems you already got the coil packs and plugs ruled out.
fsm does point to tranny control in some instances in the 2006 manual diagnostic flow chart.

 

[SanDiegoCruiser]

Today I played closer attention to putting the trans in "N". It raised the RPM slightly which gave the appearance of a smoother idle. However after about 30 secs of being in N, the rough idle came back.

The mystery continues..

 

[Igoteyedeas]

I recently had the exact same issue and it turned out to be a misfire in the #5 cylinder due to a coil that was intermittently firing. I kept thinking it was the tranny as well given that putting it in "N" would help, but really is just the fact that when the engine is under load, it magnified the misfire.

That said, if you replaced all plugs and coils it's likely something else as everyone has mentioned above. To rule out any spark/fuel issues, have you hooked it up to an OBD II reader that can read all of the sensors throughout your motor (not just pulling codes in other words)? That helped me find my issue, so maybe that's a good place to start. Seems like you've thrown all the parts (read: money) you could at it, so that's surprising it still hasn't been found.

Good luck!

 

[Rolocado]

For the past 6 months I've been trying to trace down a rough idle. It's started around the same time I had my alternator replaced. There is a ton of information on here about various things that people have done to try and fix their idle issue. I wanted to start a new thread since I've read them all and still have not fixed my issue. No CEL and everything looks fine according to my Scangage.

Symptoms: Rough idle when sitting at a stop light with the truck in drive. Less noticeable with A/C on since RPMs are higher. Shifting to N appears to alleviate the issue. At HIH I noticed less of an issue when at over 10K+ feet. RPM at stop in Drive with AC off is around 600-620. My mechanic who's well versed in all things Land Cruiser is stumped & I am tired of throwing parts at this.

Here is what I have replaced / tried - all OEM parts
1. Fuel Filter
2. APPS
3. TPS
4. 8 new Denso coils/plugs
5. Clean MAF/Throttle Body
6. Had mechanic clean fuel injectors
7. Replaced air filter
8. Mechanic checked for vacuum leaks
9. O2 Sensors




Any suggestions on what I should try next?

Thanks

SDCruiser

What grade of gas are you using? Does premo run any different than reg?

 

[savirc]

Could it be your alternator? not on land cruisers but in the past I've had bad alternators cause bad idle. Not sure if you have a spare one to test this.