Intermittent rough idle

Hi,

I have a 98 V6, 4WD, manual w/120k miles. I have had a somewhat intermittent rough idle for quite a while now. It's not that bad, but it's irritating - basically the truck starts to vibrate a little at stop lights. It runs great under power.

Initially, I did a tune up to try to cure the problem to no avail. I did plugs, cleaned MAF, checked compression, cleaned throttle body and of course checked for cracked hoses etc. I did not change the plug wires thinking that plug wire problems are more likely to show up under load.

I now have the FSM however the diag section is not very good as it recommends checking the fuel pump for a rough idle. I can do this but before I dive in I thought I'd ask what some likely culprits might be and things I can check fairly easily. I'm not throwing a check light. I do have a basic OBD II code checker.

here are some things I would check -

EGR modulator (seen these leak on other cars)
Idle air control valve - had one car where these were notorious for carbon build up which caused the plunger to jam however have not heard of this issue with Tacomas.
PCV - have never checked this

Even when the idle becomes rough, the idle speed seems to stay at spec. This seems to point away from the IAC and to a spark problem (i.e. wires).

Any other ideas?

Thanks

 

I would still say check the IAC for deposits. I would check any and all avenues to make sure to rule those out instead of speculating.

 

check for vacuum leaks they will only show up while your idilin and go away as soon as you give it gas

 

Thanks guys,

I had someone else ask me what type of plugs I put in. I put in platinums which most say are a non-no in these trucks. Now when I say platinum they are the exact part number as the OEM plugs but with the 'p' designation, and look identical to dual electrode OEM's. This supposed expert has told me that the Toyota coils are not designed to run platinums.

Well, I changed back to OEM non-plat, and the problem did not go away. Again, this expert told me that I may have done damage to the coils. I can believe that platinums are not a good idea but I find it hard to believe that the coils could be damaged by using platinums. I'm fairly knowledgeable about electronics and a platinum will have a lower resistance so at a given voltage so more current will flow during spark but it's hard to believe that it's enough to damage the coils.

Anyhow, I'll try changing the wire set since it's the original set and I'll look for vacuum leaks. Even after 10 years I noticed that the hoses (air and water) still look great except for the PCV valve hose which is a little worn.

 

Did you ever find an answer for the rough idle?

 

seafoam the motor if you havent been able ti find the rpoblem. it will clean the combustion chamber and all, plus.....maybe clean the throttle body. it should help.