High Idle Issue

My 98 Tacoma hunts for idle and high idles bad. I'm inclined to believe it is a sensor/input issue, as it will idle "happily" sometimes (1100rpm, I know that's also high but it doesn't affect driveability so I'll take it) but sometimes it will idle at 1500rpm or even 2100rpm, and sometimes it will jump between those different high idles.

I've replaced all the vacuum lines already and used brake clean to check for vacuum leaks (none). The truck seemed to like the new vacuum lines for a minute, only high idling at 1500 for a couple of days, but then it got back to its 2100 idle.

There's a box with a host of vacuum lines over on the passenger side of the engine bay adjacent to the brake booster. When I was replacing the lines it hissed. I've also heard the gas cap hiss when taking it off. I may have a clogged vent tube on the gas tank? (although it has no trouble filling so I think that's unlikely) Could a clogged vent tube cause a high idle?

I have visually inspected the MAF (looks new) and IAC. I "cleaned" the IAC (brake clean and a rag). That seemed to make no difference.

I haven't messed with my temp sensor, but the temp gauge doesn't behave weirdly so I don't think It's malfunctioning, does the idle control work off a different resistor than the gauge?

I don't like doing shotgun maintenance (replacing all the sensors) especially when the IAC is $400 at Napa (I don't have an address to ship to).

 

Have you noticed any trends as to when it 'sometimes' idles happily?
For example, I dealt with a high idle and it would idle normally when restarting the truck if it had been driven then turned off for 5-20min. Any trend could be a clue.
Do you have an OBD scanner that can read live data while driving?

If I remember correctly there are two temp sensors, one is dedicated to the dash gauge, one for the ecu. So I believe its possible to have an issue with the ECU temp sensor, and the dash gauge would look fine.

 

There doesn't seem to be any consistency with the idle issue. Sometimes it will high idle cold, sometimes it will high idle warm. Sometimes it will drop down to 1100 when I put the clutch in and stay there, sometimes it will drop and then bounce right back to 1500 or higher. Sometimes it will "hunt" for idle (bouncing around) sometimes it will sit at a relatively random high idle happily (1400, 1700, 2000) and it will change its mind on how it idles from stoplight to stoplight.

I'm going to look into the two temp sensor thing. My gut has been telling me to replace the temp sensor because the "normal" idle of 1100 is still really high, almost warming up the engine-type rpm. If the temp sensor has an internal resistor that is just a wire it could very well be getting hot and moving around over bumpy surfaces causing different resistances. If the ECU temp sensor is bad that would be a good culprit. If you have any more info on that I would love it.

I prefer doing real diagnostics and fixing based on said diagnostic but I'm getting lazy and want this truck to work. I threw a new TPS sensor at it and the high idle persisted. I'm close to replacing the MAF too. If that doesn't work I'll slowly replace all the idle related sensors and convince myself that it is an ECU issue lol

 

Cleaning the IAC it could still be bad. But plug in a scanner and check the ECU temp (separate from cluster temp). You can also look at the o2 sensors etc there are a lot of things that contribute to idle. From what I have seen, a hunting idle can be IAC, throttle sensor, or ECU temp sensor. I had a low idle and it was the IAC - replacing it made a difference didn't bother cleaning it.

 

I also ran an OHM test on the IAC and it passed that. I'm not opposed to replacing it, however, it is the most expensive sensor/part to replace and I'm a little tight right now.

 

Curious,,,, how long has this been going on, and what happened to trigger this or has it always done this to you?

 

Def check the ECU temp then, bad sensor could be sending all sorts of weird info.

 

Ok update: I replaced the ECU temp sensor & the high idle has improved (although is still quite high) I rarely see idle above 1500 and if I do it's only briefly.

One consistency (or semi-consistency) is that the idle will "bounce" off of 1000 when I put in the clutch/am coming to a stop. Do wheel speed sensors play a role in idle conditions?

I tried replacing my TPS to no avail already. I plan to hit a junkyard and rob every MAP, IAC, & temp sensor that I can find to see if that makes a difference.

 

So when I was dealing with my high idle it would bounce like that when coming to a stop. That had me really thinking I had a vacuum leak in the brake booster, bc I couldn’t think of any other explanation for it. However, my high idle ended up being due to a stuck open thermostat so the truck was always “cold”, so the idle never dropped. When that was fixed, the idle went to normal and it never fluctuated when coming to a stop again.

Thermostat and a low coolant temp could be your issue, or the bouncing idle might just be an ancillary symptom of any high idle. Due to my experience, that’s something I wouldn’t I dig into too much, because it really just leads you straight to the brake booster which is one of the most expensive parts to replace.

The reason I asked in my first post if you had a OBD2 Scanner is that moving on from here you really need access to one to avoid just loading up the parts cannon. Super easy to check things off the list of possibilities, like a thermostat, by just reading the coolant temp sensor data live while driving around.

I use an Ultragage, there’s also Scangages, these are both great truck-mounted options, and I believe you can get a cheaper Bluetooth obd2 scanner and an app that can do all of this as well.

Another thing to remember is that any fix might not show itself immediately. Assuming most these fixes will have you remove the battery terminals while you work, then the ecu is going to have to relearn the correct idle. So it may take a day or two after you fixed the broken part before the idle returns fully to the desired 700rpm.

 

Check the throttle plate how its sitting. I would still clean the iacv again, clean the maf

 

I was looking back at some old footage and about 2 weeks ago it was idling at <1200 on a cold start (~30° outside) so this issue is relatively recent and has occurred after a couple of thousand miles (mostly on the road but maybe <100 offroad)

That thermostat tip is massive, I might just go ahead and replace the thermostat because that's a relatively cheap part (at least in small block Chevy Land it always was)

the correlation of a new temp sensor & improved idle condition is making me think that the thermostat is a good shout. Although I drive this thing for pretty far distances, even with a stuck open thermostat it still gets hot and the idle is still funky. It's also worth noting: Since replacing the temp sensor, it (sometimes) idles normally (1000-1200) on cold starts and then gets barely warm & starts to high idle.

I was wondering if a bad alternator could cause a high idle. I've been working the electrical pretty hard on this truck between constantly charging phones/battery blocks & my laptop, and always running the headlights & running the interior light for 1hr + while setting up camp. I haven't put a multimeter on the battery to check the alt but if it's always idling at 1500 that reading could be deceptive anyway. I'll ask to get my battery tested at the next parts store I hit, maybe it has a dead cell.

I have an OBD2 scan tool but it was "cost-effective" in 2017 when I got it so idk if it has live data functionality. I also live in this truck rn (I may have mentioned) so I can't order any parts/tools online, I need to buy stuff at stores.

The throttle plate is sitting normally & when I let off the throttle it will "bounce" off of a normal idle which is a pretty clear indicator that the IAC is fighting some demons (IMPO). I've also tightened the throttle cable (it was gently loose) and now the gas pedal feels great.

I've gone around and checked a few grounds (engine to chassis, harness to engine) I should probably just check them all to be sure.