Gargoyle_Runner

So I just spent a couple of weeks chasing a rough idle with a miss. If I didn't have a wrench or multimeter in hand I was reading forums on the possiblities. In case you've noticed there are a "few" pages regarding this. The downside of the forums are the variables. These motors are 20-30yrs old, most of which have a good deal of mileage. Alot of them have been rebuilt or repaired with either aftermarket and sometimes parts from other vehicles some of which not even Toyota. A lot of backyard mechanics adding their special touches. All of these things effect the end result. So what has worked for one person doesn't always work for the next. It's not their fault or lack of knowledge or yours. Their motor, or yours could have a history that is very different from the other. The best you can do is try things but always give yourself the ability to undo what you've done. Make a detailed list of what you do. Some fixes might not be the right fixes but still improve what your dealing with. I've had many of adjustments help but not fix the problem. So each time I do something that I think has fixed everything I go back and start undoing the other adjustments that I felt helped. Otherwise before you know it your problem is no longer the one thing you overlooked and instead it's everything you've adjusted to fix it. What I've learned and you have to keep in mind is that you are working with a fuel injection system that was designed 30yrs ago. It has a bunch of sensors and working parts that all have to be working in harmony if you want a smooth idle. You could have a problem with one thing or a host of things, which is common. The fun part of chasing down the host of things problem is trying to discover is one item causing the host of problems or is there actually multiple defective parts.

I just put a rebuilt motor in my 86 4Runner and it ran rough for weeks as I chased the gremlins out of it. First thing I learned was that the grounding is super important. Most of the sensors ground through the motor and it's manifolds.

People will first tell you to check codes which is a great place to start. The problem with that is the ECU doesn't always display codes. Sensors can be just out of working order but not enough to throw a code. This seems to be common with the TPS, which was one of my problems. You will also be told to check your timing along with what it should be. The problem you will find if you continue reading on timing is that what it should be doesn't work for everybody. Who knows why. Some motors run better 2 degrees over and like crap at spec. So when it comes to timing experiment. Adjust it and drive it, up hill and down hill, fast and slow. Find what works for you. The same goes for the AFM. It has two settings that supposedly you shouldn't touch. However if your part is 20+yrs old and has a lot of miles on it the springs and resistance strips have worn and the stock adjustment might need adjustment. How much? Who knows? You have to do some research and understand what you are doing and then experiment to find out what works for you. Alot of sensors and relays are farily simple to test with a multimeter and the information on how to find it and test it along with the range at which it works is an easy search away.

I had a rough idle after my rebuild. I bought a complete super stock head from Engine Builder along with a 268 cam. The bottom end was a master kit 20 over also from Engine Builder. Prior to the rebuild I had a pretty smooth idle with a suttle miss that went away with my foot on the pedal. After the idle was rough with a definate miss. I had no codes offering any help. I checked vacuum. Metered everything. I adjusted the timing a few degrees advanced, ended up back at spec. I replaced and gapped plugs along with checking the rest of the ignition all well within spec. I didn't get around to putting a fuel pressure gauge on it. I did clamp off things and pull vacuum lines and came to terms that there was certainly something not working right. So it was possibly the FPR or the pump, had a new filter on it. My gut was leaning toward the pump and I guessed right. That fixed the miss but the idle still wasn't great. Went back around with the multimeter. The O2 sensor was on the edge. Swapped it with a Denso sensor and really improved the idle. Still not right though. I went to the TPS again and checked the numbers on it. They were also in range just at the very edge of it. However after setting it for the 2nd or 3rd time the code started naming the TPS. Swapped that and another improvement but still rough below 1000rpm. Then I cracked open the AFM. The cog inside I've read is to adjust the mixture while driving and the screw under the plug was for idle. They both effected idle for me. After turning the screw clockwise a full turn and adjusting the cog counterclockwise 3 notches the motor purrs like a kitten. My milage is less than prior to the rebuild,17mpg compared to 20mpg. But it runs great now.

The engine idled smooth before the rebuild for some reason given that all these problems had to exisit then also. The changes made to the motor however, larger pistons, different cam, whatever. These changes were enough to cause these other issues to show their ugly faces.

My advice to anyone chasing down the idle issue just keep in mind it could be
Timing
AFM
TPS
IACV
O2 Sensor
CSI
CSI Switch
ECU Temp Sensor
ECU
Wiring to any of these
Fuel Pump
FPR
Fuel Filter
Vacuum leak
Exhaust leak
Valves
Leaks in the Intake flow
Low Coolant
Bad Ground
Ignition - all of it
Headgasket
EGR

And it could be a combination of any or all of those things. Happy hunting.

If I missed something please list it. If something fixes your problem list it please. How to find and test all of these things is listed in detail in the forums so just search for it.

slow-mo

Unfortunately, a lot of people test electrical components via the Ohms setting on a DVOM. What this does is isolate the component from the circuit. It is always best to test the component while under load, while it is in use. And the best way to test it is with some kind of graphing multimeter. It is best to see how the component is behaving while under load; is it giving a proper electrical pattern. Or is the electrical signal dropping out, is it behaving erratic, etc.

But then again, very few DIY-ers have such diagnostic tools.