Since the original article post vanished over a decade ago, I've re-posted this content, along with some information of my own, in hopes that this will be of assistance to others with this same issue.
Do you own a Toyota Pickup or 4Runner with a 22RE? Does the idle have a strange problem when you step on the brake pedal? Does it happen ONLY when you step on the brake pedal, and ONLY at idle? I'm not talking about a vacuum leak that will cause your engine to stall if the brake pedal is pressed or won't allow your engine to run at all without lots of gas. If your truck doesn't have vacuum leak symptoms but instead has the symptoms described below, I'll bet your vehicle has a rare problem only few of us have seen.
Note: this is usually not an actual brake problem or failure of the brakes, although it could be a vacuum leak in the brake booster. It's usually an engine idle fluctuation problem originating with the ECU.
At idle speed only, when you step on the brakes (no matter how hard), the RPM's suddenly start fluctuating up and down and up and down and up and down etc. by about 400-500 RPM's. Each cycle is about 2-3 seconds from up to down to up again. But you may notice that the RPM's drop suddenly, but take the 2-3 seconds to slowly rise back up. It appears that the lowest RPM is actual idle speed (what the RPM's suddenly dropped to), and the highest speed seems to be a bit above normal idle speed. If you take your foot of the brake, the problem goes away instantly and RPM's return to normal. Put your foot on the brake again and the problem returns. I've noticed in my experience that sometimes the problem only happens a few cycles then goes away. And in other cases the problem happens every time and the entire time the brake pedal is pressed. As another symptom, be it quirky, try shifting your truck into 4WD high. Don't worry about locking your hubs. Just pull the stick straight back into 4WD high. Does the problem go away? It should have gone away if your vehicle has the same problem. Put the stick back in 2WD and the problem ought to come back (assuming your vehicle's case was not a flakey intermittent issue) Try something more. This is assuming you are parked safely somewhere and do not intend to drive the truck around while doing this next test. Put the stick back in 2WD if it isn't, and remove the fuse for the brake lights (NOT the fuse for the tail lights). Your brake lights will not light up at this point, but try stepping on the brakes again. Did the problem go away again? One more test you can do is pull the brake light bulbs themselves. Basically anything that will keep the circuit broken for the brake lights will "fix" the idle/brake problem.
Ok, every test that you tried provided the same symptoms I described, so now what? Well, I've heard of two solutions for this. I'm not sure if the solutions are related, but they both worked. The solution I tried involved bypassing the wire that normally runs from the brake light switch on the brake pedal to the computer and then out to the actual brake lights. Instead of the normal route, I bypassed the computer and ran the wire straight from the pin switch to the brake lights. Note that there are two wires on the brake light switch. One is always hot when the truck is running, and the other is only hot when the pedal is pressed (and the switch closes and power runs to the brake lights). I am not listing the color of each wire because it probably is not consistent across all years. Nor am I going to list the wire that runs from the computer to the brake lights. This wire is singular up until it gets to the spare tire cross member where it splits into two--one for each side of the vehicle. My fix was to cut the wire on the pin switch that runs to the computer, and to cut the wire that runs to the brake lights. I cut the brake light switch wire right at the switch and re-ran it through the dash to the area I cut the wire running from the computer to the brake lights. What I originally tried but failed to work was to just run a new wire from the switch to the computer--this did not work. Somehow the computer is getting a faulty signal from something.
Someone else on an online Toyota bulletin board also had this same problem but they brought it to a dealer instead. The dealer claimed the problem was the EGR valve. The dealer said that the EGR somehow sends a signal to the computer whenever the brake lights come on. I'm not sure if this is true since I've never looked closely at the EGR valve. Nor have I looked into this problem any further since I fixed it myself. Anyway, once the dealer replaced the EGR valve, the problem went away.
These two fixes could be related. It could be a fluke too. I know my way worked because one of my trucks with this problem did it all the time, every time the brake pedal was pressed. I have no recommendation on what way to try first. I also do not know the possible side effects of using my method. I have not run into any other problems since I fixed it. All other engine and electrical systems have operated normally since I made the fix a year ago.
While trying to diagnose this problem, I found out some things the problem IS NOT related to. I replaced the distributor, AFM, EFI computer, alternator, battery, and the brake light switch and nothing fixed it. I used a multi-meter on numerous wires including the brake light switch wires, the brake light wire, and the main battery posts (checking for alternator output) checking for voltage spikes or drops and found nothing.
If your Toyota truck or 4Runner has had this problem and you found a fix, please let me know. If you try one of the fixes I mention, tell me your results. Good luck! You're chasing a goofy difficult-to-diagnose problem.
On my 1985 4Runner, I can exactly duplicate this issue if I bump the idle speed up too high (>1000 RPM). I can easily do this with the A/C idle up air valve. When I do that, I get this exact behavior. What is happening is that the ECU thinks you're trying to brake to a stop, so it cuts the fuel injectors off until the engine speed drops, then fuel is restored. I feel the real fix is to just adjust your idle speed to ~750-800 RPM and then set the A/C idle up valve to slightly boost the idle RPM up to ~850-900 RPM, to keep the engine from bogging down when the A/C kicks on. No need to cut any wires or modify anything.
It may help to understand the purpose of the A/C amplifier, the A/C idle up valve and the ECU fuel cut RPM:
If you can't adjust your engine idle speed low enough to prevent this issue from occurring, find out what is the cause of the high idle speed. Might be a bad o-ring on the idle setting screw on the throttle body. It might be a misadjusted throttle cable, improperly adjusted TPS, or an air/vacuum leak, etc.
There's also a good discussion of this topic along with some other possible causes and solutions on this old 4x4Wire forum thread that's worth a read:
Here's another possibility from Alison in Wisconsin with her 1986 4Runner:
I have experienced the fluctuating idle/engine rev earlier thus summer after taking it to the mechanic for some vacuum line work that was above my pay grade. While at the shop, they noticed a piece in the coolant system that was starting to go bad and replaced it.
Driving it afterwards was the first time I had the weird engine rev, only when braking after the engine had warmed. Assumed they hooked up the vacuum lines wrong, but everything checked out good and the mechanic didn't know what to do about the rev besides adjust the idle/clean throttle body. (Neither worked)
So I'm searching 4Runner forums and I stumble across a post that mentions that the computer is programmed to cut the gas to the fuel injectors when braking. This was done for "fuel economy." Then, a bit later in my search I found a small thread about low coolant being connected to and/or causing a weird idle.
I had only superficially checked my coolant level prior. When I went to bleed my coolant system, I had to add about a quart and half, wow! I didn't really hear an excessive about of bubbles in my dash as I drove, and my thermometer was showing normal temps A small amount on of bubbles on occasion seems normal, I really struggle to get all the air bled out even with the front of the truck raised.
Voila! Refilling and bleeding the coolant fixed my idle! I guess when the mechanic changed some piece in it he forgot to refill, big whoops.
Apparently, a sensor that tells the computer to "cold idle" is normally clear of coolant when the engine is cold, and submerged when warm, but is clear in both scenarios when the coolant is very low. As you drive, this conflicts with the computer trying to cut the gas to the fuel injectors when braking, causing a low/high alternating rpm as it searches for the right idle, but only when braking, only when the engine is warm.
Not sure how or if the EGR valve is connected to my issue.
[Last updated: 25.April.2023 ]Visitor # 3005 since 15.JUL.2021