With the rebuilt carburetor installed as described in my post: “Chasing Down a High Idle in a 1982 Toyota Pickup – Replacing a Carburetor” and all vacuum lines were replaced as described at: “Chasing Down a High Idle in a 1982 Toyota Pickup – Vacuum Lines” my initial tests were really promising. The truck ran REALLY well. No more obvious hissing from the carburetor area, which was a great sign. A cold start was immediate, and driving down the road was smooth. I purposely did not make any adjustments to the carburetor because 1.) I trusted the initial settings by Carburetor Center, and 2.) I didn’t want to introduce other changes when trying to find a root cause of an issue.
While it ran much smoother, the idle was still high. Certainly not as high as it once was, where it may rap up to almost 1,800 RPM when warm, but now more like 1,400 RPM. Here is how I diagnosed and ultimately fixed the issue.
Leaving No Vacuum Port Unplugged
Now that I was certain many of the variables around the carburetor were addressed, I wanted to now revisit the vacuum system. My first step was to go back and plug every open vacuum port I could find.
My test drives showed promise, but was still holding a higher idle than I’d expect. Under the hood when it was holding an idle at about 1,400 RPMs, I checked to see if my throttle cam was fully returned against the stop. To my surprise, and contrary to previous observations, it was not. As soon as I pushed it down with slight pressure, it returned to about 1,000 RPMs. When I pushed down on the throttle cam, perhaps about 1/16th of an inch to its return, it felt like it had a bit of a notch, or an indexed feel to the return. Something that did not exist when the motor was off and cold.
So with that insight, I wanted to rule out anything mechanical (linkage, etc.) that may be preventing that throttle cam from returning fully. As a side note, I suspect I wasn’t the first to run into this issue on the pickup, as I found an old throttle spring attached to the throttle cam to supplement its spring. This may have been someone’s quick fix from many moons ago. But I wanted to do this the right way, so I took that off, and wanted to look at the first potential cause – the dashpot.
Ruling out the Dashpot
As noted in my previous post, the dashpot is a vacuum controlled mechanism to help control the throttle to prevent the motor stalling after rapidly lifting off the gas. It also helps keep the air/fuel mixture in a.more appropriate range, which can be a challenge for rapid acceleration or deceleration. My thought was that perhaps the faulty dashpot (bad diaphragm, etc.) might be preventing the throttle from returning fully. Fortunately, this one is an easy one to test. To eliminate it as a potential cause, simply unplug it from its vacuum source, and cap that vacuum source off.
The result? While the throttle was returning faster, it still seemed to be sticking still. Occasionally if I revved it up over 2,000 RPMs and let off the accelerator, it would have enough momentum to return the throttle cam to its starting position, and would drop down to 1,000 RPM as desired. So while the dashpot was not the culprit, it was perhaps exacerbating the symptoms caused by something else.
Looking at the Throttle Linkage
Next up was to look at every aspect of the throttle linkage. The cable, and the cable housing all seemed okay, and moving nicely. There was also a nice amount of slack in the cable to the throttle cam, which was also a good sign it was not too tight. Although, I did choose to reroute some vacuum lines and electrical wires to make sure the cable had a clear and direct line to the throttle cam.
My next target was the throttle cam assembly itself. I wanted to take it off and thoroughly inspect it, clean it and do whatever I could to get it working at its best.
I pulled off the throttle cam assembly, and realized this probably hasn’t been serviced in decades. Oxidation, and grime from years of use may be having an effect on the linkage. It was time to put it in an ultrasonic cleaner, followed by hand polishing any metal that contacted any other metal. The idea was that oxidation can add friction, so reducing that could be beneficial. If nothing else, I was going to have a shiny, and clean throttle cam assembly.
After the assembly was thoroughly cleaned, dried, and polished, I added just a slight tough of silicone lubricant on the plastic bushing before reassembling, and mounting back on the truck. One thing I did notice was at the end of the arm of the throttle cam that connects it to the throttle body of the carburetor, there was a hole for a cotter pin that was not used. A little research resulted in someone else having a stuck high idle because of a lack of this cotter pin. So needless to say, I added one, and you can see the two ends of it in the photo below.
With everything reassembled, the assembly and the linkage to the carburetor looked much better.
Time to Test
Next up was the real world test. To eliminate variables, I thought it would be best to initially test it without the dashpot connected to its vacuum line. This was going to be the best way to interpret if there was an improvement on the throttle linkage. The result was terrific. Every single time, it made a full return to idle at 1,000 RPM. Even if I let off the gas in a slow way, it made a full return, with no notch feel at the end.
With that working good, time to reconnect the vacuum line to the dashpot. If working correctly, this will simply slow down the throttle as it is returning to idle to prevent stalling. Once connected, I confirmed that it was working exactly as intended. It returned to 1,000 RPM every single time under all types of tests.
A Troubleshooting Post Mortem
So there you have it. The problem to my high idle was not a single root cause, but a combination of factors that had an amplification effect on the behavior of the motor. They would include:
- Potentially very small vacuum leaks from old vacuum lines and unplugged ports.
- A carburetor with a non-OEM base plate, a missing base plate gasket causing vacuum leaks. And potentially other issues with its choke breaker, etc.
- A poorly functioning throttle cam.
Once the new carburetor was installed, focusing on the other dependencies in the system helped me find the final contributing source of the issue. I never needed to adjust any carburetor settings, and in hindsight, doing so would have added more variables and frustrations.
Happy troubleshooting!
– Pete