At idle (like at a stop sign or sitting in the drive way) my '92 Toyota revs up and dies down (kind of a “brrrrvvvvvv-buh… brrrrvvvvvvvv-buh”) We have been fixing it up and got ahold of another Toyota that has a decent motor but the body, frame, and everything else is shot. We are pulling a starter out of it and was wondering if the part that causes the gallop could be replaced too.
The galloping is most likely due to a vacuum leak. With this truck being 20 years old, check and replace all the vacuum lines or cap off the ones you don’t need.
Busted is probably right. But vacuum line by the foot is dirt cheap. I might be easier to just replace all the vacuum, lines one at a time and see if the erratic idle stops. Don’t forget too that this engine has a diaphtagm ignition timing advance on the distributor as well. And, don;t forget that wandering timing from a loose (worn out) distributor shaft can also cause a wandering idle. See if the shaft has any lateral play.
PS: your vacuum gage and your timing light will probably be your two most valuable tools for diagnosing this. Sigh. I miss the old days.
It’s my feeling that reving up and down is more of a rich condition than a vacuum leak (lean condition). What do the spark plug tips indicate? If rich, check the fuel pressure regulator for leakage. Pull the vacuum hose on the regulator and check for the presence of fuel there. There shouldn’t be any.
I concur w/others that this is an air leak somewhere, which is affecting the fuel metering, causing the mixture to be soemwhat askew. This confuses the heck out of the engine computer.
It’s probably a leaking vacuum hose or a leaking vacuum operated device. All the vacuum devices have a rubber-like diaphram in them, and over time they’ll crack and start to leak. No air is supposed to enter the engine from these devices, but when they leak unmetered air gets past the MAF and confuses the ECM. The first thing I’d do it get oen of those vacuum pump/guage gadgets and test every vacuuum device to make sure it holds vacuum. The most common cause of a vacuum leak is the brake booster. But there’s lots of vacuum operated devices, and it could be any of them. Test them one at a time. If no devices are leaking, then you have to test the hoses from the manifold to the devices using the same method.
There’s one other possibility. If the idle adjusting screw has been messed with, and it is out of it’s operating limits, this also confuses the ECU. There’s no way to fix it until the idle screw is reset to it’s nominal point. The Toyota shop manual for your car probably expains how to do that.
You need something that looks like this: