My winter beater has a small hiccup or "sputter" at 2000rpm. It's a '94 Geo Prizm with the 1.6L 4A-FE motor. It's got about 182k on the odometer. The engine runs well all throughout the rev range, except for right around 1800-2000rpm when under load. It seems most notable in the 1-2 upshift, but it will do it if I downshift or on random other upshifts. It doesn't do it every single time, but probably 75% of the time I'm on the gas at that rpm. When it happens, it feels like a misfire, as the engine bucks a little bit and acceleration is almost non-existant. No CEL on. It lasts no more than 1-2 seconds tops, then it picks up again and revs smoothly the rest of the way.
I changed the distributor cap a few weeks ago because the old one let moisture in (car wouldn't start when it was damp out). Could it be plugs/wires?
I had a Mk3 Jetta that did that exact thing (yes, unrelated, but maybe helpful?). Turned out to be a bad MAF. Not sure what kind of system the Geo/Toyota uses.
I had a similar rpm-dependent stumble that was the result of a bad MAF on my camaro as well. Cleaning it made no difference but after I tried to fix everything else I was lead back to the MAF being bad.
Try unplugging it and going for a drive, the car should use a table rather than the maf.
So I'll take a look at the MAF first to see if that's the problem. Wouldn't the car run really poorly if I unplug the MAF?
Possibly, but with GM EFI, they run the MAF in addition to a speed density system. So if one or the other fails, it can "limp" home. The limp home programming really isn't much different driveability-wise then if the MAF is unplugged.
But you said it's a Geo, so you really have to be sure it is not really a Toyota setup.
Ranger50 wrote:
Possibly, but with GM EFI, they run the MAF in addition to a speed density system. So if one or the other fails, it can "limp" home. The limp home programming really isn't much different driveability-wise then if the MAF is unplugged.
But you said it's a Geo, so you really have to be sure it is not really a Toyota setup.
It is a Toyota setup.
And it doesn't have a MAF, by the way.
Klayfish, there's an IAT sensor in the intake tubing. Make sure that it's connected and making a good seal. 
