Hesitation/Surging upon acceleration from stop.
 

I have a 1995 SC400 with 203k miles. When accelerating from a stop under average/medium throttle it will hesitate or surge between 1200 and 2000 rpm. If I give it more throttle so that the rpm jumps right up to above 2k it will be fine or if I accelerate really slowly where it doesn't get above about 1500 rpm it will be okay. Only one time did it do it other than from a stop while I was doing about 40mph going down a slight hill and it was pretty violent but it's never done it again. There's no check engine light. It idles perfectly fine and seems to run great other than this. After reading about this problem on other posts i've basically narrowed it down to 4 things. MAF or TPS (but i thought a check engine light would come on for these.) or clogged fuel filter/failing fuel pump. Oh, and this might just be in my head but I swear it doesn't do it as bad or often when I have it in ECT PWR mode.

It could be ECU capacitors, but my guess is ignition coils. I'd be surprised if it's any of the four things you narrowed it down to, based on personal experience.

I sure hope it's not the ECU capacitors... Does the 1UZ have separate coils for each plug? How would I go about finding which one is bad if it's not throwing a code?

If your ECU hasn't been serviced by you or a previous owner, that's the place to start in troubleshooting anything. It's a good five years beyond its shelf life.

There are diagnostic steps for the two coils (one up near the thermostat, the other down by the A/C compressor), but an iffy coil will likely pass the tests and won't throw any codes. Best way to tell is to just try different coils.

Actually, the ECU would probably be the easiest fix for you. Crack it open and identify whether or not the caps are leaking.

Typically, if the ignition coil fails, the car will sound like a 4 banger (because, well, it is). To identify if the coil is bad, unplug it then start it up. If there is no change, the coil is bad.

That would tell you if it's bad, not if it's iffy. I don't think a dead coil would lead to the symptoms described here, but a dying one definitely does.

Well, it looks like before I start diagnosing other things I'd better get the ECU repaired because I'm 99% sure it's never been done and it's 21 years old and has 203k miles. I'll be sending it in to Tanin Auto Electronix when I save up an extra $150 (I'm too much of a wuss to do it myself). Will update with results.

Just give us a call when you are ready! Sounds like it could use some TLC after all these years.

Thanks,

Did you ever figure this out?