I am going to shoot my car...
 

<rant>

I swear I am about to commit physical violence against my car…

For the past year, I’ve been living with a slight stumble at around 2,000 RPM. The engine just sputters and coughs a bit, and hardly makes any power at all when it’s operating in a very narrow window or RPM and MAP. No backfiring, no major drama, just a serious hesitation.

Specifically, it seems to occur from 1,800 RPM to perhaps 2,100 RPM. And it’s only really noticeable at between maybe 15” to 5” of manifold vacuum. If I lay into the pedal, it accelerates out of the bog. If I simply wait patiently for the RPMs to climb, the engine finally clears its throat and smoothes out once the RPMs get to about 2,200. If when I upshift I land back in the 2,000 RPM range, the process repeats itself. Hot or cold, day or night, makes no difference at all.

I’ve got datalogs of several of these events showing that the injector duty period, ignition adjustment, AFM voltage (simulated) and A/FR are all completely normal when it happens.

As to diagnosis, I have done the following:
What really bugs me is that this isn’t really all that big a deal. In the grand scheme of things it’s a very minor problem, it just happens to be one that is proving unsolvable. Remember how when you were riding in the back of the car as a kid, and your little sister would gently tap your leg with her foot at regular intervals? It wasn’t actually causing you any harm, but you could never get mom to believe that it was happening intentionally (damn those puppy dog eyes) and hence it just pissed you off until you finally just punched your sister in the arm. And then you were done for. That’s about where I am right now… (with apologies, sis, if you happen to ever read this)

</rant>

i though of three possible things and erased everyone because they just sounded real dumb...sorry, i cant think of anything worthy

can you post a graphical representation of the datalog? say like an idle to redline run with that problem included?

Joe,

Is 1800-2100 outside the range of operation of the IAC? Do you have access to a known good IAC you could swap? I had a stumble on my 92 a few hundred RPM lower. I had overheated once (blown hose) and I think I messed up the wax pellet in the IAC, although I'm not sure. At any rate, I picked up a used IAC on m.net and all was well. Again, mine was at a lower RPM (1200-1500) so perhaps completely unrelated but after a year, it's a shot.

Curtis

its probably gremlins! i've heard of this happening a lot on motorcycles specifically american v-tiwns. for some reason theres a lot of little annoying problems that are gremlin doings. heres what you do get a gremlin bell and hang it off your car, and hopefully they go away.....or drive your car harder so that your rpms don't drop to the 2k range when you shift?

other than that i'd run one of those fuel cleaners through it.

are you using supra 440 injectors?

i have same problem but when im going into boost in first gear.

doesn't the 1.6 tps make some kind of transition in the rpm area?

Several replies in one.

I am using 440cc injectors, though not Supra parts. The engine idles fine, passes smog, and runs perfectly in all other operating modes with these injectors.

IAC- that's a very interesting thought. I will try to locate one.

Except in automatic cars, the 1.6 TPS is a simple On/Off/On switch, the two On positions corresponding to idle and WOT. The switch is correctly adjusted.

It appears that there are two such devices I could investigate.

One is located on the left side of the intake manifold (the side nearest the head) and has two coolant lines going through it. I assume this is the part you mean, though I can't quite figure out (from looking at the parts diagram, which is all I have access to at the moment) how it works exactly. meaning I don't see how air is getting to it unless there's some hidden passage inside the manifold itself. I assume however that this is the part you mean.

Then there is a larger unit attached to the bottom of the throttle body assembly. This one appears to have an electrical connection and an air hose to the intake crossover pipe, though I can't seem to find it on the Madracki wiring diagram.

Joe,

I was talking about the Idle Air Control Valve. Go here :http://web.archive.org/web/200406100...F+3+ENGINE.pdf and look at page 33. The IAC is 20-660. I think you were asking about the Air Valve (13-190A on page 34). Again, this might be a blind alley but they are both fairly easy swaps if you have access to known good parts. Good luck.

joe post a datalog! :)

I've gotta friggin' find one. My shit is scattered all over the place, and the one I came up with last night was just a few seconds' worth. I need to put the tablet PC back into the car and capture a new log. Probably do that tomorrow.

That's the exact diagram I was looking at. And I figured you meant the 20-660 because it's the only one that would logically contain a wax pellet (the other one being apparently electronic in nature.)

Sadly I don't have a whole parts car sitting in my driveway, but I'll see if I can scrounge one up from somebody here.

sorry, dont mean to nag. just anxious to help! no rush if you can't do it tomorrow. I'm goin on vacation.