need ideas on what to check - flat, high-end acceleration

[lee]

The subject line more or less says it all.

I recently changed to a 4.11 auto transaxle. Since that time the acceleration on the higher end of the RPM scale (say 4500+) has been flat. I first thought it might be due to the ECU "forgetting", but I have since been through 8 or so cold-hot-cold cycles of driving with no seat of the pants difference.

I admit to not checking anything yet except the seal of the plastic air intake hose to the MAF sensor and to the throttle body. Spark plugs, air filter, fuel filter are about 15K miles old. No blinking lights or CE lights. Gasoline is from a Chevron station that has always served me well in the past. No misfires, but there's a slight hesitation when fully cold that disappears within 2 or 3 blocks of driving in the morning. However, that all existed before the tranny swap. The only new thing I've noted is there is a mild swing in the engine feel while idling (warm). You can't see it on the tach, but it sounds like it slides smoothly and slowly up/down maybe 10/25 rpm when sitting in park - the speed of this variation is about the same as my resting breathing rate (maybe we're in some kind of twilight zone synch)..

I have searched (but maybe not the right terms). Open to suggested items to check and or threads that have already discussed this.

[thundering02]

There wa an article refering to tourque control. basically they were gounding one pin in a harness to another.

[Earthworm]

If it's only higher RPM then perhaps your IRIS valve is sticking?

[lee]

Originally posted by Earthworm
If it's only higher RPM then perhaps your IRIS valve is sticking?

good idea - it also operates on vacuum, could be as simple as I have knocked something off or cracked an old vacuum line - thanks

thundering - the torque control signal is a while-shifting thing the 5-MT guys were working - I'm still automatic and the "flatness is while accelerating in any one gear, not during the shift - still it warrants a look if the wires are improperly making connection - thanks for the idea as well

[Beav]

Uneven idle speed is many times a vacuum leak. There is a small reservoir and check valve for the IRIS just under the intake, behind the p/s pump. My check valve was cracked and leaking, you may look for the same. Check valves available at NAPA.

[lee]

thanks Beav - two votes for vacuum, so it looks like I need an underhood review

[oab_au]

Yes I'll second Beav, the leak would have to be on the engine side of the butterfly valves.

Harvey.

[thundering02]

Sorry I saw the 411 and must have skippes the auto part the I shall take back my vote to replace it with hoses.

[Beav]

Nothing like total unintelligibility.

[Earthworm]

Originally posted by Beav
Nothing like total unintelligibility.

Isn't it clearly obvious?

[lee]

OK, checked the vacuum lines and no obvious leaks...doesn't mean there isn't any, but no lines are off or broken away.

However, I did remove the exhaust to swap out the tranny. What's the chance I could have screwed up a Cat if the pipes took a good bounce on the floor...and would they (if screwed up) cause this sort of problem?

[Earthworm]

Is there a way to test if the IRIS valve is in fact functional?

[SVX-FF]

Lee did you use the SVX torque converter or the donor one?
That might make a difference. I used the SVX one during my swap. also what year and make tranny?

Steve

[lee]

Steve,

I used the SVX torque converter.

the transaxle is from a 1998 Imprezza, the rear differential is from a 1997 5-MT Legacy.

The tranny seems to shift just fine - better than it ever has in fact - even after the original was rebuilt a couple of years ago.

The problem is that the acceleration somewhere above 4K rpms isn't what it used to be. It does accelerate, just not like it should - a flat feeling is the best I can describe it given my limited ability to think of the right word at the right time.

I have a flat accel. curve between 2000 and 3500 RPM ever since I opened up my exhaust, but yours sounds suspiciously like either exhaust constriction or your CAS (crank angle sensor) isn't adjusting the (duh) angle of the crank properly. I believe the SVX forwards angle/timing/cams/whatever else at 2000, 3500, and 4500 RPM. Anyone please feel free to correct me if it seems like I'm talking out my *ss, casuse I am, but it makes some kind of twisted sense to me right now.