1994 ZR1 acceleration question

[daveshowey]

Hi all,

I'm noticing a slight hiccup in power when I'm running WOT in the lower RPM range.

It's barely noticeable, but it feels almost like a stutter.

Once I hit 4k rpm it's gone and accelerates normally.

No engine codes or anything like that, it's just not as smooth an acceleration curve as I would like.

Any thoughts? My first inclination is to change all the plugs.

[scottfab]

I'd say more info is needed. Does this happen when fully warmed up too? Does it happen with the power key off? With air filter removed?

Some SWAG on it:
1. oil build up in the map sensor tube.
2. small vacuum leak
3. an ignition wire or coil starting to go
4. an injector
5. fuel pump at marginal low psi

sky is the limit at this point.

[Franke]

Just thinking out loud...Dave, someone once told me in a previous post that at 4k RPM the ECM goes back to open loop for fuel mixture so the O2's, while providing feedback to the ECM, are ignored. I think I read that some where also. Have you heard of that too Scott? Anyway do the cheap stuff first. I bet Mr. Haibeck and the other tuners on here would know.

[scottfab]

Originally Posted by Franke

Just thinking out loud...Dave, someone once told me in a previous post that at 4k RPM the ECM goes back to open loop for fuel mixture so the O2's, while providing feedback to the ECM, are ignored. I think I read that some where also. Have you heard of that too Scott? Anyway do the cheap stuff first. I bet Mr. Haibeck and the other tuners on here would know.

I've never heard of that unless you're in in WOT the O2 sensor data is used once the car is warmed up. (unless you're in limp mode)

[rkreigh]

Originally Posted by scottfab

I've never heard of that unless you're in in WOT the O2 sensor data is used once the car is warmed up. (unless you're in limp mode)

zr-1 has a narrow band 02 and the input is ignored at wot as they can't read the richer fuel mixtures. they only work close to 14.7 stoich (hence the "narrow band")

so the fueling is all driven from the tables, map, and tps inputs

the hesitation could be many things, but for sure I'd check the plugs, wires, and make sure the secondary flaps open smoothly. might be a bit carboned up and hanging up.

vacuum leaks will do this as well. good luck finding the problem. factory service manual is the BOMB for trouble shooting and keep one calm facing the prospect of what the H@LL is it

drove the LSV quite a bit with the top off today, it was SOOOOO nice

[We Gone]

Had a hesitation on my 91 when I first got it at around the same RPM the vacuum pots for the secondary's were on backwards as has been found to be common, the linkage was hanging just as they started to open than would pop open. I found this out by using a hand vacuum pump and watching them open. replaced both.

[XfireZ51]

Originally Posted by Franke

Just thinking out loud...Dave, someone once told me in a previous post that at 4k RPM the ECM goes back to open loop for fuel mixture so the O2's, while providing feedback to the ECM, are ignored. I think I read that some where also. Have you heard of that too Scott? Anyway do the cheap stuff first. I bet Mr. Haibeck and the other tuners on here would know.

The "learn" is off at 4k, but motor still in C/L depending on the TPS% set for
PE (power enrichment) mode.

[Franke]

Thanks guys for the additional info on the question I had concerning the O2's and ECM fuel mixture at high RPM/WOT. I understand the diff with the ECM using the TPS, tables, map etc instead of the O2's feedback.