Engine loses power when starting off.

[scottfab]

You really should rule out a primary fuel pump starting to fail. That is how mine behaved until if finally just quit. No fun getting a flat bed ride home for my baby.

The test is most disruptive but needs to be because the pressure is fine most of the time. You need to get a pressure gauge taped to the windshield mounted on a long extension such that you can see it as you drive. Note what the pressure may be doing on the stumble. Mine would sometimes set a lean code but not always.

[VetteVet]

Originally Posted by scottfab

You really should rule out a primary fuel pump starting to fail. That is how mine behaved until if finally just quit. No fun getting a flat bed ride home for my baby.

The test is most disruptive but needs to be because the pressure is fine most of the time. You need to get a pressure gauge taped to the windshield mounted on a long extension such that you can see it as you drive. Note what the pressure may be doing on the stumble. Mine would sometimes set a lean code but not always.

Scott,

I'm in agreement with you. That was my first thought when he said it only happens when car is up to temp. The secondary pump is commanded on full time by the ECM until 176 deg (or 177, I can't remember the exact number). Once that temp is reached, the ECM turns off the secondary pump and only energizes it when secondary port throttles are commanded open. The secondary pump running full time while the car is warming up would mask any primary pump problems below that temp.

Jep

[HAWAIIZR-1]

Originally Posted by Paul Workman

Lee,

I'm pretty sure my AutoXray gives dynamic TPS readings (just another thought). If so, we could see if the TPS isn't glitching - or any other sensor; O2 or whatever for that matter!

P.

Good point P mentioning the TPS, I had similar symptoms when my TPS was a no go.

[Schrade]

Sounds like an 'exacerbation' of the 'hesitation' that I'm trying to resolve.

Backfire? Maybe lean gases lighting up in the out-pipes, when fuel delivery catches up.

I think mine would probably do the same, except that everything is burning pretty cleanly, and the hardware is clean. My MILD lope, just above idle, leads me to believe that fuel delivery coming off of idle, isn't in sync with the air increase, again, JUST COMING OFF idle.

Idle @ 0% TO is PERFECT rock steady, no codes. Above 2.4% TO is smooth again too. I still have the secondary-blocking TB gasket installed as well, which eliminates secondarys' leaking air for the diags...

I found my bolt (screenshot stuck in 'seeing eye' tool's LED - will post tomorrow), and will resume DataMaster with incremental idle increase with modded throttle stop screw tomorrow or Monday. SOMEone will be able to make something of it, I'm sure.

I think this is gonna' be real interesting here...

[Franke]

If its a 1990 to 92 could be a faulty or intermittent MAP or TPS sensor. The MAP is used as a direct input to the ECM to help control fuel requirements. 1990 to 92 cars didn't use mass air flow sensors. In general TPS sensors are known to go bad and cause off idle hesitation etc. Both of these can be tested easily. Best guess at this point.

[efnfast]

Bob, I've got a fuel pressure gauge. I'll bring it Saturday.-Steve

[Dynomite]

Thanks for the tables and explanations

Originally Posted by XfireZ51

I've attached a jpeg of 3 tables used for C/L operation. One shows the parameters for the ECM to consider if it was a HOT start, COLD Start or WARM start. Then there is an accompanying TIMER telling the ECM how long to wait before C/L based on the start-up temp. So anything under 29C will use 5 minutes as an example. However, in addition there is an O2 WINDOW.

So it could go out of C/L depending on whether its too rich or too lean with reference to the voltage being generated by the O2.
You also see a table for what the target AFR is when in O/L and that is referenced to CTS. So colder the coolant temp, the richer the mixture. The number in the table is how much additional fuel from the baseline. Changes in the baseline will affect this table then.

The O2 is the final word for C/L. Whatever the ECM is doing ends up as a signal from the O2. The feedback loop to the ECM causes it to modify the fueling accordingly. SA is being affected based on the RPM/MAP along w modifiers like DECEL, startup, CTS.
There is a table that defines for the ECM the amount of delay, based on airflow, before allowing the INT to change. Its an O2 INT Delay. This way the ECM correlates the effect of changes it has made w the results coming from the O2. Otherwise it would be out of sync and "chase its tail". In general, the use of headers and moving of the O2 further back has not demonstrated a need for changing the values in this table.

Most tuners will resort to O/L when larger cams are introduced into the equation. All "LEARN" happens in C/L. However, LEARN is not always ON with C/L. Several other parameters define when LEARN is active. If LEARN is not active, the ECM uses whatever fuel trims have been stored for those cells. The overall goal is to get the VE tables to be as close to optimum as possible requiring little intervention by the ECM.

The values in these tables are not necessarily stock.

[XfireZ51]

Cliff,

As an FYI, the values in these tables are not necessarily stock.

[Dynomite]

Originally Posted by XfireZ51

Cliff,

As an FYI, the values in these tables are not necessarily stock.

Copied this technical information in one spot so I can get back to it with note added if that is OK with you

Fuel Trim and Closed/Open Loop Operation

[XfireZ51]

Originally Posted by Dynomite

Copied this technical information in one spot so I can get back to it with note added if that is OK with you

Fuel Trim and Closed/Open Loop Operation

My pleasure. Glad to contribute. The issue of using the correct injector bias may be another topic you would be interested in. I am in the process of discovering just how important it is but won't be done until car is back on the road.