Engine loses power when starting off.

[Bob Lyle]

My ZR1 runs good most of the time. This only happens once in while, when starting from a stop in first gear the engine stumbles, loses power and may back fire. After several seconds the engine regains power and is ok. This only happen when taking off and the car is up to temp. Any ideas would be appreciated.
Bob

[LGAFF]

my car is doing the same, I think its o2....

[LGAFF]

As noted it may be the O2 heating up


I changed prom, ECM, etc still doing it....leaves the o2 I believe, someone with a tech 1 or scan tool should be able to ID the issue

http://www.zr1.net/forum/showthread.php?t=20069

[Bob Lyle]

What is o2

[LGAFF]

Oxygen sensor; its going bad, bad ground etc....confusing the ECM and altering fuel causaing a lean or rich condition

Laymans terms...a sensor that sniffs the exhaust, checks to see how rich or lean it is, then tells ecm to add or trim fuel

[Mystic ZR-1]

Bob
We can check for any codes on Sunday
D

[LGAFF]

mine does not throw a code when it happens, does yours?

[Bob Lyle]

I have had it scanned several times in the past, but never while it was happening.
No error codes ever came up. This has happened since I bought the car 97. Care presently has 36k on it. I can go several weeks with no issues.

[LGAFF]

Mine does not show codes, I noticed a change in the 02 readings when it happens....I have not swapped mine yet but plan to do so...

[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.

[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.

[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.

[VetteVet]

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

[XfireZ51]

Franke,

All LT-5s are Speed Density motors. Not MAF.

[efnfast]

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

[scottfab]

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

That is why I put in a "bypass" switch to force the 2nd pump to stay on in an emergency.
The idea is, if the primary ever fails again I can get home without a flat bed. This thread shows both switches I put in. The first is a switch on the fuel pump "pig tail" it is ONLY good for testing pressure with engine off. (important to remember) the second switch is shown in the very last pic in the first post at this link: http://zr1.net/forum/showthread.php?t=18224&highlight=fuel+pump
It is for forcing the 2nd pump on when the engine is running.
Sorry the pic is HUGE. You have to scroll over to the right to see it.

[HAWAIIZR-1]

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.

[Paul Workman]

That is why I put in a "bypass" switch to force the 2nd pump to stay on in an emergency.
The idea is, if the primary ever fails again I can get home without a flat bed. This thread shows both switches I put in. The first is a switch on the fuel pump "pig tail" it is ONLY good for testing pressure with engine off. (important to remember) the second switch is shown in the very last pic in the first post at this link: http://zr1.net/forum/showthread.php?t=18224&highlight=fuel+pump
It is for forcing the 2nd pump on when the engine is running.
Sorry the pic is HUGE. You have to scroll over to the right to see it.

Very nice "Kluge", Mr Scottfab! :cheers:

P.

[scottfab]

Very nice "Kluge", Mr Scottfab! :cheers:

P.

I suppose it is but technically since it is:
1. documented in the FSM that goes with my car
2. does not dismantle or damage a designed in system
3. is completely reversible

it probably does not meet the minimum requirements of a kludge as
I was taught at Rockwell. :dancing

[Dynomite]

Oxygen sensor; its going bad, bad ground etc....confusing the ECM and altering fuel causaing a lean or rich condition

Laymans terms...a sensor that sniffs the exhaust, checks to see how rich or lean it is, then tells ecm to add or trim fuel

I am assuming the individual o2 sensors (RH and LH) control individual (sets of 8) injector fueling (RH Bank of Injectors and LH Bank of Injectors) through the ECM.

And by extension the ONLY individual Bank (RH 4 cylinders or LH 4 cylinders) control feature since the Secondaries on both banks are controlled as a unit (excluding the mechanical failure of one Bank of Secondaries). The ignition (spark) is controlled as pairs (one each side). The RH and LH bank Fuel Pressures are tied together through the Pressure regulator on the Fuel Rails.

The Reason I bring this up now............a Question directly related to the function of the o2 sensors as individual sensors and individual control of Cylinder Bank (4 cylinders each Bank) Fueling came up on CF.

[XfireZ51]

Dyno,

The O2s modify the fueling by R/L bank but only in Closed Loop. The VE table is used no matter, but fueling will be the same for both banks in the case of Open Loop. This is true IF there's been no LEARN prior to use. Otherwise the learned BLM will be used, just not modified. In Closed Loop, the PW will be calc'd for the individual bank of cylinders dynamically using O2 sensor input.
One reason I like using C/L. In the case of the secondaries operating, remember the cal effectively divides the calc'd PW between the 2 injectors.

[Dynomite]

Thank you.......the information is much appreciated :thumbsup:

Dyno,

The O2s modify the fueling by R/L bank but only in Closed Loop. The VE table is used no matter, but fueling will be the same for both banks in the case of Open Loop. This is true IF there's been no LEARN prior to use. Otherwise the learned BLM will be used, just not modified. In Closed Loop, the PW will be calc'd for the individual bank of cylinders dynamically using O2 sensor input.
One reason I like using C/L. In the case of the secondaries operating, remember the cal effectively divides the calc'd PW between the 2 injectors.

[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...

[Schrade]

Dyno,

The O2s modify the fueling by R/L bank but only in Closed Loop.
Got that.


The VE table is used no matter, but fueling will be the same for both banks in the case of Open Loop.
Got that.

This is true IF there's been no LEARN prior to use. Otherwise the learned BLM will be used, just not modified. In Closed Loop, the PW will be calc'd for the individual bank of cylinders dynamically using O2 sensor input.
One reason I like using C/L. In the case of the secondaries operating, remember the cal effectively divides the calc'd PW between the 2 injectors.

Is this OEM operation? (division of PW?)

Or is it Marc's over-write operation?

In other words, does his NEW coding do this duty operation, whereas it did NOT divide PW between 1' and 2', in original configuration?

[Dynomite]

Dyno,

The O2s modify the fueling by R/L bank but only in Closed Loop. The VE table is used no matter, but fueling will be the same for both banks in the case of Open Loop. This is true IF there's been no LEARN prior to use. Otherwise the learned BLM will be used, just not modified. In Closed Loop, the PW will be calc'd for the individual bank of cylinders dynamically using O2 sensor input.
One reason I like using C/L. In the case of the secondaries operating, remember the cal effectively divides the calc'd PW between the 2 injectors.

Would you summarize the coolant temperatures and other circumstances that will switch the ECM from Open Loop to Closed Loop?

When in Closed Loop, what are the learning input sources (I know o2s).
And since o2s, there is definitely a difference in the location of the o2s in regard to stock exhaust manifolds vrs Headers......what might be the difference in ECM learning?

If running Marc's CHIP.....I assume the fuel trim is learned in Closed Loop beyond that baseline fueling set by Marc? Or maybe Marc keeps it in Open Loop ;)

Or maybe I should stick to my Open End Flex Head Ratchet Metric Wrenches :D

My two ZR1s (90' and 91') run like rockets and I am not sure why......I blame it on Marc's CHIPS :sign10:

[XfireZ51]

Chuck,

There is a constant in the calibration for the secondary injector. When they are turned ON, the ECM splits the PW. Part of the OEM software.




Dyno,

Would you summarize the coolant temperatures and other circumstances that will switch the ECM from Open Loop to Closed Loop?

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.


When in Closed Loop, what are the learning input sources (I know o2s).
And since o2s, there is definitely a difference in the location of the o2s in regard to stock exhaust manifolds vrs Headers......what might be the difference in ECM learning?

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.

If running Marc's CHIP.....I assume the fuel trim is learned in Closed Loop beyond that baseline fueling set by Marc? Or maybe Marc keeps it in Open Loop ;)

Haven't really looked at one of Marc's cals recently. I know he has stated that the LT-5 came with a bit of a rich fuel table and that headers pretty much bring it into line. So my impression is that he doesn't do much tweaking on fuel there. 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.

Or maybe I should stick to my Open End Flex Head Ratchet Metric Wrenches :D

My two ZR1s (90' and 91') run like rockets and I am not sure why......I blame it on Marc's CHIPS :sign10:

[Dynomite]

Thanks for the tables and explanations :thumbsup:

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.

http://i287.photobucket.com/albums/ll142/dynomite007/Dynomite3/4fc2efd1-7689-43a7-a3e3-991fefd35969.jpg

[XfireZ51]

Cliff,

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

[Dynomite]

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 :handshak:

Fuel Trim and Closed/Open Loop Operation (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-6.html#post1583870824)

[XfireZ51]

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

Fuel Trim and Closed/Open Loop Operation (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-6.html#post1583870824)

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.