Effects of Forcing Open Loop

[craigger]

Hi All, I have a 1994 Sonoma 4.3L vin w with a intermittent hesitation with moderate throttle in the 1500-2800 rpm range. Bought used with many issues and am mostly down to this hesitation and some knocks. After months of tearing my hair out (many problems, many parts), I'm reluctantly at the point of admitting that I can't seem to fix the hesitation. My current work-around is that I have found that if I force it to run in open loop all the time (by setting "Minimum Temperature for Closed Loop" to max), the hesitation is not there. What effects/problems/damage might happen with running forced open like this?
Thanks in advance.

[lionelhutz]

For the most part it just means you need to ensure the VE table and MAF table (if it has one) are setup fairly accurately because the PCM can't compensate for errors in those tables. If it's running rich or lean it could shorten the engine life. Rich could also shorten the catalytic converter life.

[newellshk]

Do you have a log of it happening? o2 or cts gone bad maybe?

[craigger]

For the most part it just means you need to ensure the VE table and MAF table (if it has one) are setup fairly accurately because the PCM can't compensate for errors in those tables. If it's running rich or lean it could shorten the engine life. Rich could also shorten the catalytic converter life.

I do have the VE table set up pretty well. It has a MAP sensor. Is there a corresponding table for the MAP that I should be paying close attention to?

[craigger]

I have stared at the logs for hours and can't see any correlation of the hesitation. I believe the cts is good. I have pulled it and put in in a pot of water along with a thermometer, all good. And cts temp read by pcm matches that on the instrument panel. I O2 sensor has been replaced twice. I figured that is most likely thing for my symptoms, but no joy. Thanks for thoughts.

[newellshk]

Is it CPI or TBI? Are you running stock spark advance? Are the knocks showing up around the same rpm range?

[craigger]

Is it CPI or TBI? Are you running stock spark advance? Are the knocks showing up around the same rpm range?

It is CPI running stock advance (only temp changes made during test logging).
The range of the knocks are in the 1600 to 3200 rpm cells, map of 95 to 100.
I did try reducing "Main Spark Table vs MAP vs RPM" of those cells by 5. Didn't seem to help. Nor did increasing tps ae, map ae, and pe by 10/30/50 % in those areas seem to change.
Just to note, I did change the injector fuel rate quite a bit to globally bring the trims in line, from 119.4 to 141 lbs/hr. Originally way rich.
I would like to say this is the first time I've been in this area of a tune, so any suggestions are appreciated. Good chance I am making poor guesses.

[Six_Shooter]

Have you made any AE changes?

[Hog]

I have stared at the logs for hours and can't see any correlation of the hesitation. I believe the cts is good. I have pulled it and put in in a pot of water along with a thermometer, all good. And cts temp read by pcm matches that on the instrument panel. I O2 sensor has been replaced twice. I figured that is most likely thing for my symptoms, but no joy. Thanks for thoughts.
I assume by reading this, that both ECT sensors are in working order. Both the sensor read by the ECM and the other sensor that feeds the temp gauge on the cluster?

peace
Hog

[Hog]

Are you sure that you aren't having any of the typical Centra Port Injection injector issues?

http://www.chevythunder.com/gm_cpi_and_cspi_19922004.htm

peace
Hog

[craigger]

Are you sure that you aren't having any of the typical Centra Port Injection injector issues?

http://www.chevythunder.com/gm_cpi_and_cspi_19922004.htm

peace
Hog
Which ones are you asking about? I think I've tried to check most. I don't find any vacuum leak, the spider injector assy and nut kit have been replaced. New dist, plug wires, cap, rotor. All delco. Plugs. Had many issues with this purchased, used, vehicle. Compression ok, no exhaust back pressure, vacuum gauge ok, fuel pressure good, fuel leak down ok. Intake manifold gaskets (both) replaced on recommendation of pro (no evidence, he just had no other ideas). TPS, Map, replace. EGR cleaned and tested bypassed. IAC removed, cleaned. PCM. Grounds. Hi test gas.
Any thoughts are appreciated.

[newellshk]

I have a 1994 s10 blazer with the cpi and i've added quite a bit of spark across the board and haven't seen(in my logs) a single knock yet.

I am wondering if it's possible that there is a ton of carbon buildup in one the cylinders causing detonation? I've never personally experience that on an engine, but I have read about it. It does make sense that a bad cpi unit(previous to your ownership) would cause one or more cylinders to run much richer than the rest, resulting in carbon buildup. You said compression is okay, was it actually higher in any cyls?

Another guess is that it could be false knock. However I'm not sure if you would get false knock only at higher load levels, it seems it would be rpm dependent but not load dependent, but again I'm only speculating.

Is the hesitation always accompanied by the knocks? Can you run a vacuum line from the cab into the engine bay, maybe attach to the valve cover (pcv or something) and try to listen to see if you can actually hear any knocking?

[craigger]

I have a 1994 s10 blazer with the cpi and i've added quite a bit of spark across the board and haven't seen(in my logs) a single knock yet.

I am wondering if it's possible that there is a ton of carbon buildup in one the cylinders causing detonation? I've never personally experience that on an engine, but I have read about it. It does make sense that a bad cpi unit(previous to your ownership) would cause one or more cylinders to run much richer than the rest, resulting in carbon buildup. You said compression is okay, was it actually higher in any cyls?

Another guess is that it could be false knock. However I'm not sure if you would get false knock only at higher load levels, it seems it would be rpm dependent but not load dependent, but again I'm only speculating.

Is the hesitation always accompanied by the knocks? Can you run a vacuum line from the cab into the engine bay, maybe attach to the valve cover (pcv or something) and try to listen to see if you can actually hear any knocking?

There may have been alot of carbon build up. In addition to the cpi spider causing running rich, when I bought I almost didn't get it home to running extremely rich from dead O2 sensor. I did Seafoam the motor, also a shop ran a 'professionally available only' decarbonizer treatment. And running Lucas gas treatment on general principles.

Currently I'm address the tps and map ae, in large steps, to see what are the effects. Nothing working yet at 200% each increased.

No, hesitation not alway accompanied by knocks. And knocks not always noticeable (or very very slight). Want to make a run with wife driving and me monitoring. Hard to find time as moving.

RE runnning the vacuum line into cab to hear knocks. I don't follow, can you clarify? Listing for knocks with vacuum?

Thanks!

[newellshk]

With a good o2 sensor you should be able to see a lean condition if that's the problem. You'll need to look at the actual o2 mv value(available in tunerpro) when it happens, not the slow to update blm cells. I'm wondering if increasing the injector constant may actually be causing your ae to run rich, but then again it would have to run quite rich to feel a stumble/hesitation.

The very fact that you had to modify your injector constant that much is a bit concerning. What's your fuel pressure showing with key on engine off(KOEO), and at idle? If it's higher than spec ( ~58 psi KOEO iirc), I would pull the upper plenum and adjust it. A security torx bit will allow you to modify it quite easily, or you can break off the little security nub inside the adjustment screw to fit a normal torx bit. That will also give you a chance to double check for leaks. We've actually used little cups with lids that we cut holes in the top to ensure the poppets are flowing evenly. Crank with the coil disconnected for a while and measure each output with a graduated cylinder or whatever you have available. I realize you used a new delco spider unit but i have heard stories about even new/reman units having problems.

I know it's a pretty big job but I would go ahead and pull the heads and clean any carbon buildup. Then you at least know you have a healthy engine to start with.

[Hog]

Which ones are you asking about? I think I've tried to check most. I don't find any vacuum leak, the spider injector assy and nut kit have been replaced. New dist, plug wires, cap, rotor. All delco. Plugs. Had many issues with this purchased, used, vehicle. Compression ok, no exhaust back pressure, vacuum gauge ok, fuel pressure good, fuel leak down ok. Intake manifold gaskets (both) replaced on recommendation of pro (no evidence, he just had no other ideas). TPS, Map, replace. EGR cleaned and tested bypassed. IAC removed, cleaned. PCM. Grounds. Hi test gas.
Any thoughts are appreciated.
I was pretty much referring to the leaking maxi injector and regulator leaks.

peace
Hog