Hey guys I have a 95 Firebird and have been chasing an issue with half to full throttle hesitation and loss of power. It mainly occurs from 1000-3000rpm when accelerating with about 40% or more throttle. I have checked the fuel pressure, ran codes, replaced the optispark twice, and replaced the icm and coil with no avail.
After 3000 rpm it runs smoother.

One peculiar thing I have noticed though is when reading rpm and other graphs on EEHack, there are times when the logging pauses for seconds at a time and doesn’t make data points. The logging resumes after a few seconds, so Im thinking it could maybe be the PCM.

The first optispark had similar symptoms and no codes, but the one I just put on is an original ACDelco and the car now backfires when letting off the throttle, which the other one didn‘t.

Does anyone have any ideas on what I could do to try to fix it?

 

Did you resolve your earlier problem with the intermittent valve train (?) ticking? Is that possibly related to this new problem?

 

No I couldn’t figure that one out, but Im pretty sure it’s not related because the previous optispark I had the ticking noise hadn’t started occurring yet, and it still had similar symptoms.

 

Does you data logging show any knock retard under the low rpm/40+% throttle position acceleration?

 

Yes it does, it retards ignition up to 15 degrees depending on the throttle position.

 

Almost sounds like you have a code for the knock sensor. When that sets, the PCM appears to go to a default knock retard based on things like throttle position, rpm, CLT, etc. 15° is the factory value for the maximum allowable knock retard. When the code sets, I've even seen small amounts of knock retard at idle. I would expect to see DTC 43, or possibly DTC 47.

Or maybe that elusive “tick” is being picked up by the knock sensor.

 

Hmm that is interesting. I had a code 43 a few months back but forgot how I fixed it. And I don’t think the knock retard is directly related because I’ve had it running and driving with the first opti before it had issues, without the sluggish acceleration, while it still was pulling timing.

I’m starting to think that my issue is the opti because the first one I bought worked good for a few days but I started having similar issues with sluggish low rpm acceleration, and around 3000 rpm it seemed to cut out for a second and then start accelerating again.
I bought a used OEM opti a week ago and put it on and it now backfires on deceleration and is sluggish at low rpms also.

I’ve done the voltage test on the opti harness and connectors with a multimeter and it was good, so I’m a little skeptical about buying another one, but don’t know what else it could be.

 

The "Shoebox" test for voltage on the Opti harness only proves the PCM s providing the required reference voltage. It does not verify the Opti high and low res signals. And, if the tach is working, the Opti is working. When it seems to cut out for a few seconds, does the tach needle drop to "0"?

The Shoebox test at the ICM verifies the Opti signals. But appears you have an intermittent problem, and that's not likely to show up.

If you have a data log (.csv or Excel format), I can take a look at it if you want. I don't particularly like the eeHack logs, at least the few I have looked at. Labor intensive to set up for review. Scan9495 is far superior.

A "false" lean condition, one that can be caused by an exhaust leak before the O2 sensor, can cause popping on decel. If the problem is getting worse, still need to check for an exhaust leak. Might have been there for a long time, but the ticking and popping only becoming noticeable as it gets worse.

 

No, the tach rpm behaves normally and hasn't dropped to zero.

I do have a .csv data log I will add on here, but I could not figure out what was wrong using those but again I might not know what to look for lol.

I've thought about the possibility of an exhaust leak causing a lean condition, but I would think that if that is the main issue, my car would run better in open loop, which it does not.

Back to the optispark, is it possible or likely that the cap and rotor are bad? Or that a bad plug or wire would cause the car to have these issues at low rpms?

 

A burnt plug wire on your headers is a possibility. It happened to me and I thought I was being careful. It was pretty easy to find and all my issues with power loss went away. I added heat shield protectors and no issues after that.

 

When you are trying to accelerate at low RPM, what gear are you using? The log has a column for "Transmission Selected Gear" and it always shows "3". I'm not sure how the PCM gets that data for a manual trans, unless it's calculating it by comparing engine RPM to vehicle MPH. But it couldn't always be in 3rd gear.

Did you shut the ignition key off at any time during this log? There is one frame of data where the voltage at the PCM drops to "0" and all the sensors read "0".

This log has no time scale. Makes the data somewhat useless. Missing columns that should be there.

 

I use 1st and 2nd mainly when accelerating, but I have a skip shift delete thing so that may trick the computer into thinking it’s in 3rd at all times.

I got gas so I’m pretty sure this is from that.

Yeah I used EEHack and it seems like the .csv it exports isn’t complete.

 

What grade/octane fuel are you running?

Without a time stamp on each line of data, it's very hard to interpret. It "appears" to stay in open loop for a long period of time, but it may just be that the software achieved a very high frames per second scan rate. Is take almost 10,000 lines of data to enter closed loop. Even at 10 frames a second (about the highest scan rate I have ever seen) that would be 1,000 seconds (16.7 minutes) to achieve closed loop. The factory programming allows the PCM to enter closed loop in 206 seconds, as long as the O2 sensors are heated up and active, and the coolant temp reaches a specified level. The O2 sensors seem to take a long time to warm up to operating temperature. Another weakness in the eeHack log - it doesn't show the "O2 sensor ready" status.

And you are experiencing a huge amount of knock retard at the slightest movement of the throttle. That is definitely the "sluggish" feel you are experiencing below 3,000 RPM. In open loop, in many cases, the O2 sensors are showing a relatively lean condition. You open the throttle, air flow clearly increases, but it takes a while for the injector pulse width to catch up. In general, flogging it in open loop is not a good idea, simply because the PCM cannot correct for incorrect A/F ratio.

The "drop out" line is not where you purchased gas. The engine was shut down at record 11320. When restarted ,again it's in open loop and stays there for a long time again. Odd though, that when the engine was restarted the O2 sensors had cooled down and were no longer active. And, the LTFT's reset to 128, indicating battery power to the PCM had been interrupted and it lost the stored long term fuel trims.

Fan on temperatures are not stock, so this is not a completely stock tune.

If you want further analysis, you need to try Scan9495. I just don't feel confident with the eeHack stuff. Odd labels for the long term fuel trims, which can alternatively be labled as BLM (Block Learn Multiplier). eeHack calls them "Left Block Learn Trim"..... never saw that one before.

 

I'm running 93 octane fuel.

I just replaced the optispark today with a new one from Summit Racing due to it having a warranty, and because I was still able to return the one I had on, and was hoping I could rule out the opti once and for all. After driving around for about 15 minutes, it behaved the same so I'm now very positive it isn't the optispark.

One thing I did notice though is when I first started the car up, the battery voltage gauge in the car fluctuated a decent amount, going up from 13 to 16 or so, and then back down to 13. It stopped doing that after about 30 seconds so I do not know if that is related.

The previous owner bypassed the sensors and plugged the fans into the fuse box so whenever the ignition is turned on, the fans go on as well.

I want to thank you for trying to look through the eehack log, and wanted to let you know that when I drove the car today I logged it with Scan9495 and am going to upload the .csv file along with this post. One thing I noticed with Scan9495 was that the error code for the icm went on, even though I replaced it a week ago. So tomorrow I will recheck my icm and other ignition voltages, and try to put the old one back on if necessary.

 

Your PCM still thinks it's controlling the fans. It thinks it turned on Fan #1 at 189.5deg-F (stock is 226), and Fan #2 at 196.3 (stock is 235).

When you start the engine, the starter bleeds down the battery voltage. The regulator normally appears to boost the charging voltage upwards to ~16 volts, to replace drawdown. As the battery recovers, the regulator drops the charging voltage to ~13.5 volts. If that's what you are seeing, it's normal.

I'll download the Scan9495 file, but it will be a while before I have time to look at it in detail, with the holidays coming up. I'll be on the road Friday, returning home Tuesday 12/28. In the meantime, look at post #2 in the thread linked below, showing the best procedure for running a log:

https://www.camaroz28.com/forums/com...-guide-886891/