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Closed Loop stumble underload

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Old 09-13-2015, 12:31 AM
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Closed Loop stumble underload

Hey All,


I am a new member and have been reading a lot of forums from here and Firebird Nation and have gotten some great insight and knowledge into my newly acquired Formula. However, I have been having some issues now that I need some expertise on please and thank you!


First, its a 93 Formula 6 speed with 126,000 on the original motor (stock with the exception of a CAI and 3.73's.) My father-in-law ordered the car brand new in 93 and subsequently blew a head gasket in 2004 and the car has been sitting in his garage since. Long story short, he gave my wife and I the car because he needed his garage back and it has been a never ending money pit since that time lol.


1st shop replaced the head gaskets, plugs & wires, both ECT's & O2's and serviced all the fluids. Car ran good until warm and would sputter and die until it cooled off which ended up being the fuel pump. So, replaced that and the fuel filter of course... Also, fans would not kick on and the 1st shop said it needed a new ECM, so they hard wired the primary fan to come on with the ignition so I could at least drive it for awhile. Nevertheless, after searching I finally found an ECM for a 93 and installed it in the car and although the fans work now, there is a miss under load in any gear during medium-heavy acceleration... Kind of stupid not to know, but I am not sure if the miss started before or after the ECM install, as it had the same symptoms when the fuel pump was acting up, except now it doesn't die when warm!


2nd shop advised that the miss was timing related and said I needed to get the new ECM flashed as well as replace the cap & rotor. Thanks to Shbox's site I managed to replace the cap & rotor with an MSD vented unit (I did Loctite the rotor screws) and replace the water pump why I was there. The Opti looked good; no rust, oil, or moisture seemed present and no high or low resolution codes have been thrown on the car so I rolled the dice and didn't replace it (I know I should have but I am about $6,000 into this car at this point, just to get it running!) Car ran amazing until it got to operating temp / closed loop and it was the same miss, stumble and occasional back fire underload during acceleration in any gear (worse from say 1500-3500 rpm but noticeable all the way up to about 5K.) Also, I would speculate that the car is running very rich, as when you shut it off the rear of the car smells like raw gas pretty bad. I had Chevy flash the PCM and they noted that ALL eight injectors were out of fuel trim spec and that both O2 sensors were bad, so naturally the problem still remained. Chevy did however adjust the air rate or IAC maybe (something in the throttle body) and now it idles rough once its a temp to, which it never did prior to them touching it. Nevertheless, I recently installed Accel 24 lbs. injectors and both O2 sensors again and the problem is still there! I emailed with Rob a few times about the situation and I am trying to get the ALDL cable so I can data log the car while it is rolling, but does anyone have any suggestions to try first? I have seen many posts where a heated related misfire was attributed to the coil or ICM, but I am not sure... I found a great article on checking for vacuum leaks on LT1's so I have started doing that as well, but nothing has popped up yet.


Here is a break down of the parts replaced, all within 1000 miles.
ECT sensor, twice (water pump side)
O2 sensors twice
Fuel pump
Fuel Injectors
Fuel Filter twice
Plugs, wires, cap & rotor (no light show at night)
Radiator
Water Pump and some other things not relevant to the motor.


I am beyond frustrated with this car and to be honest, if I thought I could get the money back out of it that I put into I would probably just sell it, but I suppose I am committed now...


Thanks in advance for the insight,
Josh
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Old 09-13-2015, 01:18 AM
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Re: Closed Loop stumble underload

Why did you select larger than stock, non-GM injectors? The is just going to make things worse. While it isn't possible to determine that all 8 individual injectors were running rich, what the GM dealer probably found was the both banks of injectors were running rich based on negative long term fuel trims (LTFT) for Bank 1 (left) and Bank 2 (right). At any point have you checked the fuel pressure? If so, what were the results?

Are the replacement O2 sensors AC Delco, Delphi, or some other brand?

At any point have the MAP and IAT sensors been checked? They are the key inputs to the 93's speed-density A/F ratio control.

ECM failure is extremely rare, and I doubt yours needed to be replaced and reflashed twice. You need to get the ALDL cable so that you can run a data log with Scan9495. I can review the data log for you, but it can take a while sinceI have a backlog of logs to review recently.

Oh.... And welcome to CamaroZ28.com!!!
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Old 09-13-2015, 02:27 AM
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Re: Closed Loop stumble underload

Injuneer,


Thanks for the reply and I haven't checked the fuel pressure since the injectors were installed, but after the original fuel pump and filter install I believe it was around 41lbs. at idle ish, maybe a little higher... However, The last mechanic that had the car (before Chevy) said the fuel pressure was fine at idle and under load and the only thing relevant, he noted, was the right O2 sensor reading was bouncing a little low (not sure what the reading was) but that it was still within operating range.


I got the Accel injectors after reading some of the posts from guys on here and Firebird Nation and when I ordered them from Summit the tech. advised that 93's were 24lbs., not 22lbs. like they actually are. However, I had planned on getting a tune from PCMforLess anyways so I wasn't to worried about the slight increase, but I was waiting until the car was running right to do that... I never through the factory ones away


I don't know what the first batch of O2's were, but I believe the ones I just got were Bosch's from Autozone.


I have never checked the MAP or IAT sensors and I am not sure if any of the shops have either, but no one has mentioned them being bad any of the 12 times it has been in there lol. However, I will check Shbox sites, as I think I saw something on there about checking those... I will let you know when I get to it, probably Tuesday after work.


Lastly, I am not sure the factory ECM was bad either, as the car needs an instrument cluster to. When I put the new ECM in I let the car sit at idle until it hit about 235 and neither of the fans turned on so I shut the car off for fear of over heating. However, a mechanic put his scanner on it and the gauge is reading 20-30 degrees hotter then the car is actually running... so the primary fan comes on as soon as the gauge hits the reds (Ill be getting a cluster soon as it still bothers me to see it getting that hot regardless of what the actual temp is!) I thought it was the head temp sensor but after I replaced it I noticed the oil pressure and volt gauges twitching a little bit... I know those 93 ECM's are a bit of a hot commodity, so perhaps Ill send the factory one out to get checked and see if truly is bad.


Thanks,
Josh
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Old 11-14-2015, 07:59 PM
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Re: Closed Loop stumble underload

What was the resolution of this problem?
It would be helpful to many here to know.
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Old 11-15-2015, 09:24 AM
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Re: Closed Loop stumble underload

He hasn't signed into this site since 9/17.
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Old 11-06-2018, 06:31 PM
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Re: Closed Loop stumble underload

is Threxx still here? Man I used to come here and read so many posts back in the day! I haven't even logged in and quite some time.
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