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Discussion Starter #1 (Edited)
I changed my plugs on my '94 Z (obd1) 2 days ago because the car had a slight miss. I also lowered the fuel pressure from 65psi to 50 psi (which I have since raised back up to 60 psi). It ran fine for about 15 minutes then it started to miss badly, the miss seemed to begin about the time I would have expected it to go into closed loop. I should also point out that this morning when the car was first started and still in open loop it was missing badly.
Today I changed Spark plugs and wires (2 of the plugs on the passenger side seemed to be running lean) When I started the engine it sounded great not a trace of a miss, I should point out that I just changed the Opti less than 200 miles ago. On the way home I hooked up my lap top and using Data Master did a scan, almost as soon as the PCM went into closed loop the engine started to miss again.
I suspect that I have at least one faulty injector but there are no injector faults showing and there are no DTC codes either.
Looking at the STERM % I seem to be getting some very lean conditions such as +3.1 as well as an O2mv of 75. I don't know how to interpret this data and am hoping some of you guys that are familiar with Data Master can help me.
I also got a tune from Brian At PCMFORLESS which I installed about a month ago, did a data run and then reinstalled my original tune, to do a comparison of the performance change, but have not reinstalled Brian's tune yet as I am waiting for him to make some changes.
Here are the files if any of you can look at them and give me your suggestions as to how I should go about diagnosing the problem.

Was hoping to post the zip files but I see we can?t do attachments, if anyone is interested enough I will email them to you.
 

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A +3.1% isn't that lean. It's only adding in 3.1% of the already commanded fuel. That's not bad.

My first question is WHY are you running at 60 PSI fuel pressure?? That's definitely on the high end.

How well are you O2 mV bouncing around???
 

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Discussion Starter #5
Fastbird said:
A +3.1% isn't that lean. It's only adding in 3.1% of the already commanded fuel. That's not bad.

My first question is WHY are you running at 60 PSI fuel pressure?? That's definitely on the high end.

How well are you O2 mV bouncing around???
Fastbird,
At this point I am running 60psi because anything less seriously affects performance, I did a post in the main engine forum re my Injectors,
27# which I am sure are way too small for my setup of 550hp.

As to the O2mv they seem to bounce around alot, over a 15min run they bounce between 75 and 850 mv. I have a really strong suspision that at least one of my injectors is on the way out.
 

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Discussion Starter #6
gettinthere said:
How big are they? I can attach them.
Average zipped size is 3 x 150 k or one file of 500k (at least thats what my programer tells me) Thanks for your interest
 

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cmerc said:
Average zipped size is 3 x 150 k or one file of 500k (at least thats what my programer tells me) Thanks for your interest
E-mail them to me. I'll see if I can attach something that big.

[email protected]
 

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02's are supposed to switch like that in closed loop. That's normal as long as they are roughly averaging .450mv They will switch several times a second.

Almost has to be a fueling issue. What do you have the injector constant set at in the PCM, just out of curiosity?
 

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Discussion Starter #9 (Edited)
Chris I don?t have a clue what the injector constant is set at, but I totally agree that it is a fuel problem. I pulled yesterdays new plugs and # 1 & 3 hardly looked like they has been fired, all the rest were very dark. That seems to indicate to me that injectors 1&2 are very dirty and causing a very lean condition (I am also starting to get backfiring from the exhaust), and the others are running way rich. I also noticed today that when the engine is switched off the fuel pressure falls almost immediately by 8 - 10 psi.
I assume that is a leaking injector. In any event I intend to remove the injectors tonight and soak them over night in injector cleaner, then reinstall them. As soon as I figure this problem out I intend to get bigger injectors and a tune to go with them so that I can lower the fuel pressure.
Do you agree with my analysis?
By the way great site you guys have, and I really appreciate your input on this.
 

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Glad we can help. You can help us out too by sharing the link to this site with your friends :D

IMO you gotta have new injectors no matter what. Instead of troubleshooting questionable injectors, go ahead and get the correct sized ones in there and then if you still have the problems, do you troubleshooting.

Normally I wouldn't say throw parts at it until its fixed, I'd suggest diagnosis, but in this case you have mis-matched parts on there and that should be corrected first.

You may have one or more injectors hanging open, flooding a cylinder and causing the miss. It could be they can't handle the pressure you have them set at and they are locking open. We're having the exact same fuel pressure symptoms on Chris89GTA's car, but it won't even start. FP drops from 45 to like 7 almost instantly after the pump primes.
 

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Discussion Starter #11 (Edited)
While I totally agree with you that I SHOULD change the injectors and see if that fixes it, it is unfortunately not that easy for me because.
I live in Nassau, Bahamas, this translates into:
1.No injectors available, I have to get them from Florida.
2. No one here to reprogram my PCM I have been using Brian at PCMFORLESS, and will get him to reprogram when I figure out which injectors I can get.
3. No one here with the skills or equipment to help me solve it.

Therefore I have to use my limited pre PCM skills, (or lack thereof) and the Knowledge and advice of people like you.
I'll keep you up to date.
 

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I understand, yeah that's a dilemna for sure. Hopefully soaking them will help. Good luck!
 

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this is off topic , but when i saw this engine missing i thought back to when i worked for a chevy dealer ship and a lady brought her car in and said it wasn't running right it had a skip so the service manager told her the engine was missing she said one of you'll got it cause nobody else has worked on it
 

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Discussion Starter #15
gettinthere said:
I got your zip file. It's too big for me to attach, so I'll upload it on m server and host it later.
Thanksfor your offer to post the zip files, but don't waste the space on your server, it?s defiantly the injectors,
We removed them last night and soaked them in cleaner overnight. My mechanic reinstalled them today and the car ran much better, no backfiring or missing, after a road test we lowered the fuel pressure to about 55 psi, as we still suspect that at least one of them is dripping, I will be getting new ones this week and hopefully Bryan at PCMFORLESS will be able to give me a quick new tune so I can put them in.
Thanks for all the help and advice guys, it's greatly appreciated.
 

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Discussion Starter #16
A late report, new 42# SVO's installed, yet more new plugs installed, lowered fuel pressure to 50psi.
I changed my old Hypertech program's injector size in the constants file, the car runs great just about feels it really does have the 500+ HP the guy I purchased the car from claimed.
As a bonus a guy (LT1 fool) on another forum kindly offered to tweak it for me. Can't wait to burn up some more fossil fuels.
 
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