Hey all,

I've been trying to troubleshoot this issue for a few months now, I've read all the posts I could find about issues related to my problem and I can't seem to pin point the exact issue.

Basically what I've got is in my sig. and I've data logged the event as it occurs. Basically the issue started with what I thought was a coil going out, on a warm day I'm driving down the road manually shifted in 2nd gear doing about 3 grand maybe 3500 a little higher then I usually cruise in 2nd at. Then I feel a loss of power, I open the throttle a little more to see if I was just imagining things, and it's as if the key was turned off almost. So I pull over and turn the key off/on and check my gauges, everything looks perfect, running nice and cool, etc. so I fire it back up as if nothing even happened, gave it a couple good revs and it sounded good and healthy/happy. Drove down the road half a mile, tried putting it back into 2nd gear and running it around 2500-3000 consistently and after about 30 seconds of that RPM same thing this time with a back fire, I let off on it and it picked itself back up and ran funny, wouldn't rev that good wanted to backfire, so I babied it into my work parking lot (Autozone lol) and pulled the ICM/coil plate and tested the ICM about 15 times on our Wells tester, passed every time.

However when I Ohm'd out the original OEM coil from 1994 it didn't have any continuity on any pins, I triple checked my findings with both a Duralast coil and MSD Street Fire coil which both tested within spec and almost identical. So I appeared to have a bad coil. Replaced it and the problem went away immediately, the car even sounded cleaner/louder slightly and it pulled harder then I was used to. So a month goes by and the problem comes back and this is where I start scratching my head.

I pull the coil and test it again, perfect. Test the ICM another 15 times, perfect. As a last ditch I pulled my less then a year old AC Delco wires and replaced those with the Duralast wires on it now. I Ohm'd the old wires and they were all under 6 Ohms, and no melted/burned marks, etc.. That actually seemed to make the problem go away for a day or two, but then it came back.

I checked fuel pressure at the test port on the rail return with the regulator connected at idle I was at 36psi, WOT parked 43.5ish, without regulator 43.5ish. However I didn't test it driving it, I dont have a gauge with a line that long..

Now as far as I can tell there is nothing noticeably wrong with the car, it's running rich for the most part, and sounds happy/healthy. There's tons of power at WOT and everything seems good until it stumbles/backfires and/or dies. I run Shell 91, SeaFoam every once in a while, Syntec 10w40+Lucas Synthetic Stabilizer, no vacuum and/or metered air leaks of any kind.

Here's a Datamaster log, it falls on it's face around frame 1750..

You guys rock and any help or input would be greatly appreciated..

Thanks all.

 

dStruct, noticed a few things. 1- you are pulling 5* timing at WOT starts retarding at record 1603 and increases to 5.9* at record 1620, your knock count is not moving so I am guessing it's burst knock. Although this pulls power it's not you problem and it can be tuned out. 2- You could use a tune to lower your temperature, you are running 225* stock fan setting with a 180 T-Stat you could turn on fans at 195 lo/205 hi, or go to a 160* T-Stat and run them at 175/185. Again this is not your main problem. 3 - Is the 3.23 rear in your sig stock for your car or did you upgrade '94's came w 2.73 or 3.23. I am asking because it looks like you are hitting the rev limiter at record 1640, stock fuel cutoff is 5800 RPM, in rec 1639 your PCM commanded a 1st to 2nd shift, you were at 40 MPH/5700 RPM (looks more like a 3.42 rear) and the injector Duty Cycles were at 85% (seems high). However prior to the tranny actual shifting you then hit 6K in rec 1640. So your tranny shifted to 2nd and cut your fuel at the same time. I would suggest a tune to lower shift points or up the rev limiter. 4- I also noticed at rec 1637 your RPM starts fluctuating 500 RPM up and down, hard to say if its the rev limiter cutting fuel or a crappy low resolution pulse from the OPTI. The 85% injector duty cycle is considered high it could be caused by low fuel pressure or delivery where PCM sees lean and tries to add more fuel by opening injectors. The backfiring in 2nd at 2500-3000 you mentioned in your OP sounds like high voltage coils, ICM, wires. On the trace your BLM's look OK, AFGS is good, MAP is good nothing looks way out..

 

Well it's a tired LT1, i've got 182,000+ miles on an original bottom/top end and I pulled the Hypertech tune and 160F stat just to rule those out as a possible cause to my issues. The rear end is rebuilt with hardened 3.23's and a Auburn posi.

I'm seriously suspecting a bad Opti but I just had it looked at when I was having Coil trouble and my very trusted and 40+ year Chevy race engine builder says it tests good as new and I replaced it just over a year ago. So I'm down to it being either a fuel pump or Opti and the pump looked like it tested good to me, is 36psi at idle with the regulator connected about right? When I rev'd it with the regulator connected it peaked around 43.5psi but I wasn't able to test it with high injector DC to see if I have a flow issue..

It just happened again today after I sat in a drive thru for a few minutes my temps were around 220F and when I pulled out onto the street I get about 50 yards at about 60% tps and it stumbles/backfires and starts missing real bad, and I heard some kind of weezing type of misfire coming from what sounded like the intake, I pulled over and turned it off and it didn't want to start back up without my foot a little on the peddle. I started it back up and it idle'd perfect but when I touched the pedal it would sound like a bad tps, a delay in throttle response and didn't want to rev too high without missing slightly.

It very well could be fuel related, my pump hasn't been replaced since I've had the car so it's been a few years at least..

 

I hard missfires like crazzy along time ago...I changed all kinds of things, fuil pump/filter the fuel injectors, cap & rotor, coil. none fixed it. decided to change O2s..ran like a champ. Maybe this could be it?

 

Hmm good to know.. I considered that as a possible cause but my driver side O2 is about a year old and my passenger side O2 is just a little older then that and they seem to be reading evenly and good to me. And when the stumble/backfire happens you can see in my data log the O2's appear to smoothly taper off down to about 4 or so.

Can anyone confirm the O2's appear to be working normally as I believe them to be?

Also I should mention this issue only happens when in closed loop and fully warmed up and/or when it's warm outside. It's never done it when the motor was cool or in the morning etc.

Man I hate only having 1 vehicle right now..

 

You can eliminate you O2's from the picture by putting it in SD mode and seeing if that changes your hot problem. They both look almost even on the trace , both are in good range and switching OK. I went back and looked at records 1737 -1768 , after your WOT run that I commented on previously. It looks like your engine died on the vine.. all was good BLM's were 126/124 in cell 15, you hit tps 60%, went into PE mode, O2's' switched to 900 mv as normal. But then everything bogged down.. even though you increased TPS to 77% your AFGS, RPM and injector PW were in slow motion only increasing very slightly. Like it wasn't getting air or the ign was cutting off. You can put it in SD to eliminate the O2's but I don't think that will help.. No codes and engine just dying means the problem is outside the PCM sensor control, back to fuel, high voltage or air/exhaust obstruction. Since it only does it when hot it points back to you new coil or ICM... PS: you should get your shift points fixed for the rear setup.. w/o a cam or engine mods your torque starts dropping, and is past its peak at you current 5700 shift point. You s/b shifting down around the 5000 or below mark.

 

Yeah I'm really lost on this problem, as for the rev limiter I think it just appears that I hit it because I never hit the limiter before it shifts, the rear setup is stock other then having the gears replaced with hardened 3.23's and the new Posi. but I do have my shift points and fans tweaked with my usual daily tune.. Like I mentioned I put the stock tune back on to rule out any funny business.

Can anyone confirm this looks like a bad Opti, if I lose low/high res signal I should lose ignition too right, not to mention throw a code? Also could this be something stupid like an Oil sender cutting ignition or does that only apply to the first few seconds the car is running?

Thanks all.

P.S. you can see exactly where I lose power by looking at the IAC stepping, it spikes up..

 

can you take the car completly cold, like left over night, start it, and just floor it own the street real fast? does it do it then? only reason im saying that is because at that time the O2s arnt "reading" because they arnt up to temps so that could help. And when i had the issues the O2s were only days old not years :P. POS bosch. Im runnng Ac Delco ones now.

also, I just reread your post. changing the coil "fixed" for a month. This happend to me exactually. i replaced the coil 4 tims thinking they were going bad. i went to a msd blaster coil and like you said, the issue went away for quite some time then just came back like wtf? only time it threw a code was after highway drving for 2-3 hrs strait (they looked like they were working good in TTS) Changed both O2s and it works fine now.

 

Yeah I can, if I cold start it and in either open/closed loop mode for the first 20-30 minutes at the least I can drive it however I want and as hard as I want and it'll run perfect, it's only after it warms up pretty good.

Like if I drive to work on a warm day, let it sit all afternoon in the sun, go out and leave for lunch, then I'll have trouble usually.

But here's the ultimate question, can bad O2's cause my injector DC to drop completely to 0.00 like they're doing?

One thing I did different, when I put my new MSD Street Fire coil on I mounting it on the bracket/plate with the ICM upside down from the way the stock one was positioned, just so the plug reached better but I really can't imagine that would matter in the least, just thought I'd mention it.

 

DStruck, in record 1901 looks like you flat stalled out (RPM=50, desired idle RPM=3188 are PCM default readings ign on w/o engine running) just prior to that you went super lean starting in cycle 1884 ST BLM's went from 136-168. I believe the high IAC count was due to your speed still at 53-46 with no RPM the PCM was trying to keep the IAC up anticipating a restart. The injectors (DC=0) shut off when you lost RPM as a safety feature. Looks like you restarted it at 1938 battery voltage dipped to 9.9V cranking. Also noticed your O2's never recovered after the WOT run they went to 4mv and stayed there till the stall, I can not explain that yet. Man it looks like you just lost fuel delivery but that would not be a hot only problem. Can you tune it to run in SD mode for a couple of days to prove the O2's are not the problem..

 

If you lost the low res pulse from the Opti, it would show "0" pulse width (= 0% DC ), and "0" RPM. But don't look at DC, look at pulse width. The DC is simply a calculated number from the pulse width and RPM.

Are you sure you aren't getting DTC 16?

I haven't looked at your data log yet, because its a .uni file, and my Datamaster is on my PC at work, not here at home.

 

Yeah I'm sure there are no codes, the light is off and hasn't so much as flashed. I did have a DTC 16 when my first Opti went out, but that was replaced over a year ago and it's been running great until recently.

After all the input you guys have provided I'm leaning more toward a fuel issue. What do you guys think about the fuel line inside the tank that connects the pump to the tank? If that line were to crack it would act like a straw with a hole in it and introduce air bubbles into the fuel supply line, maybe not enough to make it run bad, but at WOT it could introduce enough air in the line to provide a lean condition, not to mention the possibility of pump cavitation.

I dunno just a thought.. But that would at least explain why when I lay into the peddle it always takes at least 10-20 seconds before anything will happen. For example getting on the freeway, all the way up the ramp everything is cool, good power, etc. but by the time I get to the actual freeway it will usually fall on it's face and/or die all together.

Also I should mention since this issue originally happened I've been changing around the exhaust, went from a 3" MagnaFlow Cat + Slowmaster to a test pipe + 12" Glasspack, and now I have a 24" Glasspack on it with the test pipe. But none of this seems to have impacted anything except the test pipe + 12" glasspack I felt a little more HP/torque.

 

Well it finally died completely on me, I'm pretty sure it's ignition related.. I pulled out of a parking lot and got on the throttle maybe 60% or so, 5 seconds later it lost all power followed by a huge backfire and started missing horribly, and I could hear wheezing coming from the intake sorta.. So I pulled over and shut it off and when I restarted it revved to 2000 on it's own, if I bumped the throttle a tiny bit I could rev it, but if I step on it at all it would just lose all power sputter and then die almost immediately. I'm thinking a bad ICM?

It's doing this in Open and Closed loop modes and only seems to happen when it's fully warmed up, who knows maybe in the morning I can drive it home =(

Any ideas?

 

dStruct, What led me away from the opti was the way it stalled, even though you were moving and playing with the throttle trying to keep it running, the RPM (and injectors) just took a clean linear downward slope from 2325 to 0 RPM over a period of 4 seconds. When it stalled the injectors then went to 0. The RPM during that 4 seconds showed the OPTI was putting out low res pulses to the PCM, again no DTC codes during this period so the PCM to ICM signal was not degraded enough to throw a code. I suspected fuel going away, but it could also be in the ignition components past what the PCM can monitor like the ICM/coil/rotor/cap/HV wire to coil. Usually a rotor and/or cap failure does not show as an intermittent problem. Since it's now getting worse and doing this cold maybe time to retest the ICM..

 

So I got word from my mechanic/engine builder buddy who's looking at my car, he found the pulse going from the PCM to the ICM has low voltage and is out of spec from what it should be. Also the coil seems to be not showing correct numbers either..

It's looking more and more like a bad computer.. Sorry for such vague information, but does anyone have any thoughts on that?

Thanks all. I'll post an update when the issue is finally resolved.