jacked1

Hi,

I searched the forums for this exact issue but I can’t seem to find a match to what my z28 is doing. Was driving home from work about a week ago and out of their blue my 94 Z started running rough @ idle and proceeded to start backfiring on and off. It’s also now running rich. Now this happened out of the blue after driving about 20 miles on the highway and then driving on side streets @ about 20 mph. This issue happens when the car is cold and warm - in closed or open loop - makes no difference. When the motor starts it starts quickly but no longer starts @ a high idle and within 10-20 seconds tapers down to a slower idle as it used to before this issue started. It starts @ a low 700 rpm ide and stays there while running rough. It also surges and misses under acceleration. No DTMs are being generated. There were also none stored in the system.

This is what I have checked so far. Checked for vacuum leaks and found a couple old vacuum hoses (one off the plastic EGR tube to the intake and the other that connects under the drivers side and runs back to the evap canister - both were just degraded leaking vacuum hoses) that were bad which i fixed. Checked intake vacuum and it hovers around 20 hg/lbs. when the rough idle hits the gauge moves a bit right on the stumble @ idle. Changed plugs, checked all wires which were new as of 2 months ago. All wired look like new and are not burnt. All header pipes get hot while the engine is running. While running i pulled each plug wire one by one and saw the expected response where it runs worse with each one pulled and replace onto the plugs. I also pulled/plugged in the power to each injector one by one to be sure if all injectors were working and the engine changed each time to a rougher idle so injectors look good. I looked for arching when it was dark out both from under the car and under the hood and saw no arching.

Other than new plugs and wires, I also had to replace the opti 2.5 months ago as it was intermittently losing the low res signal and the car would be DOA in till the low res spark would start working mysteriously. The cart has 83k on it. I got it about 18 months ago. I don’t know the full history on it but the mods I can tell have been done are hooker log headers, aftermarket exhaust, cats and muffler. It’s 2 cats into a singlet 3 .5 inch pipe to muffler to dual looking exhaust tips it the back. Things that i don’t think apply to this issue are a Easton Elocker and the suspension has been lowered - someone poured a good amount of money into it to set it up for drag racing. The original owner did this - I’m the 3rd owner.

I’m thinking this is an optispark related issue as the missing and rough idle happen in both open and closed loop conditions.

I’d greatly appreciate any input as to what others here might think. This issue has bee driving me nuts as I should have been able to figure this out already.

thx in advance.
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Injuneer

What brand is the 2.5 month old Opti?

Did you Loc-tite the rotor screws before installing the Opti? Was there any evidence of an oil leak at the timing cover seal behind the Opti, or the water pump seal in the timing cover above the Opti? Did the new Opti include an attached short harness, or does it use the factory short harness that plugs into the Opti? Since your problem with the previous Opti seemed to be intermittent, did you examine all Opti harness connectors for corrosion, damaged pins, or missing wire insulation?

What spark plugs did you install, and what gap?

Misfires can cause the engine to run rich, as can leaks in the exhaust system BEFORE the O2 sensors. Have you checked the fuel pressure? Have you checked the appearance of the plugs to verify it’s running rich, and if so, which cylinders or all cylinders?

The most productive analysis of the problem is usually a data log of the PCM while the engine is exhibiting the symptoms. Download a free copy of Scan9495, a scanning and data logging program specifically for the 1993-1995 LT1 engine.

https://www.camaroz28.com/forums/com...95-lt1-874306/

Get the correct cable, export a log file in .csv format, post it here and I can take a look at it. The author, GaryDoug is a member here and can help with install or setup problems.

Manual or automatic trans - that determines the cold start idle programming - RPM vs. coolant temp?

jacked1

Thx Injuneer -

I have a data logging file created with the Scan 9495 utility you cited above. I will run a new one later today and post it. I appreciate the assistance.

With the rotor screws - yes I opened the opti and LOC-tighter them. There was an oil leak behind the opti which i fixed with a new press in gasket when I replaced it 2.5 months ago. I know it’s sealed now as the oil leak that was previous existing is now gone. The water pump gasket was not leaking oil. The opti did not include a harness so i used the old one. I inspected it for damage and corrosion and it looked all good. I believe it was the factory harness. I replaced the plugs with NKG Iridium IX TR55IX #7164 plugs and gaped them @ .040. All old plugs were a bit carboned up with the exception of the 3rd one back on passenger bank (right side) BTW - both 02 sensors were replaced within the last 2.5 months due to throwing error codes and making the car run rich. I cleaned the plugs @ the time I replaced the 02 sensors. This is an automatic transmission.

I’ll post a data log today.

Thx again .

Ok - just uploaded a datalog where the issue was happening. LMK if this will do or I can run a new log later today.
Thx

94Z28LS1toLT1

your issue sounds exactly like mine was a few years ago. My car would stall tho,it backfired also and had lack of power. I opened up the opti and there was oil in it. I changed the gaskets to stop the leak I had,so when I put the new opti in I won't have a problem with it. I hope this helps man

GaryDoug

Good for you in making the log file an Excel file for Fred. However I prefer the original csv log file if you have it. I like seeing it loaded into my program ;-)


Nevermind, I just renamed the file as a csv and it's ok. Maybe you just changed the extension without changing the format?

I will let Fred answer. He is far better at this than anyone else on Earth. At least your AC is boss! 44 degrees!

jacked1

Mike- thx for the reply. I had this issue in the past where the opti ended up with oil in it due to a leaking opti timing chain cover gasket. I think this time it’s different - well the oil part anyway. I do appreciate the reply. Thx.

jacked1

Hey Gary,

Yes the file was a .csv originally but while uploading it to the board here it looks like it changed to Excel. Thx for taking a look.

So the suspense is killing me - LMK what you guys see as the issue.
Thx

Injuneer

I missed the fact that you posted the data log, because when you added it as an edit to an existing post, it didn't show up as a "new post", and that's where I look when I sign in. When Gary added his post, it showed up as a new post and when I looked, I saw the file had been added, but I didn't have time on Saturday evening to download the file.

Odd that the extension on the file was changed to .xls when you uploaded it. I have reviewed 100's of files uploaded to this site, and all of them have opened in Excel, with the .csv. extension. When I tried to open your file, my version of Windows 10 cautioned me that the file extension did not match the file format, and that I should use caution in opening it. When I opened it, all a got was an actual .text, tab delimited with commas. Normally when I open the files downloaded from here, they have the .csv extension, but they open in Excel

Please try uploading again.

jacked1

Thx Injuneer - here it is uploaded again. Looks like it uploaded as a .csv.

Injuneer

Got it. Opened OK. I'll try and spend some time on it this afternoon.

Injuneer

I can't believe this... I just typde a huge summary, clicked "Go Advanced" and rather than get what I expected I got an error notice related to the "Ad Manager" and lost everything. Sorry for that. but I have to reconstruct this. I'll do it in Word next time..

Short summary - no smoking gun. You do have issues with idle air control (IAC) and possibly a slightly high MAP at idle, which may indicate a vacuum leak. But the PCM appears to compensate for this by adding a bit of extra fuel using the log term fuel trims.

More to follow, but I've run out of time for right now.

jacked1

Thx a ton man. I really appreciate it. I did find a vacuum hose leak - an intake elbow right above the IAC was split and was replaced yesterday The issue still persists. I'll run another data log this evening and post it if that helps.

Thx again for the time you have been spending on this!
Rick

Chimera96

OP

your MAP has a orange grommet like gasket to seal the nipple of the MAP in the intake manifold. If that is aged, cracked that can be a source of vac leak

Its the same seal used on some Harley MAPS but I can't remember the part # but a search may yield that 411. Its only a few $

jacked1

Hi - thanks for the input. I appreciate it. It is a good idea. As part of my troubleshooting efforts I removed the MAP sensor and cleaned it. I think the orange gasket was intact and in good shape but it definitely can’t hurt to double check it - I’ll take a look @ it.

Thx again man. Rick

jacked1

Injuneer,

I just ran a new data log and uploaded it. I do see what you are noting as the IAC position seems to remain too open. I'm thinking once the engine is cruising in closed loop the IAC position should be on the 7 to 20 range (depending). So this is possibly a dirty IAC or would it be determined that this is a bad IAC that i need to replace? Now I'm getting ahead of myself. I'll wait on your analysis.

Thx, Rick