JP94Z

I've loved F-Bodies for a long time and recently was able to get my hands on a 94 Z28 for cheap. My mother bought it new, and instead of her trading it in I bought it from her. It only has 37k miles but it's maint. has been neglected here and there.

For years it's had a problem with a surging idle, which she ignored. In gear when stopped or in park, the RPM will surge, sometimes as high as 1400RPM or so. This occurs only when it's up to temperature on warm days. If the day itself is cool enough, it will not do it. But I'm in Florida, so that is rare. It can happen A/C on or off, park or drive. WOT is fine. When it is doing so severely, upon initial acceleration it will stumble a moment and then act normal. Cruising or WOT seem fine.

Tried checking for vacuum leaks with propane, and it was not affected. I have not been able to get ahold of a pressure gauge to check the fuel pressure. (fuel pump was replaced once) Like nearly everything, the Opti and O2 sensors are original, and just recently the original water pump has begun to leak out of the weep hole, so I am thinking of replacing the Opti when I replace it.

There are no codes in the computer. Here is a link to data collected as it was surging, if any of you can help me figure this out I'd be appreciative. Other than that, it's going to be Opti and O2's and cross my fingers.

JP94Z

Since this post I've seen something that seems significant to me so I will share it with all of you, and if I'm wrong someone may tell me.

While watching the live scan data on a cold start, the bank 2 oxygen sensor rather quickly seemed to warm up and give readings in the 800-900mv range. On a cold start in open loop it sounds like that should be correct. The bank 1 sensor however, gave readings of around 100mv at the same time.

The bank 1 sensor stayed with these readings until I felt the car switch to closed loop. I felt it because the RPM dropped a little and it began to idle rough. I verified it had switched to closed loop on the scan tool, and for a little while the bank 1 sensor was giving readings of about 300mv, and the bank 2 sensor bouncing around high and low like they seem to be supposed to. Once the PCM started upping the amount of fuel to bank 1 that sensor began to jump back and forth in readings more like normal.

Could the bank 1 sensor be dying and/or too slow? Or is there a more likely explanation? Tomorrow I plan to pull plugs from bank 1 to see if the PCM has been running it too rich, and compare them to bank 2 which seems normal. I also have tried to see if there was a potential exhaust leak on that side, but I hear no noises or see any signs so far.

Am I heading in the right direction with this?

shoebox

The left O2 readings do look suspicious to me. I would start there.

JP94Z

Thanks to shoebox for the response. I replaced the O2's and they are reading more quickly, perhaps too much so, as they made the problem worse.

From some experimentation, it seems like the surge was the PCM's response to a misfire. And now with new O2's, it seems more aware of that. While it still refuses to throw any codes for what's happening, it's seeming to get narrowed down.

I disconnected the IAC while it was idling to stop the computer from upping the idle in the surges. This made the occaisional misfire noticable. I was able to observe the computer react to it, bank 1 misfired and the O2 must have suddenly gotten a very rich on that side and it would cut back fuel for a moment on that side. It also tried to open up the IAC.

I was also concerned with a "twitch" the tach seemed to do right before a surge, and now I think I know why. My scanner will only record readings at quickest every half second. At least one of the times it misfired I had a recorded 4350rpm from the PCM. with 650 a half second before and 700 a half second after. So we know that's not true. It may be happening each time, my scanner may just not be quick enough to catch it.

I also had a chance to speak with the Z's previous owner (my mother) and come to find out it once or twice backfired on start-up.

Would I be correct in assuming now would be a good time to order an Opti-Spark?

JP94Z

Does anyone know any reason why the PCM would see a false high RPM reading like that other than the optical sensor and wheel in the opti?

I would have removed the opti and took it apart to see it's condition already but it's been too rainy and windy. Yesterday while under the car I could see the opti has lots of oil on it. Could oil be leaking in the back side and wreaking havoc on the optical sensor?

Basically what I want to know is if this is essentially a "smoking gun" pointing to the need for a new opti.

shoebox

Could be the opti or interference on the wiring from the opti to the PCM or maybe even the PCM, itself. I have not really heard of anyone seeing this type of problem before. My gut feeling would be the opti or it's wiring.

JP94Z

Thanks shoebox. I've looked over the wiring a bit and haven't seen anything obvious wrong with it. The Opti is original and has seen plenty of Florida rain and humidity as well an oil bath from leaks never fixed, so if it is the problem it would be understandable. She always put Mobil 1 in it, which seems to be really good at finding places to come out of.

As soon as the rain lets up enough for me to try, I will go over the wires more thoroughly and move them around a bit and see if it has any effect, and look at the harnesses better than I have.

As for the PCM, I wouldn't know how to go about being certain it's okay, aside from making sure there's nothing obvious with it's connectors.

If all the wires seem okay I will begin Opti removal and see what it's like inside and get ready to order a new one Monday. I'll keep this thread updated on progress in the hope it may save someone else time if they run into this problem.

white/blackss

I Have Read About 20-30 Threads About This Same Problem On The 1994 Cars And To My Knowledge There Seems To Be Some Problem With The Icm And The Coil Getting Too Hot Causing This To Happen.

My Car Has All Brand New Plugs,wires,filters,opti, And I Have Checked The Fuel Pump Volume And Pressure And With No Luck, It Still Does This.

Soon Enough I Will Be Able To Relocate The Icm And Coil To See If Theres A Difference.

I Know What U Mean Man, It`s Horrible Driving Down The Road When She Warms Up And Start`s Surging.

Even My Friends Have Made Comments About It, That`s How U Know It`s Bad.

Good Luck With Your Work, And Keep Us Posted.

white/blackss

well, did u ever figure it out yet?

did u try to move the coil and icm?
im having the same issue with my 94 and want the magical answer to fix this thing.

JP94Z

I was unable to find any faults in the wiring, and I know the Opti has been drowned in unfixed oil leaks so there's where I'm betting so far. I also found out recently from my mother it had a new thermostat put in it a few years back, so it's possible someone wasn't so careful of it.

I ordered an Opti and have begun disassembly. I should have everything off tomorrow then I can clean all that oil off the engine. I will try to perform an autopsy on the old Opti tomorrow when I get it off. If I find anything odd I'll post pictures.

Other than that, once I get the Opti and put it all back together I'll test it out and hopefully have good news. If not, I will post the bad news and seek professional help. (for me, not for the car )

romoranger

on mine the icm was originally mounted to the head. i moved it with a bracket a bout an inch or so from the head. i have been using this icm for a while now and it works great. i can take pics if you would like

JP94Z

Okay, new GM Opti is in as well as a new water pump. It's been a real hot day so I just cranked her up and let her idle for a long long while and bled the cooling system. Just before I started taking her apart she would last 20 minutes at most max before the issues began on a hot day. I let it idle for 75 minutes...yes, 75 minutes and no problems.

While it was idling I finished off the bleeding, checked for leaks, ect. No further sign of erronious RPM readings either. Took it for a drive for about an hour, during rush hour. It's a little stronger, and definately smoother. Again, no issues.

Looks like the Opti may have done it. The fuel trims are also getting closer to eachother the longer it runs. I haven't changed the plugs yet as I didn't want to get new ones messed up and have to change them twice. But I think these are cleaning up as it runs.