I'll first start with a description of my problem, and then list what I have done in an attempt to isolate and/or fix it.

My car is a 1994 Camaro Z28. It is the "late model" of 94, so some of it's parts are apparently 95. So far, I know that the oil pressure switch is a 95.

When I first got the car 3 months ago, I did the usual tune up and routine maintenance items: oil change, spark plugs, cleaned the air filter (K&N), fuel filter, checked my lines, etc. A few of the vacuum lines had cracked elbows so I replaced them or fixed what was left the best I could. The car ran fine, but seemed to get bad gas mileage (12-14 mpg). I chalked this up to an older (100k) engine and need for more engine care and routine maintenance.

Recently, my car began surging when the accelerator is held at a steady position, keeping the rpm's the same. This is felt most when I am at 2000-2200 rpms. I belive it is occurring at other ranges, it's just not as drastic, so I can't feel it as much. My gas economy has also taken a dive, to around 9 mpg. Please note that while I am depressing the accelerator enough to cause my rpm's to rise moderately, or flooring it, there seems to be no problem. Except for the gas, hehe.

My first action was to replace the spark plug wires. My new bosch wires arrived and were installed. No improvement. Next, I replaced the vacuum lines that I had repaired, with GM parts. I also replaced the evap valve on the manifold, since it was connected to the vacuum line that was damaged. Again, no luck.

After this, I purchased a fuel pressure gauge, and checked the system according to the haynes manual. It checked out perfectly.

Next I replaced the Ignition Coil with a stock replacement unit, checked all of my spark plug wire routing, and pulled the plugs for inspection.

My plugs were all excellent, with the exception of #3 and #7. They both had more carbon build-up and were both wet when I pulled them. I had been driving the car around the block to warm it up. Also, in case anyone asks, the other plugs were dry, and had usual signs of wear on them. Nothing major, though. This unsettled me, but I continued on with the list of checks I had written up before and checked my EGR system. It functioned properly according to the haynes manual's check, which was to depress the valve. My rpm's dropped considerably, which supposedly indicates that it is working properly.

Edit: After pulling the plugs, I used a spark tester, and all of the plugs were receiving spark both while turning the engine over and while it was running.

My next path was to check sensors. First was the TPS, reference and actual voltage checked out perfectly. The IAT sensor checked out fine. The MAF appeared clean and to be functioning. My coolant fill level sensor works intermittently, but as far as I understand it, it only feeds into the light on the dashboard. Around this time sunlight was beginning to depart and I decided to pull the fuel rail off to check for leakage before the sun went down. I believe I read in the haynes manual how to check them. It said to pull the rails and turn the key to on, which pressurizes the system. None of the injectors leaked. I tried turning over the engine, and still no leaks. Since #3 and #7 were my "troubled" plugs, I tested #1 and #3 injectors side by side for a quick comparison. Both appeared to spray the same amount of fuel in the same interval while turning over the engine. Satisfied that my injectors were working, I cleaned them off (a little buildup), changed the O-rings, and reinstalled them.

EDIT: I also checked the battery and alternator voltages. Both are fine. My grounds also appear to be good.

Also, a few days prior to all this, I checked my pcv valve and air system. Both appeared to be working fine. I also tried running the car with air disconnected, and it still ran fine (still with the surging problem tho).

By now it was basically dark, so I decided to check the evap canister and lines for deterioration or damage. That all checked out fine, and by now it was dark. I spoke with one of my buddies, who is also an F-Body owner, online, and he told me that it could be that my optispark is malfunctioning. He also suggested the MAP or the knock sensor. I had no way to test the knock sensor, and for the MAP, I disconnected it and drove the car a bit. It ran much crappier with the MAP disconnected, but I was unable to test it completely and will do so tomorrow when I have the time and light to do so.

Any advice or suggestions that you guys can give me is very appreciated, as I am basically at my wit's end. If I can't figure this out in the next couple of days, I am going to have to take it to a dealership and have them run a diagnostic on it. My wallet is starting to feel lighter already. Thanks for any help, in advance

 

You did an excellent job checking things very methodically. I was thinking 2 directions either tune or test. First if the history of the opti is unknown, I would change it. Remember it has a cap and rotor that wear and corrode just like other cars. The opti, however should be replaced as a whole unit. One other thing that fails is the bearing in the rear. Use an OEM unit. Your car uses an unvented opti which unfortunately is more expensive than the vented one. I don't know why.

The other thing I suggest is having the injectors cleaned professionally. I have cleaned many on the rails with 3M cleaner and the difference is amazing.

I would also consider new O2's but only after all the cleaning and testing is done.

The last course of action I highly suggest is testing the back pressure of the exhaust. Done with a rather expensive pressure gage screwed into the O2 bung. And also a compression test of each cylinder. If any variation is found in compression, a leak down test is in order.

Hope that helps

Keep up the good work

 

Though you provided a lot of info, I did not see where you stated how many miles are on the car. Given the condition of some of the items you already replaced, I would assume quite a few. Optis don't last forever and would be a likely suspect. Another possibility would be the Ignition Coil Module, just to not leave anything out. If you had a buddy's coil and ICM to swap in, that would be good for a test, without spending any money.

Has it been scanned for any trouble codes? A low resolution code (16) for the opti does not cause the SES to come on and if merely intermittent, will still allow the engine to run. An old opti can be affected by signal resolution problems because of bearing rust dust in the optical section and then by normal deterioration of the cap and rotor. You should also check the opti harness for corrosion on the connectors.

 

In response to shoebox, My car has 104k miles on it. I don't know anyone local with an F-body, so I can't swap out the ICM, but I'll test it to the best of my ability, with whatever tests my trusty haynes manual has. The optispark looked to be in fairly good condition, the harness didn't have any corrosion and all the spark plug connectors and coil connectors looked fairly clean. I know this doesn't reflect the internal condition though. Is there any cheap way to get the codes scanned? The cheapest scanners I can find online seem to be pretty expensive, and the local stores all said they had nothing that would work. Any links for one for sale for a reasonable price online?

slopokrodriguez, I don't know if it makes any difference, but my car has a flowmaster cat-back exhaust on it. I don't know the details about it, but it does have a flowmaster logo on it.

Thanks for the advice guys!

 

Another update... today my cat cracked open! I was driving home, taking it easy on the throttle. I caught a light, and when I started to drive off again, my car sounded totally different. I drove the 300 or 400 yards to my house, got underneath, and ack! My cat was glowing red hot, and the side where the top section joins the bottom had seperated apart... the whole thing was glowing and heat was radiating off of it. I know that cats will get clogged up and destroyed from too much gas in them... so is this a symptom, or the source of my problem ?

 

yes and yes.

that is a common cause for the 2000rpm stumble

 

Sounds like you solved your problem! Replace it with a high-flow (Carsound cats can be had cheap on www.ebay.com ) and keep us posted.

 

well... unfortunately i could see no way to replace my cat without replacing the entire y-pipe, and since i couldn't be sure that the y-pipe/cat assembly was my problem, i decided to gut the catalytic converter. i riveted the seam back together the best i could, emptied it out, and put it back on. unfortunately one of my manifold-ypipe bolts broke, so now it is held on by a c-clamp. ah well. the car seems to run better, and is a bit louder, but it still has the same surging problem. plus my cat-repair job wasn't as good as i hoped it would be, so it leaks out under the car. that just means headers moved to the top of the list of future mods .

my plan now is to take the car on monday to the dealership and have a diagnostic run on it. they told me it would be $74, and that sounds fairly reasonable. hopefully they will be able to tell me exactly what is wrong before i go spending 6 hours replacing my optispark.