SLO-Z

Ok I will try to make this as short as possible. I have been searching posts for 3 days and the answeres are all over the place.

I recently rebuilt my brother in laws 94z. He purchased a slightly used short block, 37,000 miles (looked great). We had Lewis port the heads, and we put a Hot Cam kit in it. It still has the stock manifolds and exhaust running through a flowmaster, headers are on the way. After the rebuild and reassembly the car fired on the first crank, it sounded great. We took it for a quick spin and it died (no power-cut off), evidentally the cat had been clogged because it turned the manifolds cherry red. We dropped the exuast and gutted the cat for a temporary fix. The car seemed to run good but smelled very rich so I told him to drive it and keep an eye on everything. As the computer began learning, the cam seemed to quite down a bit and run smoother, but I noticed it was running PIG RICH and still smoking (all of the time) not alot of smoke but noticeable. The car increasingly became more difficult to start and was popping alot. By the time I finally got to drive it it was terrible. So I ordered a tune from MADZ28 and now it seems to start and idol fine but still very rich, if I puch it to WOT it sounds like it is hitting the rev limiter around 5k rpm's. When we did the rebuild I put in new stock wires, coil, plugs, and old OPTI. My thoughts are that the opti and o2 sensors are bad, but I dont want him to start throwing money at the problem, so any suggestions would be great. It definitely seems like 3 problems to me. 1 being the rich condition, 1 being the bad miss, and 1 the smoke. I guess the smoke and rich could be linked but Im not sure, It doesnt appear to be dark black smoke - maybe a little white/blue??

brutalbird

A missfire would allow more oxygen in the exhaust system, making the o2 think it was running lean. The computer sends more fuel in response to the o2 sensor and you get so rich the exhaust gets smokey. Could be as simple as just a plug wire grounding out. I would do a scan and see which side is showing a low o2 sensor reading. That is the side that has too much oxygen and is the side the misfire is on. If the o2s are balanced then maybe it is opti related. If the o2 readings are not balanced, then one side of the exhaust has more oxygen than the other. An exhaust leak, intake leak, or misfire on that side could cause the condition. A scan can also show a trouble code for a defective sensor or opti output signal. I'de start there myself.

SLO-Z

What is the most cost effective way to scan the vehicle?

SLO-Z

For what its worth the missfire/backfire doesnt happen until wide open throttle. It idols a little low, but the exhaust is prctically blowing out gas all of the time. The dip stick(oil) smells like gas but the oil is still nice and clean.

SLO-Z

I was just messing with the car some more and noticed that it isnt really smoking at start up when cold, and reving the motor it sounds great and has no stumble at all - only under load. I dont know if this helps or not.

Injuneer

You need a scan. You can use free software like TTS DataMaster (free for fisrt 20 uses) or FreeScan, as long as you have a laptop and the cable. They will provide a data log over a variety of driving conditions, from cold start through transition to closed loop, then some easy driving and a WOT blast. That will allow a check of the vital sensors and computer parameters, to see if there is an obvious problem.

Alternatively you can use a hardware type scanner, but data collection becomes a problem. At least you can check for codes (not all codes turn on the SES light), long term fuel corrections (BLM's) for a few cells, O2 readings at WOT, etc.

SLO-Z

Do you know where the best place to find the correct cable and software would be? Your help is much appreciated.

slopokrodrigez

If you want, I can scan it for you but you would have to bring it to Hampton.

Try this site.
http://www.andywhittaker.com/ecu/ecu_software.htm

SLO-Z

Thanks I appreciate it, I pass through Hampton often my family has a place in Nags Head. I just downloaded the freescan and will be ordering the cable tomorrow. Where can I find numbers that I can compare the acquired data to? Is there some sort of standards?

SLO-Z

I finally have a small log from a run. Is there anyone that can take a look at this for me? It is in .CSV format. I logged it in freescan. I do have a question though, it didnt show any air/fuel ration or anything in the "rich/lean" field. Is this normal?

Injuneer

The PCM does not get any measure of actual A/F ratio, except for the O2 sensor milliVolts. Those numbers are not really accurate except at stoichiometric level (14.7:1), in closed loop. Some scanners will pick up the "target" A/F ratio, but that is just the value the PCM is trying to achieve (e.g. - 14.7:1 in closed loop, ~11.7:1 in PE mode).

Have you seen my online scanner writeup?

http://members.aol.com/InjuneerZZ/ScanMast.htm

I sometime review data logs, but I'm so backed up with them that I hesitate to take any more.

SLO-Z

Thanks, I hve not seen your write up I will definitely take a look at that.

SLO-Z

After reviewing the data sheet it does look like the o2 sensors are pretty messed up. According to the notes injuneer had me look at (if I am reading them properly) the o2 sensors should be reading approximately the same numbers +/- 30, mine are WAY of from that from time to time. If anyone can look at my log and see if they see anything else I would much appreciate it.

I do have a theory about what may have happened to the car so let me know if anyone agrees with this.
The cat clogged causing the manifolds to turn cherry red, in turn I feel this may have messed up the o2's, which caused the car to begin running crazy rich, and this may have fouled the spark plugs enough to cause the misfire and hard stumble around 5k rpm.
What do you think?

SLO-Z

anyone?

Injuneer

The O2 millivolts should only be close to each other at WOT. In closed loop, each bank is controlled independantly, and there is no requirement that the O2 sensor readings be close to each other in any given data frame.