computer was replaced will that make the engine run rough?

[Red93TA]

I was talking to the previous owner today and since i've had the car had a heck of a time trying to track down a small miss in the engine and apparently the ecu has been replaced by the shop that did the water pump and stuff. Now I had the car scanned at a friends shop and he told me it was running rich, most likely because the tps was bad so i went and replaced the tps and it didn't change a thing still has this dang miss. Is there anything special they are supposed to do when replacing the computer. I mean sometimes the car runs perfect and then sometimes you try to take off quick and she backfires and spits and sputters and even before you get to redline starts backfiring and popping in the exhaust. I haven't checked the fuel pressure or changed the distributor cap and rotor but everything else has been done as far as tune up and mild performance stuff here and there but i'm wondering if maybe the computer isn't set right for that car since it was a replacement. Ohh fuel mileage sucks too which i attributed to the running rich problem. But if its runnin rich does that mean the fuel pump isn't bad? Who knows maybe someone on here smarter than me does !

[Injuneer]

Lots of things can cause it to run rich. Some possibilities:

"True" rich condition:
-fuel pressure too high
-coolant temp sensor indicating cold temperature
-MAP sensor signal indicating high MAP (low vacuum)
-inlet air temp sensor indicating lower than actual temperature
-incorrect injector size (not programmed correctly in ECM)
-EVAP canister saturated with fuel
-Leaking injector(s)
-intermittent spikes in TPS sensor voltage
-EGR valve stuck open

Since your condition appears to be intermittent, look at the harness connectors for the MAP, TPS, AIT and CLT sensors. The MAP sensor connector is notorious for drying out and cracking, causing intermittent spikes in MAP. The ECM uses MAP to determine when the engine is under load, and richens the fuel mixture accordingly. Same when the TPS voltage spikes... it figures the air flow it going to increase rapidly and richens the mixture. ECM also sets a richer A/F ratio when the engine is cold, just like the choke on a carbed engine.

Your 93 is the only LT1 that operates in speed-density mode (no MAF). It has to use the MAP sensor and the IAT sensor to calculate how much air is entering the engine. If the MAP reports high MAP, and the IAT reports low incoming air temp, the ECM calculates that the air is much denser than it really is, and adds too much fuel.

"False" lean condition - ECM "thinks" the engine is running lean and adds fuel the engine really doesn't need:
-exhaust leaks before the O2 sensors
-misfires
-faulty O2 sensors
-faulty O2 sensor wiring
-high voltage interference with O2 sensor wires.

Check the exhuast system for leaks. Look carefully at the AIR system check valves to insure they are not leaking. Misfires should be noticable if they are there.

If you can get a data logging scanner (software) on it, something like TTS DataMaster, it may be possible to determine the source of the problem.

[Red93TA]

i downloaded freescan to my laptop but i've been looking for a cable to hook it up and i'm unsure of which cable to get from the previous post i was looking through. I think this speed density is a pain but alot easier than having to worry about a MAF sensor. The IAT is new, both coolant temp sensors are new and reading properly according to my friends snap on scanner, the MAP appeared to be working properly the only thing we were getting spikes on was the tps and he said it was because it was zero'd too high like .75v or somethin like that and he said it needed to be around .60 i read the tps info stuff on shbox site and it said you might have to elongate the holes and check again but he lives out of town and since i put the new tps on there its still doing it i'm prolly gonna have to do that. o2 sensor looks clean and the voltage on the scanner appeared to be right but i do have a terrible leak in the exhaust right before the cat. I can't get to it with the welder to seal it up since its between the chassis and the top of the pipe. That might be causing it. I know exhaust leaks will make them run bad but i haven't had the oppurtunity to fix it. I tried taking the y pipe down at the manifolds but those bolts are so rusted i don't think its gonna happen without breaking them. Thanks for the help injuneer you really know your stuff.

[Injuneer]

i downloaded freescan to my laptop but i've been looking for a cable to hook it up and i'm unsure of which cable to get from the previous post i was looking through.

http://www.akmcables.com/

I think this speed density is a pain but alot easier than having to worry about a MAF sensor.

Neither system is any better than the other.

The IAT is new, both coolant temp sensors are new and reading properly according to my friends snap on scanner, the MAP appeared to be working properly

Unless you are watching a sensor constantly for intermittent problems, you may not even see them. That's why the data log is to your advantage.

the only thing we were getting spikes on was the tps and he said it was because it was zero'd too high like .75v or somethin like that and he said it needed to be around .60 i read the tps info stuff on shbox site and it said you might have to elongate the holes and check again but he lives out of town and since i put the new tps on there its still doing it i'm prolly gonna have to do that.

Not likely to be correct. The closed throttle voltage can be anywhere fomr 0.20-0.90V, and the ECM will set that as the "baseline" for 0% TPP. What kind of "spikes" were you seeing? If you rotate the blades slowly from full closed to full open, the TPS voltage should increase smoothly from the closed throttle value to about 4V higher than that at WOT.

o2 sensor looks clean and the voltage on the scanner appeared to be right

You can not physically see the O2 sensor. All you can see is the metal shield around the sensing membrane. If the membrane is coated with lead, silicone or some other materials, it will not work correctly. You can't really "see" that. What is the scanner reading that you consider "right"? In closed loop the voltage should be swinging over the full range of 0xx - 9xx milliVolts, changing about 9 times per second. Scanners can't even display the data that fast.

but i do have a terrible leak in the exhaust right before the cat. I can't get to it with the welder to seal it up since its between the chassis and the top of the pipe. That might be causing it. I know exhaust leaks will make them run bad but i haven't had the oppurtunity to fix it. I tried taking the y pipe down at the manifolds but those bolts are so rusted i don't think its gonna happen without breaking them. Thanks for the help injuneer you really know your stuff.

The O2 sensors are not near the cat in the stock exhuast setup. If the leak is after the sensors, and not within 6" of them, it isn't going to affect anything. If you think its a problem, get some exhuast pipe "tape" and slide it over the pipe to block the leak. You don't need to weld it. The tape is not permanent, but it will block the leak long enough to rule the leak out, or prove its the problem.

[Red93TA]

i don't really know he's the one that worked at the chevrolet dealership and was telling me what voltages and what things were doing i was watching him do it. Like I said I have no clue what the values are supposed to be on there thats why i need to get the cable and take the laptop out there and just constantly record it so when it starts running rough i can see what it says. The whole concept of a computer in a car is really starting to piss me off. It seems like the smallest thing can make them run haywire. As far as clean i took it out and it didn't have alot of black soot on it so i wiped it with a clean shop towel and put it back in. I've been told that if they are really covered then normally they are not going to be reading properly. Yea its the stock setup but the exhaust on the car is ridiculous. The shop that put the new cat on cut it at the Y in the Y pipe and thats where its leaking. If I had a decent digital camera and a lift i'd show you. But they put in a universal cat that was longer so they had to cut it back that far. Personally I don't like it and if I can find a good used Y pipe somewhere i'm going to replace it. There were alot of things wrong with the car even minor things but still wrong when i got it. The temp sensor in the water pump had been put in with teflon tape and the computer wasn't turning on the fans. Got that fixed and now the fans work but it shows on the guage to be in the red before it turns the fans on so I put a switch on the ground side of the relay and made it come on when i start the car. Other one still operates with the a/c properly but one runs all the time. It had been running with no air intake on it. The rubber elbow had been torn to pieces and it was setting there but wasn't hooked up for i don't know how long. The Throttle body was filthy, so gunked up inside it wouldn't idle right because the grime wasn't allowing air past the blades, among all the other things i have listed that were changed. I'm gonna order a cable this week and if you think there is a better program to use than the freescan let me know and i'll try it.