edman

Without discussing voltage too much, and talking too much about tuning for best results, I have heard that when larger throttle bodies are installed in LT1's the tps often times needs to be turned CCW for best performance. So, how long have you guys been trimming your holes into slots? A small bit ,1/32", 1/16", more even? My engine hunts in drive between 1100-2000 rpm slightly on very light load, and runs normally as throttle is opened. I would like to trick the computer a bit to richen that rpm range for better driveability. I have slotted the holes ever so slightly on a brand new tps, under 1/32", and the hunting has decreased slightly, so I want to add even more. As to voltage, I have not checked it, but if there is a desired range that richens idle/off idle, that's what I could use. thanks , Edman

Injuneer

The TPS closed throttle voltage has to be between 0.30-0.80 volts. If its between those value no adjustment is necessary. That allows a tolerance to the values that will set codes or cause idle problems. IAC counts should be between 20-40 for a stock setup. Are you running a non-stock cam and has the PCM been tuned?

edman

So your general answer seems to say that rotating the tps has no effect on adding timing or fuel, is that correct? I was under the impression that it does alter fuel and timing depending on the setting. As I stated, my idle thru 2000 rpm is a bit lean, causing mild "hunting", and you are saying that voltage .3v is not different than voltage .65v in terms of fuel the computer directs to the injectors?

Injuneer

Closed throttle voltage will not affect timing or fuel. It has to be in the range of .30-0.80 for the PCM to baseline the 0% voltage. Then it prorates throttle percent open by scaling it over a value between the baseline closed throttle value, and a value about 4V higher than closed throttle = 100%.

Sounds like you may have the throttle stop screw out too far. With the engine fully warmed up, back off the throttle stop screw until the engine RPM is at the programmed idle value. For a stock tune, M6 stock idle is 800 RPM, A4 in gear is 550 RPM, 650 RPM in neutral. If you have a cam and someone tuned it, they may have altered the target idle setting.

Whole thing works a lot better if you have a scanner. Stock tach is not accurate, PCM scanned RPM value it exact. Once you have it at the desired idle, use the scanner to check the idle air (IAC) counts, looking for a number in the range mentioned above. Again, that's with stock programming. If you have had a tune, the tuner may be looking for higher counts. If you have the throttle stop screw out too far, the IAC counts will drop to "0" and the PCM loses control of the idle speed - has no way to pull it down, if idle too high.

Once you've got the RPM and IAC counts right, look at the TPS voltage, and make sure it's between the values mentioned above.

Where you get in trouble is when the closed throttle voltage goes above 1.0V. The PCM interprets that as partially open throttle, and the idle RPM wil possibly be elevated.

Spark timing is determine by MAP (engine load) and RPM, with offsets for things like inlet air temp. Fuel is held at 14.7:1 in closed loop, steady state. A rapidly opening TPS will cause the mixture to richen, simulating the accel pump on a carb. The point at which the PCM transitions from closed loop to power enrichment mode is determined by a table of RPM vs. throttle %.

edman

Give me the brand and model of an easy to find scan tool! I have an old tool that reads codes only, and have no codes. Smog due in two weeks, and without the lean surge I have, it passed by only two to three parts per million of HC these last ten years of ownership.

Injuneer

One of our members has written EXCELLENT scan software for the 93-95 OBD-1 LT1's. Free download, you buy a cable. I've recommended this, and helped people with interpreting the resulting data logs. Far superior to anything else on the market. Scans not only the PCM, but the air bag system and ABS.

The link describes how to build a cable, but Gary indicates the quality and specs of the Chinese parts has deteriorated badly.

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