Troubleshooting 101

[roadrunner_70]

See if you can troubleshoot this problem. The answer will be published later.
Btw, it was done using an old Monitor 2000 Scanner. You can pick these up cheap on ebay, just make sure you have the cartridge for your model. These were readings taken on two different dates, one during the problem, one after correcting it. 1993 4.3L V6 Symptoms were: rough idle, poor mileage.
------------Feb.----Mar.
RPM--------675-----600
TPS---------.48-----.48
MAP---1.19-1.23----1.07-1.13Volts
CTS--------190-----192 Degrees F
IAC---------000-----007 (zero is fully closed)
ESC-----4.4-5.4----8-14 degrees BTDC
BLM---------129-----126 (Long term fuel trim)
Integrator-128-129---125-127 (Short term fuel trim)

I also had readings from before the problem started, which I used as my control. These readings did not differ much from the last readings. Although the first readings may point to several areas, look at where you would start. My point in this excercise is to look for the source of the problem rather than throw parts at it. Good luck!

[2000CAYukon]

Originally Posted by roadrunner_70

See if you can troubleshoot this problem. The answer will be published later.
Btw, it was done using an old Monitor 2000 Scanner. You can pick these up cheap on ebay, just make sure you have the cartridge for your model. These were readings taken on two different dates, one during the problem, one after correcting it. 1993 4.3L V6 Symptoms were: rough idle, poor mileage.
------------Feb.----Mar.
RPM--------675-----600
TPS---------.48-----.48
MAP---1.19-1.23----1.07-1.13Volts
CTS--------190-----192 Degrees F
IAC---------000-----007 (zero is fully closed)
ESC-----4.4-5.4----8-14 degrees BTDC
BLM---------129-----126 (Long term fuel trim)
Integrator-128-129---125-127 (Short term fuel trim)

I also had readings from before the problem started, which I used as my control. These readings did not differ much from the last readings. Although the first readings may point to several areas, look at where you would start. My point in this excercise is to look for the source of the problem rather than throw parts at it. Good luck!

To me this looks like the MAP (I highlighted it above) was either not holding a vacuum, or was bad, or the vacuum hose to the MAP was leaking, or the vacuum port on the TBI for the MAP was clogged.

This caused the ECM to add fuel causing your rough idle. Both the BLM (long term fuel trim) and Integrator are adding fuel since they are above 128 (in the case of integrator it is 128-129). After you fixed the problem with the MAP, the ECM started to remove fuel and the idle quality (as well as speed) improved.

//2000CAYukon

[2000CAYukon]

After looking at it a little more, the timing is being retarded (ESC is 4.4 to 5.4) which indicates a problem with the ESC module or knock sensor. This probably explains why the MAP reading is higher in the before case.

Interesting that it does not give you a knock count. I use WinALDL and it shows knock count but not timing.

//2000CAYukon

[roadrunner_70]

Well, I'm surprised there weren't more responses. You're on the right track, Yukon, but, keep it simple.
High idle
High MAP reading = Lower Manifold Vacuum
IAC: Good clue here;it's fully closed during the problem
ESC: Reduced during problem also
BLM and Integrator:Both showing efforts to richen the mixture during problem, and leaning it out after it's solved.
This all leads to...
A Vacuum leak! Anything else is speculation. Check hoses for the obvious, but a little detective work with a can of carb cleaner(no silicone additives),around the base of the TBI and the source of the leak becomes evident. A deteriorated base gasket on the TBI.
What I didn't include in the original is that I tested the voltage for each cylinder and found that two rear cylinders were running lean. So, unmetered air was flowing by the TBI; oxygen sensor was reading lean, computer tries to compensate by richening mixture, however, mixture is not fully dispersed as it would be during normal route through TBI, leading to uneven dilution of fuel and air, which leads to rough idle. That no codes were set indicates that the air leak was being managed by the computer, but barely, leading to some rough running. None of this would have been possible without the use of the scan tool. OBD 1 scan tools are still kicking around at very reasonable prices, and I'm not talking about the code readers, but the scan tools themselves. Just by using it and even posting the numbers on here, you can avail yourself of the collective knowledge of everyone who can understand and interpret the readings for you.