well, i have 28 ohms of resistance in my map sensor circuit. the IAT and A/C pressure switch are also on this circuit. the circuit is designed to get a 5 volt referance from the pcm. i verified that i was getting that 5 volt referance by using the scan tool and a dmm. now, power is obviously fine but when i check the ground side of the map sensor i'm getting 28ohms resistance. usually, under a half of one ohm is good for a ground circuit. i looked up the schematic and it shows that circuit being grounded in the pcm. My pcm is new and functional. i don't believe these pcm are case grounded.... so here's where i need help. i need to find out what to look for ground wise to get rid of this resistance. becuause of this problem, my car will not run in closed loop. in closed loop, the short term fuel trims cut out completely and slowly starve it of fuel until it dies. it runs fine in open loop since open loop does not make adjustments based on output signals from the sensors. help??

 

Have you tried unhooking the other sensors in that circuit to see if it helps? Just a thought........

 

no, what will that do? they are all grounded on the same wire

 

That is wrong. The only sensors that aren't referenced in open loop that are in closed loop are the O2 sensors. You may be trouble shooting the wrong sensors...

 

How are you measuring the resistance of the MAP sensor ground - between what points? ....with the connector disconnected from the sensor? When you probe the voltage from the black wire to ground, what does it read?

What are your MAP readings for barometer, voltage and kPa and does it match the local weather station, or a hand held barometer? What are your MAP readings at idle - voltage and kPa?

 

well, i pulled the pcm today and disconnected the power to the circuits. i used the battery and a jumper lead to created 12volts (12.61 volts excactly) through that isolated circuit. i put one meter lead in the ground wire in the sensor and the other in the pin on the pcm harness, and checked to see if any voltage drop was present throughout all 3 sensors in that circuit. resistance did not drop at any point, therefore verifying the wires are all good and do not have excessive resistance. i know the wires are good. I'm not real sure if i meaured the resistance of the sensors correctly, but i was getting about 2.4 mega ohms at around 91 degrees from the IAT. i did not get time to test the map and a/c sensor today. i backprobed it. injuneer, what should i be doing to accurately test these circuits??? i guess i'm not sure if those first resistance figures were accurate, but when we self grounded the map sensor a month ago, the short term fuel trims did stay constant, though a little on the low side around 108-111 on the one bank. so it did stay in closed loop more often, still switching back and forth though.

 

The MAP sensor has nothing to do with the STFT's. It also isn't going to have an effect on the car going into closed loop or not.

 

correct, no sensors, unless faulty are going to determine closed or open loop. pcm determines that off of exhaust temps last i heard. but, what i'm saying is that something is messed up enough where the short term fuel trims will not process in closed loop. they just free fall until it dies. the pcm is smart enough to know something is up and it switches itself into open loop all the time. i'm just having a nightmare of a time finding out what is making that do that.

 

The gates for closed loop are 1) "active" O2 sensors, indicating they have reached the temperature at which they will operate (~600*F), 2) coolant temperature (seems to vary over the range of 120-140*F) and a 3) timer, that seems to clock out at about 200 seconds. The PCM has no way of knowing the EGT, it just looks for activity from the sensor to indicate the sensor it the right temperature.

I've never seen any evidence of the MAP sensor affecting whether the PCM stays in closed loop.
If they are "free falling" (dropping well below 128???) its because the O2 sensors are telling the PCM the mixture is way too rich, and the PCM is responding by cutting the short terms. How does this indicate the "short term fuel trims will not process"? What does "process" even mean in the sense that you used it?
Did you mean to say "voltage did not drop at any point"?

Somehow, this seems to be way more complicated than it needs to be. Check for +5V at the hot pin of the connector, to chassis ground. Then check that you have "0" V connecting the ground pin of the connector to a chassis ground. Then verify you have +5V between the hot pin and the ground pin.

Measure the signal voltage on the blue wire. Use a scanner to see what the PCM says the voltage is.... they should be very close.
What does this mean? What did you "ground"?
The short terms should never be constant in closed loop. They should be moving back and forth above and below 128, in response to the corresponding swings of the O2 sensor voltage. If it goes into open loop, the short terms will be 128.

 

this might not have anything to do with it but could the fuel pump be an issue not supplying enough pressure????? or maybe the f/p req. i ahve found out that with my cam swap the computer wasnt tunes for the higher f/p and it didnt run right, i put the factory f/p req and seemed to help? I was just wondering

 

"I've never seen any evidence of the MAP sensor affecting whether the PCM stays in closed loop."

-me either, i'm not blaming the map sensor at all.

"If they are "free falling" (dropping well below 128???) its because the O2 sensors are telling the PCM the mixture is way too rich, and the PCM is responding by cutting the short terms."

-problem occured on 2 sets of o2's...the originals and brand new gm o2's i put in this fall. and yes, the short term blm's start at 128 when you start the car and free fall steadily until about 80-85 blm when it finally dies.

"Did you mean to say "voltage did not drop at any point"?

-yes, sorry.


"Somehow, this seems to be way more complicated than it needs to be. Check for +5V at the hot pin of the connector, to chassis ground. Then check that you have "0" V connecting the ground pin of the connector to a chassis ground. Then verify you have +5V between the hot pin and the ground pin.

Measure the signal voltage on the blue wire. Use a scanner to see what the PCM says the voltage is.... they should be very close.
I'll have to try that and see if it makes a difference."

What does this mean? What did you "ground"?


-I backprobed the ground wire in the map sensor and used an aligator clip on the other end to attach it straight to the battery ground.

 

sorry, trying to figure out how to quote nicely

 

Why are the BLM's allowed to go to 85? Unless the tuner changed the boundaries, they shouldn't go lower than 108. What does your O2 voltage look like? Do you have a log of the car?

You never did answer these questions ^^^.

 

This is getting totally confused. He said "the short term fuel trims will not process in closed loop. they just free fall until it dies. ".... I think he just made another typo when he then said "the short term blm's start at 128 when you start the car and free fall steadily until about 80-85 blm when it finally dies". I think he's still talking about the short terms (INT's).... but who knows at this point.

I really can't follow it at all.

 

no, i don't know the voltage or barometer readings and if they are right...i'll have to check when i get a chance. okay, on the tech2, the "short term fuel trim" starts at 128 and as soon as i put it into closed loop manually, the numbers start falling until it dies. the numbers fall very quickly. it probably takes 15 seconds from 128 until it dies. i was thinking the blm's were the readings for my "short term fuel trim" as it describes on the scan tool. Sorry for all the confusion. I've ben really confused. my instructor is giving me pointless busywork to do on this car and it only leads to more confusion. so far to find this problem, his only ideas were to check my injectors and the MAF sensor. he's pretty much useless.