Okay, this is my first time having to ask a question... it is a dandy!
I have a 91 Z28 Camaro with an automatic trans and a 305 TPI.
I have recently installed TWO new fuel pumps, new MAP sensor, checked the fuel pump relay, car is holding 42 lbs. of fuel pressure at idle and pressure goes up when I press the accelerator. Yesterday I had a well known mechanic to come up and he put a scan tool on the car and all sensors seemed to be in normal parameter.
The car will run good until the car goes in to close loop. As soon as the computer closes the loop in starts idling realy bad and won't take fuel. It actualy will die, sometimes it will start back and others it will take a shot of ether to get it back running. Once you get it started back it will sit and idle realy rough, but won't die.
The TPS tested okay!
I can jump over the wires going to the fuel pump relay and start the car and it doesn't change anything.
Is there anyway that the fuel pressure could be reading good, but still not getting to the injectors?
I can listen to the injectors and her all of them working.
I don't understand what the computer could be reading that is making the car run so bad.
I TRULY DO APPRECIATE ANY HELP THAT ANYONE CAN GIVE
By the way, after shutting the car off in closed loop and restarting it, it stayed in open loop!

 

Hey cwv2, If you want to check your fuel injectors all you need is a ohm meter and check the resistance of each injector while the car is warmed up and running. Unplug one at a time and measure the resistance and they should all be within an ohm of each other. Make sure you plug them back in after you measure them before you go on to the next one. I know mine is suppose to be between 16 and 17 ohms.

Your doing better than me, right now mine is having a similar problem which I will posting right away (I had to get towed home)

 

I am not sure but I don't hink it could be an injector problem because the car only does it after it goes into closed loop. When we put the scan tool on it we seen that it had went into closed loop we shut it off and it stayed in open loop.....
I thought I was going to have to get my car towed, but I somehow limped it home!

 

With scan tool check O2 voltage while engine is still cold. If it reads below 450 mV check AIR diverted valve stuck open

 

The mechanic took his scan tool back to the garage, but the O2 did not seem to be communicating with the computer.
With the O2 sensor unplugged we got the same reading as it was if it was up.
What does the O2 sensor ground too?
I have seen many O2 sensors bad and I have never seen one make something run as bad as this.
I hate to ask a stupid question... But how do I check the AIR diverted valve?
Would it show a code on the computer?

 

New O2 sensor.... did not help!

 

ECM provides 450 mV DC bias voltage on a wire (purple) that connects O2 sensor to the ECM. If the scan tool always reports 450 mV then you have a break in a harness somewhere between ECM and NBO.

Perform the following check.
Connect scan tool to ALDL connector and configure it to read O2 voltage.
Disconnect O2 sensor from harness as close as possible to the sensor (there should be a weather pack connector next to NBO).
Configure DVM to read Vdc while having one lead into ECM harness side of the O2 connection and the other connected to electrical ground. Turn ignition key into run, but do not start engine. DVM should read 450 mV and scan tool should report the same.

Start engine with O2 disconnected - DVM should continue reading 450 mV. If you let engine warm up to operating temperature (more than 2 min run time) ECM should set DTC 13 - O2 circuit open. You can clear DTC by turning off engine and disconnecting negative bat terminal for about 30 seconds.

To check O2 sensor harness for open ground O2 connection. Scan tool should read very low voltage ~ 20mV or less. If it still reads 450 you have ECM with bad buffer circuit or bad sensor ground circuit Pin D6 ( it's ok to cry). If you have open sensor ground circuit look for broken tan wire or a loose solder lug on the back of passenger side head or next to a common ground near thermostat housing.


//RF

 

Thanks, I will try this tonight or tomorrow. Yesterday, if I remember correctly, the scan tool read a very low voltage (something like 5 mV or less).
Once again, I thank you for any information that you can provide. I have always been around "old school small blocks" but this is an entire different world!

 

If and only if scan tool (confirmed by a DVM measurement) reports 5 mV look for a short to ground on O2 sensor wire (purple wire).

//RF

 

Jacked up the car and did not find anything wrong with the O2 sensor wire. Is there anyway that I can test the voltage on the O2 sensor wire without a scan tool.
I am sure that I can check it with a DVM, but I am not sure to do that.
Do you know anyway to test the ECM to see if it is bad?
I took a look at it today and found out that it had been changed before and replaced with a remanufactured one. The date on it was Nov. of 1998. Are these remanufactured ones very dependable?

 

from your first description of the problem, sounds like a possiable vacumn leak to me. I would take that ether or some carb cleaner and spray the intake and vacumn lines to see if the idle changes. If its leaking or sucking air, the idle will rise for a couple of seconds then drop back down. Its worth a shot, just something to rule out that doesnt cost $$$$$$$

 

You can measure O2 voltage directly with DVM right next to NBO (look for a connector ). Generally having a scan tool confirms what ECM sees in terms of input voltages. Checking O2 voltage just confirms what the sensor is doing, however if the ECM's input buffer circuit is hosed you will be hard pressed to determine that without a scan tool. Alternatively get yourself a ALDL cable, laptop with WinALDL (freeware) and you can monitor many parameters, and perform data logging, etc. This allows a lot more functionality then a scan tool.

Re-manufactured ECM - I have no experience - I have couple that I pulled out from JY - they appear to be OK, but I have collected a few over the years.

//RF

 

Okay, so here is where that I am at now. Another mechanic cam up and we put the car on the scan tool and it showed no troubles. O2 sensor (replaced yesterday) was working as well as the computer was going into closed loop like it should. I believe we have ruled out a sensor or computer related problem.
Could I be looking at something wrong with my distributor. Does anyone know if the distributor controls the firing of the injectors.

 

if it runs fine in OL and crap in CL then its less than likely the dizzy.

even if the module or ECU took a crap it should default to base timing.


The only real diff between OL and CL is in OL the ECU doesnt take o2 readings and maybe doesnt take ECT readings

 

Did you set base timing with EST bypass connector open to about 6 yo 8 degrees?? If ICM inside dizzy does not produce DRP pulses ECM will never fire injectors. Since engine runs in OL dizzy ICM module is working OK (providing DRP pulses). In OL fuelling is based on stored VE data (based on combination of CTS, TPS, MAP and other factors. Check CTS, and TPS - these are very easy to check if you have a scan tool or with a DVM. New MAP senosr can be a suspect - check it as well. (BTW it is very common to get a 'new' DOA sensor now days.

//RF

 

I checked my injectors today. ( I don't know if it matters but the car was cold). I had 2 injectors that read low resistance. One read 10.8 and another reaad 9.8 ohms. If I understand correctly that menas that one of the injectors is already bad and the other is on it's way out the door. The other injectors read between 13 and 16 ohms.
Oh yea, I set the base time to 6 degrees with the connector unplugged. Once again I do appreciate the info!

 

You may want to re-measure the two suspect injectors just to be sure. Generally speaking any TPI injector that measures below 13 Ohms is a dud.

//RF

 

Rechecked the injectors and came up with the same readings. Heres another question... I checked the voltage on my injectors and with the key on I have 12 volts on BOTH of the terminals that plug into the injector, is this normal? If not what could be the cause?
Thanks again for any suggestions RF Master.

One more thing, I bought a noid light and put on the injectors and it fired properly! I am going to put a couple of used injectors in it, just to see if it fixes it. If it does I will order a new set.

Do you think this could be a problem from a stopped up cat?