[tnoftsger]

Alright, i am having trouble with a GM vortec 5300 v-8...
In open loop, it stumbles and sputters and basically behaves horribly, once in closed loop it runs fine.

The oxygen sensors are not in play in open loop so we don't know it the AFR is off yet.

The ECT (Coolant Temp. sensor) is supplying info to the PCM to adjust AFR and Advance Timing
The MAP sensor is supplying info to the PCM to adjust AFR
The MAF sensor is supplying info to the PCM to adjust AFR
The IAT sensor is supplying info to the PCM to adjust AFR and Advance Timing

Do the Knock sensors influence the PCM in Open Loop?

I'm a new study to these systems so i need help here!

Are my facts straight so far?

One of these sensors must be malfunctioning and either throwing off the fuel mixture or the timing.

Or, of course the PCM is bad.

I have checked the ECT and it is within limits
I have checked the MAF and it is within limits, Soooooo
Fuel pressure and regulator are fine.
Air filter is new.
i'm left with the MAP sensor or Intake Air Temperature sensor?

Opinions please.

Thanks

[454me]

When My son got his eagle talon it acted just like that. It would run horribly when in open loop and great when in closed loop. What I found with it was the timing was about 40 degrees off. Once the computer took over it corrected the problem. I am not saying this is your problem but you need to disconnect the computer wire so it cant go into closed loop and test things until you find it. Every car has the wire in a different place. On his it was under the dash. On the chevy celebrity I had it was a brown wire on the fire wall passenger side. You might search online to find where yours is at. I would that if you had a sensor that was bad you would be getting a trouble code.

[tnoftsger]

no codes unless it's really cold and running like junk, then you get a multiple misfire, which is understandible.

it has no distributor so i don't think i can adjust the "Base" timing

[curly5759]

I can problably help, but I would like a little more info:
Year/Make?model of vehicle

How are you measuring MAF and ECT? Code reader? Scan tool? DVOM?

Are you sure you have a Map sensor? Most MAF sensor systems do not use a Map sensor.( there are exceptions).

Knock sensors adust timing when Knocking is sensed, so unless you have pingign going on, the knock sensor can be ruled out. If it is pinging, I'd be checking fuel pressure/voulme.

Have you checked the fuel pressure? Are you using a K&N filter or similar?

[bluesman123]

The Vortecs do use a MAP sensor...its just behind the big injector plug on top of the manifold. It would help a lot if you could monitor the engine while it is in open loop. A couple of thoughts:

Is the VCM the one that came with that engine? If it isn't you may need to get it reflashed with the latest and greatest. Remember, open loop is running a preprogrammed set of fuel/air ratios.

Have you checked your coil and plug wires? Its harder to fire a rich mixture which is what you have in open loop.

What do your plugs look like if you pull them out after running in open loop?

Keep us posted,

Dave

[Siggy_Freud]

What are the actual readings you are getting from the sensors? Give me the TPS sensor readings too if you can get them.

Something is causing your base fuel and or timing maps to be out of whack. Do any of the sensors readings change at idle between open and closed loops?

BTW many newer engines use both a MAF and MAP sensor. Each have certain advantages over the other which is why many cars use both today.

How long does it take to reach closed loop? A knock sensor should be working regardless of which loop. Look at the knock count. If you are getting knock, the computer will kick the system into a third mode known as Limp Home which will cause things to run like crap (normally way rich).

[tnoftsger]

all of my sensor checks have been with a DVOM and i have been checking them against the specs in the Haynes manual i have for the truck. A 2000 Suburban. The ECT i took out of the truck and measured while in ice water with a thermometer and then in hot water, it was fine.

The MAF was simple a ohm check while unplugged and not running. I trying to get a OBDII scan tool to try and monitor the sensors while the truck is running.

Everything is stock except the plugs and wire which i changed just to be sure.

The time it takes to straighten out depends on the temperature. This morning (8 degrees F) about 3-5 minutes. Saturday Morning (24 degrees F) about 90 seconds. These times are how long it takes to run well enough to leave it. Any earlier and it will probably die.

My brother-in-law has a OBDII scan tool, i will borrow it and see if i can get real-time data from the sensors.

[CCCCCYA]

I would vote for checking the MAP sensor. When your in open loop, the computer still takes readings from the map and tps sensors to determine which fueling table to use. When in closed loop, it uses everything (O2's, MAF, IAT, Coolant temp,etc..) to determine fueling.

Get it warmed up and in closed loop and running fine.. Now unplug the MAF. If the MAP sensor is bad, then it will stumble and try to die just like when it's cold. Also, you need to get a Tech II on there or something similar. OBDII cars will throw a code for just about everything that goes bad, but might not necessarily turn the SES light on..

Good luck..
Dave C.

[Siggy_Freud]

How many wires are going to your Oxygen sensor? Replacing this might help getting you into closed loop faster if your current Oxygen sensors heating element has gone out.

It would definitely help to know what your readings are. There can only be a certain numbers of things going wrong . You'll find them eventually.

[roys63]

Its amazing how bad the Vortecs will run when the fuel pump is going bad ! Have you checked the fuel pressure? They need a lot of pressure About 60 lbs on most?

[tnoftsger]

yea, the fuel pressure is fine, i have a guage for that. Still waiting on my scannner to get the sensor data.