Closed loop/open loop, explanations ?

[purplebutterfly]

This seems to the right thread to ask my question:

I have a 98 wrangler, 4.0 fuel injected, that runs terrible open loop, but runs just fine closed loop. When I attach my scan tool while the engine is running: the MAP and Throttle sensors seem to be fine. I think the PCM is trying to advance the timing before the engine is ready.

I would really like to know what other sensors are providing feedback when open loop. A control law would be nice......

[Blue Bowtie]

Welcome Aboard!

The PCM relies upon inputs from the EGO, MAP/BARO, TPS, ECT, EGR feedback, MAF, IAT, CKP, CMP, KS, and VSS sensors to trim fuel and adjust timing. Even with that, there are some things it does not do.

For example, the PCM does not monitor fuel pressure. If pressure is low or excessively high and the base injector pulse width for open loop operation is insufficient, it may run poorly. Once in closed loop the PCM can trim fuel (within reason) to increase or decrease injector pulse width to compensate and make it satisfy the EGO sensor - Thus running acceptably.

Similar phenomena can occur with other systems, such as a leaking EGR, other vacuum leaks, mis-reporting MAF, etc. No scanner will figure that out for you. It will only provide data for you to interpret. Obviously, if a sensor input is way out of range a DTC will be set and your scanner will report that as well, but that is only after something is so bad that a condition exceeds an allowable threshold to set a trouble code. And that doesn't necessarily mean a sensor has failed, but may mean that the condition it senses is way out of range.

If you can post some of the open loop and closed loop data we may be able to make some guesses as to the root cause.

Also, a bit more descriptive input might help. To say that it "runs terrible" in open loop can be interpreted in many ways. Is it barely idling or idling at 1,800 RPM? Is it misfiring and shaking or is the RPM hunting? Is it stumbling and recovering? Is there a long period of cranking to get it started?

A PW R2880 "runs terrible" when first started and it seems like it will shake itself right out of the nose, Once warmed up it will make over 2,000 HP as smoothly as anything and pull you along at over 550MPH - All from an engine that "runs terrible" at startup. See what I mean?

[purplebutterfly]

Originally Posted by Blue Bowtie

Welcome Aboard!

The PCM relies upon inputs from the EGO, MAP/BARO, TPS, ECT, EGR feedback, MAF, IAT, CKP, CMP, KS, and VSS sensors to trim fuel and adjust timing. Even with that, there are some things it does not do.

For example, the PCM does not monitor fuel pressure. If pressure is low or excessively high and the base injector pulse width for open loop operation is insufficient, it may run poorly. Once in closed loop the PCM can trim fuel (within reason) to increase or decrease injector pulse width to compensate and make it satisfy the EGO sensor - Thus running acceptably.

Similar phenomena can occur with other systems, such as a leaking EGR, other vacuum leaks, mis-reporting MAF, etc. No scanner will figure that out for you. It will only provide data for you to interpret. Obviously, if a sensor input is way out of range a DTC will be set and your scanner will report that as well, but that is only after something is so bad that a condition exceeds an allowable threshold to set a trouble code. And that doesn't necessarily mean a sensor has failed, but may mean that the condition it senses is way out of range.

If you can post some of the open loop and closed loop data we may be able to make some guesses as to the root cause.

Also, a bit more descriptive input might help. To say that it "runs terrible" in open loop can be interpreted in many ways. Is it barely idling or idling at 1,800 RPM? Is it misfiring and shaking or is the RPM hunting? Is it stumbling and recovering? Is there a long period of cranking to get it started?

A PW R2880 "runs terrible" when first started and it seems like it will shake itself right out of the nose, Once warmed up it will make over 2,000 HP as smoothly as anything and pull you along at over 550MPH - All from an engine that "runs terrible" at startup. See what I mean?

More data: This car sat for a year when I friend of mine gave up trying to fix a misfire. The spark plugs are new, new fuel pump, new wires, new distributor, new IAC. This resolved the misfire. Now what happens is the vehicle starts up just fine. However when idling the RPM decreases, the IAC tries to compensate and open so the engine surges (700 RPM to 1500 RPM). If you try to drive it when the car is doing this, it will run if you don’t try and give it gas. If you do try and accelerate it will cough and miss badly. The MAP parameters move, the temp parameters (intact and water) move, what I see happening is the timing advancing from 6 degrees to about 20 degrees when its running like crap. After the engine reaches temperature, everything is fine.

[Blue Bowtie]

Has the PCM been changed at some time, or possibly reflashed? It almost sounds as if the open loop backup parameters are wrong for the vehicle.

It might also be helpful as a diagnostic if you temporarily disable the EGR.

[purplebutterfly]

Thanks a good question. I don't know the answer to that. I guess I need to do a little research and get the software part number off the scan tool and check. I'll disable the EGR and check back into the forums when I have something to report.

Thank you