Renix open-closed loop running hot or cold?

Here is one for you Renix Experts out there? I am running a 1987 4.0 Renix.

I hear that if & when the engine is running cold that it runs the engine rich in an open loop mode. I am wondering why they did not set it to run lean until it warmed up all the way to normal operating temperature, to warm the engine and exhaust faster????

Running it leaner, after a brief 60 second rich-fuel warm up would warm the engine up faster, right? It would also warm up the Cat converter sooner with out belching unburned HCO's, right?

I once had one of the very few ever made, Chrysler Lean Burns, made in 1976. It was the last car ever made (so I was told) that came stock with out a cat converter. It passed emmissions tests buy running real lean (which had it's own problems).

Systems with a cat converter run rich as they need the extra unburned fuel to burn in the cat converter, thus heating it up enough to ensure complete combustion of all those nasties in the exhaust.

So I am wondering why they would not run it LEAN at start up, or shortly there after, in the open loop mode since it would heat up the engine, exhaust and cat converter(?) faster. Is it because the excess unburned gas heats up the cat converted faster than a lean burn condition????

This is being asked by someone else who is also dangerously over educated (a Chemical-Env. Eng., but not a combustion or automotive eng.) but still has questions.

Mike McGinness
South Houston, TX

because it's much safer for the motor to run it rich.
THe computer doesn't know the exhaust mixture until the o2 sensor warms up.
If a condition existed that the motor would run leaner than normal, setting the ECU to run lean before the O2 sensor could tell the ECU it may run too lean, and damage the motor.
A massive vacuum leak would be such a scenario. If a vacuum leak suffiecient enough to run the motor lean and raise the idle existed during a rich open loop mode; imagine what it would do if the ECU was already running the motor lean.

The engine goes into "closed loop" operation when three things are achieved: One, a timer internal to the ECM expires, and two, the engine temp is arround 130 degrees fahrenheit, and, lastly, the O2 sensor is telling the ECM that it's capable of detecting exhaust gas changes that reflect fuel mixture. The timer is the least likely of the three to prevent ECM from going into closed loop. Engine temp is the most likely parameter to delay closed loop operation; it mainly depends on how long the engine has had to cool down since the last run-time, and, of course, ambient temperature. The exhaust gas temperature is critical to O2 operation. So a cold start would obviously take the most time for the exhaust gas to reach that magical number (arround 1100 degrees, as I recall). I didn't have the means to determine when my old Renix went into closed loop, but I have monitored my Chevy ECM. From a cold start (Calif cold ) it takes my SBC about 2.0 minutes to go into closed loop. The bottom line, the engine does not need to obtain nominal operating temp (190 or so degrees) before going closed loop, and does need to be "rich" when cold, or it will stumble, fart, and etc.

"because it's much safer for the motor to run it rich.
THe computer doesn't know the exhaust mixture until the o2 sensor warms up.
If a condition existed that the motor would run leaner than normal, setting the ECU to run lean before the O2 sensor could tell the ECU it may run too lean, and damage the motor.
A massive vacuum leak would be such a scenario. If a vacuum leak suffiecient enough to run the motor lean and raise the idle existed during a rich open loop mode; imagine what it would do if the ECU was already running the motor lean."

Very interesting. That's an answer that makes too much sense to argue with. I am impressed!:clap:

Thanks,

Mike McGinness
South Houston, TX

Largely because running lean can damage an engine a lot quicker and a lot more than running rich will. Detonation is very much a factor - you'll note that the "Lean Burn" idea isn't around anymore.

The difference between "open" and "closed" loop isn't so much the engine coolant temperature tho - it's the temperature of the HEGO sensor tip (yes, ECT has somthing to do with OL/CL operation - but it's not all of it.)

Earlier EGO sensors (Exhaust Gas Oxygen) weren't heated, and they were simple one-wire units (self-grounding sensors.) Operating temperature of the EGO sensor tip is, as I recall, some 700*C - so it took a little while to get there. It was called "open loop" mode because there was no feedback signal for fuel trimming (since the EGO sensor signal was unreliable until it hit operating temperature.)

The HEGO (Heated Exhaust Gas Oxygen) sensors seeks to reduce the warm-up time - rather than "heat soaking" the sensor tip in hot exhaust gasses, it carries its own heater. OL/CL times are reduced from, say, ten minutes to down around thirty seconds - which gets the fuel trimmed faster. "Closed Loop" refers to the feedback produced by the HEGO sensor monitoring exhaust gasses - the "loop" is the total fuel/engine management system.

There are also different varieties of HEGO sensors - the NHEGO (Narrowband Heated Exhaust Gas Oxygen - also called "knuckle" - which are in common use) and WHEGO (Wideband Heated Exhaust Gas Oxygen.)

The NHEGO gives a quantifiable signal in a VERY narrow range - if stoich is 14.7:1, the NHEGO is quantifiable around, say, 14.4-15.0:1 Beyond that, it's a flat "rich" or "lean" signal - and the ECU has to guess just how "rich" or "lean" before it can apply a correction. This means a lot of dancing around to get it right.

The WHEGO gives a quantifiable signal well past the "rich burn" and "lean burn" values - say, from 8.0:1 to about 25.0:1 (the engine won't run at either of those levels.) With proper programming, that means that the ECU will know just how rich or how lean the engine is running, and can come up with an accurate correction much faster.

Why don't they use WHEGO sensors? Cost - the NHEGO averages $80 for a dedicated application, and the WHEGO starts up around $250. Given the advantage in efficiency (and economy) you'd get with effective fuel trimming, it's a workable trade - but it's not going to happen without some widespread education.

5-90

Ecomike said:

Here is one for you Renix Experts out there? I am running a 1987 4.0 Renix.

I hear that if & when the engine is running cold that it runs the engine rich in an open loop mode. I am wondering why they did not set it to run lean until it warmed up all the way to normal operating temperature, to warm the engine and exhaust faster????

Running it leaner, after a brief 60 second rich-fuel warm up would warm the engine up faster, right? It would also warm up the Cat converter sooner with out belching unburned HCO's, right?

I once had one of the very few ever made, Chrysler Lean Burns, made in 1976. It was the last car ever made (so I was told) that came stock with out a cat converter. It passed emmissions tests buy running real lean (which had it's own problems).

Systems with a cat converter run rich as they need the extra unburned fuel to burn in the cat converter, thus heating it up enough to ensure complete combustion of all those nasties in the exhaust.

So I am wondering why they would not run it LEAN at start up, or shortly there after, in the open loop mode since it would heat up the engine, exhaust and cat converter(?) faster. Is it because the excess unburned gas heats up the cat converted faster than a lean burn condition????

This is being asked by someone else who is also dangerously over educated (a Chemical-Env. Eng., but not a combustion or automotive eng.) but still has questions.

Mike McGinness
South Houston, TX

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