open loop when at idle

[BBSteve]

Hello, my 1988 K3500 with a 7.4 and a th400 trans smells rich at idle and the plugs are black and sooty. My super old NAPA mind reader scan tool says that it goes to open loop whenever I let the truck idle. It doesn't care what the engine temp is it always goes into open loop, is this normal. Best I can tell all of the feedback from the coolant temps are normal, I installed a 180 degree thermostat a few months back could this be the issue. It has a recent tune up which included a new O2 sensor as well as a new MAP sensor.

 

[PlayingWithTBI]

It doesn't care what the engine temp is it always goes into open loop, is this normal.

It is possibly normal. Some of the older GM calibrations were set that way. Another possibility is your one-wire O2 sensor is getting too cold at idle and throwing it into OL. If that's the case, a 3-wire heated O2 sensor will cure that.

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[AuroraGirl]

Hello, my 1988 K3500 with a 7.4 and a th400 trans smells rich at idle and the plugs are black and sooty. My super old NAPA mind reader scan tool says that it goes to open loop whenever I let the truck idle. It doesn't care what the engine temp is it always goes into open loop, is this normal. Best I can tell all of the feedback from the coolant temps are normal, I installed a 180 degree thermostat a few months back could this be the issue. It has a recent tune up which included a new O2 sensor as well as a new MAP sensor.

your thermostat, is that what your big block calls for? do you have secondary air? exhaust leaks? do you have a cat converter still? Old o2 should be replaced, upgrading to heated is best

what rpm at idle

also, what ambient temp is it too

what plug and gap you running
and what kind of driving do you do

 

[AuroraGirl]

#92-674-100: ENGINE DRIVEABILITY-SURGE AND DETONATION-DIAG. REPLACE PARTS - (Oct 11, 1991) SUBJECT: ENGINE DRIVEABILITY - SURGE HOT/COLD, DETONATION (DIAGNOSIS REPLACEMENT PARTS) VEHICLES AFFECTED: 1987-89 C/K, R/V AND G/P TRUCKS WITH 7.4L ENGINE (VIN CODE "N")

Condition: ---------- Some owners of 1987-89 C/K, R/V and G/P trucks with 7.4L engine may experience the following: 1. Surge on acceleration and/or at road load speeds 2. False Service Engine Soon Light - Code 32 (EGR) 3. Poor driveability during warm up 4. Detonation under load

 

[BBSteve]

It is possibly normal. Some of the older GM calibrations were set that way. Another possibility is your one-wire O2 sensor is getting too cold at idle and throwing it into OL. If that's the case, a 3-wire heated O2 sensor will cure that.

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Thanks for the responce, I had installed a heated unit and the truck didn't care. I recently replaced the heated sensor with a OEM single wire sensor and the truck doesn't care either way.

 

[BBSteve]

#92-674-100: ENGINE DRIVEABILITY-SURGE AND DETONATION-DIAG. REPLACE PARTS - (Oct 11, 1991) SUBJECT: ENGINE DRIVEABILITY - SURGE HOT/COLD, DETONATION (DIAGNOSIS REPLACEMENT PARTS) VEHICLES AFFECTED: 1987-89 C/K, R/V AND G/P TRUCKS WITH 7.4L ENGINE (VIN CODE "N")

Condition: ---------- Some owners of 1987-89 C/K, R/V and G/P trucks with 7.4L engine may experience the following: 1. Surge on acceleration and/or at road load speeds 2. False Service Engine Soon Light - Code 32 (EGR) 3. Poor driveability during warm up 4. Detonation under load

Hi, my truck had the MAP sensor mounted to the air cleaner base which made removing the base a total pain in the neck. I had fabbed a bracket and relocated it a few years ago I did not notice any change when this was modified.

 

[BBSteve]

your thermostat, is that what your big block calls for? do you have secondary air? exhaust leaks? do you have a cat converter still? Old o2 should be replaced, upgrading to heated is best

what rpm at idle

also, what ambient temp is it too

what plug and gap you running
and what kind of driving do you do

If I remember correctly the OEM is 195 degrees, idle speed is 750 in park, ambient temp has been 50 degrees F and lower. I am using a one number hotter than stock autolite plug with a stock gap. I do not know the exact number of the plug I will have to pull one out to check. My driving is mostly around town and rural areas at lower speeds. Only see's highway speeds a few times a month.