Rough Idle When Water Temp Reaches 174 F

[Merkava_4]

2000 Buick LeSabre with the GM 3800 V6

Engine runs very strong with excellent drivability. My only complaint is that when the water temperature reaches 174 F, which I'm assuming is when the OBD II goes into closed loop operation, I get a rough idle from that temperature and above. While the engine is still cold, the idle is very smooth and unperceivable. The check engine light is not on and there are no codes stored.

I'm just throwing this out there in case by the rare chance somebody else has had the same experience and knows exactly what I should do. Kind of like playing the Lottery.

 

[901Memphis]

Well i don't know the answer to your problem, but my Buick with 3.1 will go into closed loop in less than a minute no matter how cold it is.

 

[Recalculating]

I do not know exactly what you should do nor will this information make you feel you have won the lottery but good luck anyways.

Our Buick 3800 engines are excellent. I would think the OBD II would go into closed loop at a lower temperature. As you may know, the 3800 has inherent intake manifold gasket issues so watch your coolant level. Make sure your radiator pressure cap is good and the oxygen sensor is not starting to get lazy with all above interrelated. Check for a vacuum issue too. Look closely at all those little hoses especially at connections. The material is getting old and may be starting to fail.

I have not had this exact issue with mine but seems like intake gaskets will be in the cards before wanting to retire the vehicle. In hindsight, should have done it when I changed out the water pump, pulleys, belts and supercharger fluid.

 

[The_Jeff]

I think you can get closed loop colder than 174. Maybe a canister purge valve opens then, and you could have a vacuum leak somewhere in the vapor system lines. (Lots of 'em.)

 

[Clevy]

Could it be that at 174f the thermostat opens which then begins circulating coolant and there is a gasket leak into a cylinder until the thermostat closes again?

 

[Trav]

Time to get a scan tool on the job and get some real data.
The 97-2004 3800 GenII used steel Bosch injectors on the non SC engines that had some dirt issues.
The MAF sensor was also a common sore spot and easily damaged by throttle body cleaning as it is right in the throat of the throttle body on some models.

Over heating ignition control module under the coils, lower and upper intake gaskets as well as burn through of the upper plenum, slow upstream O2 and a host of other parts are all on the menu.

Generally most of the mentioned parts will throw a CEL but not always depending on whats going wrong or an intermittent fault.
This is not a problem you can throw parts at.

 

[shanneba]

It might be that as the engine warms up a sensor (likely crankshaft position) starts to give false readings. My son had a Pontiac that would start and run well until it warmed up. Then it would die, if you let it cool down it would start again, ended up being the crank position sensor. It never threw a code for the crank position sensor.

 

[Merkava_4]

Thanks for all the guesses and suggestions guys, I appreciate it.

 

[artificialist]

Sometimes the engine doesn't react to the oxygen sensor before a certain temperature. After that temperature is reached, a failing or incorrect O2 sensor can cause the engine not to run correctly.

Sometimes an O2 sensor will be failing, but it will fall within the normal range of the computer, and there will be no oxygen sensor error codes.

Do you know the age or mileage of your O2 sensor? Also, how much oil and coolant has your engine consumed? Had any misfires in the past? If it had those problems in the past, it probably took much of the life off the oxygen sensor.

Also, check the exhaust system for leaks. Any leaks near the oxygen sensor will cause false readings.

 

[Merkava_4]

Quote:
Do you know the age or mileage of your O2 sensor?


It still has the original factory upstream and downstream O2 sensors. Mileage is over 115k.

Quote:
Also, how much oil and coolant has your engine consumed?


Next to none. I check the coolant bottle at least once a week and it barely moves from the line. I'll add maybe two ounces per month to account for evaporation. Oil consumption is one quart per 5000 miles.

Quote:
Had any misfires in the past?


Yes. It had misfires before when the ICM went out. Cylinders 1 & 4 were not getting any spark.

 

[artificialist]

115K means that a new oxygen sensor is needed. Be sure to get one from the same company that made the OEM unit (Bosch, NTK, Denso, whatever) for best results.

 

[Shaman]

I know the Regal will ignore the O2 sensor up until a certain temp. It caused several blown engines. WOT with upped boost without the o2 sensor. I would suggest looking at the O2 like artificialist suggested.

 

[Trav]

Originally Posted By: Shaman
I know the Regal will ignore the O2 sensor up until a certain temp. It caused several blown engines. WOT with upped boost without the o2 sensor. I would suggest looking at the O2 like artificialist suggested.

What caused blown engines? What you are describing is totally normal operation.
Cold engine the ECM is in open loop so it ignores the O2, this model uses heated O2 sensors to speed up the process go get into closed loop ASAP.

Any failure in the heater circuit will trigger a CEL immediately.
WOT, the ECM ignores input from the O2 regardless if the engine has forced induction or not.
Newer wide band O2 sensors are used by the ECM at WOT but not on this 2000 Buick.

Increasing the boost pressure creates a whole different set of problems, most damaged over boosted engines is due to incorrect tuning that allowed the engine to detonate uncontrollably.
Wrong injector size, too wide a gap on plugs, ignition problems, MAF issues are just a few but not the non wide band O2 sensor.

A lazy O2 can cause minor operating issues but if it is significant it will trigger a CEL.
A scan tool can be used to watch the O2 in operation.
If i am working on the engine and it has 80K or more and the job makes the O2 more accessible than normal eg Ford Triton exhaust manifolds, or they are dirt cheap i will swap it/them as preventive maintenance, otherwise no.
I never swap expensive wide bands for laughs.

 

[Shaman]

My understanding was part throttle full boost without "working" sensors caused lean booms. It's been several years since I have read about it.