Could somebody please explain to me what is meant by open loop and closed loop? I had never heard these terms until I joined this forum

 

It is a common computer car "thing". Once the engine and the O2 sensor warm up, the ECM "reads" the oxygen content of the exhaust, via the O2 sensor to determine if the air-fuel (A/F) ratio is rich or lean. It uses that info to readjust the mixture to the proper ratio. This goes on continually and many times per second. THAT is closed loop. Open loop is where the engine runs off a set of preprogrammed values in the ECM, before things get up to temp. That is a very basic explanation, but it covers the jest of the question.

RACE ON!!!

 

Thank you. So if my drive to work is so short that I never get up to temp (whatever degree that might be), my engine will remain in open loop the whole time? Any problems with this?

 

CFI-EFI covered the basic concept. The coolant temperature value which triggers the switch from open loop to closed loop depends. On a winter cold start my '95 went into closed loop at 139.2 F (evident on a Datamaster scan recording I made once). On a warm day that trigger temp will be lower, and on a re-start where the engine has residual heat left over from prior run the trigger coolant temp will be lower. Thus the myth about a 160 thermostat preventing your car from entering closed loop is BS, at least on an LT1 (but I still wouldn't use a 160 tstat because it increases cylinder-to-ring wear).

During closed loop operation, the air/fuel ratio that the PCM shoots for is 14.7:1 which is the optimal air fuel ratio for part-throttle operation. When you nail it, i.e. go to 100% throttle (WOT or Wide Open Throttle) the PCM goes into PE mode (Power Enrichment), which is similar to open loop but not quite the same thing.

In PE mode the air/fuel ratio is read from tables in the PCM and the ratios are richer than the part-throttle 14.7:1 ratio, otherwise it would burn a valve if it was that lean in PE mode. Your car can go into PE mode at throttle position as low as, I think, 66% depending on some other factors. I'm not sure on the 66% but I seem to remember it being mid to high 60's. I haven't looked at PCM tables in a awhile.

 

Your coolant only needs to get to around 145deg F after a few minutes before the ECM will go into closed loop mode.

There is no problem with it running in open loop mode as it warms up.

It also goes into open loop mode when you go to wide open throttle (WOT). Here it still recors the O2 sensor values but ignores them and uses preset tables to control fuel flow via injector pulse width.

 

The problem with this pattern of operation is that you are not getting the engine to peak operating temp and keeping it there long enough to remove all the water vapor from your engine internals. If you are running conventional motor oil in the engine this will cause the probability of sludge build-up within the engine to increase. But since you are likely running Mobil 1 synthetic sludge build-up and deposit formation should not be a factor even with the moisture staying in your crankcase long-term which it probably is. However, the combustion by-products produced by the engine that enter your oil can combine with the water to form sulphuric acid, among other nasties. Very smal amounts of it, though. But this underscores the need to religiously change the oil every 3000 miles or 3 months when operating the engine in this pattern of short-hop trips, to remove the contaminants from your engine on a regular basis before they can build up and cause pre-mature wear to begin to set in.

 

Lone Ranger, you posted that running a 160 deg thermostat will lead to cylinder to ring wear. Please explain to me how this will occur. Will the oil lose some of its protective qualities at a lower temp? I truly don't understand. As I currently run a 160 deg thermostat I am very interested and will pull it out ASAP if I need to. Thanks.

90Indy

 

I've run a 160deg T-stat & fan switch for over 4yrs (now 144k miles), even in SE NY winters, and don't worry about excessive cylinder wear (same compression this year as last year) but do enjoy the extra HP.

I run a full synthetic 5W-40 motor oil that gets changed every 3mo/3k miles or sooner if darkens too much to suit me. Modern motor oil contains anti-oxidents, dispersants, detergnets, anti-acids, etc. which lube and protect the engine till they are depleted.

The moral is that if you want the extra HP from a low temp T-stat don't cheap out on oil changes...as always, power costs $$.

 

Good explanations.I prefer the 180 though.

 

The compromise with a 160, or too cool stat, is mostly in the warm up. As 65Z01 states, the engine WILL make more power (slightly) when it is cooler. Most of these cars will run at 190*, or above, under daily driving conditions, regardless of the stat. All the lower temp stat accomplishes in THAT case is to prolong, or for short hops prevent, the proper warm up of your engine. That leads to all the complications cited, above, by others.

RACE ON!!!

 

The amount of knowledge you guys have is truly amazing. I never read discussions in other forums that provided this much information. It's enough to make a stupid man scream!!!