Open Thermostat...idle surge?

[Wringer]

Can a thermostat that is stuck open cause Idle surge?

 

[kmj 4g63t]

Not that I know of it will cause your car not to reach operating temp. Most of the time it's the idle speed controller causing the surge. Also bent valves in head, throttle cable to loose etc.

 

[vandenberg33]

The coolant temp sensor controls timing so if it doesn't reach operating temp then the ecu can't reach closed loop operation. So the car is running off of a set fuel map in the computer and the air flow sensor instead of using the 02 sensor which is used in closed loop to constatly adjust the air fuel mixture.

In short yes it can cause a surging idle.

 

[Wringer]

I just had my head rebuilt, but would a compression check tell me if it could be bent valves? Its not the ISC or any of the other typical idle surge issues, I have checked.

The coolant temp sensor controls timing so if it doesn't reach operating temp then the ecu can't reach closed loop operation. So the car is running off of a set fuel map in the computer and the air flow sensor instead of using the 02 sensor which is used in closed loop to constatly adjust the air fuel mixture.

In short yes it can cause a surging idle.

This makes sense to me, sort of. But, I find myself asking why every car wouldn't surge before reaching operating temp.

 

[92AWDHX40]

This should help you out. Terry's Talon Troubleshooting Tips #3 - Fast Idle (aka. Idle Surge)

 

[Wringer]

So, I have done pretty much all of that stuff the. I went to properly adjust the idle with the BISS screw and grounding the plug by the battery resulted in a fried wire?

 

[Colt4G63 also]

You said you pretty much did all the stuff, but did you change the thermostat?

Here's step one of adjusting the BISS: "With the engine running and hot (temp>=180), pop the rubber plug off the front of the TB to expose the base idle speed set screw (BISSS). "

If your engine doesn't warm up, you're chasing your tail.

 

[vandenberg33]

To answer your question Wringer, when a car is cold it is not using all of its sensors because conditions both in the engine (thermal expansion) and sensors (Oxygen sensor need to be heated to 650degrees) have not reached operating temperatures. So open loop is a fuel map programed by the factory for the idle speed the timing and the fuel map and uses the air flow sensor and tps to ruffly adjust for idle quality and throttle.

When the engine is warm the rings have expanded as well as pistons and the engine will need a different tune than the cold start tune the factory programmed into open loop for startups. The purpose of the thermostat is so the engine reaches optimal operating temperature for the pistons and rings to seal as well as gasket surfaces and for fluids to reach the right viscosity for the best performance and reliability so if you take the thermostat out you have just removed all the hard work the factory did the build a good performance engine.

 

[steve]

This makes sense to me, sort of. But, I find myself asking why every car wouldn't surge before reaching operating temp.

Idle surging is 99% of the time causes by too much air getting past the throttle plate and causing the idle speed to rise.

The ECU looks at the engine RPM when the throttle is closed and has a table of RPM values indexed by Engine Coolant Temp of the lower bounds for turning the injectors back on when the engine decelerates. If the RPMs are above that value the ECU turns the injectors off for emissions and fuel economy and once the RPMs drop it turns them back on so the engine doesn't stall before settling into a normal idle. Since your car doesn't settle into a normal idle the RPMs rise and once above the threshold get turns off again. When it repeats over and over you get surging.

Because the car is designed to idle higher when cold the RPM trigger points vary by coolant temps.

It doesn't take much extra air to cause idle problems. Think about how little the BISS actually moves when you adjust it and how quickly the RPMs change. Or how little you have to open the throttle when still to get the engine to race.

A dirty throttle body can cause the ISC or throttle plate not to close all the way. Over tight throttle cables can do the same for the plate. A bad PCV or EGR valve can leak too much air as can bad injector seals, vacuum lines and gaskets.

 

[Wringer]

Thanks for the info guys. I have cleaned the throttle body, checked the ISC, replaced the 2 throttle body gaskets, blocked the EGR, replace the lower injector seals when I put the new head on, deleted my vacuum lines, and my throttle plate closes all the way when my foot is off the pedal. This wasn't an issue until after putting my head on, which is when I put the new thermostat in as well, which I have noticed seems to either be stuck open or opening far too soon. Guess, I will replace the ISC and BISSS o-rings next.

 

[Wringer]

Well, I did the RRE FIAV delete to no avail, so I decided to take a peak at my ECU. Some pretty decent leakage damage. Hopefully, getting this taken care of will be the end all of my idle surge.

BTW my thermostat was stuck open, changing it did nothing.

 

[jas510]

The coolant temp sensor controls timing so if it doesn't reach operating temp then the ecu can't reach closed loop operation. So the car is running off of a set fuel map in the computer and the air flow sensor instead of using the 02 sensor which is used in closed loop to constatly adjust the air fuel mixture.

In short yes it can cause a surging idle.

My engine check light came on, found out the code was related to my coolant temp sensor. During stop and go traffic my idle creeps up to two 2k. I'm getting my coolant temp sensor fixed today. Im hoping fixing the coolant temp will fix my idle surge problem.

thanks for the info.

 

[jas510]

Nope that didnt fix it!! I will clean out my TB tomorrow and see how that goes

 

[jas510]

Cleaned TB and found out that I do not have a thermostat. Maybe installing a thermostat will fix my idle creep.

 

[jas510]

cleaned out the TB, replaced thermostat, and screwed in the BISS screw (which the previous owner had tampered with)

Idle surge fixed!

Yayyy..

noW FOR SMOG...UGHH