rough cold idle
- Thread starter beastly1
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JeffBoyette
Full Access Member
The evaporative emissions control system pulls a vacuum on the tank so the fuel vapors do not get out of the gas cap and pollute. It stores them in a charcoal canistor where it waits until the vehicle is under load cruising, at operating temperature, and monitoring all sensors. It then opens a valve to release those vapors into the engine through the port underneath the front of the intake manifold behind the throttle body.
When this system goes bad it pops a number of codes. Most of them are gas cap related. It tried to use the engine vacuum to pull a vacuum on the fuel tank and it is unable to maintain that vacuum. This can be caused primarily the fuel cap, but also any of the hoses from the tank to the intake.
Rarely some of the components in the system will go bad as well causing the vaccum from the engine to pull those vapors all the time, while at idle, while cold, and at wot. But you get a seperate, specific code.
In this case he has driven the vehicle a few times, and has replicated the symptoms during diagnostic. The computer would have already popped a code if there was anything wrong with the evap system and it has not. Id rule it out.
In closed loop all sensors work in harmony like an orchastra! each sensor has a computer controlled range. if one of the parameters is out slightly the computer can adapt and maintain itself. If a parameter or more is requiring adjustment outside of the range of the computer it will do strange things trying to get back to harmony. Changing from an OEM to an aftermarket Throttle body which has a bigger throat will change the harmony . opening or closing the blades , even a tiny bit may be more than the range of the computer can deal with. the only thing you can adjust manually with this set up is the throttle body. With some patients and diligence, you can get it back into harmony ! Do small changes , RESET the computer each time, then assess if the adjustment made it worse or better. continue until perfect harmony!! make sure the blades are open enough so that they don't stick . At some point you will have to decide if you need to change the IAC or the TPS or the MAP or the O2 or the IAT ... good luck! 
There is no better feeling than getting it fixed yourself !!
There is no better feeling than getting it fixed yourself !!
JeffBoyette
Full Access Member
If i was in your shoes,
Check the hose under the front of the intake behind the TB. Make sure it is connected.
Test 1:
i would go out to the truck, unplug the IAC valve connector. The idle will settle, either high, or low, depending on where the plunger was when you disconnected it. See if that settles the idle.....
Test 2: Like in my video plug off the hole the iac hose connects to with your thumb. And get it to stay running with the throttle set screw with your thumb still blocking off the IAC. DO NOT OPEN THE SET SCREW TOO MUCH as stated previously. IMPORTANT:Set the idle with your thumb on over the IAC hose, and it idling low 500-600rpm. This will still put the IAC controlling the air enough to give another 150 rpms.
Test 3: remove your thumb. It should idle high for a few seconds 1000-1200, and then settle at 750. This will prove the IAC is working because the computer is sending signal to the IAC to feed the engine air while you were blocking it off, when you release it, it sees the idle high, then sends signal for it to close. This will prove the IAC is working.
Test 3: After this is done. Remove both cable from the battery and touch them together. Note: it has been said by people smarter than me, that it has to be touch for up to 10 minutes. For sake of argument, theres no reason not to do this.
Crank it up and see if it resolved itself.
JeffBoyette
Full Access Member
There is a preprogrammed closed loop delay, and a open loop delay. However the fuel mapping for each is based on what temperature the vehicle is. I believe ours is 192 or 195 degrees. When you started your truck, it took about 5 seconds for it to start to act up. That was the delay.
If your truck was 192 degrees and does not do this, it is because of the fuel map associated with the readings it receives from the various sensors. But if you are 140 degrees and the surging goes away. It is not because of an open loop/closed loop or the timing delay or maps..... it will be just a physical result of the expansions of the throttle body from heat, allowing more air by the blades.
That said, you can do all the test i listed above while the vehicle is luke warm. It doesnt have to be wake up in the morning cold, but as long as it is below say... 160, the mapping will be the same, as if it was 80.
If your truck was 192 degrees and does not do this, it is because of the fuel map associated with the readings it receives from the various sensors. But if you are 140 degrees and the surging goes away. It is not because of an open loop/closed loop or the timing delay or maps..... it will be just a physical result of the expansions of the throttle body from heat, allowing more air by the blades.
That said, you can do all the test i listed above while the vehicle is luke warm. It doesnt have to be wake up in the morning cold, but as long as it is below say... 160, the mapping will be the same, as if it was 80.
rare03srt10
Full Access Member
If it idles great after it warms up and drives perfect with no CEL then why worry about it. Hell my race car idles horrible when its cold to and i have to hold the gas peddle around 1800 rpm for about 30 seconds before itll even stay alive on its own lol. Once it warms up its great though.....Not like you are gonna drive the truck cold anyways. Just sayin dont fix it if it aint broke right hahaha
When was the last time you changed the plugs? take a shaft and listen to each injector and make sure they are all clicking normal. not missing. I am not sure if the engine temp sending unit has something to do with the cold start but it is cold start related. check the connection at the engine temp sending unit. even when the surging stopped it still seemed to idle rough. make sure all plugwires are connected good. when it is surging stop the engine and pull the plugs. see if yu have any wet ones. keep looking!!! don't let it win!
JeffBoyette
Full Access Member
The engine temp sending unit comes into play after the 5 second idle delay. It helps the ecu determine if it goes into the open loop mode map, or the closed loop map. If it it not connected the ecu quickly identifies it as an open circuit and will pop a code. The coolant temperature "switch" on most vehicles controls the fan and the gauge, and the coolant temperature "sensor" controls the ECU.
There is a circuit in the coil and the ecu that determines if the voltage sent to the plugs completes the circuit. IE: makes a spark. If the voltage never completes the circuit, you will first get a blinking CEL letting you know there is a misfire occuring at that time. After it had identified a repeat problem 3 occations, you will get a solid light. Depending on how it identifies it it could be a random multicylinder misfire, or if its an isolated coil, it will show which plug didnt complete the spark.
The same kind of circuit is also connected to the injectors and monitors amperage draw. If there is an open circuit it can identify it, and If the pintle doesnt move it can identify it by amperage draw. It cannot identify a clogged injector or one with a poor spray pattern causing incomplete ignition. The only method it has to identify this is through o2 sensors measuring the unburned oxygen in the exhaust. It will compensate by enrichening the fuel mixture.
my plugs are new with only may be 500 miles on them,truck only has 4400 miles, wires are on the plugs good , i did this morning cold, i pulled the hose off the intake tube, from the iac, and it started up and had a fast idle at 1000 rpm then settled at 750 with NO SURGE, ran it for 5 minutes at idle like that . sucking a lot of air that you can hear thru the iac, then connected it back to the tube and it still ran fine no surge and idle was the same, not sure what that tells !!!!! what do you think, could it be a bad IAC.
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JeffBoyette
Full Access Member
No sir, that is normal operation. Cant identify a broken part when it works. Whether its fixed or just fixed for now, it seems you no longer have the problem. Keep us updated if it comes back.
Randall488
Full Access Member
instead of intake could this be a exhaust problem? i know the 02 sensors on my 95 are giving me a similar issue, do you have any exhaust leaks or loose connections? i have a warped manifold on my drivers side and it sucks in cold air when the engine isnt warmed over. makes the 02's read lean and it sends more fuel to compensate..
I would open the throttle blades up a bit more to quite the IAC . don't want it out of it's usable range. I think it is all working good. ....must be something else. have you had the intake manifold off ?
yes to do coils and wires but it was doing the surge before that. if it were bad o2's i would get a cel. i'm getting ready to punt!!!!!! going to have to bring it and get it scanned , all i'm doing is going nuts.
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How long has it been doing this?
JeffBoyette
Full Access Member
I thought you described normal operation this morning. Has it surged today?
yeh normal, when i pulled the iac hose off the intake tube, started it and it idled 1000 rpm then setteled to 750 rpm after a bout 10 to 15 seconds , then connected the back to the intake tube and idled normal, i don't think it fixed itself , will have to try when it is cold a gain , all i did was try something different, it seems to me it needs more air, the k&n seems to be very restrictive on giving i iac air. i will try opening up the tb a little more, and see. thx for all your input guys, on trying to figure this out.
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JeffBoyette
Full Access Member
The K&N can be restrictive if it is cleaned by the user, and then OVER oiled. But not restrictive enough to effect your idle. And it wouldnt affect the IAC. 900-1000 rpm is normal.
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