Huntiung Idle

[Mjohle194]

Hey Guys,

Really need some help on this one from some of you sonic experts. My sonic has a hunting idle. Maybe 25-35 rpm, but its noticeable, I just replaced the intake manifold which cleared it up alot, the red check valve had come out of the original one, once I installed the new manifold it cleared up quite a bit, but still there. I'm an ex tech, but sold my nice snap on scanner, so I dont have the ability to diagnose this thoroughly, I just have a cheapo scanner, which nothing shows out of the ordinary. I also just notice that at idle sometimes my mph shows 1 instead of 0, do you think a faulty vehicle speed sensor could be affecting the idle condition? I'm not sure if this would affect the can bus network GM uses. The car has 130,000 miles on it so i know at that point it could be anything, but I' getting normal scanner readings, no check engine light, and drive ability is fine. Any input would be great.

 

[dbratboy92]

I don't think the speedometer reading is related mine has done the same thing for the last 50k miles. The surging idle sounds like a symptom of a bad Check valve in the intake, yes I realized you already replaced it. Did you also replace the valve cover. When your intake goes the valve cover will fail as well. If you did check for any kind of vacuum leaks at the valve cover and the intake. I have had the valve cover gasket fall out a little when I was installing before causing a vacuum leak. But the intake and valve cover need to be replaced together. If all of that is good my best guess assuming no defective parts is your idle hasn't relearned yet. The intake comes with a new throttle body and if the idle wasn't relearned with a scan tool it can take some miles of driving before the car will relearn. The proper throttle plate position. I hope this helped. If you come up with any codes I may be able to offer more insight.


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[carsNskis]

idle is when you are stationary. Maybe surging when trying to maintain a certain speed? Could be the Wastegate actuator, which causes boost flutter

[emoji593][emoji809][emoji472]

 

[dbratboy92]

idle is when you are stationary. Maybe surging when trying to maintain a certain speed? Could be the Wastegate actuator, which causes boost flutter

[emoji593][emoji809][emoji472]

It sounded like he was talking about it happening when stationary. If your not moving or on the gas your waste-gate should be open.


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[carsNskis]

It sounded like he was talking about it happening when stationary. If your not moving or on the gas your waste-gate should be open.


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I misread the rpm for mph....

Idle still isn't 2500 to 3500 rpm

[emoji593][emoji809][emoji472]

 

[dbratboy92]

I misread the rpm for mph....

Idle still isn't 2500 to 3500 rpm

[emoji593][emoji809][emoji472]

I was assuming he meant. 25 to 35 rpm fluctuation, which doesn't seem like much but when you consider how accurate the tach probably isn't. It could be a lot more of a fluctuation than it is reading. Similar to the car compensating for a vacuum leak. Why I was asking if he replaced the valve cover with the intake.


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[carsNskis]

I need more sleep lol

[emoji593][emoji809][emoji472]

 

[Mjohle194]

Thanks for the feedback guys. So hears where I am so far, new intake manifold & new PCV hose that goes from the manifold to turbo inlet, both items were bad. Manifold was missing the orange/ red check valve and pcv tube was cracked. Still have a dam surge. As for the valve cover, its not making the hissing sound out of the vent, i replaced that about 6000 miles ago. The intake I ordered did not come with a new throttle body or injectors so Im thinking it could be the throttle blade shaft,which ive seen develop a leak on the LS motors, maybe a sticking injector, or possibly even coil related? I have a boost leak tester on it way, hopefully that will help me find the vacuum leak. Im wondering even if the intercooler is cracked? has anyone had an issue with that before?

 

[cuartz7o]

Have you tried searching for vacuum leak using carb cleaner? With the engine cold and running spray carb cleaner near all your vacuum/boost/pcv hoses, manifold, intake piping and listen if the car responds or stalls. Be safe and have a fire extinguisher near by just in case. This is why cold start is safer for this, hot engine can catch on fire

 

[Maddmatts]

The cam cover doesn't always make a hissing sound when the relief valve is ruptured. Stick your finger over the hole to see if it's sucking in air.

 

[dbratboy92]

Have you tried searching for vacuum leak using carb cleaner? With the engine cold and running spray carb cleaner near all your vacuum/boost/pcv hoses, manifold, intake piping and listen if the car responds or stalls. Be safe and have a fire extinguisher near by just in case. This is why cold start is safer for this, hot engine can catch on fire

Don't use a fire extinguisher on your engine. The stuff inside will put out a fire but it is very corrosive and can cause a mess of damage. If you are using carb cleaner or starting fluid to check for vacuum leaks use a very small amount at a time in a single location so if it catches fire for can extinguish it by smothering with a rag, simply blowing it out, or water. (Yes I realize what happens when you put water on an oil based fire, however if you either douse it completely or another it with a fine mist at the source you can put it out.) have a fire extinguisher as a last resort. All that said a much safer way to check for vacuum leaks is take a 1lb propane torch, and some rubber hose like fuel line that fits over the nozzle. Turn the gas on but don't light the torch. With the engine running direct the end of the hose near any vaccine lines and connections after the MAF sensor and pay attention to the idle the same as if you were using starting fluid. If you have access to a scan tool that shows live data you can watch your short term fuel trim and O2 sensor data. If the fuel trim goes negative the engine is compensating for a rich condition by taking away fuel, if it is positive it is adding fuel to compensate for a lean condition. Your ideal fuel trim is 0. If you have a vacuum leak it will be positive. When you direct extra fuel to the leak, by either carb cleaner, starting fluid, propane the trim will drop back to zero or into the negatives. If your fuel trim is zero + or - 2 or 3% you probable don't have a vacuum or boost leak. If it is positive and adding fuel you more than likely have a vacuum Leak clogged injector or an exhaust leak before your first O2 sensor. This needs to be checked when the fuel trim is in a closed loop. Side note if you have a negative fuel trim you more than likely have an ignition related misfire. Contrary to popular belief a clogged air filter won't cause your engine to run rich because the initial fuel air mix is calculated off the MAF data a clogged air filter would just cause less air to pass the MAF and the ECM to add less fuel.


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[dbratboy92]

Thanks for the feedback guys. So hears where I am so far, new intake manifold & new PCV hose that goes from the manifold to turbo inlet, both items were bad. Manifold was missing the orange/ red check valve and pcv tube was cracked. Still have a dam surge. As for the valve cover, its not making the hissing sound out of the vent, i replaced that about 6000 miles ago. The intake I ordered did not come with a new throttle body or injectors so Im thinking it could be the throttle blade shaft,which ive seen develop a leak on the LS motors, maybe a sticking injector, or possibly even coil related? I have a boost leak tester on it way, hopefully that will help me find the vacuum leak. Im wondering even if the intercooler is cracked? has anyone had an issue with that before?

Is it an after market manifold? I wasn't aware they existed yet. Where did you get it and what did it cost. If it is an AC delco manifold you got ripped off. They come loaded with a new fuel rail, injectors, throttle body, MAP sensor, purge valve, and some other various sensors that to be honest I don't know what they all do because we never replace them alone at the dealership. With how often the intakes go bad the other parts aren't on the car long enough to fail, except the purge valve.


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[Mjohle194]

Is it an after market manifold? I wasn't aware they existed yet. Where did you get it and what did it cost. If it is an AC delco manifold you got ripped off. They come loaded with a new fuel rail, injectors, throttle body, MAP sensor, purge valve, and some other various sensors that to be honest I don't know what they all do because we never replace them alone at the dealership. With how often the intakes go bad the other parts aren't on the car long enough to fail, except the purge valve.


It wasnt an aftermarket manifold, it was Delco. My dealer gave me the option with or without the accessorys on the manifold, I think it was $175 opposed to $300 with throttle body and sensors.

Now that Ive checked for boost & vacuum leaks I know thats not the issue, I did a pressure test on the turbo and intercool and smoke test on the manifold, also sprayed it with carb clean to see if I could get it to react, got nothing. I finally had a chance for someone to drive it around while i looked at the scanner and my long term fuel trim is -4 to -7% all across the board, so Im hoping it is a sticking injector or maybe even the coil failing, but no check engine light or pending codes. Im going to do ohms reading on the injectors and see if they all match up within .5 ohms

 

[Mjohle194]

Well that was a fail, all the injectors spec'd out 13.8-14.0 ohms, so i guess injectors arent the issue, also checked valve cover again, no suction coming from the port on top.

 

[cuartz7o]

Is it an after market manifold? I wasn't aware they existed yet. Where did you get it and what did it cost. If it is an AC delco manifold you got ripped off. They come loaded with a new fuel rail, injectors, throttle body, MAP sensor, purge valve, and some other various sensors that to be honest I don't know what they all do because we never replace them alone at the dealership. With how often the intakes go bad the other parts aren't on the car long enough to fail, except the purge valve.


It wasnt an aftermarket manifold, it was Delco. My dealer gave me the option with or without the accessorys on the manifold, I think it was $175 opposed to $300 with throttle body and sensors.

Now that Ive checked for boost & vacuum leaks I know thats not the issue, I did a pressure test on the turbo and intercool and smoke test on the manifold, also sprayed it with carb clean to see if I could get it to react, got nothing. I finally had a chance for someone to drive it around while i looked at the scanner and my long term fuel trim is -4 to -7% all across the board, so Im hoping it is a sticking injector or maybe even the coil failing, but no check engine light or pending codes. Im going to do ohms reading on the injectors and see if they all match up within .5 ohms

well done, keep looking. I'm having issues with my larger 52lb injectors and am doing everything you are to rule them out. When I revert back to stock car runs fine. 4th time swapping too. I'm going to do the same measuring the resistance tomorrow.

 

[dbratboy92]

Well that was a fail, all the injectors spec'd out 13.8-14.0 ohms, so i guess injectors arent the issue, also checked valve cover again, no suction coming from the port on top.

Just because the measure correctly doesn't mean they aren't clogged. If you can't do an actual fuel injection service maybe try some fuel treatment. Sea foam and Lucas are always a good try. Are your plugs stock? If it is running lean obviously they are burning the fuel but are they burning too much or burning it too hot. I find it's usually best to avoid changing from an oem plug in any car, unless you have some legitimate modification somewhere that requires it.


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[dbratboy92]

Just because the measure correctly doesn't mean they aren't clogged. If you can't do an actual fuel injection service maybe try some fuel treatment. Sea foam and Lucas are always a good try. Are your plugs stock? If it is running lean obviously they are burning the fuel but are they burning too much or burning it too hot. I find it's usually best to avoid changing from an oem plug in any car, unless you have some legitimate modification somewhere that requires it.


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Never Mind I read it again it is running rich, whack your plugs. Idk how long it has been since you put the manifold in but it might not be a bad idea to do a hard reset on your computer. Try disconnecting both battery terminals and and touching them together for about a minute or so. And the. Put it back together and run it around for a while for everything to relearn and reset. The car might not have adjusted to a change caused when replacing the manifold


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[Mjohle194]

I havent done the ecm reset yet, but today I did notice something I really didnt like, which leads me to believe maybe I do have a bad injector or maybe even a failing maf that wont throw a code yet. What I saw was my long term fuel trim jogging around at idle, Ive never seen that before. It took roughly 2 minutes to start at -7 then it would creep up to +5, them creep back down again, it would just keep doing this in a never ending loop. Does anyone know what the MAF reading should be at idle in neutral?

 

[Mjohle194]

O and i forgot, my plugs are new, I dont remeber the part number, but it a stock acdelco replacement gapped at 0.028

 

[dbratboy92]

O and i forgot, my plugs are new, I dont remeber the part number, but it a stock acdelco replacement gapped at 0.028

How did you gap them? Not being a dick but most people gap plugs wrong, including myself for several years as a professional tech. And your not really supposed to gap platinum plugs.


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