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06 Cobalt LS. 98K. Recently (within the last month) had scheduled oil change. About a week ago, car developed a small tapping noise under acceleration. Deceleration and idle produces no audible noise. No loss of power. Car idles at about 500 rpms. Noise does not start until close to 1500 rpms and will be heard from 1500-2500 range in the first three gears. High gear, it can only be heard below 2500 rpms. Noise will rise in pitch and tempo along with the rpm range but goes away once it passes 2500 rpms in high gear. Check engine light goes on and off on it's own. Had the codes checked. Came back with knock sensor and engine lean sensor. Replaced air filter and cleaned the mass air flow sensor. No change. Could this be lifters, or could this be the beginnings of a serious rod bearing issue. Budget is tight so I'm trying to do the less costly fixes first if possible. Brother-in-law is former mechanic and he thought it was a dirty filter or MAF sensor since the noise could not be heard at idle and there was no loss of power. Also he felt that way because it only occurred during those rpms. Since new air filter and cleaning of MAF sensor engine light has not come on but noise remains the same.
 

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If it were rod knocking, it occurs even at idle. That is rather low for idle on any car. If your car is in the lean fuel setting, sounds like a Heated O2 sensor or catalyic converter is about toast.

Since your car is in "open loop" status, your PCM is on running on KAM (Keep Alive Memory) (learning this now at school *UTi*). That is whats causing your car to idle a bit low, but its doing its operation once detected a voltage malfunction through a sensor. That tapping could be either a cam knock sensor or crankshaft knock sensor. I cant pinpoint without actually mechanically diagnosing the car, but for Generic OBD II scanners, it will point you towards what system(s) are affected.

I hope this helps and you should have it checked out by a GM certified technician, not a mechanic.
 

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Chevy Customer Service
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Welcome to the forum, savy_04.

I see that you already are working with somebody to get this checked into - are they affiliated with a GM dealership? It's okay if not, but if they are and if you want for me to check into anything as a GM Customer Service Representative, please let me know!

Sarah, GM Customer Service
 

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Thanks for the welcome. To answer the last question, it does not change even at optimal operating temp. It doesn't get better or worse the longer the car runs. And based on temp gauges it is not overheating.
 

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Levee
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Not a rod bearing or it would continue that sound throughout the entire rpm band. Have the car running and have a friend help you by keeping the car above an idle and listen. Where is the sound coming from? Is it a deep sound like a diesel or a higher sound like a lifter? More info on the sound is needed for more diagnosis.
 

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Well I have an update. Went to the grocery store today. Car sounds the same but now I can hear a deeper knocking at idle after the car reached its destination. When I left the store the idle knocking did not exist but upon my return to the house the idle knocking could be heard. It's odd because when going through the rpm's it sounds like a lifter. But when the sound is made under idle, it sounds deeper like a rod. Car hasn't lost any performance but was wondering what my options would be? Should I start searching at junkyards and online for used engines now? If I'm hearing a deeper knocking sound at idle once the car has been running is it a signal that the end is near?
 

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i just thought of something... does your car start getting hot at all? over 220? i think your balance shaft chain tensioner may have failed you and your inner timing chain is slapping everywhere. when you pull your valve cover, also pull off your water pump access cover on the front cover (too many covers...) and stick your hand in there to see if the chain in there is loose too.
 

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Thanks for all the advice. Took off the valve cover this morning and the chain was tight. Rocker arms were tight as well. Next time I drive it ill see if it runs hot. Spoke to my brother-in-law (mechanic) and he feels its a bearing (his original guess after cleaning the air flow sensor and a new air filter). Is it possible to check the rod bearings by dropping the oil pan for this engine and if so is it possible to do without pulling the engine? He can do it but wasn't sure if its possible. If it is a bearing he recommends just getting a used engine because he can put one in. Said getting new bearings and machining everything that goes with that would be the same price. Plus his labor would be cheap. Carton of cigs and a 6 pack lol.
 

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