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Dr. Hamman
Dr. Hamman, Chevy Technician
Category: Chevy
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Experience:  30 Years Experience repairing, customizing, and building chevys.
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truck runs rough at idle, it runs fine at speed

Resolved Question:

 my truck runs rough at idle, it runs fine at speed.
Submitted: 2 years ago.
Category: Chevy
Expert:  Dr. Hamman replied 2 years ago.

Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. At Just Answer the deposit you pay is not payment to the Expert, I only get paid if you click on accept, we receive no other pay. I don't know if you are a pro or a novice so we may have to fine tune the answer. I only know as much about your problem as you have told me in your post, so my answer will be based on what you have posted. Feel free to add any additional info you feel is needed. If you are satisfied with the answer, and feel you can leave a positive feedback please click on accept. I don't get paid unless you do.


The first thing I would do is to check for vacuum leaks, they are the most common problem that gives this type of symptom. The injectors can be dirty and cause the same symptom. This can show up at an idle just like a vacuum leak, sometimes some fuel injector cleaner in the fuel can clean up dirty injectors, so that is worth a try. From there I would check the codes,in the on board diagnostic system and see if the computer has detected any problems. You should also look at the normal tune up items like the plugs, and filters. If you still haven’t found the problem after all of this, I would suggest performing a compression test on it to see if you had any problems like burned valves. Burned valves will give this exact symptom. If you have any more questions I am here to help. Good luck, and Thanks for using Just Answer.

I hope this helps, 100% satisfaction is my goal. Just Answer is here to serve you. If the answer is not clear, please let me know what additional help you need, and I will assist you further. When you're satisfied, click on the green accept button. I don't get paid unless you do. A bonus and positive feedback are always appreciated, good luck and Thanks.

Customer: replied 2 years ago.
it had a complete tune up, compression is fine in all holes, it is throttle body injected the only thing i have not changed yet is the fuel filter.. someone told me they thought it was the cat but wouldn't it run worse at full throttle with a bad cat?
Expert:  Dr. Hamman replied 2 years ago.

Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. At Just Answer the deposit you pay is not payment to the Expert, I only get paid if you click on accept, we receive no other pay. I don't know if you are a pro or a novice so we may have to fine tune the answer. I only know as much about your problem as you have told me in your post, so my answer will be based on what you have posted. Feel free to add any additional info you feel is needed. If you are satisfied with the answer, and feel you can leave a positive feedback please click on accept. I don't get paid unless you do.

A bad cat will normally run good at an idle, and act up at higher engine speeds. If the fuel filter was dirty/bad it should act up at higher speeds too, and idle well. I would put some injector cleaner in it, and check for vacuum leaks. After the injector cleaner change the fuel filter. Because the injector cleaner can clean some deposits out of the tank and the filter will catch them, of course it will clean the injectors too. There is no need to put any junk into a new filter, so run the cleaner first, then put a new filter on, so you will have a clean filter. To check it for vacuum leaks get a can of WD40 and spray it all around the intake, throttle body, and vacuum lines. If you hit a vacuum leak the engine will speed up for a second or two. This will allow you to find nearly impossible to find vacuum leaks. I would like to see you check the fuel pressure too. If the pressure is off, it will show up the most at an idle. Also be sure to plug in and check for trouble codes. Most auto parts stores will check the codes for no charge. It helps them sell parts, and promotes customer loyalty. The on board diagnostic system might have some info recorded that will help you track the problem down. If you have any more questions I am here to help. Good luck, and Thanks for using Just Answer.

I hope this helps, 100% satisfaction is my goal. Just Answer is here to serve you. If the answer is not clear, please let me know what additional help you need, and I will assist you further. When you're satisfied, click on the green accept button. I don't get paid unless you do. A bonus and positive feedback are always appreciated, good luck and Thanks.

Dr. Hamman, Chevy Technician
Category: Chevy
Satisfied Customers: 6047
Experience: 30 Years Experience repairing, customizing, and building chevys.
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Dr. Hamman
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