I have a 2004 gmc yukon 4.8. For about 2 months it has gradually lost power. It started on inclines, then it lost power when needing to accelerate to enter the freeway. I owned since 2007. Kept up oil changes, transmission, and radiator flushings, but never did a tune up or changed spark plugs or fuel filter. I never had a cranking problem, it never idled rough, or stalled, just a gradual loss of power until it would not run. I had a mobile back yard mechanic come to the house and checked the OBD. several codes came up. He changed the spark plugs, and wires. It would run then, but it then was really idling really rough. All the codes disappeared except "multiple misfires.". One of the first codes said something about right or left bank "lean". Also before he changed the sparkplugs he, he had notices the plugs were really burnt on one side with white or tan residue and the ones of the other side just had gas on the plugs. He called a mechanic on the phone who said it sounded like a vacuum leak or a problem with intake, possibly needed intake gaskets replaced. I had it towed to another mechanic who replaced the intake gaskets and he said he replaced the Mass air flow sensor. He was able to take the car on the freeway, but says I need to replace the Catilytic Convertors.My car passed smog within the last 6 months. I have never had a stalling or cranking problem. I don't think he changed the fuel filter cause he kind of stalled when I asked him about it and said "oh yeah." How likely is it, that it is the Cat Convertors?
Country: United StatesMake: gmcModel: YukonYear: 2004Engine: 4.8
Hello!My name is XXXXX XXXXX X can assist you with your question.Are you saying that the engine has low power and the computer has flagged the engine as running lean? The mass air flow sensor was replaced correct?What is the fuel pressure at under a load?
The mechanic says he replaced mass air flow sensor. I do not know what the fuel pressure is at under a load.
Ok.. if the mass air flow sensor was replaced and you still have a low power condition, then yes.. I would suspect a plugged converter (or both) or low fuel pressure. Here is the tests to check them both.I would recommend to start by checking fuel pressure. Fuel injected cars are very fuel pressure sensitive. Fuel pressure is very important and must be within specifications! But if the pressure is low, it does not mean the pump is bad. A plugged fuel filter, a pinched line, and low voltage at the pump can contribute to low fuel pressure. All three must be check before replacing the pump! You can get a fuel pressure gage at a local auto parts store for cheap. Fuel pressure testing is frequently overlooked but it is one of the common causes of poor performance. The test procedure must include testing that will insure normal fuel pressure under all operating conditions.Key Factors1. Fuel demands are low at idle.2. Wide open throttle at high engine revs uses the most fuel.3. Fuel pressure must be reduced when manifold vacuum is high. Vacuum pulls fuel out of the injector causing it to deliver too much fuel if the pressure remains high. Manifold vacuum is used to open the pressure regulator at a lower pressure when vacuum is high. A good way to check your fuel pump is to run an amp test on the fuel pump while it is running or while battery voltage is being applied ot it (turning the key on, cranking your vehicle or even when your engine is running. The amount of amps your fuel pump will pull will depend on how much fuel pressure your fuel pump should supply to your fuel rail. A good rule of rule of thumb for this test is as follows:14 to 15 psi 3 to 4 amps30 to 45 psi 4 to 7 amps45 to 50 psi 7 to 8 amps50 to 70 psi 8 to 12 ampsTo check for a plugged converter, you would need to use an exhaust back pressure checker. You can get this tool for any local auto parts store or even on amazon.com. You would screw it into place where the front o2 sensor goes (after removing the o2 sensor of course) and taking a reading of the back pressure with the engine running. NO more than 1.5 psi at 2500 rpm in park.Here is a link to this toolClick herePlease don't forget to rate my answer so I get credit for assisting you today. Please take a moment to get familiar with our new rating system. This is very important and this affects me greatly. There are 5 ratings that are listed below. Please rate my service as number 5 "great service" for providing you timely accurate information specific to your vehicle or use the reply button to continue the conversation. 1) I expected more-------------Does not pay me, and means you had a negative experience2) Helped a little ------------Does not pay me, and means you had a negative experience3) Got the job done------------Pays me and means you had a positive experience4) Informative and helpful-----Pays me and means you had a positive experience5) Great service---------------Pays me and means you had a positive experienceAgain, if you have any additional questions, please feel free to reply by using the "CONTINUE THE CONVERSATION" button.Best RegardsAmedee
How likely is it that it is my cat convertors. This would be very expensive if it is not the problem. I am not a mechanic. I am a 64 year old widow. Is this going to be trial and error? Should themechanic be able to say for sure that cat convertor are bad and this will solve the problem of misfiring??
Hello!IF the fuel pressure checks out ok (this needs to be checked before replacing anything), then I would strongly suspect a plugged converter. YES.. this is why I provided the test for you to check the converters before replacing them. Or... you could have someone else run the test. Feel free to print this information off and bring it with you to a garage to help them.I hope this helps!Please don't forget to rate my answer as "Great service".RegardsAmedee
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