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  1. #1

    Default High Mileage Colorado

    I have a 2005 Z71 Colorado Crew Cab with 148,000 miles on it. I have taken good care of the care with all tune ups on time and I run Mobile 1 oil only. The gas mileage has taken a nose dive and the engine is running rough. Also my engine light and ABS light is on. Before I take it to the dealer for service has anyone had the same problems? I always take it to a dealer for service but I want to get an idea of what I may be getting in to so I don't get my wallet cleaned out. Its a great truck and I want to keep it until it no longer runs. This is my 3rd chevy that I have put many miles on and it seems like these problems always happen at high miles. I have been looking at different forums online and I can do a few things first like change filters and plugs and clean the mass airflow sensor. I'm going to do that and see what happens. Next will be the auto parts store for a code scan and see what comes up if there is no change.

    I called GM and spoke to a customer service rep named Gina. She told me about a known issue with the exhaust valves and GM issued a dealer bulletin about it. GM extended the warranty on the parts to 100,000 miles. I never received this information at home. It would have been nice to know this 48,000 miles ago. I was told to take the truck to the dealer and have a diagnostic test performed to see if this is the problem. She then told me if there is a problem with the head GM and the dealer may be able to work out something to help me. If GM knows there is a problem with a component of their vehicle it should be taken care of as a recall. I made this statement to rep and I was told that she understood what I was saying but thats the best they can do.

    So now the adventure begins and I'll keep posting my progress.

  2. #2


    Could be a bad cat. If the light stays on then its a emission issue, if the light blinks on and off then its a engine/trans issue that needs immediate attention before damage occurs. I'm betting on a bad cat.

    2005 Chevy Silverado Z71 Crew Cab
    6" Skyjacker lift w/Edlebrock dual front remote resoviour shocks, single rear remote. Rancho skid-plate.
    17" x 9" M/T Classic Lock Wheels, 35" BFG A/T or M/T tires
    4.10 PercisionGears w/factory locker, T/A Performance rear-end cover
    Banks CAI intake, ported throttlebody, JBA coated headers, over-sized Magnaflow cats, Magnaflow cat back exhaust, Diablo tuned
    B&M aluminum deep transmission pan
    Warn brush guard w/Warn 9.5 XP winch, PIAA bulbs, Line-X bedliner, AWS Black tool chest, Extang tonneau cover
    Weather Tech floor mats
    Cover-Craft seat covers
    Dash mat cover
    Formula One window tint
    One black Lab in rear seat.

  3. #3
    stephan's Avatar
    Join Date
    Sep 2010
    Far West, Oregon USA,
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    Hey Steamer, since you're going to be replacing your own plugs you can do a compression check at the same time & this will tell you if your engine has the "exhaust valve" issue that the GM rep was talking about, & you will know ahead of time if they're going to be getting into your wallet a little or a lot. X2 on replacing your own filters. A dirty/plugged air filter will definitely bring your MPG way down.
    1988 Chevy C-3500 2wd (no pic)
    350 c.i. 5.7 L Stock Block, 4 Bolt Mains
    L-31 Vortec Heads, Edelbrock Cam & Intake,
    Holley 650, Flowtech Headers, Magnaflow exh.
    Jet Trans 700R4, B&M Ratchet, 4:10 gears,
    3" susp. lift kit "shadetree"
    No rev limiter, No speed limiter lol

  4. #4


    OK. I went to the autoparts store and plugged in the OBDII reader. I got several codes associated with emissions. They are - EVAP vent solenoid valve control system, Evaporative emission system leak detected, idle speed low.

    The guy at the store cleared the code and the engine light went out. After about 70 miles its back on. How do I locate a leak and what could be leaking?

  5. #5


    Could be a bad cat. If the light stays on then its a emission issue

  6. #6


    Evap system leak is usually the gas cap seal gone bad. Remedy is a new gas cap.

    If you have replaced the plugs and filters (air and fuel) and cleaned the MAF but still get bad fuel economy, try replacing your upstream o2 sensor.

    I have a 2005 with 169k miles and still get good mileage.
    2005 Chevy Colorado Crew Cab ZQ8 Xtreme

  7. #7


    I concur on the Gas Cap being the source of your EVAP codes. I had the same issue on my 04 Crew Cab I-5. Also, your low idle code is probably caused by a dirty throttle body. I have this issue all the time with mine due to the conditions we operate the truck in. It is not something a fuel additive will prevent. You have to remove the resonator box from the throttle body and clean the throttle shutter and the TB where the shutter closes. I use a good non chlorinated carb cleaner and a lint free rag to do mine. For some reason the TB builds up carbon around the throttle shutter and causes it to not fully close at idle. I will bet that if you watch your tach when the engine is running rough you will see that it is below 700rpm. The I-5 has to idle at 750 or above to stay smooth. You probably only have the rough condition when you are NOT driving above 25mph right? I was having all sorts of issues with mine and after speaking with my dealer, cleaning the TB cleared them all up. I have to clean mine about every 3 months and my mileage stays around 21-23mpg with 181,000 miles on the truck.
    Brent Seelman
    Piedmont Push Truck Association

    2004 Z-71 Crew Cab Colorado

  8. #8


    I changed the plugs for my own sanity but I cleaned out the throttle body and the truck runs like new. Such a simple thing and yet its so important!! I took the truck to Florida and driving on I-95 at 70+ I still got 20 to 21 miles oer gallon. I'm happy how the truck runs, I know have 170,370 miles on it and its going strong. I have been using Mobil 1 oil since new and I am wondering if I should stay with the current 5W30 oil or go with a heavier grade. No smoke and the truck does not burn it either.

  9. #9


    Steamer, I would not change viscosity on your oil. Even with some wear in the engine, the passages and oil galleys in the engine are designed for a certain fluidity and a heavier viscosity oil will change the flow characteristics. Stay with the 5W30 and if you feel the need (I use full Syn oil in mine also) use an oil additive with it. My Dad was an Engineering tech for GM and was involved with the development of the I-5 engine so I have a little inside knowledge of that engine platform. ;)

  10. #10


    205,000 + miles and running well. Only issue is my engine light comes on for coolant temperature yet the truck is always in the same range from when it was new. Any thoughts?

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