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2000 Chevrolet Silverado 1500:
12-14 MPG in Town, and a Miss

By
Alex Steele
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seen in Truck Trend

Question:

 

My vehicle is a 2000 Chevy Silverado 1500, 4x4, Z-71 with the 5.3 engine and an automatic. It has 98,600 miles and I've added an Airaid throttle body spacer and intake tube, K&N air filter and a Flowmaster 70 series muffler with split duals.

 

Currently I get 12-14 mpg in town and 15-16 mpg on the road, if I'm lucky. The truck used to get 19 mpg on the road and 16 in town, and I don't know what the problem is. I've changed the plugs and wires, and had the fuel system cleaned with no improvement. I had a machine plugged in and it didn't show any codes, but shows an intermittent miss on 2 to 3 cylinders at low speeds. I think I have a miss at 55-60 mph or so.

 

No one I've talked to has any idea how to improve the mpg. This is my daily driver and any extra mpg would sure be great.

 

Answer:

 

It's never easy to determinate the cause of a loss of a few mpg -- there are so many variables. But I do have a few things for you to check. Think back, did the change in fuel economy occur shortly after the addition of a specific performance add-on? Maybe the throttle body spacer which is said to increase performance by creating a "spinning action of the incoming air"?

 

The mystery misfire is a red flag. I'm not sure how the person who plugged in the machine (presumably a scanner) determined there was an intermittent misfire with no trouble codes. But if there is a true misfire, high speed, low speed or in between, have that diagnosed and repaired before you do anything else.

 

Have you replaced an oxygen sensor lately? With over 98k miles on the clock it may be time. Your 5.3-liter has four, one for each engine bank in front of the catalytic converters, and one for each bank after the cats. These sensors, especially the two up front, are what tell the Powertrain Control Module whether the exhaust gasses are rich or lean (low or high oxygen content), so it can control fuel delivery accordingly. Oxygen sensors have a significant affect on fuel economy, and they can get lazy as mileage accumulates.

 

Lastly, engine compression may not be what it once was. Hopefully that's not the case, but if were, a few mpg would not warrant major engine work.

 

I suggest spending an hour of the shop labor-time rate at a Chevy Dealer, and have them give your truck the onceover. Along with the fuel economy issue, mention the misfire you're experiencing and request a performance check of the O2 sensors.

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Comments:

TechHelp2
24 May 2017, 23:35
RE: Robert, To provide maximum fuel economy under light load driving conditions, the engine control module (ECM) will command the cylinder deactivation system ON to deactivate engine cylinders 1, 7, 6, and 4, switching to a V4 mode.

(Please Note) The engine will operate on 8 cylinders, or V8 mode, during engine starting, engine idling, and medium to heavy throttle applications.

You are telling me that you have a miss at idle, all cylinders should be firing at idle and you said that it runs well on acceleration. This tells me that you may have a problem like a vacuum leak at a vacuum hose at the manifold, brake booster or the brake booster it self. To check this take the brake booster vacuum hose off and plug it off and see if the idle miss goes away. You could also have a vacuum leak at the intake manifold gasket. Find out what cylinder is not firing then you can diagnosis the problem from that point.

A more complicated problem would be a burned valve, lifter on the canceled cylinder not functioning properly. A compression test can help guide you in your diagnosis.

Cylinder deactivation may be inhibited for many reasons including the following:

Engine coolant temperature out of range for cylinder activation
Engine vacuum out of range
Brake booster vacuum out of range
Transmission gear incorrect or shift in progress
Accelerator pedal out of range or rate of pedal application to fast
Engine oil pressure and temperature out of range
Engine speed of range
Vehicle speed out of range
Minimum time in V8 mode not met
Maximum V4 mode time exceeded
Decel fuel cutoff active
Reduced engine power active
Torque management active
Catalytic converter over temperature protection active
Piston protection active, knock detected
Cylinder deactivation solenoid driver circuit faults

Some information supplied by Alldata information website.
Robert
24 May 2017, 22:26
My 2011 Silverado, 5.3 four wheel drive has 128,000 on it. Recently I noticed via the tach, my butt and actual, deffinate cutting out but no codes showing. It has the 8/4 system. The miss is only present at slow excelleration or idle, your romp on it and it's gone like a raped ape, no miss. I changed the plugs but used diamond fire instead of A/C.... dumb move.... I have only had it for about 4 years and never a problem... till now. Serviced the trams about 4000 ago.... new filter also replaced.... As far as oil changes I change it at or above 30% oil life using Castrol 5/30, full synthetic. I've been told I'm in deep sh.., that the problems from the 8/4 system and to dump it ASAP. Can you please advise on this matter? Can I stop this problem or at least stop it long enough to get a decent trade...Love the truck but if the 8/4 is the problem it sounds like I'm looking at least 7K to fix it, don't have no where near that kind of money!!!!! HELP.....PLEASE.....
Thank you for your time

 

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