Diagnosing Poor Power

[alvocado]

I just returned from an 1800 mile vacation trip and the '02 Yukon Denali wasn't running up to par. The truck struggled to maintain a cruising speed of 75mph and often shifted down two gears to accelerate when hitting long, steady grades. It hasn't done this in the 7 years I've owned it and there has been a noticeable loss in power during regular everyday driving.

 

The plugs and wires were changed 35k miles ago using OEM parts and I've changed the air filter, PCV valve, and fuel filter (installed correctly) within the past month. I pulled the throttle body about 3 mos. ago and thoroughly cleaned it and replaced with a new gasket. I used a stethoscope this evening and checked each fuel injector with the engine idling and confirmed each one is operational. Could the injectors be working but partially clogged? At 124k miles, it should still be running fine so I'm looking for suggestions on how to track down the issue. I have some highway trips planned and would really like to get the truck running right again. Thoughts?

[txab]

Have you checked for DTC codes?

[alvocado]

Have you checked for DTC codes?

 

Not yet but I play to swing by Advance Auto for a quick scan. There are no warning lights on. Would a basic OBDII scanner pick up anything that isn't throwing a light? I did think about pre-cat oxygen sensors since they are original but I don't know if those trip a code when they start affecting performance or if they have to totally fail before registering an error code.

[2COR517]

Have you done the Seafoam routine?

[Stevens03]

Clean your throttle body, look for "coking" on the blade and intake. Also clean your MAF sensor and see if anything changes. After that, I'd suggest the seafoam route too.

[bill190]

There are quite a few electrical sensors scattered around the engine/exhaust which input information to the engine computer...

 

And the engine computer adjusts rich/lean and the spark timing based on the information provided by these sensors...

 

So get a factory service manual set and test these sensors to be sure they are working properly.

 

Then check the EGR valve to be sure it is working as it should.

 

And some things can't be easily tested "at home" unless you are an "Einstein Chemist" type. This would the oxygen sensors and these should be replaced every 60,000 miles as a regular maintenance item anyway. Remove power to battery for 10 minutes after replacing. Then drive over 45 mph as soon as you can. This helps the engine computer to "relearn" the new sensors quickly.

 

In "general" this is what the various sensors are for...

 

Timing...

 

Crankshaft position

Engine speed (RPM)

Engine load (manifold pressure or vacuum)

Atmospheric (barometric) pressure

Engine coolant temperature

Intake air temperature (retards timing when hot)

 

Fuel...

 

Coolant temperature sensor - Lean/rich

Oxygen sensor - Lean/rich

Atmospheric (barometric) pressure - Lean/rich

Manifold Absolute Pressure - More fuel

Throttle Position Sensor - More fuel

 

Then you need to use a multimeter to test the sensors. How to use a multimeter...

http://www.google.com/search?hl=en&saf...ter&aqi=g10