1998 Sierra 1500 Ext Cab Z71

Hi,

1.

Truck seems to run fine, unless I'm going uphill. Around 30 mph on arterial raods and around 60 mph (not sure rpm) on freeways, climbing a hill. Truck is in Overdrive, and begins to sputter, losing power. I downshift to 3rd, and it seems to help, and when on level ground, the engine seems to run normal again.

The Check Engine light comes on and the codes via Autozone say #4 Cylinder Misfire.
Any suggestions?


Recently had a tune-up, new serpentine belt, new fuel flow sensor. New Battery.

2. When the key is in ignition the fuel pump comes "on" and is a sligth buzzing sound...is that normal? First GMC vehicle. Had a 2001 Suburban, didnt ever hear that fuel pump buzz.

3. I'll get a "check gauges" annunciator...the temp gauge will peg momentarily on HIGH, then settle back to normal operating temp. Almost like a spike in electrical system.

Are these things related????


Thanks in Advance.

i am taking the truck had non of the problems before it went in for repairs .#1 The Check Engine light comes on and the codes via Autozone say #4 Cylinder Misfire. the code should read PO304 it may be a bad wire or a bad spark plug easy fixes. #2 fuel pump comes "on" it will buzz dut not real loud. have the fuel psi tested for a proper fuel rates and psi. #3 make sure that the battery conections are tight and clean and the ground wire at the body and eng block are clean and tight. when trucks are in the shop and things worked on the problems will happen. a good tech will test and catch this before the customer gets it back. good luck.

I've owned the truck for about 2 years. I first noticed the problem, when towing my boat. The problem repeated without pulling anything---when driving Uphills ONLY.

I took it to a 2nd Mechanic who changed the plugs, (None were fouled) Replaced the Intake Manifold Gasket (Started leaking Dexcool), and a few minor unassociated fixes.

The truck has stumped both garages.

Truck in Normal Configuration, on Freeway, doing 65 mph. Level. Start to drive up hill and it begins to 'bog down' and shutter. The whole truck vibrates. I downshift to 3rd, it seems to help a little...but powerloss is noticable and vibration/shutter is still there.

The Check Engine Light comes on. I go to Autozone...same message. #4 Cylinder Misfire. After a few 'laps' the fault clears itself and comes back at the same point....climbing an incline at highway speeds.....

Help.

when was the plug wires last changed. i had a set years ago that would let all eight plugs fire at idle but when you put a load on the motor only four plug wires were hitting.we ended up putting it on a diagnostic machine to find it.truck would run as hard and fast as you wanted to go then one day i slowed down to make a turn and it would miss when load was put on the motor.check the plug wires.

The plug wires were changed about 1 year ago, and the problem has remained. The thing is, it only happens when driving up a slope/hill. I'm think there is a short, and will check all the wires.

I have heard that the wiring harness for the Trailer lights could be causing a short and i plan to look at the connections tommorow.

The Spark Plugs are clean, no fouling (and New).