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1997 K1500 5.7/350 Rough Idle


Hi all. My truck has a rough idle that won’t go away. It’s a 1997 GMC k1500 with a 5.7/350 and about 137k miles. Just recently did the plugs, wires, rotor, and cap with all Delco stuff. Ran couple cans of sea foam and some cheaper stuff from wal mart for 6 consecutive tanks(about 1800 miles) also doubling the concentration that was recommended by the manufacturer (AKA 1 bottle for 22 gallons, I used 2 bottles for 23ish gallons). And no difference what so ever. Also, when the truck is cold, such as first thing in the morning, its starts instantly. When warmed up, like doing errands around town or something, it takes a lot of cranks to start and seems to kind of putt-putt to a start instead of solidly crank to a start. Any and all advice would be appreciated. I’ve heard horror stories about these spider injectors and fuel pumps in the mid 100k mile range, not to mention I see a lot of these trucks for sale at 150k with trans rebuilds. I’m not looking to dump a lot of money into it but she is a daily driver and overall in great shape otherwise!!!





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Put a fuel pressure guage on it. Does it hold pressure when you turn the key off? it should hold for a little while. if it drops instantly, that could make it start hard.

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.. starts great when cold... but rough start and hesitation when engine is warm ..?


this could be a function of a faulty Coolant Temperature Sensor (CTS) .. this monitors the coolant temperature in the engine and signals the Powertrain Control Module (PCM) to regulate the flow of fuel to the injector system .. (sort of takes the function of a choke on a carburetor...)


if the CTS is faulty then your engine is going to continue to receive a good rich fuel mix even after the engine warms up ... this is a common problem and very easy to fix


the CTS is located on the top front of your engine right beside the thermostat .. there is a two-pin connector for the wires that connect to it .. you could check the operation of it by measuring the resistance across the two pins on the CTS and this should change dramatically as the engine warms up ... or you could just buy one at a parts shop ($15-$20) and replace it yourself.


.. check the fuel pressure like Steve suggests above... should be within spec of 61 to 66 psi


.. and maybe replace the fuel filter if you can't remember if it has ever been replaced.. (on the frame just behind the driver's side cab)


yes.. the injectors can give you grief, but try the cheaper, easier stuff to fix first....

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