# High idle problem, 98 K1500



## 70monte (Feb 5, 2008)

I have a 1998 K1500 with the 5.7L Vortec with 130,000 miles. I'm the original owner of this truck. 

For the past few winters when the temperature gets down fairly cold, the truck idles high, even after the truck warms up. There is no CEL showing.

For example, it was 20 degrees when I first started the truck this morning to go to work. The rpms shot up to about 1,500 rpms and finally settled at about 1,000 rpms in park. It would idle at about 850 rpms in gear.

I drive 30 miles to work and when I park the truck and put it in park, the idle rpms is still at 1,000 rpms and 850 in gear.

When I left work it was 49 degrees and the idle shot up again to about 1,500 rpms initially and then settled to about 1,000 rpms. But on this trip home once the truck warmed up, the idle in gear would be at 650 and when I got home and put it in park it initially shot up to about 1,000 rpms and then settled at about 800 rpms.

I also noticed on the way to work that the temperature gauge only got to the mark right before the 1/4 mark. On the way home, the temp gauge stayed at the mark two marks from the half way 210 degree mark.

The heater still works fine.

During the warmer months of the year, the idle is normal like it was when it was new and the temperature gauge reads at the 210 mark.

The coolant temp sensor and thermostat were replaced 10 years and 42,000 miles ago and the temperature gauge sensor was replaced five years and 15,000 miles ago.

Does anyone know what could be causing this issue. It seems like it's totally dependent on what the outside temperature is. I have checked for vacuum leaks since this first started several years ago and have not found any.

Thanks for any guidance you can give me.

Wayne


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## 70monte (Feb 5, 2008)

I forgot to mention that this truck has had the intake manifold gaskets replaced three times since new and the last time was 10 years and 40,000 miles ago. The gaskets used were the next to last generation of gasket for this engine. The last generation had not come out yet. The level of coolant has remained the same in the overflow tank so I don't think it's leaking again.

I also just replaced the coolant less than one month and 1,000 miles ago.

Wayne


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## Randall Ave (Oct 29, 2014)

I would replace the therm with a factory therm, try that first.


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## 70monte (Feb 5, 2008)

Randall Ave;2059848 said:


> I would replace the therm with a factory therm, try that first.


Ok, I will try that first. Thanks.

Wayne


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## bmc1025 (Jun 10, 2009)

+1 on the thermostat. If you ever have to do gaskets again use the felpro ones with metal surrounding. These gaskets are even reusable.

I would also clean the throttle body. Just open the butterfly by hand and use a rag with solvent on it to wipe off the build up.


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## 70monte (Feb 5, 2008)

If it ever does need intake gaskets, I will use the latest metal version. This version just had not come out yet when the last repair was needed.

I have cleaned the throttle body and installed a new Delco idle control valve last year. I will get a new thermostat tomorrow if I can find a Delco one or a Motorad one which was the kind that was in the last Delco thermostat box I bought. Thanks.

Wayne


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## 70monte (Feb 5, 2008)

The new thermostat seems to have done the trick. I installed a new AC-Delco one and the idle situation seems to have been resolved. 

After driving the truck around for awhile, the temp gauge was now running high, almost to the 3/4 mark on the gauge which is the mark right before 260 degrees. I hooked my scanner up to the truck and it said my coolant temp was only 188-193 so I guess the temp gauge sensor was bad so I installed a new one of those but have not driven the truck since to see if that is resolved.

The temp gauge sensor is a Duralast brand that I was able to warranty for free so that is what I put back in it.

Wayne


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## 70monte (Feb 5, 2008)

I drove the truck tonight and the gauge now reads where it should so the new gauge sensor did the trick. I also put on a new AC-Delco radiator cap as the old one was 10 years old as well even though it seemed to still do the job. It did have a little bit of corrosion on the underside of it so I figured it was a cheap preventative maintenance item.

Wayne


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