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Old 07-16-2012, 06:25 PM   #1
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Unhappy HELP! 1994 GMC Sierra 4x4 5.7L

We lost my father in 2007 and this was his truck. It will passed on to my son in 5 years when he is 16 as pre his request. we have been scratching our heads and beating them against the wall for the past year trying to fix two problems on this truck. 1. When the truck is ran for the first 10-15 mins the temp gauge will reach almost to overheating and then it will go down to normal or sometimes below depending on how cold it is outside. We have replaced 6 (yes SIX) thermostats in the truck thinking they might be defective along with the temp sensor twice as well as the hoses. Still does it can't figure this out. 2. the ac system has had the compressor and sensors changed will only blow cold air for a day then stops. also has had the orifice tube replaced. mechanics can't find any leaks and pressures it holding but after the first day it only blows hot air. if anyone know what might be the answer to these two problems please help. this truck means the world to me and my son, the truck still has the power to smoke the back tires and let you know she'* coming. beautiful truck and we love her lots.
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Old 07-16-2012, 08:59 PM   #2
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I know someone here that owns a similar model. I'll PM him with the link to this thread.
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Old 07-17-2012, 07:32 AM   #3
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i had a 89, very similar. the temp i would say might be fine, verify proper temp with an ir gun on the thermo housing when running at normal temp. also every time you mess with the system and let air in it has to work back out. which is mostly the function of the rad cap so i would replace that too they are cheap. mostly i just think the gauges are terible, i replaced mine with aftermarket the stock one wasnt even close.

for the ac if it is holding pressure its likely electrical(switch or wiring) a good shop should have been able to put a meter on it and see whats going on, maybe take it somewhere else.
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Old 07-17-2012, 09:25 AM   #4
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There are things that will make a motor run hot, and a few that will make it run cold, but usually flucuations like that are a sign of air in the cooling system.
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Old 07-17-2012, 06:18 PM   #5
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Thank you all for the info. We will continue to attempt to find the minks. Also I have a clicking from the passenger'* side when the truck starts andshutsdown any ideas?
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Old 07-18-2012, 07:10 AM   #6
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depends what kind of noise, theres actuators for the hvac that sound like that when they go
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Old 07-22-2012, 10:23 PM   #7
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If you have a 94 Gmc with the 5.7L. When the a/c only blows warm check the compressor and see if the clutch engages. If it does not i would say its the pressure switch. On mine it screws on the line right at the compressor. You dont have to discharge the system to replace it. I have had my coolant changed or had to refill many times never had a problem with air in the sytem. You can try verifing the temp with a external gauge. But my guage has always been pretty accurate. If you have any questions or want me to check anything on mine to compare to yours let me know.
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Old 07-27-2012, 12:32 AM   #8
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Thanks rjblazer. Me and my brother that is the gifted mechanic of the family spent Wednesday going through all the main hoses and replacing o rings and etc. When we got to the new compressor that I had replaced at the start of all of this we found out that the old bolt which was a 9/16 was a few threads shy of a tight seal. Thank goodness I don't throw things away when it comes to this truck. I still had the plate and bolt that came with the new compressor which is a 15mm. Now we are blowing nice freezing cold air. Needed today with the heat index at 115f. LOL. I'm not sure if I really want to get rid of the click click noise from the passenger side dash when u start and shut the truck off. Its always done that as long as my dad drove it. He was a master carpenter and we like to say he'* still hammering away.
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Old 04-06-2013, 02:53 AM   #9
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had the temp issue with an old 95 i owned, and my 97 when i bought it, a new water pump fixed them both.
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