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Old 05-06-2007, 01:24 PM
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1991 GM 4.3 cooling issues.....need help

91 GMC 1/2 ton truck 4x4......Engine is a GM crate 4.3 with 67,00 miles on it........temp gauge reads 180ish, until you start driving then it pegs all the way to the red 260. I've changed the sending unit and put in a new radiator so far, with the same results. It will idle for a long time and not move up, until you drive it. I tried an old mechanical guage I had and it went up to 250 and then back down to 100 and I couldn't get it to do that again. Could somebody have put the wrong water pump on this thing? I think they are reverse rotation, if I'm correct. The belt is on correctly and it looks like it's moving when I take the cap off. I did use a hand held pyro to check at the head and it was reading 185 to 190. I'm lost here...........any thoughts? I just don't think this thing is running this hot.....Thanks, Brad

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Old 05-06-2007, 02:16 PM
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A quick test would be to drill a couple small bypass holes in your tstat. I had a similar issue with my 350, the addidional presure from the high volume pump was holding the tstat closed, then when it opened the water in the radiator was real cool and the temp would go way down. I ended up purchasing a Mildon high flow tstat and the problem went away.
The other thing you could be fighting is air flow when the vehicle is moving. Are all the factory air dams in place.
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Old 05-06-2007, 06:17 PM
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check the fan, is it turning the correct direction.
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Old 05-06-2007, 06:41 PM
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should be clockwise when standing in front....correct? This one is doing that. Just bought the truck and already troubleshooting........lol
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Old 05-06-2007, 06:52 PM
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look at the fan blades as someone bumps the engine over. can you feel air coming out of the grill while its running ?
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Old 05-06-2007, 06:54 PM
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It's sucking through the grille and blowing towards the engine............seems normal. I dunno.
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Old 05-06-2007, 07:04 PM
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i would run that mechanical gauge fir a little while. and see if it jives with the dash gauge. maybe the dash gauge is acting goofy.
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Old 05-06-2007, 07:10 PM
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I'll try it............I did use my hand held pyro again and also a Fluke meter that measures temp. Both the dash gauge and the old mechanical gauge were within 10 degrees of each other, at 250, while the pyro and the Fluke only showed 190 (ish). I don't think this truck is really running at 250. I just don't see how it could. Not losing any coolant or anything. THanks for the help though. Brad
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