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Ford Truck Club Forum > FORD F-100 | F-150 SERIES > 1973-1991 Ford F150



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  #1  
Old November 5th, 2018, 09:58 AM
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Default 85 351W thermostat

So I took the truck about a mile and half to Home depot on Saturday morning. When I came back out, there was a large puddle of coolant under it. I popped the hood and noticed the overflow reservoir was to the top and the coolant had come from there. It's a brand new thermostat. I've thought about running without one since I really only use the truck around town on the weekends. Just curious what would be the worst case scenario if I ran without one. I'm gonna order a new one to have on hand and a water temperature switch. The gauge doesn't seem to be showing much movement. Also curious if anyone has heard of something else causing it fill the reservoir. It didn't do this a couple of weeks ago when i took the long trip over to the coast. Just wondering if this means the water pump has quit on me.

Last edited by harrison28; November 5th, 2018 at 10:01 AM. Reason: typo
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Old November 5th, 2018, 11:28 AM
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What temperature is the thermostat?

Have you tested the pressure cap to make sure it's holding pressure, but not too much pressure?

It could be airlocked, let it idle for a while with the cap off and see what happens.

Hate to say it, but a blown head gasket can cause this.

If you run without a thermostat, the engine will never fully warm up, which will cause it to sludge and carbon up. If there are any electronics controlling anything (O2 sensor, etc.) it will run excessively rich.

2003 F-150 XL RCSB 2wd 4.2 5 speed 3.55 LS

Last edited by glc; November 5th, 2018 at 11:54 AM.
  #3  
Old November 5th, 2018, 02:55 PM
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No electronics. Always wondered what would happen if it never fully warmed up. The coolant was still green, no oil in it (like in the V-10 Excursion we had). I'll change the oil and see if there is any coolant in it. Hopefully the head gaskets are still good. I'll try the idle with the cap off. Should I wait until the t-stat opens? It's a brand new radiator cap also. If I remember correctly, it's 190 deg. thermostat in there now.
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Old November 5th, 2018, 06:46 PM
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With no electronics, I think I'd put a 180 stat in it.

Run it till the thermostat opens, the coolant level should drop. With it still running, add water slowly till it's full, then put the cap back on. Shut it off and let it cool down completely - a few hours. Then remove the cap and top it off with water. Fill the overflow to the fill cold line and you should be good to go.

You could do a hydrocarbon test on the coolant, that should tell you if the head gasket is leaking exhaust into it.

2003 F-150 XL RCSB 2wd 4.2 5 speed 3.55 LS
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Old November 6th, 2018, 10:58 AM
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Thanks for the advice. I'd much rather play with the truck for a few hours this weekend then cut the grass.
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Old November 7th, 2018, 01:35 PM
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Couldn't wait till the weekend and had some time this morning. Before I started the truck I opened the radiator cap. I filled the radiator up but left the overflow just below the the cold line. Started her up and let it run. The level in the radiator never changed. It bubbled a little. The upper hose got hot, so assumed the thermostat opened. I turned it off and started it back up about 5 minutes later. Then I noticed the overflow getting above the cold line. I shut the truck off and took the cap off the overflow to let any pressure go. It still filled up and spilled over about a quart worth before it settled back down. New thermostat and temperature switch are supposed to be here Friday. I'll change it out and see what happens.
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Old November 7th, 2018, 04:18 PM
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If the radiator itself has a pressure cap, the cap on the overflow needs to be a vented type.

Test your cap to make sure it relieves pressure within specs.

2003 F-150 XL RCSB 2wd 4.2 5 speed 3.55 LS
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Old November 7th, 2018, 05:08 PM
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I'll do that. Found a Gates test kit for about $50. Maybe the new cap I have is not functioning properly. I'll keep y'all informed.
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Old November 7th, 2018, 09:45 PM
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Before you buy a tester, see if parts stores have them for "loan" or have one they can test the cap with.

2003 F-150 XL RCSB 2wd 4.2 5 speed 3.55 LS
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Old December 3rd, 2018, 10:26 AM
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Update. Installed the new thermostat. I didn't pay attention when removing the old one,...but got to thinking. I may have had it installed backwards originally. Works great when installed properly. Also Changed the temperature switch, gauge works beautifully now.


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