Confusing engine temps
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Joined: Jun 2011
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Confusing engine temps
I recently purchased a 1979 f150 ranger with 351. The radiator was changed the day before I bought it. After a week or so of owning it, I noticed there was some antifreeze leaking behind the water pump. Ok, 23 bucks and done. A couple days later I was driving to town and the temp gauge shot up to beyond hot! Yikes. I shifted into neutral and revved the engine. The thermostat must have been stuck because the temp immediately came back down to the middle of the normal range. Ok, so I changed the thermostat.
Yesterday I took it for a test drive. The temp stayed low until I got into town. As I was driving through a parking lot, the temp again shot up to hot. Once again, I revved the engine and it came back down. Last night I took the truck to the lake to fish. The temp gauge didn't even go into the normal range. It stayed cold. Same thing this morning. I went to town, the temp barely touched the normal range. Most of the time it was below that.
Any ideas?
Yesterday I took it for a test drive. The temp stayed low until I got into town. As I was driving through a parking lot, the temp again shot up to hot. Once again, I revved the engine and it came back down. Last night I took the truck to the lake to fish. The temp gauge didn't even go into the normal range. It stayed cold. Same thing this morning. I went to town, the temp barely touched the normal range. Most of the time it was below that.
Any ideas?
Cranky Old Guy
Joined: Mar 2008
Posts: 3,562
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From: Raphine, Virginia
Is it actually overheating as in blowing coolant, or seems normal, but temp gauge just erratic? If it still has the electrical temp gauge there could be an issue there. Mechanical gauge is only way to really depend on the reading. Of course some people have infrared thermometers to isolate/verify sections of the cooling system.
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It's never blown coolant. The gauge just jumped up high then came right back down. Now though I can't even get it to hardly warm up into the normal range on the gauge. I'm just wondering if the thermostat is stuck open... But it's new. Just confusing.
Cranky Old Guy
Joined: Mar 2008
Posts: 3,562
Likes: 6
From: Raphine, Virginia
On the temp sender, take the wire off and ground to engine. With key in run position gauge should go full scale. If it does, replace the temp sender. If it doesn't, you have a problem in the wire to gauge or gauge itself. Sometimes it's just the push on end itself that becomes corroded or loose. Doesn't sound like you really have a cooling issue, though some thermostats are designed to fail in the open position. Also never use teflon tape to seal sender threads. Check that out and we'll go from there.
Hotshot
Joined: Feb 2004
Posts: 10,830
Likes: 2,611
From: The Shenandoah Valley
Using teflon tape on a sender that has a single wire lead and which works by grounding is an iffy proposition.
If it prevents solid metal to metal contact at the threads, the sender can't ground the circuit through the threads as intended.
Having said that, I still use about two wraps of teflon tape, but no more, and I watch for steady gage afterwards. I know the tape can inhibit it but I've been lucky.
Also .... just out of curiosity .... what T-stat are you using?
I use ONLY Robert Shaw 333-192 (or 333-185) on a 351M or 400 (or 351C for that matter). If the right size T-stat is used based only on diameter, it lacks the hat that shuts off the bypass as it opens. I know from experience that the RS 333-192 operates correctly and consistantly in these engines. I have seen Stants "not work" so consistantly.
If it prevents solid metal to metal contact at the threads, the sender can't ground the circuit through the threads as intended.
Having said that, I still use about two wraps of teflon tape, but no more, and I watch for steady gage afterwards. I know the tape can inhibit it but I've been lucky.
Also .... just out of curiosity .... what T-stat are you using?
I use ONLY Robert Shaw 333-192 (or 333-185) on a 351M or 400 (or 351C for that matter). If the right size T-stat is used based only on diameter, it lacks the hat that shuts off the bypass as it opens. I know from experience that the RS 333-192 operates correctly and consistantly in these engines. I have seen Stants "not work" so consistantly.
Cranky Old Guy
Joined: Mar 2008
Posts: 3,562
Likes: 6
From: Raphine, Virginia
Mornin Les. I never advise using teflon tape for the reason you stated. Hard enough doing "internet repairs" as it is. The sender threads are NPT which are tapered and cut thru the tape anyway. Teflon tape is actually used to lubricate rather than seal the threads.
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Cargo Master
Joined: Oct 2010
Posts: 2,674
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From: Ideeho
x2 on what you said...................Just an FYI on the teflon tape............2 wraps is all that is ever reccomended on ANY pipe thread fitting. And yes, it is mainly a lubricant, not a sealer. And it is only intended for use on pipe threads. Using it on anything else could lead to stripping the threads!!!!!!!!!
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