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post #1 of (permalink) Old 12-04-2002, 03:12 PM
Shilo
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350 runs hot on the highway - maybe?

Ok, this is actually in an '81 GM 3/4t van but please help me!

Recently the gauge started going up to and usually beyond 260 degrees on the highway. The funny thing is that as soon as I hit 45-50 mph it climbs up there rapidly, and once I pull over and let it idle it drops very quickly.

I have replaced the thermostat and sending unit for the gauge. When I ground the wire the gauge pegs. The wire is not touching anything like spark plug wires. Coolant is fresh and full. No coolant in oil. Fan clutch feels good.

It just started this out of the blue - one day running great, next day thinks it's overheating at hiway speeds. It doesn't boil over (haven't tested rad cap yet tho). The radiator *could* be plugged internally I suppose, but the problem happened overnight like I said.

I'm still not convinced it's running as hot as it says, and the fact that it's SOOO speed dependent makes me suspicious. Any thoughts?
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post #2 of (permalink) Old 12-04-2002, 04:21 PM
 
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Re: 350 runs hot on the highway - maybe?

I would think because it is speed dependent that it would be something like the fan clutch or something that has to do with the rpms. Have you tried getting it warm and then holding the engine at a highway speed rpm and see if it still does it? Does the van jerk at all when/before it overheats? Also, have you replaced you fan lately?
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post #3 of (permalink) Old 12-04-2002, 05:40 PM
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Re: 350 runs hot on the highway - maybe?

I haven't replaced the fan.... why do you ask?

I did try revving in Park while the motor was warm and the temp climbed but it never got into the red like it does on the hiway.

I just took it for a test drive with my multimeter on the sending unit testing resistance.... it was constantly switching, like 054 and then 200, back and forth. But I do believe the truck may actually be running warm because when the stock gauge got into the 260 degree area the readings were more like 080 and 365.

Is this what I should see from the sending unit - constant switching? It's a new sending unit, albeit a cheapie Wells from Autozone. I suspect if I put the old one back in I'd see the same behavior.

I'm beginning to think the radiator may be clogged or maybe it's running too lean. I'd still love to hear suggestions.

EDIT: no jerking to answer your other question
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post #4 of (permalink) Old 12-04-2002, 06:13 PM
 
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Re: 350 runs hot on the highway - maybe?

I asked about the fan because I bought a new fan for my 88' chev and the guy sold me a reverse rotation fan, which is the wrong kind. It would do similar things to what you are experiencing now. I'm not sure about the aending unit but I would try the old one and see if that fixs the problem. I would think that a clogged rad. would overheat all the time though. Running lean would cause the engine to overheat but prob. only in the high rpms and such. I know when when my fuel filter was clogged it would run warmer than normal because the engine was running lean from the lack of fuel getting to it. I asked if it was jerking because that would prob. point towards a clogged fuel filter from the engine running to lean. Just a question but what temp. thermostat are you running, and what types of temperature are you having weather wise?
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post #5 of (permalink) Old 12-04-2002, 07:55 PM
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Re: 350 runs hot on the highway - maybe?

T-stat's a 195 and temp didn't get much above 45 here today.

A good buddy of mine thinks it could be cavitating, esp w/the rear heat and all the heater hose running back there. Could make a lot of sense, I'm gonna bypass the rear heat (don't need it, it's just a work truck) and see if that cures the problem.

Still open to any thoughts tho, and thanx for the replies thus far.
post #6 of (permalink) Old 12-05-2002, 09:17 AM
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Re: 350 runs hot on the highway - maybe?

If you drain the radiator you'll probably find that if you look down the cap hole you'll see a bunch of stuff limiting water going through the holes. I know I have this problem. I'm afraid to do a radiator flush because chances are I'll have a leak afterwards. I'm saving for a new radiator.
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