Hope this is in the right place, didn't have much luck searching out an answer...

91 RS, 305 TBI, 146K miles. Runs like a top and performs very well. All stock I think. Since replacing fan relay, temp sender, temp sensor, thermostat/gasket and rad cap, it seems to do okay on temps. I do plan on installing an adjustable fan controller next paycheck because I'm not convinced my fan always kicks on when it should.

Situation is, while it is driveable now and in most situations cools fine, this happened twice so far. I start it up from cold and drive a little ways, achieving on the cool side of normal temperature. Then I get on the highway from a rolling start, and twice when I've romped on it (i.e 80% throttle to about 70-80mph), it hauled *** but the temp gauge needle quickly rose almost touching the red section. Continuing to cruise it went back down to normal pretty quickly but is this just fan delay or why is my temp rising so fast? I can't imagine it's good for my plugs or anything else! Only the two times I've got hard into it like that has the temp ever spiked.

Any input is much appreciated, thanks in advance!

 

if the engine is getting into detonation(AKA preignition)
when under load at high throttle opening,that can cause
the temperature to spike up rapidly-do you hear any
rattling or pinging when the engine is ran hard?
Common causes for detonation include too-much timing
advance,lean mixture at WOT,and low-grade gas.
Detonation is hard on the pistons,head gaskets,etc.
i'd check the timing first.

 

Other possible causes could be lower radiator hose collapsing under high rpm (old, soft hose).
Old/bad antifreeze foaming up (causing cavitation) when the water pump is running all out rpm's.
Weak air dam is folding back at higher speeds. In combination with what z-chev stated, it could be a cause for temp spikes, too.
Just guesses though...

 

Thanks for the input, I will start checking those items. I think the air dam's solid as it's wired to the bumper, and the antifreeze was recently replaced. I have been using premium gas. I don't hear any rattle or ping on hard acceleration.

I will read up on timing and see if it's been messed with, and give the rad hose a squeeze to see if it's real soft. Thanks!

 

I would like to add that the lower radiator hose should have a spring inside to keep it from collapsing. It's often that the new ones don't come with one and you need to transfer the old spring over.

I've also heard about head gaskets leaking into the coolant passages.

 

I second the head gasket. I had this exact same thing happen right before a head gasket failure. At low RPMS's the head gasket would hold. However, as soon as high load compression would start to leak into the cooling system and raise the temperature. The head gasket blew about 3-weeks later.