Hello and welcome to another unsolvable standard cooling issue thread!

Discussion in 'General Chevy & GM Tech Questions' started by twisted954, Jan 21, 2011.

  1. twisted954

    twisted954 New Member

    Sooooooooooo....Hello ALL!

    Happy to meet you all. Hopefully there is someone around with some insight or ideas.

    Truck: 1989 Chevy Silverado 1500 Z71
    Engine: 350 TBI

    Bought this truck a few months ago from a craigslist add. Truck was actually well maintained except there was a blown head gasket.

    Drove the truck home, popped off the heads, pressure tested them, cleaned em up, did a minor valve job and threw them back on using fel-pro gaskets and everything new. While I was in there, I replaced the water pump, t-stat and of course the usual tune up stuff.

    Fired right up BUT now I have the standard mystery temperature spike and then crazy fast drop. Thought it had to be some air bubbles...but It has been all crazy for a couple hundred miles now. Thought I should try a new t-stat, but i yanked mine, tested it and it tested good. Cooling system pressurizes, nothing leaks, no coolant being lost...I mean there is nothing happening here that leaves me to believe my repair is not good to go.

    Now there IS one dumb thing I haven't corrected yet and I am hoping that is it. When I threw the heads back on, well I flip flopped them. I was tired, at the time, and so I simply routed the temp sender wire around to the back of the passenger side head rather that switch the sender to the correct location (the plug was being a pain to get out and I was exhausted at the time).

    Soooooooooo...I am thinking that could likely be the cause, but I am uncertain if it would account for such radical temp variations. I currently have a 195 t-stat in there. Weird thing is the colder it gets, the hotter it gets on initial start up.

    Also I feel I should talk about the symptoms a little bit. If I let the car just run in the driveway, it will never fluctuate. If I drive it down the highway it is all over the place...tonight it ran up to 240-250 or so and then immediately shot back down to 190.

    I dont know...the ups and downs literally freak me out enough to ask people here for input. As you can see, I have already tried most of the common stuff. I keep reading over and over about similitude stories tho and no one ever posts up the results or any fixes...the topic just simply dies.
  2. Are you sure that you do not have low coolant? Have you checked it running with the cap off to see? Another thing to try would be to drill a small 1/8-3/16 hole in the thermostat body so there is slight circulation when the thermostat is closed which allows the heated coolant to reach the thermostst easier. Also The fact that the temp sensor is in an entirely different location could be causing the fluctuation in temperature. The worst case scenario is that the heads can not be swapped side to side. The easiest way to tell would be to look where the temp sender is supposed to be and see if there is a plug which could be removed and put the sender back where it belongs. If there is no plug the heads are different and probably can not be reversed. When you swap them from side to side the water passages that were in the front are now in the back and vice-versa. It may be that there are restrictions in the water passages forcing coolant flow to different spots in the engine to get consistant cooling, and if this is the case you now have some serious hot spots in the engine. I know that this was NOT the case with earlier small blocks but I am not familiar with the later ones. Also head gaskets are frequently directional, are you sure yours are not or if they are are they in correctly? The only way to know this for sure without another disassembly would be to pick the brain of someone who removes heads frequently. Generally knowledgable people will answer questions like this freely so a call to a local dealer or machine shop that does engine rebuilding might be helpful. Good luck, The Grump
  3. Tagerman

    Tagerman Member

    If its not that temp sensor I would be willing to bet money you have installed the Intake manifold gaskets incorrectly. I have seen that happen many times with a temp spike the result.
  4. stephan

    stephan Rockstar 4 Years 5000 Posts

    Welcome to the truck club. I put a temp sensor in the passenger side rear btw #6 & #8 & my temp guage doesn't spike like you describe yours as doing. I'm running 2, one electrical sender in the normal spot btw #1 & #3 & a mechanical temp sender back by #8 in the spot you describe. The only thing it should do is register a few degrees higher because that is the last spot to get coolant flow & only after it's been heated by the block on its travel to the back. Mine reads 3 degrees hotter on cold days & 5 degrees hotter on 80+ F days.
    I would check the easy things first. Park your rig with the rad cap uphill at the highest point, warm it up, turn on the heater & double check that it is bled of all air. This can take some time to do it right. If that doesn't cure it then buy another thermostat. They're cheap compared to head gaskets & a lot easier to replace. Your stat tested good in your pan of water, but I think it could be sticking before it opens & that's why you're seeing the spike. Another good test is just to take it out & try it without one for a couple days. It should run cooler across the board & no spiking if the stat was the problem.
  5. phoebeisis

    phoebeisis Epic Member 5+ Years 1000 Posts

    Do what Stephan said first.These are FREE and only take 20 minutes of work. Always do free, cheap, and not too time consuming first.

    Guess it makes most sense to air bleed first- 5 seconds to remove cap maybe 20 seconds to park it nose up. STILL SCREWED UP??
    Then remove stat- maybe 20 minutes of work.
  6. Bigbomber

    Bigbomber Rockstar 4 Years 5000 Posts

    Welcome to the club!

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