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  1. #1

    Unhappy '94 5.7 TBI overheating on start up until thermostat opens

    Hey all you 5.7 TBI Guru's, I need some help. My engine is overheating at start up until the thermostat opens, then it runs normal. the problem is it runs up to 240-250 degrees before the intake manifold gets hot enough for the thermostat to open at operating temp. The temp at the head temp sensor has been verified with a mechanical temp gauge installed in place of the temp sensor feeding the dash temp gauge as well as an infrared temperature sensing gun. Any ideas on what could be causing the run up when cold? Help!

  2. #2

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    I have had plenty of this type of problem on other vechicles.

    I think you should check your thermostat. Take it out, put it in your wife's nicest pot and heat it with a thermometer and check where it is opening. It should be opening up before 195F.

  3. #3
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    X2 on Pops post. The thermostats sticking.
    1988 Chevy C-3500 2wd (no pic)
    350 c.i. 5.7 L Stock Block, 4 Bolt Mains
    L-31 Vortec Heads, Edelbrock Cam & Intake,
    Holley 650, Flowtech Headers, Magnaflow exh.
    Jet Trans 700R4, B&M Ratchet, 4:10 gears,
    3" susp. lift kit "shadetree"
    No rev limiter, No speed limiter lol


  4. #4

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    Hey Guys, It is a little more complicated than that. Here is what I have already done and how it started. There was a leak at the intake manifold gasket above the number one cylinder. It was sent to a local shop for replacement of the gasket. Driven off and on for a couple of months through the summer and then when the weather started getting cold, we noticed it took forever to heat up and never really got up to operating temperature (center of temp gauge at approximately 200 degrees). I figured the thermostat was stuck open. We had driven it out to Arizona and when we got there, I pulled the thermostat housing and found that the thermostat was gutted. Just the casing was installed. No spring or sensor. I installed a new 195 degree thermostat, filled the system and when I started it up, the temp slowly ran up to 240-250 degrees. The radiator did not get hot though. After several minutes the temp gauge indicated a steady decrease in temp down to about 170 and then back up to about 240 and after several minutes the temp came back down to approx 200 and stayed there. I was on a short trip when all this happened and when I got back home I monitored the radiator and it was not overheated. I figured it was an air lock and cured itself. The next time I cranked it, it did the same thing. This time I monitored the upper radiator hose and it was not hot initially. I thought maybe I had an erratic temp gauge so I used a digital infrared sensor gun and a mechanical temp gauge to monitor the temp at the head temp sensor. It was corresponding to the dash temp gauge. I monitored the temp at the intake manifold next to the thermostat housing and it was initially about 130 degrees when the head was indicating 230. The head temp increased to 240-250 and after several minutes the intake climbed to 195 and the thermostat opened and head temp receded to approx 170 degrees. The thermostat obviously closed and the head temp climbed back up to 240-250 degrees. In a shorter period than before the intake temp rose to 195 and the thermostat opened, stabilizing the temperature until the engine was shut down. I put a 180 degree thermostat in and the cycle was the same with the exception that the max temp only reached approx 230-240 and only cycled once before stabilizing. Of note, the ambient temp outside was 65 degrees. I back flushed the system and the results were the same with the 180 degree thermostat installed. When the initial ambient temp was lower (50 Degrees) the high/low cycle was sometimes two or three times before stabilizing. I am currently running a 160 degree thermostat to optimize engine operating temperature against an initial run up of 220-230 degrees. It only runs up once with this configuration.
    I tried to talk to the mech who did the head gasket replacement to find out what he did and why he didn't install a 195 degree thermostat (which appears to be obvious) but he no longer is employed at the garage and the owner has no knowledge of it.
    So, now that you know the rest of the story, can anybody give me a clue as to what is causing it. As I understand it, the intake manifold gaskets can't be installed backwards which might block the head to manifold coolant portals and I believe I have run it enough to eliminate air locks. Any suggestions would be greatly appreciated.

  5. #5

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    I would look for a bad head gasket I have seen these go bad on these motors before . sound like the engine is pushing exhaust into the coolant passage at start up but as the engine worms up the gaskets seal because of the head and block expanding

  6. #6
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    This is just a shot in the dark Mike, but since this all started when the shop put new intake gaskets on it, is it possible that he routed your serp belt wrong & has your water pump running backwards? Or could it have always been that way since you bought it?

    Also since you found that the original thermostat was gutted, it seems like the previous owner also had this problem, & the gutted stat was his attempt at fixing it. I have seen problems similar to this when the water pump impeller was starting to slip on the w.p. shaft. Also the previous owner could have replaced the rev rotation pump with a standard rotation pump so IF the serp belt was/is routed properly, the pump impeller would still be running backwards. Is there any way for you to contact the previous owner & get some history on repairs that he might have done?

  7. #7

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    Stephan, We are the original owners. My wife bought this as a demo in '95 and put 180,000 miles on it. The truck was sitting idle on and off for the past three years as we traveled, but her brother drove it occasionally to keep things functional. We have every service record ever performed on it. So, to follow up on the thread, there is circulation through the engine and heater core as evidenced by flow into the top of the radiator at start up and continuing once the thermostat opens. I don't believe basic circulation is the problem but I have noticed that the front of the intake manifold tends to be 20-30 degrees cooler than the rear at start up until overall temperature stabilizes. This leads me to believe that there might be a blockage in the head from the 1-3 cylinders area to the intake manifold. Does anybody have a diagram showing the coolant flow path on the 5.7 TBI? I sure am interested in reviewing it. My mind is still open to all possibilities that I have not pursued already. Later.

  8. #8

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    What brand of stat are you using? I am new to Chevys but saw something silmilar to your problem on a dodge. The water pump put so much pressure on the stat that it would not open. When a small hole was drilled in the stat, it took longer to heat up, but the stat could open properly. I wonder if a better stat would work?

  9. #9

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    I have installed two different 195 degree thermostats, one 180 degree and now I am running with a 160 degree. THe 195's were Duralast and Valucraft. The 180 a Duralast and the 160 a Valucraft. I checked both the 195's in a pot of water and they both opened at 195. I eliminated the possibility of a bad thermostat after that test.
    I checked the Serpentine belt to make sure it is correct and it is.
    What is the possibility of an air lock that refuses to burp out? I have read several threads from other people that refer to that as a possibility. I have burped the sytem, backflushed it, but was that enough? Is there a special procedure to positively eliminate all air pockets from the engine? I'm still looking and I appreciate everybody's comments and recommendations. Thanks. Later, Mikel65

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    Hey Mike, I've re-read all your posts & since you've owned this rig all it's life & this problem started after the mech replaced the intake gaskets, do you think that he is the one who gutted the T stat because he saw the problem & this was his quick fix to try & solve it? Did you ever turn it over to a shop previously for overheating problems & they might have gutted the T stat? If not then he (the gasket mech) has to be the culprit who gutted it & that brings you back to the intake gaskets. I'm thinking the passages in the intake gaskets are too small for enough circulation from the front of the heads into the intake manifold & over to the water neck/thermostat, & when he saw that the engine was getting up to 240-250 F he may have gutted the T stat since it was an easier fix than re-doing the gaskets with the right ones.
    As far as a diagram for the coolant flow, I don't have one, but (when the T stat is open) cool water flows into the wp & into the front of the block. At that point some of it flows to the rear of the block & on the way some of it flows up into the heads through small to medium ports in the block deck up through the head gaskets & through holes into the heads. At that point it flows to the front of the heads where it goes into the intake mani & over to the water neck & T stat, & back through it to the radiator to be cooled.
    I don't think an air pocket is a problem at this point since you have already addressed this & if your rad cap is working correctly they will self bleed over time as long as you keep the coolant recovery tank filled to the proper level.

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