The Stovebolt Site Forums The Shop Area Engine Shop maybe trouble (?) with the first drive with rebuilt 250

I just began driving her last week. I drove to town today, when I switched it off the motor nicked & spun backwards. I had left the air filter off accidentally. The temperature was 182 ish & had only driven 50 -55 mph. The timing was set at 3 or 4 degrees.
That’s the only time it has done that. Anything I need to check?

How did you determine that the motor spun backwards?

Check the idle speed, maybe too fast, I don't know what a stock 250 should idle at but somewhere in the range of 550 - 750 rpm. How about the heat range of the spark plugs? if they are extremely out of the specified heat rang it may be creating a hot spot in the chamber and "dieseling"

Running rich can cause that.

Its called "dieseling"- - - - -slow down the idle speed.
Jerry

I had a 72 gmc with the 250 six and that truck carb had a idle stop solinoid that when you turn the key off the throttle solinoid dropped the throttle down to prevent run on.

Yes, dieseling is the term. I’m familiar with that action from lawn mowers etc.
it was in gear when I switched it off so I released the clutch & it pulled the truck backwards to stop it.
I don’t have a tachometer but it idles nice & slow.
It’s odd that the initial temperature runs up to around 200 before the thermostat opened (while driving) then quickly comes down to 180. It actually went up again then back down. I was thinking it might need burping? Previously with the old motor it never went up that much.

The mounting point of the mechanical temp sensor in my GMC is in the head between cylinders 5 and 6 which I've been told is the hottest area on the head, while the thermostat is off the front end of the head where I have an electrical temp sensor (see Segal's Law) mounted in the thermostat housing. It is not unusual to see a 10 to 20 degree difference between the front and back (the back being hotter) during conditions that cause a change in operating conditions of my truck (long hills up or down, initial start up, even slowing down for a turn after a few miles at steady highway speeds).

Or it may just need burping like you said.

I only use the temperature probe at the thermostat housing. So that is actually the temperature changing.
It would be interesting to run another thermometer to the head but where would I mount it?

I think new thermostats are not as accurate as old school parts, like everything else these days, just poor quality. Mine does the same thing, 2 new stats and the same scenario. The gauge will rise to 180 - 185, then drop off to 170 and stay there, I block off part of the radiator with cardboard and I can regulate it quite well.

Just going by a couple of your previous pictures, it looks like you have 2 PCV valves in the valve cover? One of them should be an open hose vent to the air cleaner. If that's the case the engine isn't venting right, which could cause some oil consumption problems. I'm not sure how an engine would run with 2 PCV valves, but probably not good.

The GMC heads have a threaded fitting on the top surface of the head between cylinders 5 and 6 that can be used for a mechanical (bulb type) or electrical gauges and another threaded fitting in the thermostat housing which are only used from the factory for electric gauges. When I bought my truck it had a set of electric replacement gauges with the temp sensor in the head. I moved that electric sensor to the thermostat housing and installed the mechanical gauge in the head. I don't know if you have both these mounting points available in a Chevy head.

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I wouldn't trust the dash gauge unless you've verified it with another measuring device.

Thermostats don't have anything to do with engine operating temperature. The engine will operate at the same temperature regardless of what thermostat is installed.

The thermostat only determines how long it takes for the engine to reach its normal operating temperature.

Take the thermostats you have and put them in a saucepan on the stove. Put a thermometer in the water as well. Turn on the heat and watch what temperature the t-stat opens and at what temperature it is fully open. A 180 T stat will begin to open at around 175 and won't be fully open until around 185.

Blocking off the radiator doesn't do the same thing as a thermostat. Blocking a radiator will raise engine operating temperature. A thermostat will not.

The thermostat regulates the GPM through the radiator which in turn regulates how many BTU's the radiator can disperse. If you run with no thermostat, the system will never reach a correct temperature due to the radiator being able to get rid of more BTU's than the engine is transferring to the coolant. The thermostat is designed to keep the engine temperature at no lower than the stat rating, the radiator and associated hoses, pump, etc. are designed to keep it from overheating. I'm relying on an AutoMeter mechanical temperature gauge to monitor the temp.

78buckshot. Very well stated! I have attempted to explain these same sentiments, just nowhere near as concise and eloquent as you just did.

I've had the opposite experience, running without a thermostat could cause you to overheat because the coolant is passing through the radiator faster than it can efficiently give up it's heat...by regulating the flow with a t-stat you give the radiator time to go it's job. It's one of the first things I ask when someone says they are overheating..."do you have a thermostat installed?"

Mike B

Mike B, I have heard of that situation but have not personally experienced it. The water pump will move A LOT of volume when unrestricted. Probably the same is true in the water jacket around the cylinders and head/s, if the water is moving too fast it may not be able to transfer the heat from the iron to the water.

We ran a big flat washer instead of a thermostat on the race engines and varied the size of the hole in it to control the temp we wanted the engine to run. If the radiator is efficient enough it will over-cool without some type of flow restrictor. I can look at the wear pattern on the piston skirts of a high mileage street engine and tell whether or not it has been run with a thermostat installed.
Jerry

I think a flat washer would be better than todays thermostats. I had a 2000 Chevrolet 6.5 diesel for 8-9 years, twin thermostats started acting up and running too cool, I put in 2 new Napa stats, still too cool. I then put in 2 new Delco stats and it took care of the issue. New parts are a crap shoot.

Using a washer in a daily driver would significantly increase engine wear from a significant extra amount of time the engine would take to warm up.
Racing? No problem with rapid heat up with the go pedal mashed from the get go.