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Choking and sputtering after overheating

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ScottyREwing

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Hi folks... could use some brainstorming from you smart types :)

Monday my 74 GMC K1500 4x4, Automatic transmission, 350 chev, decided to overheat. It's been running pretty hot for ages and it's definitely not due to fluids as they are fine. I'm thinking the source is the thermostat in the main rad hose at the engine connection. Ran into that before. Will get that done soon. This however isn't my problem... only when my true problem began.

After I let the truck cool down, I hopped the curb and went the other direction out of traffic and got the air blowing through the rad and brought the temp down. I was about 20 minutes down the road in top gear running about 50mph when it started sputtering and coughing and bogging down to the point that I could only drive under gentle fuel peddle application and under 40mph. It continually got worse to the point where it now does this the moment I am under load and the truck shifts into 2nd gear and above. It will remain even when I slow down and drop to first gear but it corrects itself if I take the load off the engine for a moment or two. In Park, neutral it rev's just fine... no stuttering and choking itself out. I can even blast around in first gear all over a parking lot like a hooligan and no problems. Only once the shift takes place does the problem happen again.

I checked all levels on Monday and the tranny was dangerously low so my gut tells me its the torque converter (?) that controls the shifting inside the transmission. I have topped up the tranny since this realization. I have also read recently that it could be the EGR valve?

I'm looking for all kinds of thoughts from you guys to assist in my diagnosis so lay it on me.

Thanks in advance
 
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Basics first, timing Points/condensor if u have them still.
Fuel filter , Plugs/wires. Coil , cap/ rotor, proper fuel supply.


Idles and revs ok prob nothing broken, could of lost a cam lobe.
 
Unfortunately all of these things are out of my scope of general knowledge. Cam lobe? Never heard that one before. Now I'm off to google to check that out.

I'm going to replace all plugs later but something tells me it isnt the plugs. All 8 are firing I can tell that much because I had number 5 crap out on me on occasion and know the feeling of dropping to 7 cylinders. # 5 has a ring issue so blows blue on startup.

My gut is telling me a diaphram off the carb has died but there are several and I dont really know what any of them do. Later I will try and get a picture of what my machine looks like under the hood so you guys will have something to start with.

Thanks
 
Interesting thing I just found on the web... last week I put a new fuel filter in the carb and after coming across this page... I'm wondering if its the filter I put in.

http://67-72chevytrucks.com/vboard/showthread.php?t=325241

I dont have that chrome housing you see in the thread but I do have that exact paper filter. I tossed my old one already but when I seen this I remember thinking about that rubber end and then not thinking any further about it. Sounds like its got too much resistance and not letting enough fuel through. Could be the overheating was the trigger to cause the spring diaphram to alter? I'm going to do a mod on that filter this evening just like he did in this thread and see what/if that fixes or does anything. I like to start with the easy stuff first. Its the cheapest. ;p
 
It was mentioned earlier but what your describing is a fuel issue. Low fuel pressure would cause the issues you described. A bad diaphragm in the pump or
A clogged filter could cause your problem. I would disconnect the fuel line between the pump & carb and check for fuel flow/ pressure. If you have a see thru filter it should be full all the time when the engines running but if it empty's at any time that would indicate low pressure. Low fuel pressure could cause the engine to run lean so an inspection of your plugs to check for detonation is in order. Running lean will also raise your cylinder temps so this could be why your running on the warm side.
The egr is easy to check. With the engine cold and running reach your fingers around to the back side of the valve and press on the diaphragm to open the valve. The engine should stall. If not it's clogged. If the valve was stuck open then your engine wouldn't idle due to a vacuum leak. Note if the egr valve is stuck closed it will raise the combustion temps also.
The torque converter doesn't dictate when your shifts happen. It uses fluid pressure/ friction to apply torque from the engine to the trans to make your truck go. If it fails You wouldn't be able to drive around In 1st gear like a hooligan! Your trans uses fluid pressure,modulator valve and tv cable (throttle valve cable) to know when to shift. A low fluid level in an old truck is likely from a leak. There is also a vent tube on the top of the trans. If it were to overheat it could push fluid out.(again a leak) last is the modulator valve witch is connected to the engine by a vacuum line. If the seals fail in the valve the engine would suck fluid thru the line like a straw witch would be obvious because of heavy white smoke coming from the tail pipes.
 
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Thanks earnett for your time and information shared. I have much to do on this old truck and any info of any kind is gold in my opinion.

cheers
 
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