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Chevy 350 Misfire

12K views 14 replies 3 participants last post by  EragonSuperH 
#1 ·
The 350 in my 1978 Sierra Classic is misfiring. The weather is a bit colder now but it still does it once it warms up. It will continue misfiring as I go into town, but then it will usually stop and run perfectly smooth on the way home (14 mile round trip). Any ideas as to why this could be happening? The choke also sticks sometimes but I don't think that's the cause.
 
#4 ·
since you have an electric choke.. and i would think it is a stock choke set up..

you might want to look under the distributor.. you may find 2 oil pressure switches/senders.. there will be a single wire version for the gauge.. there may be a 2 wire version.. when the oil pressure comes up. .that pushes the switch contacts closed and powers the choke coil fully.. when the choke light is glowing on the dashboard with the engine running.. the choke is only getting a fraction of the current needed to heat the choke thermostat and cause it to open.. what.. yep... turn the key on.. power flows thru the choke light to the switched side of the oil pressure switch.. since the choke thermostat is pulling the voltage low. the light will turn on.. when the engine starts.. the oil pressure comes up and closes the switch.. so the power on the switched side of the oil pressure switch circuit is equal to the power from the ignition switch so the light goes out..

the wiring to the switch can fail.. the terminals in the oil pressure switch socket can open up from being removed and installed or just thermal cycling and fail to make good contact.. so the choke thermostat will stay closed.. causing you to foul out the spark plugs.. it will take several miles of operation for the spark plugs to clean off. .


there could be other issues... depending on if your model came equipped with a secondary choke pull off... lack of the vacuum hoses that are on the back of the air cleaner housing being properly hooked up can result in a stuck closed or stuck at fast idle issue..

i don't recall if the 1978 still have the vacuum operated trap door on the passenger side exhaust manifold outlet.. if equipped it will have a large vacuum advance looking device with a long rod that goes down toward the end of the passenger exhaust manifold.. that should have vacuum applied after about 30 to 45 seconds on a cold start.. pulling the trap door open when the engine coolant gets to 110F or so.. its usually hooked up to a big orange ported vacuum switch..

on the ignition system.. the rear 2 spark plug wires on the drivers side.. must be over an inch apart .. they must NOT be touching or tie wrapped together.

would you like to go farther..
 
#7 ·
Thanks for all of that great information, but I've never seen a choke light in this pickup. It could just have been burned out since before I was born though. Is it beside the light that says fasten seat belts when you start the truck? I think I solved the issue with the misfiring though. It's condensation/moisture under the dist. cap since it sits outside all the time.
 
#15 ·
I hate to bring up this ancient thread, the engine it pertains to is long gone and the truck is currently being put back together, but I wanted to clarify something. The issue I was having in this post was not condensation under the cap like I had previously diagnosed so many years ago. It was oil fouled plugs! Number 6 would foul so bad that I couldn't even see the electrodes. I think it happened when a plug wire shorted out on the manifold and misfired that cylinder for a long time before replacing the plugs and wires and it washed the cylinder with gas and ruined the ring seal. This then resulted in extreme oil consumption which made it necessary to clean or replace the spark plugs every 1000 miles or so. The valve guides seemed fine because it never smoked on startup. Just figured I'd rectify a past misinformation because I had seen this thread I posted so long ago.
 
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