truck missfire

karl klein

Senior Member
Location
omaha,ne
i own a 1976 chevy c-20 with a 400 in it. lately i have noticed that when you are driving with a load or up hill the engine seems to stutter or missfire

i was wondering if anyone new what this could be thank you
 

75

PlowSite.com Addict
M-a-n-y possibilities here, the most obvious one that comes to mind is how recently has there been a tune-up and is the fuel filter(s) and fuel pump in good shape?

If the fuel filter(s) are getting clogged there will be enough fuel flow for idle and cruising, but not enough for the engine under heavier loads like going uphill or a full load in the truck as you describe.

That's probably a good place to start anyway.
 

75

PlowSite.com Addict
That goes back to the question of how recently has there been a tune-up? Plugs certainly could be a factor, I have a feeling it may be a combination of several items.
 
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K

karl klein

Senior Member
Location
omaha,ne
i just bought it about amonth ago the guy said he replaced the plugs wire and distributer a year ago but the plugs do look pretty bad on the outside
 

75

PlowSite.com Addict
the guy said he replaced the plugs wire and distributer

I think they all say that............................

What is the overall condition of the engine? If it's in decent shape then a tune-up may be the first thing you should try. Plugs/wires/cap/rotor/fuel filters are a good place to start if the engine is in decent shape mechanically.
 
OP
K

karl klein

Senior Member
Location
omaha,ne
i pulled the plugs today and one of them was very gunked up the others looked fine so i replaced all of them and drove it about 5 miles and so far so good. but i was wondering if it really is fixed
 

Chuck Smith

2000 Club Member
Location
NJ
Karl, the many times I have had the same problem it was spark plug boots. Once when I was trying to find the miss, I found out I had no rear brakes in the process.
Get yourself a squirt bottle, like an old windex bottle and fill it with water. Put it on "mist", have a helper get in the truck, start it up, put it in gear, hold the brake down, and give it a little gas to put a load on it. You spray the spark plugs and plug wires with the water mist. The water will bring out any arc problems. Believe me, I thought I had none until I tried this at night once, man it looked like lightening under the hood, arcing all over the place once I added water.

What happened one time with my rear brakes was a line let go while we were doing the mist test. Luckily the truck didn't lurch forward. I was sitting there in park after testing one side and seeing an arc, waiting to do the other side. I put my foot on the brake to put it in gear and the pedal went to the floor. My buddy said he saw a stream of brake fluid go squirting past his leg. Needless to say we replaced only the one bad spark plug boot. Luckily I was at his shop, and we were able to fix the brakeline there.

It seems (in my cases) that the 90° spark plug boots develop cracks, I would guess from heat. Inspect the boots carefully, mine were only about a year old when I had to replace a boot on both my trucks. In 13 years I have replaced boots more than plug wires.

~Chuck
 

John DiMartino

PlowSite.com Veteran
Location
Walden,NY
Spark plug wires are the main cause of a miss under heavy load,the higher cyl pressures require more KV at the spark plug to light off the spark,if the wires are weak,electricity takes the path of least resistance,the spark energy will find it easier to jump right thru the insulation of the plug wire,usually by the hot exhaust mainifold,and ground out to the mainifold,than it is to fire the plug under pressure,the instant it does this,you feel the motor miss.The wires may be only a year old,but if the motor leaks oil,and they got soaked,the insulation doesnt last as long,and if they were routed poorly,happens a lot,they might have rubbed thru on the manifold somewhere.if he used cheap wires,ive seen them last only a year too.A majot tunr up is your best be,and should cost around 85-110 bucks for that truck in parts.Your plugs are all full of stuff from oil fouling,a carb/choke problem running rich,or possibly you do a lot of short trips,and light loads where the enigne temp never gets hot enough to burn anything off the plugs,and out of the cylinders,short trips,and light loads load engines up with carbon deposits.take is out ,get it fully warmed up,and work it a little ,once in a while,clear its throat,so to speak.
 

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