Slant 6 with random miss and bad MPG
#1
12-10-2014, 02:59 PM
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Slant 6 with random miss and bad MPG
Gents,
I have a 86 D100 that's my daily driver. It's a short bed, 225, auto, with 3.21 gears. I've had it across the scales and it weighs 3360 with me in it. I bought the truck back in July and have been chipping away at fixing things on it and I've ran into something that I'm having trouble figuring out.
Randomly, probably 3 or 4 times a week, I will get a dead misfire. This always happens after I've been cruising along at 50-60 for a while and slow down to a stop. While I'm stopped, say at a traffic light, it will miss, shake, chug, and sound like its going to stall. Once I step on the gas and pull away from the light, the miss goes away and it seems to run fine again. I don't notice anything out of the ordinary, sound wise / power wise / vibrations / etc, while I'm speeding up or cruising.
In the past few months, since the random miss has started happening, my gas mileage has went down a lot too. I check MPG at every fill up. Over my first 14 fill ups, covering 3,400 miles, I averaged 16.72 mpg. Over the last 7 fill ups, covering 2,400 miles, its averaging 14.89. The routes, loads, driving style, idling time, etc has always been consistent.
Since I bought the truck in July I've rebuilt the carb, replaced the fuel filter, had a valve job done which included a new intake gasket of course, replaced the timing chain, set the timing to spec, adjusted the carb with a vacuum gauge, and put in a new thermostat, air filter and PCV.
Once it started getting worse mpg and doing the random miss thing, I've done the following without anything helping either issue. Converted to HEI, new plugs, wires, cap, and rotor, checked and adjusted the pick up coil air gap, checked all vacuum lines for leaks, checked for fuel leaks, checked for proper choke operation, re-checked ignition timing, switched back to the factory ignition system, checked all 4 wheels for dragging brakes, checked tire psi, and checked the compression which had 120-130 psi on each cylinder.
What am I missing? Thanks in advance
I have a 86 D100 that's my daily driver. It's a short bed, 225, auto, with 3.21 gears. I've had it across the scales and it weighs 3360 with me in it. I bought the truck back in July and have been chipping away at fixing things on it and I've ran into something that I'm having trouble figuring out.
Randomly, probably 3 or 4 times a week, I will get a dead misfire. This always happens after I've been cruising along at 50-60 for a while and slow down to a stop. While I'm stopped, say at a traffic light, it will miss, shake, chug, and sound like its going to stall. Once I step on the gas and pull away from the light, the miss goes away and it seems to run fine again. I don't notice anything out of the ordinary, sound wise / power wise / vibrations / etc, while I'm speeding up or cruising.
In the past few months, since the random miss has started happening, my gas mileage has went down a lot too. I check MPG at every fill up. Over my first 14 fill ups, covering 3,400 miles, I averaged 16.72 mpg. Over the last 7 fill ups, covering 2,400 miles, its averaging 14.89. The routes, loads, driving style, idling time, etc has always been consistent.
Since I bought the truck in July I've rebuilt the carb, replaced the fuel filter, had a valve job done which included a new intake gasket of course, replaced the timing chain, set the timing to spec, adjusted the carb with a vacuum gauge, and put in a new thermostat, air filter and PCV.
Once it started getting worse mpg and doing the random miss thing, I've done the following without anything helping either issue. Converted to HEI, new plugs, wires, cap, and rotor, checked and adjusted the pick up coil air gap, checked all vacuum lines for leaks, checked for fuel leaks, checked for proper choke operation, re-checked ignition timing, switched back to the factory ignition system, checked all 4 wheels for dragging brakes, checked tire psi, and checked the compression which had 120-130 psi on each cylinder.
What am I missing? Thanks in advance
Last edited by chief915; 12-10-2014 at 04:31 PM.
#2
12-11-2014, 12:19 PM
#3
12-16-2014, 12:46 PM
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Location: walla walla, wa
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Hi Chief--you and I are in a similar situation: we both have near identical trucks and we are noobs needing good advice. Does your truck have about a dozen ultra-tiny vacuum lines going from the pass-side rear of the engine bay to the intake manifold area? I will be following your posts and hope to learn something about this kinda-odd model.
I bought my truck about 72 hours ago, it will start, I heard the engine run for a few minutes but as it warms up it wants to die, even a touch of the throttle makes it die, then I can start it again and it will die again.
I am just starting to get it running and am still figuring out what it needs to make it a reliable daily driver. I know those tiny vacuum tubes are all open, connected to nothing at all, but they used to be plugged into something near the intake.
Sorry I have little to add on your situation, it sounds like you are pretty knowlegable, more so than I am. Good luck on your rig--we may both learn something here.
I bought my truck about 72 hours ago, it will start, I heard the engine run for a few minutes but as it warms up it wants to die, even a touch of the throttle makes it die, then I can start it again and it will die again.
I am just starting to get it running and am still figuring out what it needs to make it a reliable daily driver. I know those tiny vacuum tubes are all open, connected to nothing at all, but they used to be plugged into something near the intake.
Sorry I have little to add on your situation, it sounds like you are pretty knowlegable, more so than I am. Good luck on your rig--we may both learn something here.
#4
12-16-2014, 12:58 PM
