i have a bad miss under strain
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Junior Member
Joined: Jan 2005
Posts: 4
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From: Nova Scotia
i have a bad miss under strain
i have a 92 f150 4x4 with a 302 and overdrive tranny,,,it has a bad miss when under strainit does it more so when going up a hill,,as soon as the rpm;s drop it starts to buck,,but if i give it more gas to shift down another gear it seems fine.it does it more so when in overdrivehope u guys can help ,i'm on my last leg with this 1
Senior Member
Joined: Jan 2005
Posts: 130
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From: Dublin, OH
Three very common problems really show up under load (1) secondary ignition (coil, wire, boot, plug), (2) fuel delivery, (3) pinging. If it were pinging (knocking), you would have heard it and mentioned it.
Under heavy loads, the demand for fuel is the highest. If you pump is starting to go or the fuel filter is starting to plug, it will probably run fine except under heavy load. This could be it. It may be accompanied by lean symptoms like bucking or surging.
Under heavy loads, higher voltage is required to fire the spark plug. This is because as you push the throttle, the vacuum is reduced, so the pressure in the cylinder (before combustion) is higher, so there are more molecules of air/fuel to ionize in the plug gap. Bottom line is more stress is placed on the coils, wires, boots, and plugs. Most secondary weakness run fine until under load and then fail. Usually when this happens, there is just a loss of power and very slight roughness, a normal miss.
You said two things, bucking and only in overdrive that lead me in different directions. Bucking is a lean symptom that makes me suspect the fuel delivery, the pump or filter. But unlike fuel where the combination of RPM or throttle position cause a problem, spark is independent of RPM (with distributorless systems like these) and is only a function of throttle position. That is, going up the same hill at the same speed, fuel delivery problems would be about the same in any gear whereas ignition problems would be better in lower gears and higher RPM and worse in higher gears.
Try tuning to an AM station that is only static. Find one that is fairly quiet. Then turn it up medium loud. As you drive, let off the gas. Start adding throttle listening for a distinct buzz that sounds like the continuous firing of a spark plug. Keep adding throttle until you're wide open or you hear it. If you hear it, downshift and make sure it goes up with RPM. If you hear it, you have a secondary problem. The problem is then what to do about it. Changing coils/wires/plugs can be expensive so I'd perform a visual inspection looking for burned boots or spark burns on parts adjacent to coils and look at the plugs. If I had a spark tester (looks like a spark plug with a ground clip), that would tell me which cylinder(s). Or I'd consider having it diagnosed with an ignition scope.
If you think it's your filter, take it off. If you blow through it, it should have NO resistance. If your cheeks puff out, it is badly plugged. If you just feel some resistance, it will cause this only at high load problem. If it's fuel and the filter's good, try unplugging the MAF sensor and see if it is a fuel calculation problem. If it's better with the MAF unplugged, clean the MAF. This can cause a WOT fuel delivery problem too.
Let us know.
I just reread your post. If you have a traditional electronic distributor and coil, and you suspect the ignition, check under the cap for rotor or tracking between the coil tower and plug towers.
Under heavy loads, the demand for fuel is the highest. If you pump is starting to go or the fuel filter is starting to plug, it will probably run fine except under heavy load. This could be it. It may be accompanied by lean symptoms like bucking or surging.
Under heavy loads, higher voltage is required to fire the spark plug. This is because as you push the throttle, the vacuum is reduced, so the pressure in the cylinder (before combustion) is higher, so there are more molecules of air/fuel to ionize in the plug gap. Bottom line is more stress is placed on the coils, wires, boots, and plugs. Most secondary weakness run fine until under load and then fail. Usually when this happens, there is just a loss of power and very slight roughness, a normal miss.
You said two things, bucking and only in overdrive that lead me in different directions. Bucking is a lean symptom that makes me suspect the fuel delivery, the pump or filter. But unlike fuel where the combination of RPM or throttle position cause a problem, spark is independent of RPM (with distributorless systems like these) and is only a function of throttle position. That is, going up the same hill at the same speed, fuel delivery problems would be about the same in any gear whereas ignition problems would be better in lower gears and higher RPM and worse in higher gears.
Try tuning to an AM station that is only static. Find one that is fairly quiet. Then turn it up medium loud. As you drive, let off the gas. Start adding throttle listening for a distinct buzz that sounds like the continuous firing of a spark plug. Keep adding throttle until you're wide open or you hear it. If you hear it, downshift and make sure it goes up with RPM. If you hear it, you have a secondary problem. The problem is then what to do about it. Changing coils/wires/plugs can be expensive so I'd perform a visual inspection looking for burned boots or spark burns on parts adjacent to coils and look at the plugs. If I had a spark tester (looks like a spark plug with a ground clip), that would tell me which cylinder(s). Or I'd consider having it diagnosed with an ignition scope.
If you think it's your filter, take it off. If you blow through it, it should have NO resistance. If your cheeks puff out, it is badly plugged. If you just feel some resistance, it will cause this only at high load problem. If it's fuel and the filter's good, try unplugging the MAF sensor and see if it is a fuel calculation problem. If it's better with the MAF unplugged, clean the MAF. This can cause a WOT fuel delivery problem too.
Let us know.
I just reread your post. If you have a traditional electronic distributor and coil, and you suspect the ignition, check under the cap for rotor or tracking between the coil tower and plug towers.