Engine miss under hard acceleration
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Junior Member
Joined: Jan 2005
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Engine miss under hard acceleration
I own a 99 short wide w/ a 4.6 and about 65,000 miles. It has ran well for many years but lately I have noticed a miss under hard acceleration. That is the only time it misses. Under normal driving conditions it runs fine. One of the coil packs was replaced a few years ago. I'm not sure if it is the coil packs or spark plugs??? If any of you guys have any ideas they would be greatly appreciated.
Senior Member
Joined: Jan 2005
Posts: 130
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From: Dublin, OH
The heavy acceleration hides a lot of faults. Vacuum leaks are overwhelmed, the engine can soak up too much EGR, small misses are easy to overlook. The big two heavy acceleration misses are secondary ignition and fuel pressure.
Secondary ignition. The volts required to fire a plug are roughly proportional to the number of molecules of air and fuel between the center electrode and ground strap. The higher compression, the more air. At no load (idle or rev'ing in neutral) it only takes 3 to 5 KV to fire a plug. At heavy load, full throttle, it takes upwards of 20 to 25 KV. If the coil is tunneled, a weak spot in the plug wire insulation or COP boot, water in the boot, any high voltage weakness, it will run fine at idle but miss under load.
There are two simple effective tests. (1) tune to an AM station that has no signal, just static and turn up the radio. Ignore the whirs and hisses. Listen for a very fast crack or sparking noise, you'd know it if you heard it, of secondary ignition sparking. You probably won't hear it off throttle, but put it in a high gear and push the accelerator. When you hear sparking sounds, it is a secondary ignition.
The other test is a spark tester. Costs <$10, looks like a plug with an alligator ground clip and no ground strap above the electrode. It will have to jump from the center electrode about 3/4" over to the side of the tester. That much gap in the atmosphere is equivalent to the .035 to .055 gap of a plug under heavy load. Put it on not running, start it and watch. If the spark isn't there or isn't consistent, that coil, boot, or wire is bad.
The other common miss under load is fuel pressure. Your pump and filter may deliver enough pressure and volume at low demand but under load the demand goes up dramatically. If the pump is weak or the filter starting to plug up, it will occur then. You could rig a fuel pressure guage in the cab but if you have a scan tool watch either the Bank1Sensor1 or Bank2Sensor2 O2 sensor. At wide open throttle it goes into open loop and runs rich so the O2 sensors should stop toggling low and high and read over 600mV (high voltage). If they go low instead, under 300 mv, the truck is running lean when commanded rich. Since it runs fine when not under load, we don't expect the fueling problem to be a plugged injector, it would be the pump.
Your fuel pressure may be fine in the shop but inadequate under heavy load. Since under wide open throttle it goes into open loop (using the MAF instead of the O2 sensors it normally uses for fuel calculations), your WOT test for rich could be a bad MAF. If you're curious unplug it and try again. It will use the TPS instead of the MAF as a backup so you can eliminate the MAF.
Hope this helps.
Secondary ignition. The volts required to fire a plug are roughly proportional to the number of molecules of air and fuel between the center electrode and ground strap. The higher compression, the more air. At no load (idle or rev'ing in neutral) it only takes 3 to 5 KV to fire a plug. At heavy load, full throttle, it takes upwards of 20 to 25 KV. If the coil is tunneled, a weak spot in the plug wire insulation or COP boot, water in the boot, any high voltage weakness, it will run fine at idle but miss under load.
There are two simple effective tests. (1) tune to an AM station that has no signal, just static and turn up the radio. Ignore the whirs and hisses. Listen for a very fast crack or sparking noise, you'd know it if you heard it, of secondary ignition sparking. You probably won't hear it off throttle, but put it in a high gear and push the accelerator. When you hear sparking sounds, it is a secondary ignition.
The other test is a spark tester. Costs <$10, looks like a plug with an alligator ground clip and no ground strap above the electrode. It will have to jump from the center electrode about 3/4" over to the side of the tester. That much gap in the atmosphere is equivalent to the .035 to .055 gap of a plug under heavy load. Put it on not running, start it and watch. If the spark isn't there or isn't consistent, that coil, boot, or wire is bad.
The other common miss under load is fuel pressure. Your pump and filter may deliver enough pressure and volume at low demand but under load the demand goes up dramatically. If the pump is weak or the filter starting to plug up, it will occur then. You could rig a fuel pressure guage in the cab but if you have a scan tool watch either the Bank1Sensor1 or Bank2Sensor2 O2 sensor. At wide open throttle it goes into open loop and runs rich so the O2 sensors should stop toggling low and high and read over 600mV (high voltage). If they go low instead, under 300 mv, the truck is running lean when commanded rich. Since it runs fine when not under load, we don't expect the fueling problem to be a plugged injector, it would be the pump.
Your fuel pressure may be fine in the shop but inadequate under heavy load. Since under wide open throttle it goes into open loop (using the MAF instead of the O2 sensors it normally uses for fuel calculations), your WOT test for rich could be a bad MAF. If you're curious unplug it and try again. It will use the TPS instead of the MAF as a backup so you can eliminate the MAF.
Hope this helps.