Help misfire and bucking
#1
01-19-2019
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Location: Fredericksburg, Virginia
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Help misfire and bucking
I’ve got a 98 4.0 4x4 with a manual trans aprox miles 228,000. About three weeks ago the truck threw a cylinder 4 misfire code and began misfiring and bucking when ever the truck was put under a load and when letting off the clutch. After letting off the clutch the truck did decent in 1st and 2nd gear from 3-5k rpms But once I hit 3rd through 5th any time I give it any kind of throttle it bucks and skips awful. Since it started doing this I’ve replaced plugs, wires, coil pack fuel filter. I put a gauge on the fuel rail and it has 64 lbs of fuel pressure consistently even while driving and bucking. Anybody have any clues? I’m thinking either fuel injectors or intake gaskets next. Just wondering what you all think before I tear into the intake.
#2
01-19-2019
#3
01-19-2019
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#4
01-19-2019
Its possible its an intake issue, air leak, but you should have Check Engine Light(CEL) and a Lean code , with misfire codes
Easy test for vacuum leak
Warm up engine, let it idle
Unplug the 2 wire connector on IAC Valve
RPMs should drop to 500 or so or engine may even stall, either is GOOD, no leaks
If RPMs do not drop then there is a leak
If exhaust was partially blocked, you could be getting back pressure misfires, and they can come and go, with loss of power under higher load
Can be tested using vacuum gauge, $20, actually vacuum gauge is one of the "old tools" that's still one of the best diagnostic tools for all gasoline engines.
Tests here: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine
simple and easy to test
Fuel injectors are a long shot, possible, but not at all a common problem in Rangers, I still have my stock injectors in my 1994 4.0l, never been touched. over 300k
If old filter was really badly clogged or fuel that drained of of the line that goes to engine looked bad then yes, could be fuel rail and injectors are clogged up
Easy test for vacuum leak
Warm up engine, let it idle
Unplug the 2 wire connector on IAC Valve
RPMs should drop to 500 or so or engine may even stall, either is GOOD, no leaks
If RPMs do not drop then there is a leak
If exhaust was partially blocked, you could be getting back pressure misfires, and they can come and go, with loss of power under higher load
Can be tested using vacuum gauge, $20, actually vacuum gauge is one of the "old tools" that's still one of the best diagnostic tools for all gasoline engines.
Tests here: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine
simple and easy to test
Fuel injectors are a long shot, possible, but not at all a common problem in Rangers, I still have my stock injectors in my 1994 4.0l, never been touched. over 300k
If old filter was really badly clogged or fuel that drained of of the line that goes to engine looked bad then yes, could be fuel rail and injectors are clogged up
#5
01-20-2019
Join Date: Jan 2019
Location: Fredericksburg, Virginia
Posts: 12
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Its possible its an intake issue, air leak, but you should have Check Engine Light(CEL) and a Lean code , with misfire codes
Easy test for vacuum leak
Warm up engine, let it idle
Unplug the 2 wire connector on IAC Valve
RPMs should drop to 500 or so or engine may even stall, either is GOOD, no leaks
If RPMs do not drop then there is a leak
If exhaust was partially blocked, you could be getting back pressure misfires, and they can come and go, with loss of power under higher load
Can be tested using vacuum gauge, $20, actually vacuum gauge is one of the "old tools" that's still one of the best diagnostic tools for all gasoline engines.
Tests here: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine
simple and easy to test
Fuel injectors are a long shot, possible, but not at all a common problem in Rangers, I still have my stock injectors in my 1994 4.0l, never been touched. over 300k
If old filter was really badly clogged or fuel that drained of of the line that goes to engine looked bad then yes, could be fuel rail and injectors are clogged up
Easy test for vacuum leak
Warm up engine, let it idle
Unplug the 2 wire connector on IAC Valve
RPMs should drop to 500 or so or engine may even stall, either is GOOD, no leaks
If RPMs do not drop then there is a leak
If exhaust was partially blocked, you could be getting back pressure misfires, and they can come and go, with loss of power under higher load
Can be tested using vacuum gauge, $20, actually vacuum gauge is one of the "old tools" that's still one of the best diagnostic tools for all gasoline engines.
Tests here: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine
simple and easy to test
Fuel injectors are a long shot, possible, but not at all a common problem in Rangers, I still have my stock injectors in my 1994 4.0l, never been touched. over 300k
If old filter was really badly clogged or fuel that drained of of the line that goes to engine looked bad then yes, could be fuel rail and injectors are clogged up
#6
01-20-2019
Yes, that rules out any vacuum leak
It's possible MAF sensor is failing, try unplugging it and running the engine without it, CEL will come on after a bit, maybe right away, lol.
Computer already "knows" its running a 4 liter engine so already knows the amount of air that will come in at any given RPM and throttle position, that's just simple math
What it doesn't know is the WEIGHT of that in coming air, which is very important
Air:fuel mix ratio for gasoline is 14.7 to 1, and that is a WEIGHT ratio, not volume
14.7 POUNDS of air to 1 POUND of gasoline
14.7 grams of air to 1 grams of gasoline
Thats why 200mpg carbs and "pre-vaporizers" were/are a bunch of Hooey, lol.
If MAF sensor is not reporting correct WEIGHT of the incoming air, then engine will be running lean and rich, back and forth
Reason is that the MAF only "sees" about 10% of the incoming air, so if its off by 1% thats a 10% overall error, if its off 2% that's a 20% error in computer's calculations
So it is an important sensor, but..............not a common failure, not rare but not common
So unplug your MAF and go for a drive, wait until it warms up for best test, as long as outside temp is not real cold or real hot(big change in WEIGHT of the air) and you don't live above 2,000ft elevation engine should run OK
It's possible MAF sensor is failing, try unplugging it and running the engine without it, CEL will come on after a bit, maybe right away, lol.
Computer already "knows" its running a 4 liter engine so already knows the amount of air that will come in at any given RPM and throttle position, that's just simple math
What it doesn't know is the WEIGHT of that in coming air, which is very important
Air:fuel mix ratio for gasoline is 14.7 to 1, and that is a WEIGHT ratio, not volume
14.7 POUNDS of air to 1 POUND of gasoline
14.7 grams of air to 1 grams of gasoline
Thats why 200mpg carbs and "pre-vaporizers" were/are a bunch of Hooey, lol.
If MAF sensor is not reporting correct WEIGHT of the incoming air, then engine will be running lean and rich, back and forth
Reason is that the MAF only "sees" about 10% of the incoming air, so if its off by 1% thats a 10% overall error, if its off 2% that's a 20% error in computer's calculations
So it is an important sensor, but..............not a common failure, not rare but not common
So unplug your MAF and go for a drive, wait until it warms up for best test, as long as outside temp is not real cold or real hot(big change in WEIGHT of the air) and you don't live above 2,000ft elevation engine should run OK
#7
01-20-2019
Join Date: Jan 2019
Location: Fredericksburg, Virginia
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Yes, that rules out any vacuum leak
It's possible MAF sensor is failing, try unplugging it and running the engine without it, CEL will come on after a bit, maybe right away, lol.
Computer already "knows" its running a 4 liter engine so already knows the amount of air that will come in at any given RPM and throttle position, that's just simple math
What it doesn't know is the WEIGHT of that in coming air, which is very important
Air:fuel mix ratio for gasoline is 14.7 to 1, and that is a WEIGHT ratio, not volume
14.7 POUNDS of air to 1 POUND of gasoline
14.7 grams of air to 1 grams of gasoline
Thats why 200mpg carbs and "pre-vaporizers" were/are a bunch of Hooey, lol.
If MAF sensor is not reporting correct WEIGHT of the incoming air, then engine will be running lean and rich, back and forth
Reason is that the MAF only "sees" about 10% of the incoming air, so if its off by 1% thats a 10% overall error, if its off 2% that's a 20% error in computer's calculations
So it is an important sensor, but..............not a common failure, not rare but not common
So unplug your MAF and go for a drive, wait until it warms up for best test, as long as outside temp is not real cold or real hot(big change in WEIGHT of the air) and you don't live above 2,000ft elevation engine should run OK
It's possible MAF sensor is failing, try unplugging it and running the engine without it, CEL will come on after a bit, maybe right away, lol.
Computer already "knows" its running a 4 liter engine so already knows the amount of air that will come in at any given RPM and throttle position, that's just simple math
What it doesn't know is the WEIGHT of that in coming air, which is very important
Air:fuel mix ratio for gasoline is 14.7 to 1, and that is a WEIGHT ratio, not volume
14.7 POUNDS of air to 1 POUND of gasoline
14.7 grams of air to 1 grams of gasoline
Thats why 200mpg carbs and "pre-vaporizers" were/are a bunch of Hooey, lol.
If MAF sensor is not reporting correct WEIGHT of the incoming air, then engine will be running lean and rich, back and forth
Reason is that the MAF only "sees" about 10% of the incoming air, so if its off by 1% thats a 10% overall error, if its off 2% that's a 20% error in computer's calculations
So it is an important sensor, but..............not a common failure, not rare but not common
So unplug your MAF and go for a drive, wait until it warms up for best test, as long as outside temp is not real cold or real hot(big change in WEIGHT of the air) and you don't live above 2,000ft elevation engine should run OK
#8
01-20-2019
Should be instant change
With no MAF computer will run engine Richer because that is safer than Lean, so colder air temp wouldn't be much of an issue, heavier air weight
But no CEL is very very ODD, very, lol, MAF is a MAIN sensor, leave it unplugged and if possible check for pending MAF code now.
Does the CEL bulb work?
It should come on with Key on, then go off after start up
With no MAF computer will run engine Richer because that is safer than Lean, so colder air temp wouldn't be much of an issue, heavier air weight
But no CEL is very very ODD, very, lol, MAF is a MAIN sensor, leave it unplugged and if possible check for pending MAF code now.
Does the CEL bulb work?
It should come on with Key on, then go off after start up
#9
01-20-2019
Join Date: Jan 2019
Location: Fredericksburg, Virginia
Posts: 12
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Should be instant change
With no MAF computer will run engine Richer because that is safer than Lean, so colder air temp wouldn't be much of an issue, heavier air weight
But no CEL is very very ODD, very, lol, MAF is a MAIN sensor, leave it unplugged and if possible check for pending MAF code now.
Does the CEL bulb work?
It should come on with Key on, then go off after start up
With no MAF computer will run engine Richer because that is safer than Lean, so colder air temp wouldn't be much of an issue, heavier air weight
But no CEL is very very ODD, very, lol, MAF is a MAIN sensor, leave it unplugged and if possible check for pending MAF code now.
Does the CEL bulb work?
It should come on with Key on, then go off after start up
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