Hesitation under load ,creating a misfire code that clears in three drive cycles
Engine runs great except when regaining speed after a downshift to pass or go uphill. hesitation and loss of power triggers a misfire code that clears in three drive cycles. have changed plugs ,wires, cap, rotor, entire sfi, fuel and air filters.
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Re: hesitation under load ,creating a misfire code that...
Problem is almost always plugs, wires, or coils/ICM. I have seen all three, they all act exactly the same with the symptoms you all describe. Sometimes CEL and code are present, other times not. Start with cheapest solution and work your way up (plugs & wires, coils, then ICM) The coils can be tested with a spark indicator individually, plugs visually inspected. Most auto shops can test the ICM.
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You either have a bad catalytic converter causing too much back pressure or you are running rich. I think cylinder 3 and 6 are on the same side of the engine (except opposite banks) right? Locate the fuel pressure regulator. Is it on the same side? I've seen those go bad and allow fuel to be sucked into the vacuum line causing a rich condition/ misfire on two or three cylinders on that side of the engine.
Since you changed the sensor we will assume it is working. When you disconnect it you put the computer in limp mode which stops it from using sensor values. If code 300 is the only code we will assume the coolant temp sensor and others are ok. I would check fuel pressure with a gauge next.
The O2 are not ready because it takes some time for them to complete there self check after clearing trouble codes. This will involve a few drive cycles. Once the evap problem has been resoled, the check engine light cleared, you'll have to drive it for a couple of days to get everything in the "ready" status.
An engine that
hesitates, stumbles or misfires when accelerating or when
it is under load is an engine that is either sucking too much air, not
getting enough fuel or misfiring. If the Check Engine Light comes on,
you may find any of the following codes:
• P0171, P0174 Lean
fuel condition codes • P0120 to P0124 Throttle position sensor codes •
P0222 to P0229 Throttle position sensor codes • P0400 to P0409 EGR
If there are no misfire codes, a common cause of
acceleration stumble is a bad throttle position sensor (TPS). The TPS
tells the computer how far the throttle is open. The computer uses this
information to determine how much fuel is needed to maintain the correct
air/fuel mixture and when extra fuel is needed if the throttle suddenly
Another common cause is dirty fuel injectors. If
varnish deposits have built up in the tips of the injectors, they won’t
spray as much fuel as they normally do, or will “dribble” fuel instead
of spraying a fine mist. This creates a lean fuel mixture and
conditions that are ripe for stumble and hesitation (also misfire). Look
at short term fuel trim (STFT) and long term fuel trim (LTFT) with your
If the numbers are high, it tells you the engine is running lean and
the injectors need cleaning. Treat mild cases with a high quality
fuel-injector cleaner additive. Severe cases require professional
Also if this sensors apply to your
vehicle, camshaft sensor or
crankshaft sensor if sensors bad you don't get any spark. Also
run a scan on the car.
Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 122°F (50°C) and within 11°F (6°C) of the ambient air temperature at startup.
Do not leave the key in prior to the cold start or the heated oxygen sensor diagnostic may not run.
engine must be run for two and a half minutes with the air conditioner
on and rear defroster on. The more electrical load you can apply the
better. This will test the O2 heater, Passive Air, Purge "No Flow", Misfire, and - if closed loop is achieved- Fuel Trim.
Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 55mph (88km/hr) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
Hold Steady Speed.
Hold a steady speed of 55mph (88km/hr) for 3 minutes. During this time
the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim
diagnostics will be performed.
Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch.
It is important to let the vehicle coast along gradually slowing down
to 20 mph (32km/hr). During this time the EGR, Purge and Fuel Trim
diagnostics will be performed.
Accelerate. Accelerate at 3/4 throttle until 55-60mph (88-96 km/hr). This will perform the same diagnostics as in step 3.
Hold Steady Speed.
Hold a steady speed of 55mph (88km/hr) for five minutes. During this
time, in addition to the diagnostics performed in step 4, the catalyst
monitor diagnostics will be performed. If the catalyst is marginal or
the battery has been disconnected, it may take 5 complete driving
cycles to determine the state of the catalyst.
Decelerate. This will perform the same diagnostics as in step 5. Again, don't press the clutch or brakes or shift gears.
Sounds like a plugged catalytic converter or in rare cases muffler. This will set codes but try this; remove O2 sensor closest to front of catalytic converter, it will be noisy but drive again, if performance is better, suspect plugged catalytic converter,replace cat and reinstall O2. Also check air filter and mas air flow sensor. Have had some luck carefully cleaning sensor with throttle body/air intake cleaner after removing from vehicle with key off. Hope this helps ! Rate me please.