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What makes a rough idle a misfire condition?

What makes a rough idle a misfire condition?

What complicates a rough idle/misfire condition is having the PCM not register any specific misfire codes. In my neighbor’s Blazer, the PCM wasn’t even registering a P0300, but it was obviously running very rough (at idle that is).

How often does a 94 S-10 4.3 drop rpm?

After you take it out and drive it, it’ll idle fine for a couple minutes, then will drop a couple hundred rpms every 30 seconds or so for just a count, then back to normal. Any ideas? I’m pulling my hair out.

When to use rough idle when in drive?

Q: Rough idle when in drive..but idle is fine when in nuetral or park. Truck idle is rough when in drive with no acceleration (stop signs, red lights, traffic) but idle is fine when in park or nuetral. . Recommended Services. Car idle is rough Inspection.

How is my 1994 S-10 4.3 running?

Starts fine, even today when it was -15, fuel pressure 52 running, holds after shut down, newer plugs/wires/cap rotor, tested MAP sensor and tests good, tested resistance changes in TPS (with sensor out) and changes were smooth along range, unplugged every sensor I could find individually looking for changes, no luck.

What should the idle rate be on a 1997 GMC Truck?

Underdog, I don’t know the 1997 5.7, but 1000rpm sounds like a high idle, not where it should “bottom out”. A V8 should idle at 500rpm. GoNDown, did you recently disconnect your battery? After I left mine dead in the yard for a while and didn’t charge it, when I started it up it ran like you describe for a few days and then it was fine.

After you take it out and drive it, it’ll idle fine for a couple minutes, then will drop a couple hundred rpms every 30 seconds or so for just a count, then back to normal. Any ideas? I’m pulling my hair out.

Starts fine, even today when it was -15, fuel pressure 52 running, holds after shut down, newer plugs/wires/cap rotor, tested MAP sensor and tests good, tested resistance changes in TPS (with sensor out) and changes were smooth along range, unplugged every sensor I could find individually looking for changes, no luck.

What’s the idle RPM on my Chevy Suburban?

My 1997 Chevy Suburban (5.7) is acting up on me. When I start it up the RPM and idle seem fine. As I let it sit for a couple of seconds the RPM’s slowly begin to drop until they finally bottom out at about 1K. As soon as I put it into Reverse/Drive etc. the RPM’s drop below that mark and it struggles to stay running.

Is there an engine misfire code on a GM 4.8L?

Engine Misfire or Rough Engine Idle on GM 4.8L, 5.3L, 6.0L is becoming; more common and getting harder to diagnose. Because, GM V8 engines are very prone to a rough idle and engine misfire when cold. The check engine light may also come on, with codes P0300, P0171 and PO174.

Which is cylinder fails in a mis fire?

Sean, don’t panic, your mis-fire is cylinder #5, the AFM lifters that fail are #1, 4, 6 and 7. At this time, you have no AFM worries. Did you read the thread that @Rebel_Bowtie linked into this thread? There is a reference to a service bulletin (a TSB) within that thread, read it and see if it describes your problem.

Why does my GM V8 engine misfire when cold?

Because, GM V8 engines are very prone to a rough idle and engine misfire when cold. The check engine light may also come on, with codes P0300, P0171 and PO174. But, as the engine warms up, the engine misfire or rough idle tends to go away; which seems confusing.

How to troubleshoot a misfire ( GM 4.8L, 5.3L )?

Engine Mechanical: Misfire Troubleshooting. You can find this tutorial in Spanish here: Cómo Diagnosticar y Resolver Una Falla en Cilindro (GM 4.8L, 5.3L, 6.0L) (at: autotecnico-online.com ). The core symptom of a misfire is rough idle.

What complicates a rough idle/misfire condition is having the PCM not register any specific misfire codes. In my neighbor’s Blazer, the PCM wasn’t even registering a P0300, but it was obviously running very rough (at idle that is).

What do you need to know about cylinder misfire?

This means you need to remember the engine load, throttle position, RPM and of course, road speed. For a cylinder misfire problem, you need to change components, such as coils, spark plugs, etc. to isolate the root cause of the misfire. Also, make sure you jot down the other codes, as misfire can be caused by a malfunctioning component or system.

Why is my GMC Sierra running rough at idle?

These packs are fired by the engines computer at exactly the right time to provide the optimal ignition timing. If a coil pack goes bad, it’ll cause a misfire and rough idle condition. Really, your Sierra will be running rougher at any RPM, but a misfire will be most pronounced at idle.

What kind of engine does a Chevy Blazer have?

Although this case study is about my neighbor’s 1995 Chevy Blazer with a CPI equipped 4.3L engine, the info easily applies to any CSFI ‘Spider’ equipped 4.3L, 5.0L, and 5.7L equipped GM vehicle.

How can I tell if my Blazer cylinder is dead?

If the cylinder is NOT ‘dead’, then this will have a negative effect on engine idle since it will get worse. Repeat this test on the remaining 4 spark plug wires (testing 2 at a time). The result? The result of the cylinder balance test was that cylinder #1, on my neighbor’s Blazer, was dead!

Can a GM 4.3L Vortec engine be tested?

This isn’t possible on the GM 4.3L, 5.0L, and 5.7L Vortec engines with a ‘Spider’ fuel injector (the fuel injector is inside the intake manifold plenum and its outside electrical connector doesn’t allow for such a test).

Where is the idle control valve on a Chevy Vortec?

The idle control valve sits on the throttle body and is responsible for adjusting the amount of air that enters the engine at idle, ultimately controlling engine idle speeds. At idle, the valve blocks a small amount of air from entering the engine, keeping idle RPM’s low.

What are the symptoms of a Chevy Vortec engine problem?

Symptoms of Failing Idle Control Valve and Throttle Position Sensor 1 Rough idling 2 Engine stalling 3 Hesitation during acceleration 4 Hard shifting and lack of power 5 Lack of accelerator responsiveness 6 P0122 engine code (TPS sensor)

Although this case study is about my neighbor’s 1995 Chevy Blazer with a CPI equipped 4.3L engine, the info easily applies to any CSFI ‘Spider’ equipped 4.3L, 5.0L, and 5.7L equipped GM vehicle.

Why is my 7.4 Vortec rough idle leaking fuel?

Get a scanner on it and check the LTFT’s. If they’re pegged at +25 then the FP regulator is leaking. Very very common issue on the 7.4’s. If you don’t have scanner access connect a FP gauge, cycle the fuel pump a couple times and then watch the gauge for drop off.

What causes an engine to misfire on all cylinders?

Low fuel pressure will cause a lean mixture in your engine which will result in misfires on all cylinders. If you have trouble codes for misfires on all cylinders, you will want to check your fuel pressure. 5. Injector Problem Another problem, which was more common five years ago, is injector problems.

How can you tell if your car engine is misfiring?

Rough acceleration is a typical sign that your engine is misfiring. Sometimes, the engine will misfire on idle also; your engine sensors will get faulty values, and the air-fuel mixture will get messed up. This can cause a very uneven idle, which can jump up and down, and the engine may also shut off on idle.

Get a scanner on it and check the LTFT’s. If they’re pegged at +25 then the FP regulator is leaking. Very very common issue on the 7.4’s. If you don’t have scanner access connect a FP gauge, cycle the fuel pump a couple times and then watch the gauge for drop off.

Can a 2000 Chevy Monte Carlo be misfired?

This is a real life case study that will help you diagnose a hard to find misfire. Although this case study involves a 2000 Chevy Monte Carlo (with a 3.4L V6), you can apply this info to any 3.1L or 3.4L GM equipped vehicle (see the Applies To box on the second column).

What’s the idle RPM on a 99 Vortec?

Truck recently had an exhaust manifold replaced. When its cold (as in first thing in the morning), it fires right up and high-idles for a few seconds and runs fine. After the first few seconds, its idles rough. The engine struggles to keep it above 7-800 RPM.

What happens when the EGR valve gets stuck on a GM 4.3L?

This tutorial addresses a very common problem I’ve seen on the 1987-1995 GM 4.3L, 5.0L, and 5.7L engines with throttle body fuel injection system: a stuck open EGR valve. And when the EGR valve gets stuck open, it’ll cause a rough idle condition that may hard to diagnose.

How is cylinder balance used to diagnose misfires?

Over the years, this test has helped me solve tons of hard to find misfires on pre-OBD II and OBD II vehicles. The idea behind a cylinder balance test is to ‘short out’ an engine cylinder to see if the cylinder is ‘dead’ or not (at idle).

How to diagnose hard to find misfire cases?

But there were no trouble codes at all. Just to make sure, I asked him for the scan tool and checked myself too!

What are the misfire codes for GM 4.3L?

P0303 Cylinder #3 Misfire. P0304 Cylinder #4 Misfire. P0305 Cylinder #5 Misfire. P0306 Cylinder #6 Misfire. P0307 Cylinder #7 Misfire. P0308 Cylinder #8 Misfire. Sometimes, even tho’ the engine is suffering a bona-fide misfire, no misfire codes are registered in the computer’s memory and/or no check engine light (CEL) on.

What is the code for a 1996 Chevy k1500?

Have a 1996 chevy k1500 has code p0300 (random miss fire) have replace cap, rotor, wires, plus, coil unit, mass air flow Have a 1996 chevy k1500 has code p0300 (random miss fire) have replace cap, rotor, wires, plus, coil unit, mass air flow – Answered by a verified Chevy Mechanic

What happens if you misfire the cylinder 4 on a Silverado?

It also can cost money to ignore it, since driving with a misfire can damage the vehicles catalytic converter. The engine is also not firing all of the fuel, so raw fuel is also passing through Cylinder 4 into the exhaust. Mileage may suffer as well when your Silverado has P0304 as well.

What are the symptoms of a misfire code?

Whether your GM vehicle has a V6 or a V8, if one cylinder or two cut out, it won’t run on 100% engine power and this will cause some of the following consequences (symptoms): The check engine light (CEL) will be on. One or more misfire codes (P0300-P0308) will be stored in your pick up’s PCM memory.

How to diagnose hard to find misfire case study?

But, simply not trusting the fuel pressure leak down test results, he had also removed the intake manifold’s plenum to visually confirm that there were no leaks (there were none). What complicates a rough idle/misfire condition is having the PCM not register any specific misfire codes.

Over the years, this test has helped me solve tons of hard to find misfires on pre-OBD II and OBD II vehicles. The idea behind a cylinder balance test is to ‘short out’ an engine cylinder to see if the cylinder is ‘dead’ or not (at idle).

But, simply not trusting the fuel pressure leak down test results, he had also removed the intake manifold’s plenum to visually confirm that there were no leaks (there were none). What complicates a rough idle/misfire condition is having the PCM not register any specific misfire codes.

If the cylinder is NOT ‘dead’, then this will have a negative effect on engine idle since it will get worse. Repeat this test on the remaining 4 spark plug wires (testing 2 at a time). The result? The result of the cylinder balance test was that cylinder #1, on my neighbor’s Blazer, was dead!

What causes rough engine idle or TDC code?

So, should the engine misfire, rough engine idle or (TDC) code; point to an air leak in the intake manifold gasket, following the below directions. A very common cause of a leaking intake manifold is the gaskets and bolts used from the factory.

Engine Misfire or Rough Engine Idle on GM 4.8L, 5.3L, 6.0L is becoming; more common and getting harder to diagnose. Because, GM V8 engines are very prone to a rough idle and engine misfire when cold. The check engine light may also come on, with codes P0300, P0171 and PO174.

So, should the engine misfire, rough engine idle or (TDC) code; point to an air leak in the intake manifold gasket, following the below directions. A very common cause of a leaking intake manifold is the gaskets and bolts used from the factory.

Because, GM V8 engines are very prone to a rough idle and engine misfire when cold. The check engine light may also come on, with codes P0300, P0171 and PO174. But, as the engine warms up, the engine misfire or rough idle tends to go away; which seems confusing.

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Ruth Doyle