P0300 Chevy 7.4 Vortec

Justin Technician California Posted   Latest  
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Driveability
2000 Chevrolet C3500HD 7.4L (J L29 290hp) 4-spd (4L80-E)
P0300 — Random/Multiple Cylinder Misfire Detected

So I have an rv cutaway that has 30 k and runs good . 4 years ago /15 k miles ago the previous owner replaced engine for a siezed engine . Previous owner had shop install a gm crate engine . I have what I feel is a lean misfire but what that really means is it is almost not obvious misfire even to a trained tech . I have performed the following data review at idle and just off idle which is where the misfire counts occur on every cylinder eventually but number 2 / 1 / 7 /8 are the biggest offenders. Also when it shows more then one misfire such as cylinder 2 and 8 at the same time I could not feel misfire . It will show misfires up to 1100 rpm but after that nothing . - viewed engine data and this is what I have at idle - maf-7.4 gs map 35 kpa ( baro 101 - 35 = 19.49 in hg which matches my vacuum gauge ) cam sensor -12 degrees tps 0 percent baro 101 . Long term fuel trim 2-3 percent negative both banks and short term 128 ( zero ) . Compression on all cylinders 150-155 and running comression 50 on the 3 I checked . So I first started with plug and wire move to see if worst misfire count changed and it did not . Then installed new wires , plugs .cap and rotor due to wires breaking at plugs . Next adjusted cam sensor sense timing was at 12 degrees instead of zero along with removing distributor to inspect gears - ok but did find aftermarket rotor screws scrathcing cam sensor but away from magnet since they were too long or meant for cap !! No change with shorter screws . I will say that unplugging map sensor this speed density engine will not run but reconnect map and unplug maf and she runs and shows zero misfires. Also I have smoke tested intake, ohmed injectors ( 12.9 ) , flow tested with scan tool ( all dropped evenly from 50 psi to 21 psi ) . It acts like a lean misfire or water in fuel( took sample and a couple of very tiny specs but why is flow test ok ) but why is the misfire going away with maf unplugged ( disable monitor ??) . The one thing I have not done is move injectors around or scope maf or ckp . If I move injectors around im worried its not going to be a valid test because of misfire counts not happening on just one cylinder . I am thinking of scoping cam and crank sensor but do not have a pressure transducer.

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Phillip Educator
Ontario
Phillip
   

Has this got the vortec type distributor? If so the dist is out of sync. Cam offset should be 0 +/- 2 degrees. Try syncing the dust. Out of sync results in a wider gap between the rotor and plug wire resulting in a shorter burn time and higher peak firing voltages. Curious to see some ignition waveforms. Sorry just read the whole post. Well I'd have a look at ignition waveforms. Could be a weak…

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Justin Technician
California
Justin
 

performed variation relearn with no change. I will try a leakdown even though compression was good. this is a non spider web vortec big block . I smoked intake and no smoke external . I WILL SCOPE IGNITION .

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Paul Diagnostician
Tennessee
Paul
 

Variation relearn does not work on engines with distributor. The distributor has to be turned by hand and engine started and revved above 1500 rpm to see offset of distributor. Should be 0 +/- 2 degrees.

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Caleb Diagnostician
Indiana
Caleb
 

If you can not “feel” the misfire seems easy enough to try a CKP Variation relearn if available on this vehicle.

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Jim Technician
New Jersey
Jim
 

Right injectors in the right holes? Crank relearn done?

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Victor Mobile Technician
California
Victor
   

Most likely valve seats are leaking so do a cylinder leak test . also could be a intake manifold leak that p0300 happens when engine is cold due to hard rubber intake plenum gaskets they are not sealing till engine warms up. And the valve seats thats at any time. if you have a pressure transducer use it that will help alot.

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Robert Technician
Michigan
Robert
   

Is it actually misfiring that puuuurs like a kitten but chasing something because bogus scan tool says GM is misfiring monitor …. if its not misfiring wait my bad sorry P0300 Ive ran across this for years on that Era GM I dont freak unless misfire monitor looks like a tachometer 3 misfires in History and it snot actually misfiring instead of 0 on a 2000 and the Iridium plugs seem real touchy…

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Rusty Owner/Technician
Massachusetts
Rusty
 

What happens if you disable EGR?

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Tom Owner/Technician
Tennessee
Tom
   

I have a '99 7.4 in my personal truck. I recently had to replace my EGR valve. It was causing an off idle misfire while taking off from a stop with the slightest throttle openings. It only bothered the engine at very low rpms and minimal throttle opening. Try unplugging it. Also, cam sensor offset should be 0 +- 2 degrees.

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Rusty Owner/Technician
Massachusetts
Rusty
 

A hyperactive EGR is my suspicion also. That's why I suggested he disable it. The fact that its all the corner cylinders that are counting up makes me think that the cause is mixture related and may be simply a density misfire. I wonder if running it in S/D mode disables the EGR system . I think that EGR valve may have internal backpressure regulation that modulates the opening based of exhaust…

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Tom Owner/Technician
Tennessee
Tom
 

I think this valve is strictly influenced by duty cycle. I don't believe it uses backpressure at all. But I could be wrong.

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Sherman Owner
Texas
Sherman
 

P0300 is so misunderstood and misdiagnosed! So, start the engine, let the idle stabilize. Take a piece of notebook paper and put it up to the tail pipe. Does the exhaust suck the paper back into the stream up against the pipe for an instant? If not, you do NOT have a misfire! No computer necessary, jus a .02 cent piece of paper. You might find a loose piece of metal vibrating that WILL trigger a…

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Justin Technician
California
Justin
 

Sucks paper . I have determined the number 8 Cylinder is the weak cylinder by using injector kill test. The most counts I am getting in current misfires is 12 . If I short plug wire it counts up in the hundreds. I can feel very little change in cylinder 8 at idle . Off idle the misfire is a little more apparent .

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Tom Owner/Technician
Tennessee
Tom
 

Another way to check for an intake leak in addition to the methods already mentioned is to pull the PCV, and block the PCV port on the valve cover and the breather side port. Pull the dipstick and put a vacuum gauge on the dipstick tube. Start the engine. If you're registering vacuum then the intake gasket is leaking on the valley side. I have seen this on 7.4's before.

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Glenn Owner/Technician
Texas
Glenn
 

Check for a broken valve spring on #8. They break at the base and wind into them self. They will pass a static compression test. You will need a good light and a pocket mirror to find this.

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Rusty Owner/Technician
Massachusetts
Rusty
 

I think this is the most likely fault.

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Sherman Owner
Texas
Sherman
 

A 10 dollar vacuum gauge will show it, too.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Justin, This sounds like a vacuum leaking intake or vacuum hoses. Smoke tests seldom work unless it is a gross intake leak. What I have found that works best is using a spray bottle of water. Shoot a stream of water at the intake mounting surfaces, then watch for steam to increase out of the tailpipe. In some cases, if you listen real carefully you may actually hear the water being sucked…

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Rusty Owner/Technician
Massachusetts
Rusty
 

A misfire caused by a vacuum leak would be worse at idle and improve with throttle opening. His symptom is the opposite.

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Justin Technician
California
Justin
 

MY MISFIRES ARE AT IDLE

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Zachary Mobile Technician
Texas
Zachary
 

A lot of great information already given so consider this just additional food for thought. When you run a compression test with a conventional gauge, what you're measuring is peak cylinder pressure. This is because of the Schrader valve in the test hose. So if you have a valve that's not sealing intermittently you're likely not going to see a significant difference when you perform that test…

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Justin Technician
California
Justin
 

Any advice on a low cost pressure transducer ? I also installed a low cost pizo sensor on intake with injector 1 as trigger . See below .

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Glenn Owner/Technician
Texas
Glenn
 

Hi Justin, When looking for a pressure transducer, cheap non calibrated ones are easy to find on Ebay. It is best to research and find the calibrated ones if possible. Automotive Test Solutions (ATS) offers individual ones. Usually for gasoline engines, the 300 psi one is all you need for good in cylinder pressure wave form testing. Below is a link, I have one of these I use with my Autel…

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Justin Technician
California
Justin
 

Thank you glenn . What would you recommend for scope extension leads so I can measure exhaust psi and crank sensor at one time ?

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Glenn Owner/Technician
Texas
Glenn
 

Hi Justin, AESWave has a number of extensions. You might call them and discuss options.

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Justin Technician
California
Justin
 

Also how do I create a math channel on crank sensor wave form to detect speed change in crank speed to see if I really have a misfire ?

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Zachary Mobile Technician
Texas
Zachary
 

rotkee​.​com/en A buddy of mine uses these and says they work pretty good. I believe the one you'd want is the PS 16. Just a hair over $100 if I'm not mistaken. I was also told about an industrial pressure transducer by a colleague recently for around the same price. He showed me scope captures of it VS the ATS500 and it was very similar. ebay​.​com/itm…Do…

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Victor Mobile Technician
California
Victor
 

Are you using the labscope to check compression? Or vacuum ?or where its connected.

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