Trying to diagnose code P0300, random multiple misfires?

Tiny
SWALDO
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  • 2005 CHEVROLET MONTE CARLO
  • 3.8L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 225,000 MILES
The engine is a 3800 series 2 L3. I’m trying to diagnose code P0300 – random multiple misfires. This behavior began recently.

I can re-create the misfire or avoid it depending on driving style.

Attached is scan data that is a good representation of the behavior. Numerous scans yield about the same results. The attached data includes some warm idle, and two ~0 to ~60 runs divided by pause/turning around. The first run is semi-spirited driving but not enough to re-create the behavior; the second run is far more spirited, and misfire then occurs, at time 105 seconds. The freeze data of the event is also attached. The engine mal-performs at this point and resumes normal by laying off the gas. The code is always P0300, and never is cylinder specific.

I’m a big fan of using scan data to diagnose the exact problem, rather than throwing parts at the likely candidates. So here are some facts on such candidates:

Newly replaced all 3 ignition coils
Inspected/confirmed all plug wires for continuity
Visually inspected/confirmed spark plugs for normal appearance
Measured fuel pressure with engine running – 48 PSI, + 8 more PSI when vacuum hose on FPR is disconnected. The pressure is then sustained with the key on, engine off.
Fuel filter, plugs and wires are all 7 years old, 50,000 miles. Fuel pump is original. Injectors are original. Ignition module is original.
Front O2 is 10 years old, 100,000 miles; rear O2 is 7 years old, 40,000 miles
The EVAP hose from throttle body to EVAP device is capped off and not attached to EVAP device. EVAP has historically had code P0449 due to the canister in the rear, and therefore was disconnected. (I do not reside in an emissions state). Code P0449 does not coincide with the current problem
From the scan data, I’m imagining all sensors are reporting correctly – pending your review

Observations of the data

You’ll see the LTFTs are double-digit-negative just about any time the car is moving, beyond the -10 but never reaching -25 that would throw rich code. This suggests correction for a rich condition. Too much fuel or not enough air. I verified the airbox and air filter were not clogged and re-created the misfire with air filter and box removed. Of course, the LTFTs are this way just the same in the first run when no misfire occurs.

The LTFTs then obey the +10/-10 range when idling.

From many observations, it seems that the misfire occurs whenever MAF of 15 to 20 is achieved (MAF beyond 20 has never been achieved) and does not occur at lesser MAFs 13 or below. So, it is not a simple matter of misfiring at a certain RPM.

So, as you might imagine, a precise diagnosis eludes me. Perhaps an expert 2nd opinion will be revealing.

Thank you
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Monday, October 16th, 2023 AT 8:18 AM

38 Replies

Tiny
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I can't add the all-important scan data and freeze data to this form. I'll attach these to email when someone emails me. Thanks
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Monday, October 16th, 2023 AT 8:25 AM
Tiny
AL514
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Hello, we don't communicate through email, it's just here, posting back and forth. When you say the MAF is reading 15 to 20, are you referring to 15grams per second (g/s) and are the front oxygen sensors fluctuating between 0.2v and 0.8v at a relative fast frequency, meaning neither of them are hanging up in the 0.8v (800mv) range? If one of them is stuck at a rich indicating voltage level, the PCM will start to reduce the fuel from the mixture and fuel trims will go negative when they shouldn't. Basically, tricking the PCM into thinking the mixture is rich when it is not.
I agree on finding the issue as well instead of throwing parts at it, that is not the correct way to diagnose an issue.
And if you can post the freezeframe data here it would be good.
And does this vehicle have a super charger?
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Monday, October 16th, 2023 AT 9:04 AM
Tiny
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Hello,

My MAF figures are in LBs/min. The front O2 is frequently switching and not hung up in the.8V range. There is no SC.

My scan data is a CSV file and the freeze data is a PDF. Each results in 'file type not allowed'. How can I get these to you?
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Monday, October 16th, 2023 AT 9:35 AM
Tiny
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Freeze data attached.
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Monday, October 16th, 2023 AT 9:42 AM
Tiny
AL514
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Your short term is correcting there somewhat at a higher rpm, Total fuel trim when the code set (-15, 6 - 7.8= -7.8) so -7.8 total is not that bad, I put these numbers into a VE calculator which Ill post below, so it's a Volumetric Efficiency test that shows how well the engine is breathing, so if you had a catalytic converter that was restricted it would show up here.
And according to the test everything looks good. What are your fuel trims doing at idle? And does the MAF read around 4 g/s (or 0.53 lb/min)?

On this freezeframe data you're at 100% calculated load, with about half throttle, 54% so I assume this is a cruising speed type of run. I would be interested in knowing what the Long term and Short term are at idle. ECT looks a little low here, and the -15% is being corrected for but something is causing it to initially drop down that low, I think with seeing these numbers (and you are correct, the sensors look to be giving correct data)

I would do a fuel injector balance test. It might be that they are not all flowing the same amount, hence the -15%. But then at higher rpm since the engine is using more fuel at that time, the short term starts to correct, because the engine would now be able to use any extra fuel.

Do you own a fuel injector pulse tool? With doing a fuel injector balance test, you'll have a fuel pressure gauge on the rail, and prime the fuel system with key On, and then pulse the injector, write down how much pressure drops from your 48psi, then move on to the next injector, re-prime the fuel system up to 48psi again, and repeat. Each injector should be within 1-2psi drop of each other. Once you have done all 6, start the vehicle and let it run for a few minutes to get the fuel out of the cylinders. And run the test again,

I will usually check the injectors like this twice to see if the numbers all stay the same. Some injectors will act up at higher temperatures. So, you could test while cold, go for a ride and retest to see if anything changes. Because besides the ETC looking at little low, which you may want to see if it gets up to around 205f range, at a hot idle. Everything else looks good. A low engine coolant temp sensor will enrich a mixture, you would have positive fuel trims though.
The calculated load looks a little high as well, but that can be scan tool data. I would also check at key On engine Off, that your TPS goes higher at WOT. 54% looks low for that rpm, but it's also measuring absolute throttle position, so I'm not 100% sure what else that takes into account, Ill look through some service info on that, but try the injector balance test, make sure they are all flowing equally. If so, then next you would be on to some possible compression issue. Sorry for the long post, I'm just looking at what you posted and your data at the same time here. If you have any questions on doing the injector balance test, I can find a video for you.
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Monday, October 16th, 2023 AT 10:51 AM
Tiny
AL514
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Here are some extra Technical Service bulletins for cylinder misfire codes setting.
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Monday, October 16th, 2023 AT 11:15 AM
Tiny
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Here is a snapshot of a couple frames of idle to answer your idle question. The TPS does indeed range to 100% when pressing pedal to floor with the key on, engine off. So, I'm looking at the injector pulse test. I do not already have such a device. Thank you
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Monday, October 16th, 2023 AT 11:36 AM
Tiny
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It's a really great tool to have, even the cheaper $30 unit works just fine, it has 3 different settings, where it will pulse an injector for 50ms at 2.5ms pulses if I remember correctly. It's really the only way to measure injector flow, besides buying one of those thousand dollar injector flow bench testing units. It's even good for checking new injectors, because sometimes aftermarket fuel injectors will flow differently than the OEM ones on the vehicle now, which will through off fuel trims as well.
I'm not sure if you have used an oscilloscope before, but you can do a relative compression test with a scope. This is a relative compression test I did on a 4-cylinder Honda CRV, this is the starter motor current flow, but you can see where one cylinder here has a little bit lower compression hump, The next one after it is a little low as well. This was taken with a Pico scope and amp clamp on the main battery cable. Very effective test to check engine mechanical really quick when dealing with cylinder misfires.
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Monday, October 16th, 2023 AT 12:06 PM
Tiny
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What would the VE look like if the exhaust was restricted? Thanks
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Monday, October 16th, 2023 AT 12:53 PM
Tiny
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This is one of the VE calculators you can use, it's best to do a wide-open throttle run and record the data while doing it and use those numbers to input in. And with either a restricted exhaust, or extreme air intake issue, even a valve adjustment issue, the EST_VE section will be low, (estimated volumetric efficiency) a restricted cat can reduce the VE to 50% or even lower to where it ends up being a no start. On the website there is a button up top that reads "Show VE Calculator Instructions". It shows the best conditions and ways to get the most accurate results. If you ever suspect a restricted exhaust, I will pull out one of the front oxygen sensors and do a back pressure test with the sensor out. Just starting the vehicle for a couple minutes and see how much pressure there is will tell you if the cat has melted down the substrate and blocked off the exhaust, exhaust back pressure max reading should not be over 1.5-2psi, which is barely anything. I'll also check the condition of a cat using a borescope camera and looking at it through the front oxygen sensor opening. I take pictures sometimes to add to a customer's post repair order so they can see for themselves that it's in good condition.
Another method is to use a pressure transducer and go in-cylinder and take a look at the waveform, cranking and then running. This waveform below is an in-cylinder waveform, you can see the compression stroke, the exhaust stroke where it plateaued, and then the next compression tower. A restricted exhaust would have higher pressure above the zero line in the middle.

https://atgtraining.com/atg-volumetric-efficiency-calculator/
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Monday, October 16th, 2023 AT 3:20 PM
Tiny
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Okay. Thanks for the VE explanation. I see what you're saying there. I will do the injector test near future.

I do indeed have a non-automotive brand inexpensive laptop USB oscilloscope. I find the scope compression test you show interesting, as I'm only familiar with the per-cylinder physical gauge compression test. Is it just as effective to scope the volts at positive battery terminal while cranking? (As I'm lacking an amp clamp)

Thank you
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Tuesday, October 17th, 2023 AT 5:21 AM
Tiny
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Yes, you can do battery voltage instead of cranking amps, you just need to invert the waveform, because it will be the opposite of the amperage waveform. So, flip it upside down when on the battery terminals, it's a great fast way to see if there is a mechanical issue vs fuel or ignition when dealing with a misfire. You're really just seeing the crankshaft's speed on compression strokes. So, if you happen to get a hump in the waveform that is low on compression and then the next hump may appear a little bit higher sometimes because the crankshaft is speeding back up at that point. So, it's not necessarily another bad cylinder after the low compression one. If you have 2 channels on your scope, you can also use the other channel to grab a sync, such as a fuel injector firing or coil firing so you know where the effected cylinder is through the firing order. YouTube has some good videos if you search for "scope Relative compression test", it will go into some more detail, but just by your question about the using the battery instead, I can see you have a good grasp on things. Most of the time I have to explain to people how to take a voltage reading with a multimeter. Which takes a long time, but I like the VE test because you can put in some data numbers from a WOT run and see how well the engine is breathing without having to tap into the exhaust for back pressure testing right away. I would do that after to verify a bad cat if needed.
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Tuesday, October 17th, 2023 AT 9:25 AM
Tiny
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I'll first perform the injector balance test near future and provide results.
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Wednesday, October 18th, 2023 AT 5:26 AM
Tiny
AL514
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Okay, whatever you think is the easiest route to take, I'll do some of these tests just to keep them fresh in my mind as well. Once you run through the fuel injector balance test, you'll really get a sense of the results and see even the slight differences between injector flow. I'm adding this video on the relative compression test at the battery posts because I forgot to add the section about AC coupling the signal when grapping a capture at the battery. He also goes over using a high amperage clamp and low amp clamp as well. If you scope has BNC connections, you can get a Hantek high amp clamp for pretty cheap, I have one and still use it, it takes a 9volt battery, but has large enough jaws to go around the thicker battery cable, with an amp clamp you can use either the negative or positive lead when watching starter current. But check this video out, it will help with getting your settings correct for voltage levels. It's not very high when only measuring voltage vs the amp clamp which will be monitoring a few hundred amps on a starter draw.
Let me know how you make out with the injector test, I'm always interested in test results.

https://www.youtube.com/watch?v=Sg7HfvQsHG8
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Wednesday, October 18th, 2023 AT 10:03 AM
Tiny
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I believe the injector balance test revealed something significant.

Performing the mode 3 low power test (100 pulses @ 3.5 ms pulse width), I have 4 injectors that all dropped 19-20 psi; 1 injector that dropped 18 psi; and 1 injector that was 23-24 psi (Cylinder #1). I performed the test 5 or so times on each injector to get good data (at a constant engine temp). Cylinder #1 is definitely beyond 10% from the others. The fact that cylinder #1 injects more might explain the negative double digit LTFTs whenever the car is moving. And the misfire only at higher load.

I also noticed that as engine temperature changes, so does the psi drop; it appears a warmer engine has a lesser psi drop across all cylinders. Very interesting. Even so, cylinder #1 always dropped 4 psi more than the others regardless of temp.

It was also a case whereby performing mode 1 low power test (single pulse of 255 ms) all 6 injectors behaved the same. Cylinder #1 did not show itself to be abnormal. If this was the only test performed, I would’ve concluded "all good".

So now I’m thinking what is the conventional approach in this case? How would it be handled with a customer at a shop?
All injectors are original, that’s 18 yrs and 225K miles. Perhaps it is prudent to replace all 6. I've also heard never need replaced, just clean them. I've also heard just clean or replace the bad one. Of course, I only want to do the work one time, removing fuel rail etc.

In the case of replacement, I’ve also heard the "less expensive ones are all junk" and I’ve heard the OEMs are unnecessarily expensive.

So, assuming you agree with the diagnosis, how to proceed?

Thanks
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Monday, October 23rd, 2023 AT 5:33 AM
Tiny
AL514
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Okay, glad you got some results from those tests, did you hopefully start the vehicle in between testing so as not to flood out the catalytic converter with fuel. It should be started in between each set of tests.
But yes, injectors will definitely act differently at varying temperatures. When they are hot they expand like everything else, that can cause them to stop working fully sometimes, but really anything can happen. You're correct, It will cause the fuel trims to go negative because that cylinder is running rich. and depending on if the injectors are Bank fired or sequential, the ECM will reduce fuel to that entire side of the engine if they are bank fired all together. I always like to see what the difference is when it comes to flow rates. It is best to replace them all at once. because many fuel injectors can be at different flow rates, especially after market ones. They all need to be flowing the same, be the same manufacturer, and OEM is the best. They might cost more, but if you have to go back and end up replacing them all again because misfires codes don't go away, it will cost more in the long run anyway. And you shouldn't have to replace them again in the life of the vehicle, unless one comes with a factory defect.
Of course, replace all the seals as well, there's a top and lower one. I'll see if there are injector seats for this vehicle. I don't know off hand if GM has injector seats in the intake manifold. Ford does, they just sit in the intake manifold and the injector sits in that. Here's a guide to help. and if there's anything in service info on seats ill post it as well as any other important info.

Also do an oil change after the repair, I'm sure the oil in contaminated with fuel by now.
Ok they are very specific when ordering new injectors for this vehicle. Below is the service info.
I recommend taking pictures as well before taking everything apart, so you have a reference to where everything goes after and be really easy on the wiring connectors to each injector, they are high mileage and have been sitting like that for a long time, so the wiring will be brittle.

https://www.2carpros.com/articles/how-to-replace-a-fuel-injector
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Monday, October 23rd, 2023 AT 10:11 AM
Tiny
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Thanks for the info. I'll change them and let you know.

I did indeed start the car after each and every injector test.
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Tuesday, October 24th, 2023 AT 8:38 AM
Tiny
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Okay, sounds good. Just curious, what is the max voltage level for your scope? And does it have BNC connectors for each channel? If you have 20:1 attenuators, you can capture some injector voltage waveforms that can show a lot too. Even watching ignition coil primary or secondary voltage waveforms can show if a cylinder is running lean or rich by the height of the voltage spike. This is the waveform I took from my own vehicle, it's the ignition coils current ramps, I know you don't have an amp clamp, but just showing some examples, you can see that first coil ramp has a downward spike at the end, that's indicative of a secondary ignition problem with the plug or plug wire.
The 2nd waveform is an injector voltage waveform, where you can see the pintle hump. But you need an attenuator on the scope due to the high voltage that coils and injectors spike to. Coils can fire up to 400 volts on the primary side. So, without an attenuator it will fry the scope. I just wanted to show some of this info because we rarely get people who use scopes.
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Tuesday, October 24th, 2023 AT 9:21 AM
Tiny
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It is +/- 5 volts, and the probes have a 10X switch on them. It does indeed have BNC connectors. I see how I could attach an inexpensive 20:1 attenuator, along with probe on 10X, and therefore be protected for both interesting tests you show. I also looked up the amp clamp and generally a number of things to make the scope more versatile, nothing too expensive there. Thanks

On one occasion I measured RPM of a lawn mower by attaching probe (set on 10x) to ignition coil ground wire, with a 2200 ohm resistor in series just in case and got my wave form. So, I have innovated on this topic, but getting the right thing is far better.

So is it the case whereby a richer cylinder would have the higher voltage spike, or the lower?
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Tuesday, October 24th, 2023 AT 10:42 AM
Tiny
KEN L
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If I could jump in here, when the sensing wire inside the MAF sensor gets worn out it will cause this problem. I would replace it with an AC delco unit. Cleaning it will not help.
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Tuesday, October 24th, 2023 AT 10:44 AM

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