Resolved New DSM P0300

[GhostTypeP0300]

What’s up! I finally found another DSM.
What are some of the things I should check out?
What maintenance and repairs do I need for daily driving?

Thanks

 

[dwb]

Timing and balance shaft belts including pulleys, tenisioner, and waterpump. This will also give you an opportunity to flush the coolant system and replace any questionable hoses. Serpentine and power steering belts too. I like to do some oil seals while I'm in there - cams, and a few in the front case like front main, balance shaft, oil pump.
I like to ensure the battery and charging system are up to snuff. You know, things that might leave you stranded.
Overall, just be super vigilant about addressing issues as they crop up. There's nothing worse than ignoring a simple problem that spirals into something much more serious. And that way when it does come time for some more work, it's not so daunting because you already took care of the simple things.

 

[GhostTypeP0300]

Description
Selling my project 99 Eclipse GST with the 4g63 engine, 5speed manual
Pros
New battery
New alternator
New lower control arms
New front and back shocks
New motor mounts
New ACT light weight flywheel
New Clutch Max Pro stage 1 clutch
New timing belt and pulleys
New Megan racing short shifter
It has intake/cat back exhaust and
1g Bov

 

[GhostTypeP0300]

Ok so I’m definitely a firm believer in doing my maintenance first but I think my main goal is to get it to pass smog and light inspection first.I test drove the vehicle and it had no check engine light car drove great.The next day I noticed the check engine light was on.
With a p0300.

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A P0300 code indicates a random/multiple cylinder misfire. For a 2G DSM 4G63, this typically stems from failures in the wasted spark ignition system, unmetered air leaks, or faulty sensors. Because misfires can damage your catalytic converter, begin diagnosis immediately.

1. Ignition System (The Most Common Culprit)

• • Spark Plugs: Check for fouling, cracks, or improper gaps. 4G63s are notoriously picky about plugs; many DSMers run properly-gapped NGK copper plugs (like BPR7ES). [1]
  • Plug Wires: Inspect the wires for arcing or high resistance, especially at the boots where heat degrades them. [1, 2, 3, 4, 5]
  • Ignition Coils: The 2G coil pack is prone to internal cracking. Test the primary and secondary resistance, and look for hairline fractures that cause spark to jump to ground.

2. Air and Vacuum Leaks

• • Boost Leaks: A 4G63 under boost will blow unmetered air out, causing a massive lean misfire. Build or buy a boost leak tester and pressurize the system from the turbo inlet to find leaks at the intercooler piping couplers, throttle body seals, or BOV.
  • Vacuum Lines: Check the brittle plastic and rubber vacuum lines running to the intake manifold, FPR (Fuel Pressure Regulator), and BOV. [1, 2, 3, 4, 5]

In 2nd Generation (2G) DSMs (1995-1999 Mitsubishi Eclipse, Eagle Talon, Plymouth Laser), Type A and Type B refer to the severity of an OBD-II engine misfire detected by the Powertrain Control Module (PCM). [1, 2, 3, 4, 5]

Type A Misfire (Catalyst-Damaging)

• • What it is: A severe misfire that causes the catalytic converter to exceed safe temperatures (usually ≥ 1500° F).
  • The Symptom: The Check Engine Light (CEL) will flash continuously while the misfire is occurring.
  • The Action: Stop driving the car immediately. Continued driving can permanently melt or destroy your catalytic converter.
  • Testing Interval: Triggered in a single "trip" (200 engine revolutions). [1, 2, 3, 4, 5]

Type B Misfire (Emissions-Related)

• • What it is: A less severe misfire that causes exhaust emissions to exceed federal limits by 1.5 times, but does not immediately damage the catalytic converter. [1, 2]
  • The Symptom: The CEL will turn on, but it will remain solid, not flash. [1, 2]
  • The Action: Can be diagnosed and driven to a repair shop, though it should be addressed to restore vehicle performance and pass inspections. [1]
  • Testing Interval: Only triggers the light if the misfire occurs during two consecutive driving trips. [1, 2]

Common Causes & Diagnosis in 2G DSMs
Whether you have a Type A or Type B misfire, the PCM will log a P0300-series code (e.g., P0300 for random, or P0301 through P0304 for specific cylinders). Common areas to inspect include: [1, 2]

1. • Ignition System: DSMs are notorious for ignition breakdown. Check for worn spark plugs, cracked spark plug wires, or a failing Ignition Coil Pack / Power Transistor Unit (PTU). [1, 2, 3]
   • Fuel Delivery: A clogged or dead fuel injector can cause a lean misfire. Listen to each injector with a mechanic's stethoscope or long screwdriver to ensure they are all clicking. [1, 2, 3, 4, 5]
   • Mechanical / Compression: Perform a compression and cylinder leak-down test to rule out common 2G issues like burnt valves, failing piston rings, or dropped valve guides

Some people have recommended spark plugs and wire or ignition module and sure it is easy to change so why don’t I go spend money on them? Well first off what if I Change them and have the same problem? So what at least I now my plugs and wires are good I was told.As far as I know there is a difference between a parts changer and a technician and it’s not a god idea to just throw some parts at a car.In my opinion why change ignition parts on a car that needs a rebuild? Or why even do a boost leak test on a car that needs a rebuild first? So. Decided to check the engine mechanical condition first rite?- so im thinking the best thing to do first is a compression test. Especially since the odometer readings are almost 300k….

• Mechanical / Compression: Perform a compression and cylinder leak-down test to rule out common 2G issues like burnt valves, failing piston rings, or dropped valve guides

Or head gasket I noticed it was not on there.so like I was saying why do an ignition system tune up or boost leak test without checking-wet/dry compression to rule out engine mechanical issues like -rings,valves,head gasket.

If a compression test reveals low compression in one or more cylinders, your misfires are likely caused by internal mechanical failures rather than ignition or fuel issues.

How to Perform the Compression Check
If you are comfortable doing this yourself, auto parts retailers like O'Reilly Auto Parts often let you borrow a compression tester through a loaner tool program.

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O’Reilly Auto Parts +1

1. Prep the engine: Run the vehicle until it reaches normal operating temperature, then turn it off.
   
   
2. Disable the fuel and ignition: Pull the fuel pump fuse or relay and disconnect the main ignition coil pack to stop fuel delivery and spark.
   
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3. Remove spark plugs: Take out all the spark plugs. Keep them in order so you can inspect their condition later.
   
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4. Insert the gauge: Thread the compression tester gauge into the first cylinder's spark plug hole.
   
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5. Crank the engine: Floor the gas pedal (to fully open the throttle) and crank the engine for 4 to 5 compression strokes (about 5 seconds). Record the highest reading.
   
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6. Repeat: Test all remaining cylinders and note their PSI values.
   
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Analyzing Your Results

• Healthy Engine: All cylinders should read within 10% to 15% of each other.
  
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• Single Low Cylinder: If one cylinder reads significantly lower than the rest, you likely have a bad valve, a worn piston ring, or a damaged piston in that specific cylinder.
  
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• Two Adjacent Low Cylinders: If two cylinders side-by-side show low compression, it heavily points to a blown head gasket leaking between them.
  
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Next Steps If Compression Is Low
If you find a weak cylinder, perform a wet compression test by adding a tablespoon of engine oil directly into the low cylinder’s spark plug hole and re-testing:

• If the PSI increases drastically, the oil is sealing the leak, meaning you have worn piston rings.
• If the PSI stays the same, the air is escaping from the top end, meaning you have a burnt/bent valve or a blown head gasket.

Next Steps If Compression Is Low
If you find a weak cylinder, perform a wet compression test by adding a tablespoon of engine oil directly into the low cylinder’s spark plug hole and re-testing:

• If the PSI increases drastically, the oil is sealing the leak, meaning you have worn piston rings.
• If the PSI stays the same, the air is escaping from the top end, meaning you have a burnt/bent valve or a blown head gasket.

Ok guys! I ordered my compression tester it should be here soon.Before doing the compression check I decided to hook up my scan tool and look at the long term and short term fuel trim to check if I had a boost/vaccine leak.

Does anyone have experience looking at fuel trims to check if I have a vacuum/boost leak? Thanks

To confirm a vacuum or boost leak on a 2g DSM using a scan tool, monitor your Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT) at warm idle and during a drive. High positive values at idle that drop closer to zero at higher RPM indicate unmetered air entering the engine. [1, 2, 3, 4]

1. Monitor Trims at Idle

1. • Warm up the car: Ensure the engine is fully at operating temperature.
   • Observe idle trims: Connect your scanner (such as an OBDLink or laptop logging with ECMLink) and look at STFT and LTFT. [1, 2, 3]
   • Analyze values: If there is a vacuum leak, the computer will detect extra unmetered air and add fuel to compensate.
     • STFT + LTFT total: If the combined trims are highly positive (e.g., above \(+15\%\)) at idle, your engine is running lean. [1]

2. Monitor Trims Under Load/RPM

1. • Hold RPMs: Rev and hold the engine at roughly 2,500 RPM while parked or driving. [1, 2]
   • Watch the trims: Watch how the fuel trims react.
     • The Leak Signature: If a vacuum leak is present, the higher manifold vacuum at idle draws in more unmetered air, resulting in higher trim percentages. As you increase the engine speed (load) and open the throttle, the effect of the vacuum leak becomes proportionally smaller. As a result, STFT and LTFT will decrease significantly and move closer to \(0\%\).
     • MAF Failure Indicator: If the trims remain highly positive at higher RPMs, you likely have a failing Mass Air Flow (MAF) sensor or low fuel pressure, rather than a vacuum leak. [1, 2, 3, 4, 5]

3. Pinpoint the Leak
Since DSMs are turbocharged, you should verify any scan tool diagnosis with a physical pressure test. [1]

1. • Construct/Buy a tool: Use a Turbo Boost Leak Tester or build an endcap with a tire valve stem.
   • Pressurize: Attach it to the turbo compressor inlet and pressurize the system to roughly \(15 - 20\) psi using an air compressor.
   • Listen and Spray: Spray a soapy water solution on all vacuum lines, intercooler couplers, and the throttle body. The location of any bubbling reveals exactly where your leak is. [1, 2, 3, 4]

fuel trims at idle and under load.

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[GhostTypeP0300]

At idle, your Long-Term Fuel Trim (LTFT) is +12.5% while Short-Term Fuel Trim (STFT) is near zero (-0.8%). Under load, your LTFT stays at +12.5% while your STFT drops significantly to -9.4%.

Because your total fuel trim (LTFT + STFT) drops from +11.7% at idle down to +3.1% under load, your scan tool data strongly indicates a vacuum leak, not a boost leak.


Why This Indicates a Vacuum Leak

• At Idle (+11.7% total): Engine vacuum is highest at idle. Unmetered air enters through a vacuum leak, leaning out the engine. The computer adds fuel (+12.5%) to compensate.
  
  
• Under Load (+3.1% total): When you open the throttle, engine vacuum drops drastically. Air rushes through the main throttle body instead of the small leak, making the leak negligible. The computer pulls back fuel (-9.4%), bringing total trims back toward normal.
  
  
• Why it is not a boost leak: A boost leak under load causes metered air to escape outof the intake. This forces the engine to run rich, causing fuel trims to spike highly positive under load to add fuel for air that isn't actually there. Your trims do the exact opposite.
  

How to Pinpoint the Leak Using Live Data
To locate the exact source of the leak using your scan tool, follow these diagnostic steps:

1. Monitor Total Trim: Set up your scan tool graph to display both STFT and LTFT.
   
   
2. Propane or Brake Cleaner Test: With the engine idling, safely spray small bursts of brake cleaner or unlit propane around suspected areas.
   
   
3. Watch for the Drop: When you spray the leaking area, the engine will draw in the chemical fuel. Your STFT will immediately plunge into deep negative numbers (-15% to -25%) as the oxygen sensor detects a suddenly rich mixture.
   

Common Failure Points on a '99 Eclipse Spyder
Inspect these high-probability areas on the 4G63 (Turbo) or 4G64 (Non-Turbo) engines:

• Intake Manifold Gasket: Spray around the cylinder head mating surface.
• Throttle Body Gasket: Check the seals around the throttle body and BISS (Base Idle Speed Screw) O-ring.
• Vacuum Lines: Inspect the small hoses going to the fuel pressure regulator, EGR valve, and boost control solenoid.
• PCV Valve: A stuck-open PCV valve acts as a massive internal vacuum leak.
• Brake Booster Hose: Check the large vacuum line running to the brake booste

 

[ItsTwan309]

Im lost here.
Are you answering your own questions on this thread

 

[GhostTypeP0300]

Ok I’ve been trying to get this DSM to pass smog and light inspection. I'm trying to diagnose a p0300.

The first thing I want to do is check the engine mechanical condition so I ordered a compression tester. In the meantime I hooked up my scan tool to look at fuel trims to see if I have a vacuum/boost leak or bad sensors. I posted pictures of my LTFT/STFT at idle and around 2k RPM. I felt like my fuel trims and live data are telling me something but I didn’t understand so I googled my exact symptoms and fuel trim readings - AI gave me an answer. So maybe that’s what your referring to. My last post was an AI generated answer.

If you have any questions feel free to ask. Thank you

 

[ItsTwan309]

That's it.

Yeah do a boost leak test, and yes your trims are off. ECU is pulling 12% fuel to make it correct.

I take it you don't have DSMLink.
AutoZone will lend you one, you didn't have to order. I'm sure Amazon has them pretty cheap though.
Check your ignition timing, check your plugs, change wires, check coil pack, resistor pack. Could be a bunch of things

Is your BOV still recirculating?

 

[GhostTypeP0300]

No I sold my v3 when I parted out my GSX.

Yes BOV is still recirculating because I have the stock MAF.

The P0300 Code: This is a random/multiple cylinder misfire code. When a charging system fails or drops below the proper threshold, modern vehicle computers (ECUs) and ignition coils do not receive the steady, clean voltage they require, which can easily trigger misfires

 

[ItsTwan309]

Stock injectors?

 

[GhostTypeP0300]

Yes

 

[ItsTwan309]

good voltage?
check everything spark related

 

[GhostTypeP0300]

Check this out!

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If your voltage is solid-14.2 at idle and you still have P0300 issues, the issue likely lies within the ignition or intake.

This is what I found online - A charging system voltage below 12.5 is a severe voltage drop that can easily trigger a P0300. Is the check engine light solid or does it flash during acceleration ? - it’s solid.
In a DSM, a steady check engine light (Type B misfire) indicates that the ECU detects intermittent, random drops in electrical voltage. This prevents the ignition coils from properly transforming the power into a strong enough spark to ignite the fuel. A healthy charging system should read between 13.5 V and 14.5 V while the engine is running. A steady 12.5 V reading means your alternator is failing or your battery is draining. If bringing the battery to a full charge clears the code the low charging voltage is the culprit. If restoring the charging voltage doesn’t fix the problem its likely mechanical, related to parts or vacuum leak.

The 2g DSM warning light is wired to the alternators internal voltage regulator. If the main charging fuse blows (The fuse is god) the alternator cannot push power to the battery, but the warning light on your dash will not illuminate (my warning light did not come on during test drive.) because the sending circuit itself did not fail. Since the fuse is still intact it’s likely a failed internal voltage regulator a bad wiring connection or failing ECU. The power light may not be on because your exciter wire is not getting power or the ECU is not grounding the circuit to trigger it.

Weird factoid about DSM’s the battery charging circuit is not complete if the dash bulb is burned out or missing. Does your battery light come on when you click the key? (yes) if it doesn’t then you have a bad bulb.

 

[ItsTwan309]

take battery, alternator, and starter to local O reily's, advanced auto or autozone for testing.

 

[GhostTypeP0300]

Will a bad generator relay cause the problem?

 

[ItsTwan309]

If that relay was bad I would assume it wouldn’t work at all

 

[GhostTypeP0300]

Thanks, according to the last owner the battery and alternator are new. So I didn’t expect charging system to be a problem during the test drive because the light did not come on. I honestly do not want to uninstall anything until I get the proper diagnosis. Although I can have the parts tested for free at CSK Auto , sorry they call them O’Reillys now. That was my first job when I was going to UTI. Anyway I rather buy a digital multimeter and check the wiring because it would not be fun to do all that work and have them tell me I have a good battery and alternator and it’s possible a wiring issue.

 

[ItsTwan309]

Never hurts to walk through the exercise

My bad on stated above, starter don’t need tested. Just batt and alt

 

[GhostTypeP0300]

Yes if the generator relay was bad the alternator will stop charging the battery, the power would not make it from the alternator to the battery even if the alternator was good.

It definitely doesn’t hurt to take them off and it’s free but the idea is to work smarter not harder. So I’m going to get a multimeter asap.

Ok back to that generator relay!

Note - while 2g DSM’s pre-99 have the generator relay many late 99 models don’t use it from the factory!

Ok so what is the location of this generator relay? I was told that it’s in the engine compartment under the airbox and next to the coolant reservoir.

 

[99dsmer4g63]

99’s do not have a generator relay. Just because you are at 12.6 at idle does not indicate a charging system failure.

Do some research on how the g terminal controls the alternator internally through the ECU.

Low voltage mode = battery voltage (12.5v)
High voltage mode = 13.5v or higher.

Does this car have a 6 bolt swap?

What cam sensor are you running?

-Daniel

 

[GhostTypeP0300]

Ok I checked for the so called generator relay. I hear it’s a diode and resistor actually.
Is it supposed to be next to my boost control solenoid? I didn’t see it there I did find an empty plug but I’m not sure if it’s missing or didn’t come with it.

It has a 2 g cas stock 7 bolt as far as I know.

yes 12.6 at idle does indicate a charging system failure.

 

[99dsmer4g63]

yes 12.6 at idle does indicate a charging system failure.

You should read what the people that designed ECMLink say. Stop using AI.

ecualternatorcontrol [ECMTuning - wiki]

www.ecmtuning.com

 

[GhostTypeP0300]

ok thanks yes 12.6 at idle does indicate a charging system failure according to google..but yes definitely need to do my research on the g circuit I also read the ecu can cause this and yes I would definitely take Thomas Doris advice or someone from ecm tuning over ai…

 

[dwb]

The relay you speak of, if you have one, is only in case the battery dash light is blown (as I understand). Does that light illuminate when you turn on the ignition?

Verify with a dc volt meter across your battery what actual voltage your battery is seeing. Give it some throttle and see if voltage increases. Put some load on it - headlights, blower, rear defroster and do the same test.

Show us a clear picture of your engine bay. Sometimes we can spot something that would otherwise be overlooked.

Please stop relying on AI and and do some old school research. FSM, this forum, even a Haynes manual should be your go-to resources. You will understand the process much better and be better informed when it comes to repair. AI often throws random shit out there that is completely irrelevant or incorrect and makes you chase ghosts.

 

[GhostTypeP0300]

2G DSM Alternator Control​

This page provides a basic description of how the two wires running from the 2G DSM ECU to the alternator (G and FR) are used.
There are four wires running to the alternator. Two wires are G and FR from the ECU. The other two are a voltage sense line (from the headlight circuit) and an “S” terminal tied into the gauges somehow.
If you have the DSM manuals on CD, check out the “generator” section of the 2G technical manual. It provides a really good explanation of how the ECU controls the alternator. I'll briefly summarize below.
The alternator has two “modes” - low voltage/low load and high voltage/high load. The ECU controls which mode the alternator should be in using the “Generator G” pin on the ECU harness (pin 33). When G is grounded, the alternator is put into low-voltage mode (around 12.3v). When G is left floating, the alternator is in high-voltage mode (Around 14.4v).

Thanks for the help 99Dsmer!

You should read what the people that designed ecm link say. Stop using AI.

ecualternatorcontrol [ECMTuning - wiki]

www.ecmtuning.com

The relay you speak of, if you have one, is only in case the battery dash light is blown (as I understand). Does that light illuminate when you turn on the ignition?

Verify with a dc volt meter across your battery what actual voltage your battery is seeing. Give it some throttle and see if voltage increases. Put some load on it - headlights, blower, rear defroster and do the same test.

Show us a clear picture of your engine bay. Sometimes we can spot something that would otherwise be overlooked.

Please stop relying on AI and and do some old school research. FSM, this forum, even a Haynes manual should be your go-to resources. You will understand the process much better and be better informed when it comes to repair. AI often throws random shit out there that is completely irrelevant or incorrect and makes you chase ghosts.

ok I’ll check and Post voltage at battery asap thanks dwb-I was actually thinking of checking the b+ first to see what voltage is at the alternator but google says check g circuit first like 99Dsmer was saying.

The relay you speak of, if you have one, is only in case the battery dash light is blown (as I understand). Does that light illuminate when you turn on the ignition?

Verify with a dc volt meter across your battery what actual voltage your battery is seeing. Give it some throttle and see if voltage increases. Put some load on it - headlights, blower, rear defroster and do the same test.

Show us a clear picture of your engine bay. Sometimes we can spot something that would otherwise be overlooked.

Please stop relying on AI and and do some old school research. FSM, this forum, even a Haynes manual should be your go-to resources. You will understand the process much better and be better informed when it comes to repair. AI often throws random shit out there that is completely irrelevant or incorrect and makes you chase ghosts.

Yes the light works I did read it was for back up only

on the way to get a Dmm thanks !