PO 300 Multiple Cylinder Misfire - JeepForum.com
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post #1 of 45 Old 01-26-2016, 12:51 PM Thread Starter
Mjczj
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PO 300 Multiple Cylinder Misfire

'97 4.0 GC 177K

CEL codes PO300, PO301, PO303, PO304 PO306

Ok, so it's not a new subject, I've read most of the posts here and on the internet. Looking for overlooked solutions. My wife want's me to call Fr. Joe's for donation or send it to the crusher. I love it, and believe it has another 150K if I can resolve this.

Here's the problem: Time to pass CA. smog, persistent CEL preventing this. It's clean enough to pass, but any active or pending codes prevent this with CA.'s new computer system. Clearing the code before the test won't do it either, system sees this.

I have solved stalling with rebuilt computer, it runs great, no detectable misfire.

Here's what I've done.

Rebuilt ECU
Full tune up, plugs, wires, rotor. rechecked gap at .35
Cleaned Idle air control.
new upstream oxygen sensor.

Independent mechanic detected low vacuum of 14, said it should be 20.

So, just did a Valve job with machining at respected machine shop.

I suspect a vacuum leak at HOV blend doors because they fail to hold under conditions of lower vacuum ie. acceleration.

I'm heading to HF for a vacuum gauge, does anyone know correct vacuum?
Any ideas to save this noble GC?

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post #2 of 45 Old 01-26-2016, 01:12 PM
MrRoundel
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I think that my '98 4.0L pulls a vacuum of between 15-16" mercury, and it has no codes presently. What are the codes it's throwing now, (all of the above?)and what are their definitions? Thanks.
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post #3 of 45 Old 01-26-2016, 01:20 PM
MrRoundel
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OK, I see that the codes you listed are for misfires on all cylinders. Does your code-reader give you live-data readings? If so, you might want to see what's going on with the fuel-trims, etc. New O2 sensor NTK or Mopar, I hope? It can make a difference. Maybe cam-sensor? It's inside the distributor, and is also called the pickup coil. Or the coil? Or failing fuel-regulator/pump?
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post #4 of 45 Old 01-26-2016, 08:37 PM
belalerede
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Quote:
Originally Posted by MrRoundel View Post
OK, I see that the codes you listed are for misfires on all cylinders. Does your code-reader give you live-data readings? If so, you might want to see what's going on with the fuel-trims, etc. New O2 sensor NTK or Mopar, I hope? It can make a difference. Maybe cam-sensor? It's inside the distributor, and is also called the pickup coil. Or the coil? Or failing fuel-regulator/pump?
What he said.
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post #5 of 45 Old 01-26-2016, 08:45 PM Thread Starter
Mjczj
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Quote:
Originally Posted by MrRoundel View Post
OK, I see that the codes you listed are for misfires on all cylinders. Does your code-reader give you live-data readings? If so, you might want to see what's going on with the fuel-trims, etc. New O2 sensor NTK or Mopar, I hope? It can make a difference. Maybe cam-sensor? It's inside the distributor, and is also called the pickup coil. Or the coil? Or failing fuel-regulator/pump?
Thanks Mr Roundel. My code reader is the basic variety, no live data. Did fix one vacuum leak to HVA. Checking all connections including vacuum reservoir. Will monitor OBD II to see if fixed.

Any other ideas welcome, save a GC!
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post #6 of 45 Old 01-27-2016, 07:43 AM
jrobeson18
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try the CPS i have a CEL with pending codes of p300 (multi cylinder)and p303( cyl 3) missfire codes. but mine goes on and off. im gunna change the sensor when i have time. but as for me thats my problem.
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post #7 of 45 Old 01-28-2016, 11:22 AM
MKCoconuts
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Start with the basics. Plugs and wires, especially if they haven't been replaced in a while. You can read the plugs to give you an idea on what's causing the misfire and you can ohm out the wires to see if they have high resistance
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post #8 of 45 Old 02-14-2016, 04:37 AM Thread Starter
Mjczj
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Saved from the Crusher (at least for 2 yr. when the next smog check is due)

Everyone likes a happy ending right? I'll cut right to the chase: CPS, Crankshaft position sensor.

I borrowed a fuel pressure tester from Autozone and found fuel pressure to be slightly in spec at 45psi.

I decided to go back to Auto Guys in San Marcos, CA to see if they had further ideas. These guys are Vietnamese immigrants that I found to smart, thorough and totally ethical. they checked vacuum again, found it had risen to 18 after my valve job. They drove it for 2 days, found it still threw the PO 300 usually with a few other specific misfire codes in the mix. They were still stumped, guessing maybe camshaft, cam chain, etc. Said he pulled CPK, looked old and aftermarket, said I could try that next. Refused to charge me anything for the diagnosis.

Was it worth the $84 for the Mopar part? Maybe, but I decided to roll the dice on the $42 Napa Echlin part. First start up was worse, it missed going uphill. I had to guess on the bolt torque the first time, so off to Harbor freight for an inch pound torque wrench. I pushed the CPS in further to seat it and torqued down to 60 inch lbs.

Began to run well after seating. 2 other problems solved:
1. Transmission that was reluctant to up shift to third, now worked great.
2. Idle performance much improved.


But still throwing PO 300 after 20-30 miles. Kept driving it, got it to go 80 miles with out CEL and headed for smog test. Failed again, this time slightly too high on NOX. Walked over to O'Reilly's next door, gave it the Seafoam treatment through the throttle body, judging by the smoke after start up, removed much crap from the Cat. Re test found Nox dropped from 511 way down to 348. Pass and Victory! She lives!

Learned a few more things:
1. It's always the CPS, (maybe not, but a PO 300 is so non specific, but seems to be the most common solution on PO 300)
2. It takes some miles for the ECU to adjust to a new part.
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post #9 of 45 Old 02-14-2016, 05:00 AM
ZeeJay1997
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Quote:
Originally Posted by Mjczj View Post
Learned a few more things:
1. It's always the CPS, (maybe not, but a PO 300 is so non specific, but seems to be the most common solution on PO 300)
There are 20-30 other things that can cause misfire DTCs.
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post #10 of 45 Old 02-14-2016, 07:20 AM Thread Starter
Mjczj
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Quote:
Originally Posted by ZeeJay1997 View Post
There are 20-30 other things that can cause misfire DTCs.
Good list, but consider adding CPS for PO 300, this was the fix after Tuneup, Valve Job, Vacuum leak fix, Fuel pressure test etc.
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post #11 of 45 Old 02-14-2016, 07:31 AM
ZeeJay1997
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I'm tickled pink that you stopped the DTC but there's no need to scream CPS every time you type it out.

CPS controls timing and they do get lazy.
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post #12 of 45 Old 02-14-2016, 02:56 PM
Uniblurb
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I was real big on blaming the devil crank sensor on all the world's problems! Reason I have 2 crank sensor tests in my signature.

But will say I've become a lot more open-minded after reading all Zee's info and know now there's lots of other things which can cause misfire codes.

Plus since Mopar crank sensors have become so pricey over the past few years I even refer members to additional CKP testing rather than just those in my signature.
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post #13 of 45 Old 02-14-2016, 04:04 PM
tylerzj96
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I have the same issue with a multiple misfire P0300 and P0302 cylinder 2 misfire. New plugs, wires, cap, rotor. Fuel pressure is 49 psi always. Injectors are working fine. No issues with spark. I haven't replaced the ignition coil or the cps. I'm reluctant to change the cps because I know how much of a PITA it is to change. The misfire has just come up out of nowhere yesterday. If I remember correctly from my compression test, #2 was the low compression cylinder,. It was about 90psi I think and the others were around 150
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post #14 of 45 Old 02-14-2016, 04:59 PM Thread Starter
Mjczj
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Quote:
Originally Posted by tylerzj96 View Post
I have the same issue with a multiple misfire P0300 and P0302 cylinder 2 misfire. New plugs, wires, cap, rotor. Fuel pressure is 49 psi always. Injectors are working fine. No issues with spark. I haven't replaced the ignition coil or the cps. I'm reluctant to change the cps because I know how much of a PITA it is to change. The misfire has just come up out of nowhere yesterday. If I remember correctly from my compression test, #2 was the low compression cylinder,. It was about 90psi I think and the others were around 150
On the 4.0, it's not a bad job at all. 10 minutes max. Get under the car with a long extension on an 11mm socket. Remove from under the hood. Replace with new. Place bolt in socket, carefully thread and torque to 60 inch lbs. It took a few miles before my ECU played nicely with the new part. Most noticeable was how well my auto trans with 177K now shifted. MY jeep had good even compression. Your results may vary.
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post #15 of 45 Old 02-14-2016, 05:02 PM Thread Starter
Mjczj
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Quote:
Originally Posted by Uniblurb View Post
I was real big on blaming the devil crank sensor on all the world's problems! Reason I have 2 crank sensor tests in my signature.

But will say I've become a lot more open-minded after reading all Zee's info and know now there's lots of other things which can cause misfire codes.

Plus since Mopar crank sensors have become so pricey over the past few years I even refer members to additional CKP testing rather than just those in my signature.
I agree, I hate to throw parts at a problem, test when you can.
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