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Unread 01-26-2016, 01:51 PM   #1
Mjczj
Registered User
1997 ZJ 
 
Join Date: Mar 2014
Location: Carlsbad, Ca
Posts: 17
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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Unread 01-26-2016, 02:12 PM   #2
MrRoundel
Registered User
1998 ZJ 
 
Join Date: Jan 2007
Location: California
Posts: 1,601
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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Unread 01-26-2016, 02:20 PM   #3
MrRoundel
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1998 ZJ 
 
Join Date: Jan 2007
Location: California
Posts: 1,601
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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Unread 01-26-2016, 09:37 PM   #4
belalerede
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1997 ZJ 
 
Join Date: Dec 2010
Location: kenna, wv
Posts: 152
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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Unread 01-26-2016, 09:45 PM   #5
Mjczj
Registered User
1997 ZJ 
 
Join Date: Mar 2014
Location: Carlsbad, Ca
Posts: 17
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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Unread 01-27-2016, 08:43 AM   #6
jrobeson18
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1997 ZJ 
 
Join Date: Oct 2015
Location: Gastonia, North Carolina
Posts: 177
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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Unread 01-28-2016, 12:22 PM   #7
MKCoconuts
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1997 ZJ 
 
Join Date: Jun 2014
Location: Saint Marys, Pennsylvania
Posts: 87
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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