1999 Passport P0304 won't go away

02-24-2010, 07:59 PM
Here's my story :

I have replaced the fuel pump, o2 sensors, spark plugs, swapped around the coil packs, changed the injectors around, replaced the intake manifold gasket, and checked the vacuum lines.

A few questions, What is the sensor connected to the vacuum lines, it is on the drivers side on the back of the engine attached to the intake manifold.

And second could a bad crank position sensor cause this problem?

The truck runs fine, I don't notice any of the misfiring. The code won't stay off for even a day. 20-30 miles max when I reset it.

Any ideas would be appreciated!

Little Update :
Plugs : Autolite Double Platinum
O2 sensors : Bosch direct fit on the Driver Side, 1 Bosch Universal and 1 direct fit (not sure of the Brand) on the Passenger Side

02-24-2010, 09:35 PM
Just my 2 cents, but I sure wouldn't think so. If it is always P0304, indicating the misfire's always on #4, I wouldn't expect the issue to be only on #4 with CPS failure.

Since you say you changed out the coil packs with the other 5 cyls (#6 is different of course), I assume you swapped the plug wire with the coil pack so that shouldn't be the problem.

The only thing I can think of is possibly the #4 injector is not operating up to snuff (sealing O rings are weak/compromised, or injector slightly stopped up), or possibly a bad valve or a bad ring on #4. A compression test on numerous cyls with comparison should determine if that is issue. If it's running good, can't be too bad.

BTW, I'm not aware of any Autolite plug that Isuzu believes will work with the Ion Sensing ignition on this car. Veh will run fine on numerous plugs other than those recommended I know, but usually results in continuous P0300 random misfire codes that are impossible to totally eliminate. Since your misfire is always on #4, however, that's not likely your issue here.

Just not sure what sensor you are asking about on back of engine. Can you post a pic? You're not talking about the fuel pressure regulator are you? It's in the spot you describe, & operates on engine vac.

02-24-2010, 10:25 PM
All 6 of the Coil Packs are the exact same. I can get the exact model in an hour. The coil packs have not been replaced since purchasing the car and this problem wasn't always present. It ran good at one point, many months ago.
(did a quick search on a parts site and 99 does not show a different coil pack for #6 while 2002 did, so i believe that also was not implemented until the 2000 models)

I read at the link that Ion Sensing was not used until the 2000 models.

The Sensor is the one above the Fuel Pressure Regulator, on the metal bracket.

As for the plugs.. yup looks like I didn't get the right ones... I thought those were listed. I am having trouble finding Double Platinum in the recommended brands.. http://isuzufaq.ibctech.ca/#faq15 ... I recently tried the Denso Twin Tip and those also seemed to give the CEL. I might try to swap in some older iridium plugs I still have kicking around just to be sure it isn't from the plugs... as to why it would isolate Cylinder #4 is still where I am getting stuck on the plugs issue.

Cat Fuzz
02-25-2010, 11:53 AM
99's are not ion sensing and all the coils are the same. Swap number 4 fuel injector with another one and see if the code follows.

99 Fronty
02-26-2010, 05:44 AM
The Sensor is the one above the Fuel Pressure Regulator, on the metal bracket.

This is an electrically activated vacuum control valve that operates the variable length inlet actuator on the back of the manifold.


02-28-2010, 08:07 PM
This is an electrically activated vacuum control valve that operates the variable length inlet actuator on the back of the manifold.


Should this make a sucking sound? Would it have anything to do with getting a misfire in the certain cylinder...

Anyways going out to the garage to swap the injectors now, will update after. If i can find the compression tester I will also do a few of the cylinders.

02-28-2010, 11:03 PM
Alright just finished swapping the Fuel Injectors and reset the CEL. Drove it about 10 miles without the CEL coming on again. Before it would come on with 5 minutes of it idling.

I think I found a leaky vacuum hose while I was poking around, replaced it, going from the electrically activated vacuum control valve that operates the variable length inlet actuator so that could have had something to do with it. I thought I heard a leak back there but must have missed it the first time I did the vacuum lines...

Only time will tell now...

03-09-2010, 01:12 AM
Alright been a little while, few updates...
Code P0304, CLeared
Changed the fuel injector clip on the #4 injector, no difference.
Started running rougher.
Code P0304, cleared
Coolant Leak on Right side hose under Throttle Body, Cut a piece of the hose because it was bulging and causing the coolant to pass the nipple area.
Still Rough, shaky like one cylinder isn't working correct, not always but about 4-6 times a minute
Code P0304, cleared
Compression Test 1 & 5 @ 125 psi, 2,3,4 & 6 @ 100-105 psi (not the actual test with the throttle all the way open, just a quick start and shut off)
#6 plug was gassy so I switched the plug & coil pack with cylinder #1 to try and isolate that problem.
Cleared all codes caused by not having plugs/coils in

Testing tomorrow, Any ideas? Thinking about swapping the #6 injector and doing all the seals on the injectors. Getting a new connector for the #4 coil pack? Who knows... Getting sick of having the same code, wish a different one would come up already

03-12-2010, 01:38 AM
Hmm Alright, Pulled the #6 injector, Put in an older, but confirmed working injector.
Glass Media Blasted some Old NGK Iridium IX plugs (under 40k miles) and put them in.

Light went on. Reset. Light went on within 5 miles. After 3 more restarts light went off.

Maybe we are nearing the end, I hope so.

Ordered some Coil Boots, as I didn't comprehend that these were replaceable until a week ago. Should be in Sunday.

If the light comes back on and the boots don't help, the last resort in a pressurized injector clean on Monday or Tuesday. After that I am ready to give up

12-28-2011, 12:18 AM
Haha, Still having the same problem. Just got the Passport back after 6 Months at the dealership for the Trailing Arm Bracket Recall... My driver side seperated in 2009 and we welded it so good that they wouldn't buy the vehicle back :( I hope the other side breaks loose after their Stage 1 Spray Wax wears off.

I was reading my own thread hoping to find an answer and then noticed it was me having the problem in the thread.. oops :headshake:

Still P0304 and P0440 now if anyone has any ideas it would be appreciated. :confused:

For a while it went to P0300 so I am glad it is back to cylinder #4 at least it gives me somewhere to look closer

01-31-2012, 06:47 PM

Did you ever figure this out?? I have a 2001 Rodeo with a very similar issue (except mine is P0301)

To date, I've done the plugs and wires. Dont remember which kind of plugs I bought, just know they were recommended by Oreilly after a quick lookup on their system.

I've also done the camshaft position sensor. The fuel sending unit. Valve cover gasket. Air filter. Fuel filter. And in unrelated news: the alternator and battery.

I just ordered a new coil pack (mine's the 4cyl) ... and that pretty much tapped my account. I hope to get it on Friday and install same day.

Sometimes... before my truck warms up ... there's a puttering noise when I accelerate. Regardless... P0301 never wants to go away.

How did you situation end up??

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