# Cylinder 4 misfire P0304: advice needed.



## 63554 (Jan 5, 2007)

Ok ladies and gents... heres the deal. I've got a 97 HB A/T 2WD. I'm moving back to texas from california in a week decided to give the truck a tune up before I go. The truck was throwing a p0400 EGR code so I knew i had to take care of that while i was at it. I will post in order of what I did. 

Checked EGR and diaphragm moved freely and didn't stick.
Found a melted vacuum line from EGR valve to EGRC-BPT valve. Replaced the vacuum line and Viola! No more code. 

So I continued with my tune up. 

Started with an oil change, air filter, cleaned TB. Then I moved to brakes front and back. 

After doing my brakes I ran bottle of sea foam through the engine like the bottle said 1/3 pint in the crankcase, 1/3 in the fuel tank, and 1/3 into the vacuum line through the brake booster. Smoked out the neighborhood and the truck was running great. Few hours later went to the auto parts store and I got this nasty hesitation/stumble around 1500RPM - 2000RPM only in first gear. Then my CEL started flashing for about 20 seconds (Which i've never seen it flash) then goes to solid. I got home read the code P0304 cylinder 4 misfire..... No problem. I know sea foam is notorious for roasting plugs so

New spark plugs. - didn't fix
New ignition wires - didn't fix
New distributor cap and rotor - didn't fix
New fuel filter - didn't fix
Ran a bottle of Techron fuel injector cleaner through it (in desperation) - didn't fix
Cleaned MAFS - didn't fix 
Compression test - checked out normal.. 175ish with a OEM spec of 190 standard... minimum 142. all cylinders within 10PSI of each other. 
Pulled injector harnesses one by one while idling... - engine slowed on every harness I pulled. 

Now i'm stuck... not sure where else to go.. fuel pressure? maybe perhaps something still with EGR or EVAP? I need help! Sorry for the long post but I wanted to be clear.


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## smj999smj (Jan 1, 2006)

Try spraying carb cleaner around the intake gasket at the #4 runner with the engine running to check for a possible leak. If that's okay, it's possible that you have a bad #4 injector. You could try swapping two and see what happens. If that's not it, I would remove all plugs and perform a cylinder leakdown test.


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## 63554 (Jan 5, 2007)

what's the process of getting the fuel injectors out? I would like to swap two like you said but how do they come out? it looks like they just have two screws and pull out? anything else i need to know?


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## smj999smj (Jan 1, 2006)

It's best to remove the fuel pump fuse and start the engine, let it run until it stalls, then crank a couple more times in order to depressurize the fuel system. The fuel injector cover is held on by 2 screws and lifts straight off. There's a spacer under the cover, so be cafeful not to drop it. Also, be careful not to damage the "O" seals in the injector; if they get damaged, they will need to be replaced.


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## 63554 (Jan 5, 2007)

swapped injectors... reset OBDII ... no go...

still getting p0304.

sprayed carb cleaner everywhere around the intake and no change in engine idle.... 

any other ideas??


i guess leak down test is next... uhgg.. everything was running fine until that damn sea foam!


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## 63554 (Jan 5, 2007)

i think i may have some sort of exhaust clog.... the engine will start to misfire around 1000-1500 rpm... but if i rev up to around 2000 -2500... the entire engine smoothes out.... if it were some sort of exhaust clog... how could i check that?


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## Tech2452 (Feb 7, 2021)

Did you ever figure this out? Mine is doing the exact same thing. Thanks in advance


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## rogoman (Dec 16, 2004)

ajan246 said:


> swapped injectors... reset OBDII ... no go...
> 
> still getting p0304.
> 
> ...


You probably should never used an additive such as sea foam. Some of these additives cause more problems then doing good. If you've got a good running engine, leave it be. Just perform the standard PM like replacing wear components such as plugs, filters, lubricants as needed.

Do you have spark on #4 plug? The #4 fuel injector may not be firing. This can be tested with a "noid" light probe on the injector harness connector.


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