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Gary, Chevy Technician
Category: Chevy
Satisfied Customers: 16877
Experience:  15 years GM master tech
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P0304 Misfire cylinder 4 on a 2002 express 3500, 77,000.

Customer Question

P0304 Misfire cylinder 4 on a 2002 express 3500
JA: How many miles are on the car? Is it a steady misfire? Or random?
Customer: 77,000. random infrequent flashing of check engine light. Might flash once every 35 miles. Tuned up and injectors replaced still doing it, but not as much. Always cylinder 4.
JA: What is the model of your '02 Chevy?
Customer: 3500
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: tune up and injectors replaced. Changed them over to MPI. I have a mechanic
JA: Anything else you think the mechanic should know?
Customer: runs good. do not notice anything in performance when the code comes out. No pattern. random when it happens
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Chevy Mechanic about your situation and then connect you two.
Submitted: 1 year ago.
Category: Chevy
Expert:  Gary replied 1 year ago.

I am sorry for the long wait

what size engine is in this van ?

Customer: replied 1 year ago.
Expert:  Gary replied 1 year ago.

new cap and rotor ? and the new cap and rotor made no difference after you installed it and you never feel it misfire other than the light on and or blinking ?

Customer: replied 1 year ago.
Correct for all
Expert:  Gary replied 1 year ago.

with the new updated injection system this issue usually comes down to a sticking valve guide hanging a valve just enough to trigger the crank sensor to see that cylinder to not have worked as much as the others or a bad crank sensor

the lower distributor gear is NOT worn down at all ?does the rotor have excessive side to side play in it at all showing signs of a bad or work bushing or that the lower gear is worn down at all ?

Customer: replied 1 year ago.
The original problem involve cylinder 4 misfiring and cylinder 7 misfiring a little. After injector replacement cylinder 7 did not misfire and cylinder 4 continued to misfire but at a lower rate.
Customer: replied 1 year ago.
Would a bad crank case position sensor affect only one cylinder?
Expert:  Gary replied 1 year ago.

yes it would

the 5.7 and the next years 5.3 crank sensors were causing misfire issues like this where you didnt feel it misfire but would blink the light

if the distributer is tight and nothing loose and the lower gear is fine then change the crank sensor if it still acts up the only thing left that we have seen is the $ 3 cylinder laves sticking in the valve guides causing this exact issue


you can always get me back for FREE for any follow up any time with this question even after you rate me here at Justanswer

if you have any questions please ask as i do not get credit for my work until you rate me here at Justanswer

The vehicle may exhibit a SES Light due to a P0300 and misfire. If the misfire is related to the information below, it will typically happen while cruising uphill, pulling a trailer or on hard acceleration and then stop misfiring shortly after returning to an idle. Typically, cylinders 3, 4, 5 or 6 will be the ones to experience this.


If the P0300 SI diagnostics did not isolate a concern, the following may help:

Remove the valve cover, valve springs and valve seals on the effected cylinder or cylinders. A small wire tie or rubber bands can be placed in the valve stem keeper groove to prevent the valve from falling into the cylinder. Rotate the valve while moving it up and down in the guide to see if it binds. If a binding valve is found, remove both cylinder heads and use either of the following methods to increase the stem to guide clearance:

^Send the head out to a machine shop and advise them to increase the valve stem to guide clearance to 0.002 inch per guide.

^Hone the guide with a 9 mm hone, such as Snap On BCG249, until a clearance of 0.002 inch is obtained. This will usually take about 4 strokes of the hone.

Reassemble the cylinder heads using new valve seals.

Expert:  Gary replied 1 year ago.

how are you making out ?

you can always get me back for FREE for any follow up any time with this question even after you rate me here at Justanswer

if you have any questions please ask as i do not get credit for my work until you rate me here at Justanswer