Misfire PO304 Diagnosis

[sonny]

Would like your thoughts on diagnosis. I've read through the FAQ. 1997 with 280,000 miles. Check engine light. PO304 code. Rough Idle. Vehicle begins misfiring bad under load at 45 mph, flasing check engine light. This is a recurring problem. Each time I've changed plugs (wires too) and the problem goes away. The plug appeared fouled. Same exact thing happens about 2-3 months later.
New subtle issues. The a/c compressor began to cut off. Realized running slightly hot and coolant low. Filled up with coolant and truck runs fine and a/c works great. No obvious leaks. Exhaust smells rich but no sweet smell. No obvious smoke.
What is causing misfire on #4? I've checked injectors and wiring harness. All look good. Could this be a small leak in head gasket at #4 that leads to misfire? Head gasket has never been replaced to my knowledge. Thanks for your help.

 

[clx16]

Do you have any pictures of your plugs and maybe any bore scope pictures inside your cylinders?

When you hit the starter is it slow to start? Any galloping while turning over?

How is the oil level and when you changed it last what did it look like when it first started coming out of the plug?

 

[Kernal]

"Realized running slightly hot and coolant low"

"Could this be a small leak in head gasket at #4"

How much coolant have you been adding, any external leaks, overflowing or significant bubbling/boiling fluid in the radiator tank?

As mentioned above, post up photos of your old plugs, especially #4

 

[sonny]

"Realized running slightly hot and coolant low"

"Could this be a small leak in head gasket at #4"

How much coolant have you been adding, any external leaks, overflowing or significant bubbling/boiling fluid in the radiator tank?

As mentioned above, post up photos of your old plugs, especially #4

Thanks for the response. I have added coolant twice in past six months. Each time between 1 and 2 quarts. I failed to get picture of spark plug. It was very fouled and burnt. I was in heavy traffic a couple of times when misfire was the worst. Took a mile or so to pull off highway. I suspect the heat from pre-ignition misfire caused the worst damage to the plug.

 

[sonny]

Do you have any pictures of your plugs and maybe any bore scope pictures inside your cylinders?

When you hit the starter is it slow to start? Any galloping while turning over?

How is the oil level and when you changed it last what did it look like when it first started coming out of the plug?

There has been some galloping at start up. Oil low light comes on a lot but each time dipstick looks good. I recently replaced valve guide seals without removing head (compressed air method). That stopped excessive oil consumption.

 

[sonny]

There has been some galloping at start up. Oil low light comes on a lot but each time dipstick looks good. I recently replaced valve guide seals without removing head (compressed air method). That stopped excessive oil consumption.

The oil hasn't been obviously milky. But I have ordered an oil sample kit and will have it tested. I suppose I'm trying to figure out if the suspected head gasket leak could be related to misfires every 2-3 months. I guess the answer is to just do the dreaded head gasket replacement job and find out.

 

[Outsane]

You can test foe exhaust fumes in the coolant with a kit it's very easy.

Also check the cap and rotor.
Good luck

 

[clx16]

The oil hasn't been obviously milky. But I have ordered an oil sample kit and will have it tested. I suppose I'm trying to figure out if the suspected head gasket leak could be related to misfires every 2-3 months. I guess the answer is to just do the dreaded head gasket replacement job and find out.

Nah man.... don't be like that haha. Statistically cylinder 6 is mostly where the headgasget goes. I would test the coolant as stated above before I would test the oil but nothing wrong with testing oil also. They have little cameras that you can stick in there and turn back to look at your valves and cylinder walls.

Not seeing your plugs or pistons I can't see if they have been steamcleaned with coolant....sounds like they havent from your statement of fouling. Also you don't know if you have burnt valves. You also don't have leakdown tests or compression values. All those are cheaper and easier Than a headgasket.

 

[sonny]

Nah man.... don't be like that haha. Statistically cylinder 6 is mostly where the headgasget goes. I would test the coolant as stated above before I would test the oil but nothing wrong with testing oil also. They have little cameras that you can stick in there and turn back to look at your valves and cylinder walls.

Not seeing your plugs or pistons I can't see if they have been steamcleaned with coolant....sounds like they havent from your statement of fouling. Also you don't know if you have burnt valves. You also don't have leakdown tests or compression values. All those are cheaper and easier Than a headgasket.

Thanks man. I'll perform those checks before taking drastic action.

 

[sonny]

You can test foe exhaust fumes in the coolant with a kit it's very easy.

Also check the cap and rotor.
Good luck

Thank you. I'll get a coolant kit and check it.

 

[LC4LIFE]

The a/c compressor began to cut off. Realized running slightly hot

If the a/c is cutting off because of engine temps, you are definitely not slightly hot. You are actually hot.
I'd also get the coolant kit and test it.