Pontiac Grand Prix code P0304 misfire

07 Pontiac Grand Prix 177k miles. Engine misfire code P0304 indicated I change spark plugs, wires, and coils which I did. No fix. Then the reading recommends replacing fuel injectors, the shop I went to is reliable… but they said my cylinder 4 that’s misfiring has a pressure of 60 psi compared to the others which are fine. My car does not like to start up without giving it gas and they recommended me to either junk my car or spend money on replacing whatever is wrong with the engine (valves, rings, etc.) not sure what to do I really want to keep my car it’s my baby. What do you recommend is best? My car is in good condition and has everything I need.

I’m not a mechanic but you have pretty much eliminated the electrical part. If you swap injectors around you can see if the misfire follows the injector or not and eliminate that issue. The only additional work I would do if it turns out to be mechanical is to pull the valve cover for that side and inspect the rockers for possible wear. Is it making any valve clatter. I guess the worst would be pulling the head to repair a valve issue but that is dumping more money into it.

Mechanics may have a better answer.

That 60 psi is the compression pressure for that cylinder (#4). That is a fraction of what it should be. Your engine is worn out.

The solution, from most expensive to cheapest, is to replace it with a new engine, remanufactured engine or a used one from a junkyard car with fewer miles. All will cost thousands. If the rest of the car is in good shape, it might be worth the expense.

A far cheaper alternative might be to pour a can of Restore from the auto parts store into the oil and see if it helps. It can’t hurt. It might buy you some time.


This is why the engine mechanical condition should always be checked before spending money on the peripherals such as plugs, coils, and so on.

As Mustangman correctly stated, that engine has a problem at 60 PSI although it would be interesting to know what the numbers are on the ones said to be “fine”. I tend to think those are not as fine as you may think they are.


A leak-down test will reveal why cylinder #4 is low on compression.



The leak-down test will tell you where the compression is being lost. If the problem is valves or a bad head gasket, I would repair the engine. If it is worn piston rings, I would look for a working used engine.


I really do not understand someone calling a vehicle ( my baby ) .

I’ve had it for so long

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Does anyone recommend or think it’s not a good idea to get a used engine and put it in?

I’ve installed many used engines/transmissions over the years and each one worked fine.

But I trust the people that sell me these components because I know they’re not going to sell me junk.


It’s not your “baby”, it’s an appliance like your washing machine, and it’s time is up. Junk it.

You need to get some clear facts together so you can make a decision.

First, what is the condition of the rest of the car? It is in relatively good shape? Does it otherwise meet your needs for a car?

Second, get some quotes for a new/used/rebuilt engine. Compare the cost of the new engine vs. the value of the car if it were running already. Can you afford to buy a new car right now? If not, it may make sense for you to get the engine replaced.

Like I said, you need to remove emotion from this, gather some firm numbers, and make a decision. Good luck.

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I have done several headgasket jobs on similar gm rigs lately. Pulling a head is pretty simple. Could you do it? Not worth paying a shop $1500 to do it.

I think this is really the first step to determine where the problem is and then go from there. Sure engines have variable life spans but a well maintained GM engine should be well able to make 2-300,000 miles. But then there are a lot of them around too if you need to pop a used one in.

If a leak down test shows the problem is just a valve, makes sense imo just to have the valve repaired. Could be a broken valve spring for example. Even more so if the rest of the cylinders show per-spec compression, the car has been driven gently, and the oil and filter changes have consistently been done on time.

My gut feeling is it may need a valve job

All we know is #4 is at 60psi

So far, we haven’t been told where the other cylinders are at

If they’re at 120psi, for example, that is not very good

Does anyone think it’s worth to put a used motor in with 65000 miles or try and fix what’s wrong w the engine that also needs a thermostat replacement
My car has 177,000 on its engine.

Not so fast. I agree that a car is an appliance, but guess what–appliances are expensive and in short supply these days. As long as the rest of the car is in good condition, it makes sense to repair it, rather than to pay grossly inflated prices for a new or newer used car.

If it were my car, I would replace the cylinder heads with remanufactured units as it is likely that there is a leaking valve. Thermostat replacement is insignificant. The problem must be diagnosed before a decision can be made.

With used engines I question whether the advertised mileage is correct, could be much more than listed and the labor is more to replace the engine than to repair it.

I don’t junk good cars that are repairable. In my state the sales tax and first year registration fee on a new car would be greater the repair cost for this car.

The OP needs to answer a couple of questions; one of which has been brought up multiple times and that is the compression numbers of the other than 60 cylinder.

What are the numbers on the other cylinders?

Second question since thermostat is mentioned. Does this mean the thermostat was stuck closed and you continued to drive the vehicle in a seriously overheating condition?