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Can't figure out engine misfire

(This is a continuation of a previous thread: )

I have a 2005 Nissan Sentra with 75,000 miles on it. Several months ago (see above link) the check engine light came on with a P0304 cylinder 4 misfire code. The only symptom appears to be a rough idle/shaking every 3rd or 4th start for the first 10-15 seconds, but runs smoothly afterwards. My mechanic replaced all my spark plugs (I had new plugs put it at 50,000 miles for the same issue) of which the one from cylinder 4 was worn down much more than the rest. The problem persisted so they replaced my fuel pump and fuel filter and switched the spark coils from cylinder 4 to a different cylinder, but the code still read cylinder 4 so it wasn’t the coil. That was back in October.

I took my car back in last week for the same problem and without starting my car (because the problem is worst on the first start) they pushed my car into the bay and did a smoke test, no leaks in the fuel lines. Then they checked the cylinder and said that there was “excess” fuel in the cylinder and replaced the fuel injector. Then after ~3 days the check engine came back on with the same P0304 misfire code. Previous repairs/diagnostics rule out spark plugs, coils, fuel pressure, and injectors. What else might it be? Are there other symptoms I should pay attention to aside from the trouble starting?

Thanks for any help!

@mihalybaci‌, I reopened the old discussion if you would like to put the update at the end of that discussion. I can close this one if you prefer.

Misfire on startup is often an intake vacuum leak that closes up when hot or coolant in the cylinder from the last hot soak. See if the number four plug has discoloration or ash on it.

Get the car hot and let it sit overnight, remove all the spark plugs and cover the number four hole with a paper towel and crank the engine. Look for coolant on the paper towel.

I agree with @rattlegas‌

In our fleet, there was a truck that would have a fouled spark plug and misfire every few months, always the exact same cylinder

I said enough is enough. I smoke tested it and found nothing. But I didn’t stop there. I removed the upper plenum and found a torn intake gasket. After replacing the fouled plug and the intake gasket, the truck was permanently fixed.

It’s been back several times since then, and the misfire is now just a distant memory.

@cdaquila‌ - This discussion can be here or on the old thread, which ever is most convenient.

Thanks for the replies. I will definitely make sure my mechanic checks the intake gasket. I will post an update (either here or on the original post) when the repair has made.

Thanks again!

Was the compression ever checked? I wouldn’t keep hunting until you know that compression is reasonable and balanced across all 4 cylinders. Chasing misfires is often as easy as spark plugs. But you always just check compression at the outset to make sure you’re not going to spend a lot of time chasing your own tail.

I believe the compression was checked, but I would have to look back at the records to be sure.

I got my car back from the mechanic who started it 5 or 6 six times during the day and couldn’t get problem to show up for them. They switched coils 3 and 4, which I know they did before but apparently wasn’t in the computer records so they did it again, so now I just have to wait for the light to come back on before they’ll do anything else.

Is there any chance coolant is getting into that cylinder? It could come in thru the head gasket or on dome cars an intake manifold (bad gasket or cracked gasket). It is also possible for air to get into the intake manifold but that would raise your idle speed.

I haven’t noticed any white smoke in the exhaust, which I assume would be a clear signal of burning coolant, unless its just enough to cause the misfire without discoloring the exhaust. Although, I did have to refill my coolant not too long ago. It was just below the “min” line, and the last time it was filled would likely have been ~7500 miles ago. Is that fast for coolant?

You said the #4 plug was “worn down much more than the rest.” Was it also very clean (no carbon, etc)? If so, it’s very likely there is a coolant leak into that cylinder that is “steam cleaning” the spark plug and causing the misfire at startup.

You also apparently are losing coolant slowly, and it’s maybe being burned in the #4 cylinder, but not enough to cause white smoke. The only time you might notice the smoke is at startup when the pooled liquid in the cylinder gets burned. Otherwise the leak is too small to smoke all the time.

Since you replaced all the plugs several months ago, you might be able to detect the signs on the new plugs as well. If you pull all the plugs and compare, the #4 may look cleaner than the others, but the difference may be subtle since the new plugs don’t have many miles on them yet.

I’m assuming you are saying you are experiencing about 8 ounces of coolant loss in 7500 miles? That may be normal for you car, esp if the coolant overflow tank vents to the atmosphere. In any event, even if the coolant is getting into the cylinder, at that minimal amount, I doubt that it would be the cause of a misfire. But it could be indicative of a problem that would cause a misfire I suppose. Mechanics have various cylinder leak down tests they can do to confirm or rule this possibility out.

@cigroller‌ - I checked, and the compression was tested and found normal.
@jesmed‌ - I don’t recall whether the worn plug was dirty or not. I might still have the old plugs, but I’m not sure.

Yesterday afternoon, I started my car to leave work and it had “the shakes” again for about 10 seconds. After a few blocks I reached a stop sign and smelled some gasoline, either from my car or an older SUV (mid-90s Isuzu?) I can’t be sure, and then my check engine light came on. I drove straight to mechanic, P0304 misfire, and mentioned that my coolant was low and I had to fill it up. He thinks it might be a head gasket, and I got a quote of $1450 for a “head job”.

When the mechanic says “head job”, is that the same as replacing the intake gasket that @db4690 suggests? Or is it something more involved? Also, if it is a $1400 head gasket problem, can I skip the fix for a few months or a year, or will that damage more than just the spark plugs?

If it’s a gasket leak (intake or head), it could eventually cause the engine to overheat and possibly warp the head. So this is something you should fix sooner rather than later.

But there’s no reason to guess about this. Let the car sit for a day or two, then have the mechanic pull the spark plug on the suspect cylinder and do a borescope inspection. He should be able to see pooled coolant in the cylinder. If he doesn’t have a borescope, find a mechanic who does.

I doubt the intake gasket could be the cause, as OP’s car has an inline engine

Intake gaskets can cause overheating problems on V engines

The mechanic said that there was excess gasoline pooled in the cylinder, so I assume that required the borescope. He didn’t mention coolant though.

OK, now I’m confused. If there’s pooled gasoline in the cylinder, that’s a leaking fuel injector, not a “head gasket” or “head job.”

Was it the same mechanic who saw the pooled gas and then said he thought it might be a head gasket? Because that makes no sense.

+1 to jesmed’s post.
There are too many unknowns here.

A compression test is good, but it does not check for a headgasket leak. A “leakdown test”, sometimes called a “compression leakdown test” because it tests the integrity of the cylinders to hold compression, is the head gasket test. We don’t know if that’s been run.

A breeched head gasket does not cause pooling of fuel in a cylinder. I’m going to bet my morning muffins that an injector is leaking.

If an injector IS leaking, the sparkplug will be wetted with gas when removed. What was the condition of each of the plugs when removed?

For the record:
The intake gasket and the head gasket are entirely different gaskets.
IMHO 8 oz of coolant loss in 7500 miles is a sign of leakage. Unless there’s an obvious leak path, I favor the compression leakdown test I mentioned earlier. You could be drawing the coolant into the cylinder and vaporizing it through a head gasket breech.

Honestly, I think this engine needs to be thoroughly evaluated by a competent shop. Perhaps it has been and what we’re reading is an interpretation of the diagnosis. Can you post exactly what the mechanic wrote on your copy of the shop order(s)?

Sorry, the timeline has gotten confused because of all the different work. This has all been with the same mechanic, too, and the only engine code has always been P0304 and nothing else.

First I had all four spark plugs replaced, then the coils and compression was checked. Those came out fine so they replaced the fuel filter and fuel pump. Then I decided I couldn’t afford more repairs so I took a few months off with the engline light on. When I brought it back a few weeks ago, I left the car with them overnight and the next morning they pushed it into the garage - without starting it - took out the plug from #4 and found excess fuel in the cylinder. So then they replaced the #4 injector, and a few days later I got the same #4 engine code. The mechanic switched coils #3 and #4, but the code didn’t change and that’s when he suggested that the head gasket needed to be repaired.

I have all the records, but I will have to post them tonight after I get home from work. The mechanic has not mentioned anything about the current condition of plugs. Once I look at the records I can see hat’s mentioned.

OK, so the #4 injector was replaced and it’s still misfiring. I’m wondering if the mechanic mis-diagnosed the liquid he saw pooled in the cylinder. If it was in fact coolant and not gas, the new injector wouldn’t have fixed the problem.

So have him pull the #4 plug AGAIN (after sitting overnight) and check for more liquid. If there is some, it’s probably coolant and a gasket job is in order. If no liquid, the head gasket is not the issue. Either way, it’s a 10 minute job to either confirm or rule out the need for a head gasket. Guessing is not acceptable.

If there is no coolant in the cylinder, something else is at fault…a slightly misadjusted valve, a burned valve, etc.

I would run from this mechanic. He already stuck you with a fuel injector you didn’t need. Head gasket does seem like the likely culprit.