Low compression

You need to list which model you have. Also, please list the exact fault code number. The place to start with misfire codes is usually spark plugs and wires. At the mileage you listed, those are the best suspect.

It is an SE and I checked the spark plugs they had rust on them and I forget the exact code but it affected piston two.

Based on the mileage, we would replace the spark plugs and wires without wasting time testing or inspecting them. It is only after that, in the unlikely event the problem is not solved, that we would spend our customer's money on the time needed to dig deeper. If you do not want to start with that, or if those parts were replaced recently, switch the spark plug from cylinder two with one of the others, erase the fault code, then drive the vehicle to see if a new fault code sets for the cylinder you moved the suspect spark plug to. You can do that with a third cylinder for the wire, and even another cylinder for the injector. Moving parts to different cylinders can be an effective way to find a part with an intermittent problem.

You have not listed a model yet. I do not know if you have a truck, a minivan, SUV, or a car.

It's a 2010 Dodge charger SE V6 3.5L with 93,000 miles

Did you do a compression test if so what were the readings ?

If you have low compression and the plugs had rust on them I would get a bore scope and look inside the cylinder to see if the cylinder has any rust pitting or roughness or damage. Then run a leak down test. That will give you an idea if the compression loss is due to the valves (could have rust on a valve stem holding a valve partly open) or due to the piston rings. It's possible that the engine was running and it took in enough water to hydro-lock that cylinder and it bent the connecting rod or did other damage. Just need to test and eliminate the possible issues.

Hi guys,

I have a similar problem with a 14 Challenger R/T but on cylinder 8 (P0300 and P0308)

Leak-down compression test revealed 210-215 PSI on all but one cylinder. #8 had 185 PSI. So the problem is definitely on 8.

Sparks plugs and coils checked out fine.

What would be the logical next steps for diagnosing #8?

What sort of outcomes should I expect?

Hi Cantona,

Your compression readings on the other cylinders that are over 210 PSI sounds very high. I would expect all of them to be around 190 but if number 8 is about 10% off then you need to investigate that further. The next thing you need to do is run a leak down test. This will tell you where you are losing your compression. Could be the rings, one of the valves, or the head gasket.

You need to regulate the air pressure down to around 25 PSI and then put the piston at TDC on the compression stroke so that the valves are closed. Then you need to find where the air is escaping. It will be from one of the following.

1. Out of the intake - intake valve is leaking
2. Out of the exhaust - exhaust valve is leaking
3. out of the coolant bottle - head gasket is leaking
4. Out of the oil fill cap - piston rings

Once you find where you are losing your compression then you can address that issue. Please let me know if you have questions. Thanks

Excellent advice!

I’ll work my way through the leak criteria and let you know.

Out of curiosity, which area should I fear the most ($$$) the head gasket?

Well if it is a valve then you will need to replace the head or at minimum have a valve job done. If it is the head gasket that just needs the head removed and replace the gasket unless the head is warped then it needs to be replaced. If it is the rings then the engine needs to be torn down to get the piston out and re-ring it.

In short, none of them are cheap but the rings are the most intrusive. This is why we need to make sure this is what it is before we go too much further.

On that engine I would expect a bad rocker arm or dropped valve seat before a head gasket.

Thanks Steve, what sort of effort and cost should I expect from either:

- Replacing/repairing the rocker arm?
Or
- Replacing/repairing valve seat?

If it's a rocker they are not hard to deal with other than getting the mess off that covers the area. If it's a dropped seat it would depend on what it did and how it failed. If you want to see some examples go onto YouTube and look for "MotorCity Mechanic" David has a lot of videos showing the work involved.

Hi guys, updates:

Suspecting It’s an intake valve seat, leak-down test almost confirmed it, haven’t head the heads off yet to get at it and verify.

From there i’ll either have the valve job done on #8 or I might even consider taking care of all the valves while
I’m at it?

I’ve spoken to my dodge dealer and they have suggested we could do a percentage split on repairs or a discount. I’ll investigate that option but I’d imagine with their mark up etc. I wouldn’t be getting much of a “discount” vs taking care of it locally or on my own steam.

I am interested in learning if this #8 misfire has been going on for a while. Is there a way for me to pull historic codes, with dates etc?

Maybe look at Rock auto for replacement heads, they get a core charge but you might be able to get them cheaper than through a dealer. If you have them done you will want to do all the seats.
As for the codes, you can pull freeze frame but it won't have dates.

Hi guys,

Updates:

I took my RT to my local dodge dealer and they insisted that because there were no codes they can’t begin an investigation (irrespective of the history I provided them).

The codes were cleared by my local mechanic after he performed the leak down test.

Now I guess I have to drive the car for a little while and wait for a check engine light to appear before the dealership can start work (with a bit of luck, under warranty).

My concern is that I have to continue to drive the car with a leaking valve.

How long does it usually take for the check engine light to re-appear?

I’ve read that it takes a few “drive cycles” and that there are certain criteria to constitute a drive cycle.

Is there a way for me to accelerate the process? Or certain conditions I should be applying to the car to ensure a “drive cycle” eg: driving on the freeway for 20 miles over 60 mph etc?

As always, thanks for the advice.

They should be able to read the misfire counters and see which cylinder(s) are the issue. Sounds more like they don't want to be bothered. Not much you can do to set a code unless you were to pull a coil wire and trigger it that way, but I wouldn't do that as you want them to get the correct cylinder on the first try.

Thanks Steve,

Looks like I’ll have to drive it around the place for a few days and wait for the check light to re-appear.

I’ve read that takes multiple cold starts along with 50-100 miles.

Misfire codes are a constant monitor so it should trip relatively fast.