1994 Ford Mustang engine miss

Actually, the computer can detect compression, but I don't think 80 psi would cause that noticeable of a misfire. Needed spark voltage will decrease under lower compression, but cylinder misfire is detected by the fact the crankshaft slows down when the misfire occurs. If it didn't, you wouldn't feel it. This is really more of an OBD2 thing, (on-board diagnostics, version 2), which was mandated starting with '96 models, but in the early '90s, Ford was getting carried away with their computer controls.

Pop an inductive pickup timing light on plug wire # 3 to see if it's firing.

Caradiodoc

Forget the timing light as long as you found the plug firing. Next I'd head for a cylinder leakage test. That blows compressed air into the cylinder when it is at top dead center with the valves closed. Listen for air escaping. Tail pipe - exhaust valve, air intake - intake valve, oil cap - piston rings, radiator bubbles - head gasket. Based on the 80 psi, I'd vote for a burned exhaust valve. I had two '70s Chryslers that had burned exhaust valves. Other than sounding like an air compressor, they ran fine.

Caradiodoc

I would hope, (only because I don't want it to be something worse), that the fuel smell is coming out of the tail pipe. Well, now that I said that, I take it back. I would assume if the misfire is due to a burned valve, the mixture is still burning in the cylinder. Bosch makes really good injectors. It's why you rarely hear of an injector failure on a Chrysler product. My comment was heading in the direction of anything spark-related causing a misfire would send unburned oxygen down the exhaust system where it would be detected by the O2 sensor. That would tell the computer to add fuel, but to all of the cylinders equally. No matter how much fuel is added, O2 sensors only detect oxygen, not fuel. It will keep on seeing that oxygen, but you will smell the raw fuel out the tail pipe. Besides the rotten fuel mileage, the catalytic converter is going to try to mix the unburned fuel and oxygen and burn them. It will likely start to glow orange. The color might be considered "pretty", but the excessive temperatures will cause the catalyst to melt and become plugged.

I thought about unplugging number 3 injector, but that will not make the engine computer very happy, and you will still have the air going through the cylinder. At least this way you're still getting a little power from the sad cylinder.

To switch gears for a moment, a drop from 30 to 10 mpg seems unusually high. Based on readings from the O2 sensor(s), the computer typically can only modify fuel delivery by plus or minus ten percent. The computer learns or adapts to the repeated fuel metering adjustments it has to continually make. If it is under a condition right now that requires a little extra fuel, that information will get stored a one of many "cells" in its "short-term fuel trim memory. The cell refers to all the conditions the computer monitored to make that decision. After a while, if the computer sees that it is always having to add fuel under the same conditions, it will move that information into its "long-term" fuel trim. That's the information is uses for its starting point every time you drive the car. Again, these things shouldn't affect fuel mileage as much as you're seeing.

If anyone else can add to or correct my story, please jump in.

You also have a sensor called the DPFE sensor. Can't remember what it stands for, ... "Dual pressure something something". It's Ford's answer to GM's and Chrysler's MAP sensor. It does more than just measure intake manifold vacuum, and I believe it has a big effect on fuel delivery to the engine. You also have a Mass Air Flow (MAF) sensor which has a really big effect on fuel. I'm not an expert on them because Chryslers are my specialty, and they are the only company that never used them. What little I CAN share is any air that sneaks in through a leak does not get measured by the MAF sensor so the computer doesn't know to add enough fuel to go with all the incoming air. The mixture would be lean and would be picked up by the O2 sensor. A dirty sensing element will also prevent it from accurately measuring airflow. I remember from training classes that the MAF can cause a very rich condition but I'm not ready to admit I can't tell you how or why.

While I'm sitting here thinking, it occurred to me there is a common problem on Ford trucks that cause a single cylinder misfire. I don't know if it applies to their cars. There is a rail or a set of tubes that delivers EGR gas to the cylinders to reduce emmisions. Come to think of it, I think that's where the DPFE sensor comes in. Rather than allowing the group of cylinders to accept as much exhaust gas as they want, the system provides a more carefully calculated amount, then sends that exact amount down the rail to all the cylinders to share in equally. Eventually those tubes become plugged with carbon. Any cylinder with a plugged tube will now only receive fresh air and fuel so it will run just dandy. As more tubes become plugged, the system is still sending the calculated amount of exhaust down the line, so all of it has to go into the cylinders that don't have plugged lines. Since all the exhaust gas goes into only one or two cylinders, they are the ones with the clear EGR tubes, but they are the ones that misfire because the fuel mixture is so diluted with inert exhaust gas. It's the good performing cylinders that need their tubes cleaned.

Again, I don't know if that applies to cars, but it seems to me the 80 psi of compression should not be causing THAT much trouble. If the fuel mileage dropped right after the new injectors were installed, I'd wonder if they were the right ones.

Caradiodoc

Didn't realize you had an injector apparently stick open. I just assumed a valve problem as the cause of the low compression. A cylinder leakage test would confirm leakage past the piston ring. This would make sense too because metals are beter today. No one really has trouble with burned valves anymore.

If you have headers, are you still using the oxygen sensor? Ford did have some problems with coolant sensors in the early '90s, but I can't believe a bad one would have that much affect.

As for the new injectors, did you get them from a company in Indianapolis, IN? They do a really good job, and they match them up in sets according to flow rate. The wrong injector would spray more fuel each time it fires, but the computer would see the constant lack of unburned oxygen from he O2 sensor's readings, and would cut back on the amount of time it held the injectors open. I think the supplier of the injectors has been doing this long enough that they can be trusted to give you the right injectors for your application.

Caradiodoc

You need to do the cylinder leakage test. That will rule out all the other guesses and identify exactly what is causing the low compression. I'm still leaning toward the exhaust valve, but I can't imagine how that is related to the other symptoms.

Caradiodoc

The exhaust valves and seats are made from hardened steel that is much better quality than from years earlier. I can't see a piece of injector becoming embedded in the seat, but I suppose it could get hammered out and wrapped around the valve face or stuck to it due to the heat. That would be weird, but not out of the realm of possibility. If the valve was really hot, debris might have simply deformed the valve head as it sailed out.

The easiest way to test the O2 sensor is to watch it switch from rich to lean on the scanner. The transitions are called "cross counts" and are necessary for proper operation of the catalytic converter. The fact the converter is missing has no effect on the O2 sensor's performance. If you find the reading switching rapidly many times per second, the fuel / air mixture is correct.

Next, check the short and long term fuel trim numbers. If they are high positive, the Engine Computer sees a need to deliver extra fuel. That could potentially be due to an exhaust leak ahead of the O2 sensor. Between the pulses of exhaust flow, the momentum creates a slight vacuum that pulls air in through the leak. The O2 sensor sees that air so the computer commands extra fuel. No matter how much extra fuel is delivered, all the O2 sensor ever sees is the unburned air.

If fuel trim numbers are high negative, the computer is trying to cut back fuel delivery. That usually corresponds to an O2 sensor constantly seeing a rich condition. Typically this is due to a mechanical malfunction, such as the leaking injector.

Caradiodoc