I have a '92 Ford Aerostar Extended with a 3.0. It has a mild missfire, and if I connect a code reader, it will give a code 538, when the engine is running, and there are no stored codes. If I do the cylinder balance test, it passes, but you can hear the missfire clearly. There is also vibration coming from the engine, that matches the missfireing.

 

I show error code P0538 as A/C evaporator temperature sensor circuit high

Cant see that it would be a source of your problems

Is it under load or coasting, at idle or while driving...

Mileage decrease? Smoke? Using oil?

Anything else that might help to narrow this down?

 

Error code 538 with OBD-I diagnostics is: "Insufficient RPM during KOER dynamic response test/Invalid cylinder balance test due to throttle movement during test."
Either one means the test was performed improperly.

90% of misfiring problems reported here are plug/wire related. The other 10% are burnt valves, leaking intake manifold gaskets or vacuum line leaks.

 

You have to hit the brake first and then the gas while retrieving codes.

 

Ok, thank you, I will retest. Still, any ideas as to why the cylinder balance test passes with flying colors? There is a missfire, causeing an engine shudder, and you can really hear it out the tailpipe, and it is very consistent and rythmatic, so one of the cylinders is definately missfireing consistently. It was working very well about 2 months ago when the plugs were all changed. I did not change the wires, as they were changed at the previous wire change, about 20,000 miles ago. Any ideas on a plug wire that will not give me any trouble?

 

On my '91 4.0 the OEM set lasted to around 100,000 mi. with no problems. I replaced them with a generic set and began to have engine problems 25,000 miles later. I resisted any idea it could be wires because they were relatively new but it turned out they were bad. The current Bosch set is approaching 100,000 miles with no problems to date. Just last week, I had the same thing happen with our Escort-GT. I had just replaced the wires 20,000 miles ago with a quality Autolite OEM set but they were the reason for an off idle stumble. Again, I couldn't believe it was the recently changed wire set.

You might try disconnecting one wire at a time, starting the engine and see if you can locate the misfiring cylinder that way. If changing the wire and the plug do not solve the problem then consider more serious issues like a burnt/chipped valve.

 

THe set of wires I have is a Bosch. Up until now, I haven't had any problems, but they are about 25,000 miles old. They may be due for replace ment. While I'm at it, I may as well inspect the cap and rotor. If that doesn't fix it, I'm gonna have to pull a few plugs. Its hard to tell where sounds are coming from, but it sounds like the missfire is on the passenger side.

 

Khan:

How long has it been since you replaced your O2 sensor? I have a '93 3.0L Aerostar and my factory O2 sensor went bad at about 60K mile. It never threw a code, just fouled up the spark plugs causing a misfire, that's how I found out.

 

I can't see why a cyl balance test would be good with a dead or missing cyl at idle, but then I'm not where you are

I had something similar on my '91 3.0l (new wires/cap/rotor/plugs) and it turned out to be a vacuum leak at No. 6 injector. Pulled the upper plenum (easy), carefully cleaned around all the injectors of all the crap that had accumulated, pulled that fuel rail plus the injectors, installed new o-rings, re-assembled and all was pretty good.

However, my cyl balance test (used a shorting scope for this) clearly showed No. 6 was not contributing much at idle.

The idle was still pretty crappy, then I ran a MotorVac fuel system cleaning against it (special machine, connects in place of the fuel rail feed and tank return lines) and after 10 minutes it idled smoother than my '89 3.0l! I wouldn't have believed it if I hadn't done the work myself, but this specialized fuel rail/injector cleaning really does work (I thought it was bogus).

Anyway, think about a vacuum leak. I tested using an aerosol volatile (I used 3M Brakleen and extended the nozzle using a length of shrink-wrap tubing, but anything flammable will do: carb cleaner, etc.) but that is a fairly dangerous thing to do on the Aerostar, and you definitely want to have a fire extinguisher handy if you try it -- on stray spark from a faulty plug wire and you get a nice fireball back at you. Not the for the unlucky.

Regards,
Al S.

 

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WEll, I do have a vacuum guage, and if there is a vacuum leak, it should cause a slightly lowered reading. I will have to look into it again. Thanks fo the help, I'll let you all know how it goes.