4.0 liter Aerostar misfires on cylinders 2 and 5.

[eemer600]

I have a misfire and rought idle that is very repeatable after about five to ten minutes of warmup. No DTC codes are set.

I bought the ScanXL software from Palmer Performance Engineering (PPE). I was surprised to discover the data collection (USB) device that I bought from Amazon was relatively cheap, compared to hand held scanners. The Ford OBDII PIDs are proprietary, and not readily available on hand held scanners.

Anyway, I was finally able to see my O2 sensor voltage, and my injector pulse width via the Ford PIDs. I also loaded and read the Ford misfire PIDs for all six cylinders.

The rough idle that I experience after warmup is not due to a change in injector pulse width due to a lean condition. These stay steady while cylinders 2 and 5 begin random misfire after about ten minutes of running. Injector pulse width at idle is between 50 and 60 milliseconds. O2 sensor, while I could not graph for realistic view, I did see it toggle between around .1 volt and .7 volt. And if it was a vacuum leak or fuel pressure problem, I would expect other cylinders to display misfire.

I researched and found that the ignition coil pack uses cylinders 1 and 5, 2 and 6 and 3 and 4 as the pairs for the three coil pack. The firing order is 1,4,2,5,3,6, which makes sense, because this order goes through each coil and repeats for each cylinder.

So if the misfire on cylinders 2 and 5 is not the coil pack, what is it? The only other thing I can think of is the intake manifold. Is the middle of the manifold loose and leaking and causing 2 and 5 to misfire after warmup? I replaced the oxygen sensor. It was orange, and not heavily coated. I checked manifold vacuum and it was steady, nearly 20 inches Hg.

I really do not see mechanical symptoms that relate to the cylinders 2 and 5 misfire, so I am baffled. I really want to say it is the coil pack, but the wrong cylinders are misfiring. I am really looking forward to feedback from the esteemed Aerostar forum. Thank you all in advance.

 

[Jose A.]

if it is a 4.0 liter, and it is a 1997, it could be the intake manifold Gaskets shrinking that causes the same Codes and Symptoms. There is a REVISED set of gaskets and factory procedure at ALL-DATA for the 1997 4.0 liter engine.

Don't throw parts at the problem, I replaced my spark plugs, the coil pack, EGR valve, and the symptom continued until I was made aware the intake manifold gasket and the so-called "o" rings were the cause.

Not an easy job, a multitude of sensors and other parts need to be separated from the plastic intake manifold, then you need to follow a de-torquing procedure to lift the intake manifold, then you need to remove the lower intake manifold bolts.

 

[BrooklynBay]

Did you buy the ELM327 or another type of USB scanner which is similar? Why did you have to purchase the software separately?

 

[don-ohio]

I'm wondering how many miles those coil packs have on them? If you have over 100K on them I'd replace them AND the plug wiring anyway. Can't hurt. don-ohio ^)

 

[Jose A.]

I think the coil pack works or it doesn't, no middle-point.

 

[don-ohio]

No,Jose,coil packs can misfire and cause probs while vehicle still is running.Been there..suffered that! LOL! don-ohio ^)

 

[Jose A.]

I just eliminated my misfiring and rough idling by replacing the 3 upper intake "O" rings, and the fuel rail gasket, it's running like new.

some job, but I did it. And I ended putting back the original Coil Pack thinking it was THE problem, which it wasn't. The replacement coil pack (Made in China), was not satisfactory, the Terminals or "Posts" are angled and they are slightly narrower, maybe by 1/16th", when compared to the OEM Coil Pack, but definitely smaller, so the plug wires were "dancing" on the terminals.

 

[eemer600]

Not an intake manifold leak

I did a smoke test on this Aerostar, yes, here it is a year later, with the car sitting most of the time, and I dd get some smoke out of the EGR valve. It was leaking out of the shaft, but not through the valve or the diaphragm. No smoke any where else out of the engine or intake manifold.

So after replacing the EGR valve today, I got exactly the same symptoms of cylinders 2 and 5 misfiring at idle, but the engine runs strong, and putts down the road with little throttle needed. Nice and smooth. Rough idle to me is usually a vacuum leak or weak spark. I think I have ruled out the vacuum leak, so I guess I will replace the coil pack.

Unless...Jose, you said something about o-rings. Are there o-rings in the intake manifold that isolate specific cylinders? I don't understand how this would work. Isn't air present for all cylinders at all times? How would an o-ring cause cylinders two and five to get either too much or too little air during idle, when thee are no vacuum leaks?

 

[BrooklynBay]

Jose was referring to the 00M12 kit. Spray intake cleaner on the side of the engine where the intake manifold gasket is located while the engine is running to see if the idle changes.

 

[eemer600]

OIC, well it doesn't make sense that a mechanical problem could just come and go. Spraying intake cleaner I'v tried. There isn't enough room to get in there, plus the fan moving air. I could see trying it if it was a Ranger with a hood for better access.

Any way, it pretty consistently will have a rough idle at start, but after a few minutes of warming up, the rough idle goes away. Perfectly smooth for about 10 minutes, then goes rough again. It makes sense that something related to temperature is being turned on and off. It repeats on cylinders 2 and 5 only.

I am going to put my PC scanner on it again tomorrow to see if I can correlate any controls to temperature. The problem I have with it being the coil pact is that 2 and 5 are not on the same coil.

2 and 5 are in the middle of the intake manifold, which supports a gasket problem, but this rough idle at start, then smooth, then rough as the engine warms up doesn't seem to fit a leaking manifold problem. I did buy another coil pack and will probably put it in tomorrow. Tight quarters. Not fun.

 

[Jose A.]

they call them "o rings" but they are actually 3 rubber or nylon squared "rings".

the illustration shows them as part number 9E436, then below them you have the revised paper gasket which is also shown as the same part number.

I would change the *spark plugs FIRST, if that doesn't eliminate the roughness, then go to the Coil pack, if that doesn't do it, then change the spark plug wires. Use the Motorcraft parts, nothing else. I am not a fan of buying at the Ford Dealer but I have learned that the 4.0 engine is a pita to get right and it only seems to work right with the FoMoCo parts.

*the 4.0 engine prefers the Autolite Irridium spark plugs, anything else gets wasted quickly, ask me how I know. And even new spark plugs can be bad, so don't buy 6, buy 8 and keep a spare or two.

*make sure the air intake plastic tube is not cracked at the "accordion" ripples, and that the rubber gaskets at each end are sealing well.


http://www.use.com/BtjyV

 

[eemer600]

Thanks Jose. I read the manifold vacuum yesterday. It was a very steady 19.1 Inches of mercury. The symptoms are that it will misfire when started, then after about five minutes run smooth, then after about 15 minutes of running smooth will begin to misfire again.

Wouldn't you expect this to be electrical more than mechanical. I changed plugs and wires with no change. The coil pack is nest. I will let you know how it goes.

 

[Jose A.]

let's hope the coil pack changes everything, but if not, I'm afraid you're looking at the intake manifold gaskets, the symptoms are there, I did mine in May and the whole problem is gone now.

this is a typical issue with the 1997 4.0 liter engine.

 

[eemer600]

OK, well I did do the intake manifold gasket a while back, but the problem didn't stay gone. I can't recall it too well, so I'm not sure how long the problem went away if it went away, and if that is the case, did I do it incorrectly? I guess so, or is this a once every two year thing?

 

[Jose A.]

once you did the revised intake gasket job, it should last a good number of years, unless the intake bolts were not properly torqued, and I doubt it, so if you already did it, then the problem is elsewhere.

you seem to have a good sense of the problem, and you said: "something related to temperature is being turned on and off". Yes, I have the same thought. See Numero 3 below.

1) idle air valve is one that comes to mind. I think it was BrooklynBay or don-ohio who pinpointed it in another thread.

2) MAF sensor is another. and the small sensor after the MAF senses intake air temp.

3) but in my experience with the Jaguar XJ-6, the Coolant Temp Sensor which in the Jag starts to send erratic signals when bad, and the engine responds with rough idling trying to correct itself.

Try these two tests: (keep hands away from fan and belt)

1) find the wiring plug for the CTS, (located under the aluminum intake/butterfly in the 4.0 engine, the sensor is mounted vertically at the front of the engine, to the right of the Temp Gauge Sender). Disconnect the plug and run the engine. No cigar? same thing? worse still?

2) find a jumper wire and jump the two wires at the connector itself, run the engine, any difference?