Need help with a Misfire...'99 SOHC

[paul99explorer]

Ok, I have a misfire that I can't seem to get rid of. I bought my x about a year ago and it had a minor misfire. I immediately replaced the plugs and I could see that one of them was actually cracked. I replaced them and the miss was still there. Since then, I have replaced the plug wires, tried seafoam, cleaned my MAF, etc. There is no CEL/codes whatsoever. There also does not seem to be any arching around the engine (checked it at night with a mist of water) nor does there appear to be any vacuum leaks. I have pulled the plugs out again and checked them. All of them appear to be in perfect shape. No carbon buildup, very little wear, and all of them appear identical, which tells me that most likely the problem is not with any particular cylinder. The only thing I can think of is maybe the coil pack is bad. I don't think there is any surefire way to test it, but I could be wrong. Any ideas? Also, am I overlooking any other possibilities?

 

[pmcmaster]

The other item you need to check is the Crankshaft Position Sensor (this sends TDC to the PCM which then triggers the relevant pair of coils in the coil pack. Testing the coil pack Ignition off:
Disconnect plug cables(check they are marked) & electrical connection block (on side of coil pack).
Primary Resistance
On 4.0L sohc Looking into connection block on coil pack the left terminal is positive, each of the other three terminals is the negative for each pair of coils (cylinders 1-5, 2-6, 3-4). with ohmmeter check resistance of positive with each negative terminal should be 0.3 to 1.0 ohms.

Secondary Resistance.
Insert probes into each pair of plug lead sockets, resistance should be 6.5 to 11.5 K-Ohms'

Note these are in pairs as the 'wasted' spark method is used i.e. plugs 1 & 5 spark together but as only 1 or 5 can be on compression stroke (the other will be on exhaust stroke) one of the sparks is 'wasted'.

 

[paul99explorer]

Thanks. Will test it as soon as I get a chance.

 

[Runnin'OnEmpty]

Another possibility is a sticking injector. This type of miss won't trigger a code, because the ignition is sparking OK and the injector is getting a firing puse, but no fuel is actually being delivered to the cylinder. The way to diagnose is with a mechanics stethoscope. The good injectors will click, the bad one won't.

 

[paul99explorer]

Well, I thought of that possibility, but I figured you would be able to tell which cylinder(s) weren't firing and which cylinders were. If this was the case, do you think you would be able to tell by pulling the plugs out and inspecting them?

 

[paul99explorer]

One more thing....about that crankshaft position sensor...I looked at it last night. I guess I didn't realize that it was exposed the way it is. Does anyone know why they go bad? I mean, do they just get dirty? If you take it off, can you inspect it and tell what the problem is? Can they be cleaned? I think they only cost around $10, so I may as well buy a new one. I am curious, though.

 

[Runnin'OnEmpty]

Paul, you'll get a code with an ignition related misfire. The fact that you get no code leads me to believe it's something other than ignition. A sticky injector will not show a code, because there's no way the PCM can tell if the injector is actually opening or not. Try using a section of old garden hose or something similiar as a stethescope. You need something small enough to place close to the injectors. If you can hear them clicking, then they are working. If you find one that isn't making noise, then you've found the bad one.

 

[paul99explorer]

I think that's one of the reasons I didn't consider the crankshaft sensor to begin with. I presumed a bad one would throw a code. However, I read on a previous thread that a problem with the ignition coil would not give any codes. One thing I forgot to mention is this: When the engine is cold, it seems to run well. It does not start missing until it heats up to operating temperature. Also, the hotter it is outside, the worse it seems to get. I used to have an old mustang with a similar problem. Although it was much less complicated than the inner workings of a late model vehicle, it turns out that the ignition coil was "breaking down" under higher temperatures. This stands to reason, especially since the coil sits atop the engine, where it can absorb heat. Does this still sound like it could be a sticking injector?

 

[Runnin'OnEmpty]

Paul, your description of the symptoms does suggest a coil pack problem. However, I believe an ignition misfire will show up as a code, giving the specific cylinder. But....there may be some instances where a coil might be bad and not give a code, I just don't know.

Since an injector is a minature electrical solenoid, it could also be affected by heat. My guess is that you have a bad injector, but about the only way to confirm it is to listen to them.

Probably the next step would be to do the coil pack check pmcmaster described. If it checks out, then listen to the injectors. This is a tough one, since there are no codes to help diagnose the problem......

 

[RickOTR]

An ignition missfire will not always throw a code. I had a bad coil pack and spark plug wire on the X and it never threw a code. It depends on how bad the missfire is. Take off the camshaft position sensor and clean it. A dirty one will not allow a good reading from the camshaft, causing the injectors to function incorrectly. You may want to try and keep the engine around 3000 rpm's for about 1 minute with it in park, this will open up the injectors and clean them out. This is what I have to do from time to time.

 

[paul99explorer]

Is the camshaft position sensor on the front...similar to the crankshaft sensor? After work this evening, I'm gonna check those injectors and maybe both sensors, if it doesn't get dark on me.

 

[RickOTR]

The Cam sensor in on the drivers side valve cover right next to the oil cap, just pull it out after removing the bolt and clrean it with a lint free towel. I think 99 is the same as 97 and 98, but not sure. It could also be a dirty or on it's way out MAF sensor.

 

[pmcmaster]

Have you tried dosing up the petrol with one of these injector cleaning chemicals? Dont forget the air inlet also has an intake air temperature(IAT) sensor (next to the MAF) the PCM uses this to control fuel flow & ignition but in theory this would throw an error code. According to my OBD-ll code list a primary coil fault is listed but not a secondary coil fault but it does list misfire errors.

 

[paul99explorer]

Ok, the coil pack tested fine. Now...about those injectors...maybe I'm just a little hard of hearing, but it's really difficult to tell for sure if they are clicking. I mean, I can hear clicking, but it it's hard to tell exactly where it is coming from. I tried a few different injector cleaners earlier on in this ordeal, but to no avail. This miss is EXTREMELY consistent, which would lead me to believe that it is something electrical or possibly a badly clogged injector. Besides the stick-in-the-ear process, is there any other way to test the injectors to see if they are working properly? I'm getting ready to go back out and clean the cam shaft position sensor. We'll see what happens. This miss is driving me nuckin' futs.

 

[paul99explorer]

well, i spent a little more time on those injectors. As it turns out, all 6 of them are clicking. I cleaned the cam position sensor, replaced the fuel filter, and did another sea foam treatment. She's really running good now....except for that misfire. Anyway, I still think that it couldn't be a clogged injector. By looking at the spark plugs, it appears that every cylinder is firing consistently. Any chance this could be a fuel pump going out? It's not noisy at all.

 

[capb1508]

i have the same problem, engine runs fine when is cold but after its war for about 10 min it start to misfire, and get worst on hot days. I change the coil pack, resistance out of spec according haynes manual, that does not work. Later i change the spark plug wires and sparks, that help a little, idle is more stable but not fix the misfire. I can tell you that maybe is not the injection system because my car is converted to natural gas, and when the engineis running on gas injector are shutoff and a emulator sent the dummy signal to the ECU. So same problem but injection system off, misfire when engine hot. I guess is a vacuum leak, i need to check the lower intake seals. I have no codes. That misfire will drive me crazy.

 

[2000StreetRod]

engine compression?

Have you checked the compression?
Engine misfire procedure

 

[capb1508]

Engine was rebuild after chain guide fail, valve guides, pistons, rings, time chain system, oil pump, bearings and cylinder were change. The only thing not change were the valves but valve were adjusted properly. One compression test in the cylinder #4 shows 170psi, the truck feel with a lot of power even working with natural gas when the engine is cold, when the engine is hot a misfire make the engine lose power, it feel more when i try to go up in a hill or A/C on.

 

[waskly]

Engine was rebuild after chain guide fail, valve guides, pistons, rings, time chain system, oil pump, bearings and cylinder were change. The only thing not change were the valves but valve were adjusted properly. One compression test in the cylinder #4 shows 170psi, the truck feel with a lot of power even working with natural gas when the engine is cold, when the engine is hot a misfire make the engine lose power, it feel more when i try to go up in a hill or A/C on.

whats compression on the other cylinders?

 

[capb1508]

i do not have that data yet, i will posted soon.