I've got a 96 4.0 Ranger with 60,000 miles on it. The engine has a rough idle only when the engine is hot. It shakes the truck pretty good when sitting at a stop light. When I drive off, the trucks acts fine. I've changed the spark plugs, plug wires, coil pack, fuel filter, and IAC with no luck. Now, I'm getting trouble code P0300 or multiple cylinder misfire. I hate to take it do a dealer to get looked at. Any thoughts? Thanks.

 

Well given the parts you've replaced, apparently without results, a vacuum leak down stream from the MAF, comes to mind right away, as that can affect multiple cylinders.

Given the vintage of your ride, things like rubber vacuum hoses, fittings, accessories that use vacuum (like the brake booster, evap cannister, PCV system, ect) all would be good suspects to check out.

Also seeing as how it's a 4.0L, I suppose the lower intake manafold gasket should be on the suspect list too, as it's a fairly common problem on the 4.0L

Did this problem begin suddenly, after some event, or slowly over time????

Do you hear any spark knock/ping on accelleration, or uphill pull, or while cruising????

Did you have the rough idle, BEFORE you replaced the parts you listed????

 

The rough idle has gotten worse over the past year. It used to act up only intermittently. Now it occurs all the time. This is when I tried changing all the parts. And, now the check engine light has lit up.

The engine doesn't knock or ping under accelleration or crusing. The truck seems to run fine under these conditions.

I'll check all the vacuum lines. I know a vacuum leak will cause a rough idle. But, will it cause a misfire?

What about the camshaft positioning sensor? Someone suggested this to me. Also, where is it? I heard it is on top of the intake manifold towards the firewall. Do I need to take off the upper half of the intake manifold to get to it?

I used an Autoxray scanner to read the code. It also reads the oxygen sensor voltage signals. Two of the three sensor voltage signals jump around quite a bit. The signal can range from .050 V to .800 V in an instant. The third O2 sensor's signal will vary, but stays relatively constant on the low side. I was just wondering if it was normal to have two O2 signals jump around quite a bit, while the other remains relatively constant. Knowing the O2 signals dictate the air/fuel mixture, this may be a possible cause. Or, am I way off base.

Thanks.

 

Check your plugs. What are they "telling" you?

Obviously, being probably a long way aways from you, it's tough to diagnose this stuff over the internet.

You might check into the intake manifold gasket. As noted, it is a common problem with the 4.OHV engine.

I fyou don't have a code for the Cam Pos Sensor, I would not bother right now. Of course, if you have the money to spare, why not, but then why check for codes in that sense...keep guessing and tossing money.

It's too late for you, but for those who will need reference in teh future, the coil can be checked before tossing it; the plugs give tell-tale signs of what is going on in the engine; and the IAC can be unplugged and if the idle smoothes out then the IAC is bad.

The O2 sensors are not the problem, so don't bother with them. About their only problem is if they get dirty or the harness gets cut and they can't sense anything. That's pretty much all with those...

Well, good luck. Done working the graveyard....bed time.

 

Well I suppose at this point, most everything could be considered to be on the suspect list. LOL

Right now I don't think we have enough clues to put the CMP (Camshaft Position Sensor) on TOP of the list though.

It would seem to me, if the CMP were acting out, you'd likely have other diveability problems or a CMP trouble code.

My manual says it's located to the rear & atop the cylinder block surface, behind the intake manafold, where the distributor was, on earlier engines.

If you really think it's bad, I suppose you could heat it up with a hair dryer, before a cold engine start, to see if it'll cause the engines COLD idle act up.

But it seems to me if it had a temperature problem like that, at warm idle, you'd likely have higher rpm & other driveability problems too, but hey, it's a possibility & it would be easy to heat it up, BEFORE a cold start & see if you can re-produce the failure mode.

Seems to me though, seeing as how you say things are ok, until the engine warms up & then the problem is ONLY at warm idle, but acceleration and cruising are ok, it's likely something else.

So that, what ever it is thats causing the P0300 & rough idle, is temperature sensitive & ONLY affects WARM engine idle.

Perhaps this would suggest something like a lower intake manafold vacuum leak, (caused by loose fasteners) which the 4.0L is known for.

Or maybe a low grade vacuum hose leak, thats enough to upset things AFTER the engine goes into open loop & the O2 sensors come online & the A/F mixture naturally LEANS out.

But the unmetered leak is then bad enough to lean the idle A/F mixture enough to cause idle misfire, but not bad enough to cause ping when the engine is under load or cruising at higher rpm.

Or it's something to do with components that sense, or control the throttle position, but only at idle.

Seeing as how you don't apparently have any codes other than the P0300, it would suggest to me, the probem is one the computer isn't able to monitor, as the computer apparently isn't unhappy with any of it's sensors outputs, because it's not finger pointing with other codes.

So right now, seeing as how the engine seems to be affected only after it's warmed up & only when it's at idle, we are left looking at what ever CONTROLS the warm engine idle speed, or AFFECTS the warm engine idle speed & A/F mixture.

Perhaps electro/mechanical things, like the throttle position motor, sensor, or it's adjust/positining screw, or some kind of temperature sensitive low grade vacuum leak, should be higher up on the suspect list.

So to recap, I'd first probably look real close at all the vacuum lines & vacuum operated components. If all check out ok, then I'd retorque the lower intake manafold fasteners to spec, in the called for sequence.

Then if that didn't have any affect, I'd move on to the TPS, throttle position solenoid/motor & any adjust/position set screw it may have & test/inspect them thoroughly.

Then let us know what you've found.