My 98 Explorer V6 sohc seems to be misfiring at idle, sometimes so bad that it stalls out- especially when cold. It also bogs down pretty bad under heavy throttle. My midrange is lacking until the tranny kicks down into a lower gear.

Motorcraft plugs and wires have 40,000mi on them- vehicle and original Coil have 140,000mi. No codes are present and I have spent the last six months trying to isolate this problem. One thing I am absolutely positive about is it is not the fuel system as a new pump, relay, housing and engine diagnostic has been done on it.

Funny thing the engine seems to bog around 3000rpm for a second or two then overcomes the bog and continues to rev. The idle is also incredibly weak as if it is going to stall out at any minute.

I am turning my focus to the ignition system, the problem is definately intermittent as it runs good 20% of the time and shows thes poor symptoms the rest of the time. Could bad spark plugs or wires cause intermittent problems or would it be bad all the time?

I am wondering what causes misfire and what I should look at next?

 

Ever replaced the intake manifold gaskets? They cause vacuum leaks which leads to a rough idle, especially when cold, and can cause other problems too.
Is the MAF sensor clean?

 

I'd start with the wires. Maybe get one good wire and start swapping them out one at a time. Have you tried unhooking them one at a time to see if unhooking one of them didn't make the engine run worse? That would help lead you to a specific wire/plug/coil.

 

The MAF is clean and I had the gasket recall done. I have never taken the wires out or the plugs to check them since they were replaced about 40k mi ago.

Also just had the Crank sensor replaced but it did nothing. What causes misfire and how can I narrow it down a bit?


Thanks for all the help.

 

You mentioned everything in the fuel system but the filter. Did you change it? Also, have you checked the pressure, as the regulator could be a problem (be sure to check the vacuum line from the regulator for fuel). Have you checked all areas for vacuum leaks (including hoses and brake booster)? Have you cleaned the IAC? (could be dirty or sticking). You could also pull your spark plugs and check them. You may see something amiss with one or more. Be certain to note which cylinder each plug comes from so if something is noted you can be sure which cylinder has the issue.

 

Go ahead and change the plugs and wires, if that doesn't do it go to the catalytic converter next.

 

Could be the cats but why does it have serious misfire at idle and sometimes stall out? Under light pedal acceleration it goes very well, but put it down to half and the show is over- bog city.

I had the fuel filter replaced in late November- it did not make a difference as I have had this problem since September and it does seem to be getting worse. IAC is brand new, my mechanic went over the fuel system thorougly and could not find anything wrong. He did not get a chance to measure the ignition system yet.

I could have two Magnaflow direct fit Converters installed for $350 but what if that is not the problem.

Thing that puzzles me is sometimes it is worse than others, wouldn't clogged cats be there all the time?

Thanks for all the great responses. Unfortunately this is a tricky problem.

 

Alright man. Listen to me. That EXACT SAME THINGS HAPPEND TO ME. WITH A GIRL!. yeah. it was sooo lame. had never done that, but OMG it was emberassing. It had the check engine light on. I went to Autozone and it turned out to be the two oxygen sensors before the catalyst. Alright, i know ur gonna say that urs doesn't have the Check Engine light on YET. but. take it to a shop and have them checked individually. Mine RPM would go low and it would feel like it was bout to stall. When i was w/ the girl it stalled bout 7 times (imagine that!). It feels very weak when in idle, and it will eventually stall. Also, notice that when it feels like that the battery kinda goes down as well. CHECK THE OXYGEN SENSORS. They also make u get a poor fuel econemy when they're not workin well. Before you even say it. YOUR WELCOME.

 

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Thanks for the tip Spartan. I did have a dealership run it on the Diagnostic machine and they said thx O2 sensors were running properly. But it is worth a look, thanks.

 

Testing A Catalytic Converter
Unlike the old-fashioned muffler, the catalytic converter cannot be considered in working order simply because there are no rust holes.
Catalytic converters are fitted on cars using unleaded petrol to clean exhaust gas. Apart from shaking it to see if it rattles, the Motor Vehicle Repair Industry Authority says there are some tests which must be done by a licensed repairer with instruments.
Connect a vacuum gauge to a manifold vacuum source on the motor and rev the engine to 2500 rpm. The vacuum gauge should drop momentarily and then return to normal. If it drops and does not return, then the converter may be damaged and pieces of catalyst can block the exhaust system.
Or, use a pyrometer which is a high temperature thermometer by placing the probe on the inlet of the converter, about 50 mm in front of the unit and read the temperature. Then place the probe on the outlet side of the converter, about 50 mm to the rear of the converter, and again read the temperature. The outlet should be at least 40 degrees Celsius hotter than the inlet. If there is no difference, the converter is faulty and should be replaced.
Next use an emissions analyser. Check that the carbon monoxide and hydrocarbon readings are close to normal. Enrich the mixture with the addition of propane to the inlet air. The converter should hold the CO and the HC readings down to normal. If not, then the catalytic converter may be faulty.

 

I just had my truck inspected in MA and it passed with no problem. My tuner did use a gauge to measure back pressure but he could not get conclusive numbers.

I will have the cats tested.