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'98 5.0L Engine "Chugging"


kamikaze_k77
10-19-2009, 08:29 PM
Alright, lets see if you guys can beat 2 seperate Ford garages.....
Specs: '98 Mercury Mountaineer - 5.0L, AWD, Miles:202,000 (ouch, i know)
Problem: My parents have owned this since 2001, and it was an excellent vehicle until it hit 100,000 miles. When the engine is at low RPM (below 2,000) and is under torque (slow acceleration or uphill driving), it hesitates and for lack of a better term "chugs." The engine starts great and never dies, but just runs very rough when trying to accelerate under normal conditions. Seems as though it is misfiring or just missing on one cylinder. When driving, it really feels like the trasmission is slipping, but I assure you that it is not the transmission. Some days it's barely noticeable, other days, I pray it doesn't die. It seems to run better the more humid it gets, BUT if it's raining it's pretty rough. Air filter, fuel filter, spark plugs are all new. Been through 2 Ford garages and they had no idea. Since this began, my parents have put another 100,000 miles on it and just ignored it. I have now acquired it for a winter vehicle, and I would like to see if it's something I could fix relatively cheap... if you have any ideas or more questions about the issue, please let me know! Thank you!

shorod
10-19-2009, 09:15 PM
You mention the spark plugs were changed, what about the spark plug wires? If they weren't changed, then you should replace them for no other reason that they're likely due. Same goes for fuel filter. How about the oxygen sensors, have they every been replaced? Seems like something a good tune up would likely fix.

Since it's a '98 it should have OBD-II diagnostics and may have a pending code for a cylinder misfire. A decent scan tool with a datastream mode should be able to isolate the issue, did either of the shops drive the vehicle with a scan tool connected? They should be looking at the individual cylinder misfires, the O2 sensor voltages, and the fuel trims.

-Rod

kamikaze_k77
10-19-2009, 09:45 PM
I believe the wires were replaced with the sparkplugs and didn't change anything.. I've been wanting to do the O2 sensors, but at over $100 a piece, I'd like to have some sort of educated assumption that it would work. (I don't have a ton of money to throw into this thing right now) Fuel filter was replaced several times also, no change in performance.

shorod
10-20-2009, 07:04 AM
The datastream and component parameters listing from good scan tool during a road test could tell you quite a bit. When I had my 2000 V8 I had a bad coil that caused a misfire on a particular cylinder. It wasn't enough to trip a diagnostic code, but enough that I knew it was there. The component parameters of my scan tool showed that I had several misfires on a particular cylinder, a few on another cylinder, and zero on the remaining 6. The two cylinders that showed misfires happened to share a coil. I moved plugs and wires around, the misfire counts always stayed on the same cylinders. Compression tests showed the cylinders were as healthy as the others, so I replaced the coil and fixed that issue.

Another time I had a random and sporadic chug, which felt almost like a very brief stab of the brakes. It felt and sounded as if something were in the transmission and every once in awhile caused the gears to bind up. Soon the check engine light came on and the code was for an intermittent MAF sensor. I inspected the MAF and wiring, no obvious issues, so I replaced the MAF. That fixed that issue.

If one of the shops you trust has a professional scan tool and someone that understands how to interpret the data, the diagnostic fee for a road test may be worth it at this point. Like you suggested, you don't have a lot of money to shotgun parts at this point. A $100 diagnostic fee (or thereabouts) may save you money in the long run. Did either of the shops hook up a scan tool and check for pending codes and datastream issues? Since there seems to be a tie to moisture in the air, it seems reasonable to focus most of your effort on the ignition system. Was there any change when new spark plug wires were installed, better or worse? Has anyone gone back through and made sure the wires were snapped on to both the coil and the plugs? Were they the least expensive wires available, or a good set?

For a very inexpensive attempt of reducing this issue, you might try some SeaFoam or BG44k in the fuel system to see if that has an effect. Also, if you are comfortable in doing so, you might try the 'cylinder decarb' process described on the can of SeaFoam. If nothing else, it should improve power and maybe even fuel economy slightly once you get the misfire fixed.

-Rod

kamikaze_k77
10-20-2009, 10:40 PM
Thanks Rod, the diagnostic road test sounds like a good place to start... I did try the sea foam both in the fuel and twice into the vacuum lines. Felt like a little more HP, but didn't touch the chugging. Ignition system has been something I've been thinking about as well... Might just have to bite the bullet and try something. In regards to your question about the shops hooking it up to the computer and doing diagnostics, I have no idea. My parents still owned it at that point. I've only had it this last year... You've given me a direction to head at least. Thank you! If anyone else has an idea, please post!

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