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engine miss has me puzzled. Thoughts?

trouttrooper

trouttrooper

Big Blocks ROCK!!!
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Location
Caldwell, ID
This is a '95 Taurus GL 3.0. 217,000 miles. If you've read my other post, this was the $300 deal I found for a friend of ours that's in a very tight financial situation. I've been able to get it useable for her for very little investment in parts and overall this car is in very good shape, but I'm trying to wrap my head around this one issue without throwing parts at it or having to pay a shop to diagnose it.

The car starts right up and will idle perfectly smooth all day long. I can rev the engine and it sounds great. However, put it in gear and the engine will erratically stumble. It will be fine a few seconds, then stumble a little for a few seconds, then run fine etc. etc. Under normal acceleration and cruising at highway speeds you can feel the car stumbling.

Here's the part that's got me a little puzzled. If I floor it the car will light up and get you up to 65mph in no time. There is no hesitation, stumbling, or missing going on. It's only once it settles down to cruise along when it starts stumbling again.

There are no obvious vacuum leaks. Wouldn't a bad plug, wire, fuel filter, MAF, cap and rotor, or fuel injector show up under full throttle? In fact wouldn't it be even worse than normal cruising?

I'm open to others opinions/thoughts. Of course they live just far enough away that I can't make quick trips to their place or vice versa. The car is working for her the way it is now, get the kids to school and go to the grocery store. I think it's bothering me than it is her, but like I said I'm trying to keeps costs as close to 0 as possible. Her husband got a job in CA and is starting to make decent money so they are hoping to get rid of this temporary car soon.

OK, I'll admit it. I enjoy a challenge like this. :D
 
Does it have an EGR valve on it? If so it may be opening up at low throttle levels and causing it to miss.
 
Argh, just when I think I've covered everything you ask more :D :D :D

Car had one code for bad EGR. I replaced it with a new one. Didn't change anything.
 
Was it a vacuum or electric EGR valve? if it was a vacuum style EGR valve did you replace the solonoid and transducer with it or just the valve? EGR valves can cause all kinds of havoc under low load situations. When it does it shut it off and pull the plugs to see if it is running lean or rich.
 
It's vacuum. I just replaced the egr. Not sure what other parts you're talking about. So probably have a vacuum leak somewhere then?

It's kinda a shame with this car. It's overall in really good shape but obvious that the past few years its been neglected. Just a bunch of little things that by themselves are easy fixes, but add all of them together and the cost goes up quick.
 
Poking around under the hood this afternoon I discovered the exhaust gas manifold pressure sensor's hose that connects to the EGR tube was broken right at the base of the sensor. She couldn't stay long, but I grabbed a "new" one at the junkyard and will put it in her car tomorrow. Crossing my fingers this may be the fix.
 
I was just about to mention that there is a differential pressure sensor on some F*rds that control egr function...guess you found it. :D
 
I would pull a plug and see how worn they are. Hopefully you found it with the EGR.
 
replaced the pressure sensor, it smoothed it out a little, but not much. I pulled a plug awhile back and it didn't look all that bad. Of course there's 5 others that could tell a different story.

The car is doing what she needs it to do, so for now I just poke around under the hood whenever I get the chance and see what else may be causing it. Wonder if a bad PCV valve would do it.
 
Has her fuel mileage changed? On a 88 dodge truck my O2 sensor crapped out and my mileage dropped to about 1/4 of normal. It ran like crap too.
 
You watch a scanner for cross counts to tell how good an O2 sensor is. Cross counts is going rich to lean and back. The engine, GM anyway, goes rich to lean very rapidly to generate power rich and lean to store O2 in the cat that burns/cleans up the rich cycle mixture. They get lazy (not as sensitive.) a long time before they trip a code. They have to be really bad before they trip a code.

As to the OP issue. I recall a crank or cam position sensor wire failure causing this issue on a 89 olds. Wiggled the wires enough and it finally tripped a code. Had to replace the wire running to the crank sensor.
 
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