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97 Taurus Misfires

My 1997 Taurus wagon (3.0L) with 165,000 miles started to run very roughly and threw a P0402. Once the engine got up to operating temperature (after at least a half hour of driving) the car would misfire left and right. It would miss badly both while driving and while idling.

Well, I replaced the DPFI sensor, and that cleared the P0402 code which has not come back after several hundred miles of driving. The driveability was just as bad, though, with the engine continuing to misfire badly.

The plug wires looked bad and the plugs were fair at best so I went ahead and replaced the plugs (gapping the new stock plugs to .044 per the spec), the plug wires, and the coil pack.

This resulted in a pretty dramatic, though not complete improvement.

The car no longer misses while driving. Runs nice and smooth. As a matter of fact, the car does fine for the first hour of driving. It does not miss at all.

However, after an hour of driving, if I shut the car down for an hour or so and then start it up and drive again, the car will miss badly while idling in drive. It does a better (though not perfect) when idling in neutral. It does not miss while driving. Once the ar gets up to operating temp again it does a lot better, though it still misses at idle a little bit.

A mechanic friend has checked the vacuum, and checked for leaks in head gaskets, checked the fuel pressure, and all that checks out fine. The PCM has not thrown any more codes. Though I would have expected cylinder misfire codes based on all the misfires, but none have been thrown.

Anyone have any ideas on what I should check next?

I would pull and clean the EGR valve and its intake passages and check the solenoid (if equipped which I’m pretty sure it is). The DPFE sensor is one part of that system and that may have been a good call - but there is the valve itself and the solenoid.

Don’t pull the EGR without a new gasket handy. The old one will likely crumble to near nothing.

I’m usually suspicious of a computer that has no code when there is a problem. I just missed my chance to be right for a change on a 1990 Grand Prix. A temperature sensor could be telling the computer that the engine is fully warm. That could result in the wrong fuel mixture. Too little fuel to idle right. Those are my first guesses. They are not better than the previous post.

The crankshaft position sensor (CKP) usually tells the engine computer (PCM) when the engine is misfiring. It may not be telling, or the PCM may not be listening. Here are the instructions, and info, for the CKP:

Maybe a contaminated mass air flow sensor. Have you tried cleaning it?

Maybe not enough coolant flow causing possible steam pocket in the engine, especially when it sits, and gets heat soaked. This could cause a cylinder to run hot and misfire. Any pinging? Did any of the plugs you changed show signs of running hot? Has the water pump been replaced recently?

That Vulcan engine can run pretty rough before it sets off a misfire code. These engines are definitely not the smoothest at idle.

Hmmm. IN my experience (I own several of these era Tauruses with the Vulcan engine). They ball purr like kittens except for this one, my problem child.

I guess a bit of an update is in order. I have not had much chance to work on this, other than attempting to remove and clean the egr valve. I did not get very far (stopped on Step 1) because if I attempt to turn the bolt holding the egr valve no matter what I do (tried pb blaster, heating bolt, etc etc etc repeatedly all to no avail) I am going to wreck the pipe that it is connected to. And replacing that pipe means big bucks at a dealer as it is a dealer only part. Thus I am reluctant to remove the egr valve on just a guess that it might be the problem.

So, I have kept driving the car on my daily 130 mile commute. The P0402 has never come back. These days, it idles rough and misses pretty badly while it is cold. Once the car warms up, it misses only occasionally at idle. Every once in a while it misses while driving at highway speeds. Right arund 55mph it misses the most, but it does not miss anywhere near as much as it does at cold idle. More of a minor nuisance warm at 55. It is not that big a deal, except for the gas mileage. I have another car identical to this one, and this one gets about 20% poorer gas mileage than its “twin”.

What about a leaking fuel pressure regulator? This could cause rough running and poor gas mileage. Remove the vacuum hose and see if there’s fuel inside of it. You could also suck on the fuel. press. reg. hose (a la soda straw with the tip of the tongue) and note if the reg. holds a vacuum.

This could also explain the rougher running at idle and somewhat smoother at speed since the throttle plate is opened much wider and any excess fuel is not as much of a factor. Hope that helps.

Some more thoughts.

The heads hopelessly carbon up.

Small leak at head gasket.

Small vacuum leak intake manifold.

With the timing jumper plug removed (located in the big wire loom at firewall, gray in color maybe half covered over with tap). will this make the engine run smooth? This will retard the timing slightly.

Next thing I’ll check is the fuel pressure regulator. Also the timing jumper plug.

I suppose it is possible but I do have some doubt it is the valves, head gaskets, or manifold leak. Last year the head gaskets were replaced, valves ground, intake manifold gasket replaced, etc. Of course they could have failed again, but I’ll rule out those other things first.