Misfire & EGR test

1998 3.8l 204,000 miles

Recently I've been getting intermittent misfire codes, for cylinders 1 & 4.
Searching the forum, there are a number of posts, mostly by wiswind, that
suggest a procedure to see if the EGR system might be at fault. Here's
one of them:


One thing that you can do.......to see if it is related to the EGR system.......TEMPORARILY remove and plug the vaccum line that goes to the top of the EGR valve.......take the vehicle for a drive.......does the symptom go away?......if so, then the EGR system is the likely suspect.
If this test makes no change in the symptom, then the problem is not caused by the EGR system.
Remember to reconnect the vaccum line to the top of the EGR valve.
This test should cause the CEL to come on, with a code indicating a lack of EGR flow.......which is not something to worry about......as this test disables the EGR valve

Questions: If the symptom does not go away (and an EGR flow code comes up), will the misfire codes also come up?

And what could happen if I forget to reconnect the vacuum line?

Thanks for any help!

Questions: If the symptom does not go away (and an EGR flow code comes up), will the misfire codes also come up?

And what could happen if I forget to reconnect the vacuum line?

Thanks for any help!

Of course the misfire codes can come up if your symptom doesn't go away, because you still have the symptoms ;). And also because it would mean that you don't have a problem with your DPFE sensor that open the valve all the time. You can still have a stuck EGR valve, to check it, touch the valve after 5 mins of idling, it should be cold.

If you forget to reconnect the vacuum line... Well, I espect you will not forget, cause you can make the engine overheat and engine will ping because pre-ignition. You'll not pass emission test. The goal of EGR is reduces combustion chamber temp and prevent pre-ignition. Also reduces NOx emmision.

You would also test your ignition coil resistance between 1 & 5 output and between 3 & 4. Check also input resistance to see if it's the good the value. Don't forget to check your ohmmeter offset by short-circuit it. Check also n° 1 and n° 4 plugs and wires condition.

Usually, a stuck open EGR valve cause random misfire code. Mean all cylinders misfire randomly. And no start issue.
I doubt the EGR system is your problem.

Check also your injector by unplugging all injectors wires. Then check fuel pressure when engine is off.. Then apply 12V directly on injectors n° 4's pins. If pressure drop is not enough, this injector is dirty. Prime again the fuel pump (key ON) and compare with injector n° 5's value. Prime again and test n° 1. Two dirty injectors is very rare, but it's a possible cause.

With that vaccum line disconnected, your EGR valve will not open, and there will be NO EGR flow.
If you still have the SAME driveablility symptoms, then the problem is NOT caused by the EGR system......because there is no EGR flow.
If the problem goes away, then you know that the EGR system is causing the problem.

As you know, there are a LOT of possible causes of the problem you are having.
This is an easy test to check the EGR system off the list of possible causes.
This saves you from going to all the work of removing the upper intake manifold to clean the EGR ports, and have them not be the cause of the problem.

If you forget to connect the vaccum line back up, you will be putting out more emissions, the cylinder temperature will be higher, although you will not likely see it on the temperature gauge in the car.
As Serge_saati mentioned, you could notice increased "pinging" (partly due to the increased cylinder heat temperature).

Of course, with the vaccum line to the EGR valve disconnected, you would keep getting the CEL light with code(s) stored for lack of EGR flow.

You would also test your ignition coil resistance between 1 and 4 output. Check also input resistance to see if it's the good the value. Don't forget to check your ohmmeter offset by short-circuit it. Check also n° 1 and n° 4 plugs and wires condition.
.

What should the resistance be more or less on the coil pack between 1 - 4 output? Im reading an open on mine, having a misfire on #3 that is driving me nuts.

What should the resistance be more or less on the coil pack between 1 - 4 output? Im reading an open on mine, having a misfire on #3 that is driving me nuts.

Sorry, actually it's between 1 & 5 output and between 3 & 4. It's my fault, I just confuse with GM V6 since numerotation is different from Ford.
Check between 3 & 4.

Follow up on this post:

I disconnected the vaccum line from the top of the EGR valve. (not the solenoid control). After driving about 200 miles, including long trips, I no longer get the misfire codes, and I don't detect any misfire.

But I also don't get the faulty EGR code (0400), which other posts predicted.
I do get a 1131 code-- bank one upstream oxygen sensor problem-- which
I was getting before disconnecting the EGR vaccuum line, but, if I recall
accurately, not before the misfire appeared.

I also don't detect any pinging; is that always audible, or able to be felt?

Thanks for any further help.

1998 3.8L 204,000 miles.

You're insane? Driving that long with EGR vacuum disconnected is not good for the engine. We talked about 1-2 miles driving test.

Listen, it's not because your engine code didn't come back after 200 miles you should think the problem were the EGR system. Sometime a misfire code can appears for serious reason on a Ford engine and then disappears by itself. Especially if the drivability didn't change after the line was disconnect.

Maybe your P0301 & P0304 code was a due to an intermittent vacuum leak. Check here for more info: http://www.obd-codes.com/p0300.

Saati-

Thanks for your quick reply. Re doing the test for only a few miles--the reason I kept the vacuum plugged for so long is 1) the misfiring never took place that quickly, and 2) I thought the EGR flow CEL was supposed to come on. Any
thoughts on why it didn't?

Also, the driveability did change with the disabled EGR-- no more misfiring.
Which still makes me think it is the EGR system.

I'll see what happens after I restore the vacuum line.

If it comes back when you reconnect the vaccum line to the EGR valve, the most likely causes are clogged EGR ports in the lower intake manifold....which requires removal of the UPPER intake manifold to clean. (don't remove the lower intake manifold).
The other likely cause would be the DPFE, which is what measures EGR flow......telling the PCM how much EGR flow there is.....and the PCM adjusts the EGR valve to obtain the desired flow....as measured by the DPFE.

Question-

Should I be able to feel a vacuum on the disconntected EGR valve vacuum
line, when I take it off. (I don't).

1998 3.8L
204K miles

The EGR system activates only when engine is hot and you press on the gas.
Wait that engine is hot and open the throttle plate a little bit while you do the test, cause you should feel vacuum when it activates.