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P0402, is it my EGR valve?

2010 Ford Mustang v6

There’s not much as far as obvious symptoms. It does shudder a bit after coming to a stop from the highway, but only for a second. The RPMs bounce a tiny bit at idle, but only very slightly (like from 800 to 825-850 or something).

So I got this error code and started looking around at diagnosing the problem. I read there should be no suction from the EGR valve when the car is idle and warmed up. I pulled the hoses, though I’m not positive which one is not supposed to have any suction on it.

The hose going to the manifold has suction, and there’s suction where the other hose connects to the EGR valve (the metal stem on the EGR). Disconnecting it causes a rough idle, and plugging it causes it to return to normal.

Does this mean my EGR valve needs to be replaced?

The code indicates that there’s excessive exhaust flow thru the EGR valve.

The EGR system in your vehicle is a Delta Pressure Feedback EGR.

Some things that can cause this code is a dirty/defective EGR valve, where the EGR valve is stuck open.

A restriction in the exhaust system causing excess back-pressure on the EGR valve.


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How do I go about verifying whether it is or isn’t the EGR valve though? Because I’m not an expert, but it seems to me like the valve is indeed stuck open when it shouldn’t be. And if it is stuck open, is there any other part of the system that could be causing it to remain open? Or does it being open at all times mean it’s for sure the valve?

The EVR’s vacuum source is the manifold vacuum and its output (which controls the amount of EGR ) should be much less than that, or even zero vacuum, at cold idle. Have you measured both the vacuum source to the EVR and the vacuum output which goes to the EGR at idle? What do you measure? I’d expect 18-20 inches from the manifold, and less than 5 going to the EGR at idle. Have you verified the EGR diaphragm holds vacuum to 20 inches? Have you applied vacuum (using a hand-held vacuum pump) directly to the EGR to verify it stalls the engine?

When measuring vacuum level it 's best to t-in to the line so you can leave everything connected, rather than attempt to measure it disconnected.

The DPFE gadget used to be a pretty common source of EGR problems in Ford products. We don’t seem to hear much about it these days though. You might type DPFE in the forum search feature, see what you can find, above right.

Well I went and bought a vacuum pump, it will only pump to 10in though.

That’s my EGR, on the side not shown is the lines that go to the manifold. If what is shown you can only unplug the line near the bottom. If I unplug that hose and connect the pump to that black hose, it will hold a vacuum at idle, but I can’t verify it will hold 20in as I said the pump I was able to get only goes to 10. (The meter goes to 30 but after 10 pumping it doesn’t increase the vacuum anymore, even plugging the hole on the pump itself). When I connect a hose to the pump and connect it to the metal nipple visible in the pic, the reading shows 20in, what’s being drawn (I assume) through the EGR valve from the manifold line. I have not connected the pump directly to the manifold line.

Pulling a vacuum on the places I connected the pump does not cause the car to stall out, or really idle any worse than it does just from having the line disconnected.

It was dark when I got home from work tonight so I just did all of this really quick to see what would happen.

That silver nipple is pulling 20in while the car is at idle, and that black hose holds at least 10in vacuum. At no point was I able to make the car run noticably worse.

The place to test to see if the egr holds vacuum is point 6 in the diagram above. No experience w/Ford EGR systems – at least the modern version – but I believe the EVR is the gadget on top in your photo link, and point 6 is where the rubber vacuum hose connects to the metal nipple. The computer programs the EVR to output a certain % of the manifold vacuum to the EGR’s diaphragm input (metal nipple).

So to test if the EGR diaphragm holds vacuum you’d remove the rubber hose and connect your vacuum pump to point 6. this test is done w/the engine off. I expect your vacuum pump is faulty if it only pumps to 10, but usually if the egr diaphragm holds to 10 inches then it will hold to 20, so I doubt that’s the issue.

The test you did where you connect a vacuum gauge to point 6 with the rubber hose disconnected and your read 20 inches with the engine running, not sure what that means. I wouldn’t expect you’d get any reading at all unless the diaphragm wasn’t intact. However if you connected it first, then started the engine you might could get a vacuum reading there b/c the engine vacuum might pull downward on the diaphragm and result in a vacuum above the diaphragm. Most vacuum pumps have a release valve so you could release any vacuum from that effect, then close the valve and if you still were getting 20 inches, that could indicated the egr diaphragm is incontinent. The diaphragm is that thin horizontal black line above the pintle in the diagram above.

If the EGR is working and you connect a vacuum pump to point 6 and pull 20 inches of vacuum at idle, it should stall or nearly stall the engine. A vacuum above the diaphragm creates a force that pulls the pintle upward, opens the valve at point 3, and allows more exhaust gas to flow into the intake manifold.

The testing you did is a good start, but isn’t quite the tests needed. The silver nipple register 20 inches of vacuum makes me suspicious however that your EGR’s diaphragm is faulty. Suggest to return that new vacuum pump you bought and ask for one that actually works. I have one made by mity vac (as I recall), have had it for 20+ years, and it works great. Harbor Freight has a considerably less expensive version of the same idea, but no experience with that one. It’s a good idea to purchase a separate vacuum gauge too. It’s possible to measure vacuum with the pump alone, but if you want to continue diy’er diagnosis you’ll run into limitations without having a separate vacuum gauge. A vacuum gauge is quite inexpensive.

Harbor freight is where I got it lol. They had the mighty vac but it was 15 dollars more and I’m on a budget. If I need to I’ll return it and go ahead and spend the extra money. I’ve had that vacuum before but I almost never used it (not my first go around with p0402, just first time on this car) so that why I opted for the cheaper one.

But no, at no point was I able to make the car even begin to stall. In fact messing around with all this stuff, to an untrained ear they probably wouldn’t even be able to tell the difference in the idling.

Anyways I will try what you’ve suggested and see what I can find out tomorrow.

It might not be possible to effect a stall without getting to 20 inches. You might be able to fix your pump so it will pump to 20 inches by coating the bore with a thin layer of silicone grease, aka plumber’s grease. here’s actual pictures of my car, 3rd picture has a numbered diagram.

Okay so If I disconnect the hose from 2 and connect the vacuum to it (meaning the hose is still connected to 1) it holds up to 25in. Both with the car off and at idle. It doesn’t cause the car to stall though.

5 runs between 1 and 4, but I can’t get it off. 4 also holds a 25in vacuum. 3 doesn’t hold any.

The only time the car seems to stall is if I take the hose off of 1 (leaving the hose connected to 2). When I put the hose back on 1 sometimes it stalls pretty hard, almost dies completely.

[quote=“Iamsupernova5891, post:9, topic:155304”]
If I disconnect the hose from 2 and connect the vacuum to it (meaning the hose is still connected to 1) [/quote]

Your description is a little unclear. I believe what you are calling point “2” is point “6” in the diagram above; i.e. the vacuum input that controls the EGR pintle. Have you removed the hose from point 6 (diagram above) and connected your hand-held vacuum pump directly to point “6”? With the engine off does it hold vacuum to 20 inches? With the engine idling and you apply vacuum to 20 inches, does it stall? You may have to plug the end of the hose you removed.


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The numbers I used are based on the picture I included in the last post. I want to make absolutely sure I’m doing it right so I labeled pics of my actual EGR valve.

I mean yeah it seems like it would be number 6 from the diagram, but I don’t for sure. I figured it was best for someone who actually understands all of this to see pictures of what I’m doing to confirm I have the right idea.

Did not plug the hose, I will go do that now.

Okay point 6 (2 in my picture) and plugging the metal nipple the hose came off of, I can make the car run noticably worse by pulling a 20 in vacuum. It still doesn’t die though. It just goes from smooth rpms to fast repeated surging and dropping. It’s headed towards stalling out but it still doesn’t.

And yes with the car off it holds the vacuum just fine.

I took the EGR off today too see what I could see. It didn’t look very dirty but I sprayed it out anyways.

A second code came up today, p0106. Which is apparently an issue with the MAP sensor, and I read that the EGR and MAP on my car are one unit.

I also tried putting vacuum to point 6 to see if I could see that rod inside moving (can’t get to the diagram itself, cuz it’s a solid sealed unit) and I really don’t think it’s moving even when the vacuum it’s holding at 20+ inches.

Time to replace the EGR?

With the EGR valve removed from the engine I’ve always been able to observe the pintle or rod moving as I apply vacuum to the vacuum input. When it is working correctly I mean. Usually it isn’t working correctly, which is why I removed it in the first place. I’ve never had a vehicle where the EGR and MAP were the same unit, but such a thing is possible I suppose. The EGR has a connection to the intake manifold, which is where the MAP connects.

Well I did some further research today. From what I can find, my EGR is electrical, and it’s EGR, MAP and DPFE all in one. Can’t find much when your search for mustang engine, but if you cast a wider net of 4.0 v6 Ford, you start to get some results. Now I’m trying to find back probes for my multimeter to run some electrical tests but I’ve been to like 10 stores (harbor freight, AutoZone, orielys, advanced Auto, multiple instances of them) and cannot find them anywhere.

With what I read, it pretty much means I have to replace the unit no matter what the issue really is, but I’d really like to get a solid confirmation before dropping 100 bucks. This isn’t my first time with this code and last time it wasn’t the EGR. In fact last time i never even find the issue. Just replaced one thing after another. Be

Back probe.

Or a safety pin.


I thought about that, but didn’t know if the metal they’re made of or anything like that might mess with my results. Besides, I’m rebuilding my tool collection and backprobes are something that actually really came in handy. Was hoping to get the real deal for my tool box. But if I must then I will.

As far as I can tell, what I really need to know at this point, is that the DPFE is functioning as it should. If it is, then the problem is with the EGR itself, if the EGR is functioning fine but the DPFE is bad, it still means replacing the same thing.

No matter what it seems I need to replace the EGR unit, but I’d really like to be able to confirm this 100% for once first.

The service data suggests there’s two types of EGR valve used. Presumably which one a particular car uses depends on the engine and options. The first type uses vacuum force to move the egr pintle. The second type uses an electrical stepper motor rather than vacuum force to move the pintle. The second type – it sounds like this is your configuration – uses an ESM module which has the EGR and MAP function combined. Attempting to diy’er debug the ESM without access to the appropriate scan tool may be close to impossible.

Well when I took it off and cleaned it out with carb fluid, I reset check engine light to make sure it was still going to come back. It has not, and all the systems monitors on the obd reader are set.

The EGR didn’t appear to be dirty, but it seems like cleaning it fixed it.