Thread: Stumbles, Hesitates upon heavy acceleration

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Hi guys,
I have a 94 thunderbird with a 4.6L V8, and I have a stumble/hesitation upon heavy acceleration. It feels like a slight missfire, and it happens when i go uphill under steady acceleration, or on level ground giving it 2/4 3/4 throttle. It doesn't seem to make a big difference if its cold or hot, however when its hot, the missfire is more pronounced. No engine light is on, however, when i scanned it i got a P0402. That points out the egr system and says"excessive flow detected in the egr". So far, I have done plugs, wires, coils, cam/crank sensor, throttle positioning sensor,dpfe sensor without a result. I bypassed the EGR, and it still does it. The dealer said i would be wasting my money if i get an egr, and i'm better off getting a dpfe, because they are well known to break. $100 bucks later, it is not a result of my problem. Might it be the EGR valve itself, because I am trying to work around it because it is phisically impossible to change it without removing the intake. If I was to have a vacuum leak somewhere, wouldn't the symptoms be the same(but then it wouldn't throw me the same code)?

Originally Posted by vdorobantu

Hi guys,
I have a 94 thunderbird with a 4.6L V8, and I have a stumble/hesitation upon heavy acceleration. It feels like a slight missfire, and it happens when i go uphill under steady acceleration, or on level ground giving it 2/4 3/4 throttle. It doesn't seem to make a big difference if its cold or hot, however when its hot, the missfire is more pronounced. No engine light is on, however, when i scanned it i got a P0402. That points out the egr system and says"excessive flow detected in the egr". So far, I have done plugs, wires, coils, cam/crank sensor, throttle positioning sensor,dpfe sensor without a result. I bypassed the EGR, and it still does it. The dealer said i would be wasting my money if i get an egr, and i'm better off getting a dpfe, because they are well known to break. $100 bucks later, it is not a result of my problem. Might it be the EGR valve itself, because I am trying to work around it because it is phisically impossible to change it without removing the intake. If I was to have a vacuum leak somewhere, wouldn't the symptoms be the same(but then it wouldn't throw me the same code)?

i'd check the fuel filter and fuel pressure, before i spent a lot more money.

I just replaced the fuel filter 2 weeks ago, and it actually runs worse with the new one. Thing is, I am getting an EGR code, not a fuel system code. I will check it tomorrow. I will also check the vacuum lines to and from the electronic vacuum regulator. I'm thinking that maybe if a vacuum line is busted from the intake to the regulator, its not going to get vacuum to pull the diaphragm up. But the code description is "egr valve open at idle". If the diaphragm would be broken, wouldn't that be a major vacuum leak, cause my surge and hesitation, and then my non illuminated code?

Originally Posted by vdorobantu

I just replaced the fuel filter 2 weeks ago, and it actually runs worse with the new one. Thing is, I am getting an EGR code, not a fuel system code. I will check it tomorrow. I will also check the vacuum lines to and from the electronic vacuum regulator. I'm thinking that maybe if a vacuum line is busted from the intake to the regulator, its not going to get vacuum to pull the diaphragm up. But the code description is "egr valve open at idle". If the diaphragm would be broken, wouldn't that be a major vacuum leak, cause my surge and hesitation, and then my non illuminated code?

with a vac. leak or EGR valve open at idle, poor idle should show up first.

it idles good, but when i put it in gear, it shakes a little, then it becomes smooth, it shakes a little more, and so on. I can see the rpm go up and down by 100-200. Say you are doing 20km/h, and you punch it, it backfires and barely goes. Once it's in the higher RPM's it stops doing it. As the RPM drops below 2k, if you punch it again, it will hesitate and stumble some more, until its at the right RPM.

How do I test the egr and the vacuum pressure regulator on this thing? I put a vacuum pump on the egr, and it doesn't leak or anything, so I guess it just sticks. Just pure speculation. My idea is that if the pressure regulator doesn't know how much pressure to give it, it will keep the egr open. What do you guys think?