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?

Originally Posted by vdorobantu

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?

i think one of those Honda's has chocked your chicken.

ok if you're getting an EGR code theres 1 of 2 things that will make it throw that code, either the EGR is dirty or the EGR amplifier is malfunctioning.... the EGR amp is a common problem on fuel injected fords and very easy to replace.... shouldn't cost more than $40... if you had a scope meter or a vantage, you could check the amplifier and see if you are getting a proper signal pattern from it.... clean the EGR first with brake clean though.... then clear the code and see if it still happens, if so.... its gonna be that amplifier... the actual EGR hardly ever actually fails

By amplifier, you mean the egr vacuum pressure regulator? I have 1 vacuum line that goes from the throttle body, to the regulator, and the other one goes from the regulator to the egr(both hoses are not cracked, and the hole in the TB is clean). Here is a link, maybe you will have a better idea of the system - http://www.fordscorpio.co.uk/egrmonitor.htm . Isn't the egr spring supposed to keep the valve closed, and upon backpressure from the exhaust, it should open it, and that valve regulator applies pressure to keep it open? I would like to clean the valve, but there is no way i can spray it while its on the car(unless i do it from the vacuum hole on top), because if i have to take it off, the intake has to come off too. It's in between the firewall and the engine, righ by the heater core tubes. It can't be done without taking the intake off(unless i pay the dealer a fortune and I don't have to worry about it).

I also forgot to mention one thing, my cat has a rattle in it, and I'm positive its not a heatshield, because with it on a hoist, i banged on it, and i hear rattle rattle. I yanked on the shields, but, they were pretty stiff. If it would be partially plugged, wouldn't that be the key to my answer? Wouldn't a plugged cat make too much backpressure, and cause the EGR to be open at all times and throw me a P0402 code(EGR sys excessive flow)?

An aftermarket universal cat is like 50 bucks, so i'll just replace that before thinking about anything else. I'll do it monday and let you know how it went.

to verify if the cat is any good... hook up a voltmeter to the o2 sensor after the cat and watch what happens, if you are getting a constantly rich condition (high voltage, greater than 450mV) then the cat is probably not doing its job

I can't believe what a dork i am. Wow! I did a pre and a post cat today, and its fine, it only has a rattle on the inside. Out of curiosity, I wanted to see if i have bad plugs/wires, and so on.So I pull all plugs, and i found plug #4 and 5 soaked in gas. I cleaned them all(stupid me, never cleaned the plug holes when i installed a new set and they were full of rust crumbs), took her for a spin, guess what, missfire gone. I drove her on the highway for about 1 hour, giving it heavy gas, letting the rpm drop to 5, then slamming it again, and the missfire was in fact gone. After 45 mins of driving, guess what,the missfire gradually came back. At this point I am thinking deffective wires. I sprayed them, and they are not arcing, but if the boots are bad, I can't do anything about it. Warranty works i hope(not even 500km's). I got motorcraft plugs to replace the old ones. I'll install them together. Hope NAPA is open

corrosion around the plugs will definately cause a misfire.... they weren't getting a solid ground.... i'd try and wire brush the heads nice and good in the plug holes.... see if that helps a little more