EGR Tube--How to get it sealed on the exhaust manifold?

[DPP]

I have a 1994 3.8 L V6 mustang and have got it running pretty good, but I have an issue with it spiking rpms every now and then on slow downs--driving 40-60 mph and then decelerating in speed zone hanges. The EGR differential pressure sensor threw a error and so I put the smoke machine in the sensor line to see if there was a leak and I noticed that smoke creeps out around the nut where the EGR tube connects to the exhaust manifold.

Correct me if I am wrong, but a leak here wouod allow unmetered oxygen to get into the motor to cause the rpm spikes, correct?

My question is how should I go about getting a tight and sealed fit of the EGR tube onto the connection site of the exhaust manifold?

The EGR tubes are not sold anymore and I actually had mine repaired, so a new tube is out of the question.

Thanks for the feedback.

Dustin

 

[shovel]

Isn't the bottom end of the EGR tube a flare fitting? I no longer have a V6 SN95 so I can't go look on mine but I think it's a flare which means it's compression fit, the tube smashes against a 45 degree cone and the nut just pinches it down to that. In other words the seal isn't even at the nut. Ensuring it's clean and giving the nut portion (threads and rear of the flare) a little grease might be all that needs.

There really shouldn't be enough oxygen getting in through there under any circumstances to cause substantial rpm spikes.

 

[DPP]

Yes, it just has flare fitting like you said, but smoke leaks from it--what I was trying to describe is that the smoke comes from around where it connects.



I had an P1406 error that egr valve was was forced closed by the differential sensor and it suggested an obstruction, but the tube was placed on last year so I didn't think the tube would be obstructed. I did the smoke test and it showed the slight leak, so I thought maybe oxygen could leak in. Any thoughts?

 

[cobrajeff96]

When I had aftermarket headers on a 4v block a long time ago, I had to close off the same port on one of the primary tubes. I didn't have the flared cap fitting to properly close it off so I reamed it out, tapped it for threads, but a bolt on it with a copper washer so it would seal it shut. Worked perfectly.

 

[shovel]

Yes, it just has flare fitting like you said, but smoke leaks from it--what I was trying to describe is that the smoke comes from around where it connects.



I had an P1406 error that egr valve was was forced closed by the differential sensor and it suggested an obstruction, but the tube was placed on last year so I didn't think the tube would be obstructed. I did the smoke test and it showed the slight leak, so I thought maybe oxygen could leak in. Any thoughts?

The key part of this is that the EGR tube is "upstream" of the EGR valve and the EGR valve is never commanded open while idling, so removing the EGR tube entirely should have no functional effect on idle quality - i.e. no effect on RPM.

That isn't to say leaks shouldn't be fixed, just that a leaky tube or fitting shouldn't affect idle. If you have substantial amounts of soot in your exhaust it's possible some of that has clogged the orifice in your EGR tube which would give you a DPFE code - the orifice can be cleaned (off-vehicle) with a bath in solvent of your choice. If the EGR valve itself has been stuck open because of soot then it can also produce that type of code and also affect idle qualty by being stuck open, plus it's possible to heat damage the diaphragm by constantly allowing exhaust through even at high pressure and volume (stuck open despite being WOT when it would never be commanded open)

As a side note, if you do have a lot of soot that's usually going to be because 30 year old valve guide seals are blown out, they're cheap to replace if you can turn a wrench. Easiest to do by removing the rockers, filling each cylinder with compressed air to hold the valves up, pull the keepers, pull the springs, swap the seals, reassemble. Maybe intimidating the first time but it's an afternoon job if you've got the right tools and then you get better mpg, less oil consumption, longer life outta your cats and oxygen sensors, etc

 

[DPP]

The EGR valve is brand new and the rpm rev is not an idle issue--it is while I am driving 40-60 mph, usually acts up on a slow down. I had the EGR tube off last year to get one of the sensor connections brazed back on and a crack repaired. I didn't see any blockage or soot problems.

So does a minor leak at the EGR tube connection not pose a problem of rpm spiking occasionally?

Thanks,

Dustin

 

[shovel]

The EGR valve itself is only commanded open under specific conditions, I don't know the precise digits for a 94 V6 but it's going to be in the neighborhood of this: operating temp over 160F, throttle above 15% and below 85%, engine RPM above 1200 and below 3000, closed loop on.

So if the behavior you're seeing is outside of those conditions then it wouldn't even be possible for it to be EGR because it would be closed.

Stuff to check - your EGR vacuum solenoid has two ports, the one farther from the electrical connector should go to manifold vacuum. If you get those backward it will still technically work but it's also possible for manifold vacuum to suck the valve open a little bit and that would open the EGR when it's not being commanded open.

Timing - I think your 94 still uses a distributor but I don't know if it uses a SPOUT lockout jumper or if there is another way to lock the timing and check it with a light; rapid changes in timing from a sloppy worn out chain will cause rpm surges at random times, particular when engine RPM or load is changing. Wouldn't be a bad idea to verify that timing is consistent. Look up the spec for your engine, lock out the distributor and it should stay right on that spec not only while idling but also on the way up and down revving the engine. Usually with a worn chain you'll see it bounce around at idle, lock in when you rev and then bounce when you let off the rev.

I have to imagine that the leak you've described is still fairly small like just a "crack" around the flare fitting right? Even if that amount of air was getting into the engine it wouldn't change AFR enough to do anything especially above idle.