Disabled EGR; truck gains power. Expected?

[xsdg]

So, I'm finally getting some more time to debug other issues that I'm having with my '85.  At this point, I am trying to determine whether my EGR system (including the ignition system) is working properly or not.

 

One that I've noticed is that with everything warmed up, the engine will bog slightly at every shift.  (At first warmup, it's pretty extreme, but I figure I have other issues to deal with in the cold-engine case).  Also, the truck seems to lose power in the mid-RPM range; I know that folks have said that is indicative of a timing issue.

 

Anyway, on a whim, I disabled the EGR (by unplugging the EGR control vacuum line from the BPT).  I took it around the block and that post-shift bog was _gone_.  The truck felt a little more powerful, which could be all in my head, but the absence of the post-shift bog was definitely a quantifiable change.

 

I plugged the BPT back in and pulled the fuse for the exhaust coil.  This time, the truck had noticeably less power than normal, which means that the coil is at least doing something.  In this case, if I'd step on the gas (but not floor it) in a really high gear, it would miss pretty badly, which I'm guessing is related to the mixture and/or the timing.

 

I've left the truck with the EGR disabled.  Tomorrow morning, we'll see how much of a difference it makes for the cold engine situation.  I also should have checked how it feels with EGR disabled and exhaust coil disabled, but I forgot.
 

 

Again, the question here is if the EGR system as a whole seems to be working properly or not.  I'm in California, so "rip that stuff out" is not a helpful suggestion.

[datzenmike]

Properly set up there is no EGR when the motor is cold, idle or at full throttle. At all other times there is some EGR and the amount is load dependent. EGR increases with throttle. Remember that exhaust gasses are deficient in oxygen and simply inert, taking up space in the combustion process. This reduces the peak temperatures and pressures where oxides of nitrogen are produced. If there is a leak in the tube from the exhaust manifold, oxygen will get in and the engine will run slightly lean.

[JoeCool]

If the EGR valve is clogged up, the vehicle will run better without it. If not, little gains are to be expected.

[ggzilla]

Agreed. Properly working doesn't make your engine run bad. But defective egr will.

[ggzilla]

Yes, if the EGR was defective, disabling it will gain power.

 

Defects include

* a stuck open valve

* vacuum.leaks (egr valve, bpt valve, hoses, manifold valve)

* wrongly connected hoses

[Z-train]

This is where the urban myths start."I disco-ed the EGR and th etruck ran better"So there for EGR's hurt engine performance.W-R-O-N-G.

[thisismatt]

Not sure how your egr system is set up, but on my other japanese brand vehicle I had an egr bog around 2200 rpm due to clogged egr ports in the intake. Had to slide hammer out some plugs, clear the ports, and install new plugs. Then everything was good.

[ggzilla]

That's how urban myths start. New plugs alone can fix the bog. The clogged EGR may or may not have had anything to do with it?

[thisismatt]

That's how urban myths start. New plugs alone can fix the bog. The clogged EGR may or may not have had anything to do with it?

If your response is to me, then no, plugs would not fix it.  It's a pretty clear problem & solution.

[xsdg]

So, the consensus seems to be that the EGR system is malfunctioning in some way.  Progress.

 

Things I can say for sure:

— EGR valve is new.  I replaced it in the process of getting the truck to pass smog back in November.

— BPT works as expected and does not leak.

— During the replacement, I noticed a small amount of carbon buildup on the interior surface of the intake manifold, where the EGR valve bolts to the manifold.  I just took a look at the old EGR valve itself and it looks like it's less than 0.25mm of buildup.  That said, given that I notice a significant difference between EGR disabled (forced closed) and EGR enabled (sometimes open), that would suggest that carbon buildup is not the issue.  It it were, I'd see very little or no significant difference between the disabled and the enabled states.

 

FTR, here's a Z24 intake.  The EGR bolts to the studs on the left side, away from the head:

 

 

— Vacuum leaks: very unlikely, at this point.  I replaced all vacuum hoses, and I had to hunt down every last leak to get the truck to idle properly.  The only faulty hose left is for the brake booster, but the booster will still maintain vacuum overnight, so that's clearly not an issue either.

 

— Exhaust leaks, leading to extra oxygen through EGR: I do not know how to diagnose this.  Suggestions?

 

— Misconnected hoses: this seems like the most likely culprit at this point.  That said, logically, each line seems to go to the right places (I'm part programmer, part mathematician, so it's natural for me to think about the vacuum system from a logical perspective).  The main piece that I don't really understand in the whole system are the vacuum ports on the carb, so I'll plan on triple-checking those as well.

 

— Spark plugs: I replaced all plugs in November, and all the ignition wires within the last month.  As mentioned, truck runs worse than normal with the exhaust coil disabled (removed the appropriate fuse), so it's clearly doing _something_.  Whether it's working properly or putting out sufficient voltage, I don't know yet.  Suggestions on how to check?

 

 

Thanks for the ideas so far

[datzenmike]

If the EGR does not close properly it will idle poorly or not at all. The idle mixture is so small it can't afford to have any dilution with added exhaust gasses.

 

Is oxygen getting in? Inspect that stainless? tube from the EGR back behind the head to the exhaust manifold. Exhaust is very corrosive and these pipes can be full of holes. If outside air gets in it will cause a lean mixture.

 

Timing on a dual plug head is always less because the burn time is shorter. A single plug has only one ignition source and takes longer to burn completely. When you dissable one plug the engine will act severely retarded. Dual plug timing is 3 degrees +- 2 degrees. A single plug would need 8 to 10 degrees at least to run properly.