94 suburban k2500 454cid stumbles after changing egr valve

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#1
Ok so apparently i like to chew concrete. Sorry to throw another issue so closely related to my last b4 completeing w/ a confirmed fix... That being said, so my wifes burb decided to trump my sunday what started out as just a simple egr valve swap out has turned into a nightmare. So in the process of egr swap t-body has to b removed, so changed gskt and fuel line o-rings, didnt disturb regulator, tps, or iacv but now with it all back together the dang thing stumbles on acceleration(bogs out completely then slowly picks back up) only code coming up is 35 for iacv but engine idles fine, maybe 100-150 rpm above normal. So question: when throttle is opened up (wot) shouldnt iac close completely only allowing air thru tb? Or is it possible that the regulator just crapped out at the same time? Sorry if this is choppy a little frustrated right now
 
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#2
Kinda thinkin it could b another crappy part... Possibly defective egr out the box? Sounds weird but wouldnt b the 1st time
 

billr

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#3
I wouldn't expect the IAC to close when the engine is out of its idle range. More than likely it just stays where it last was. I assume this is a simple "speed-density" system with no MAF sensor, so it really doesn't matter too much if air comes in through the TB throttle or IAC.
 
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#4
94 454 with a TB.
I do not remember R&R TB for EGR but its been awhile a long time :)

No the iac does not close.
Do the simple spray trick for a vacuum leak.
- Was this an aftermarket EGR with a shim as needed? very problematic design
Is the EGR pintel seating? Maybe some carbon
Then for testing remove and plug vac line to EGR.

Any chance you have old EGR? If diaphragm was issue replace and see if RPM goes normal
 
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#5
System has electric egr so no diaphragm. Cleaned manifold passages while removed. Tbi has to come off along with pass side valve cover bc mounting bolts get tucked too tight to vc and tb to remove.
 
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#7
No change in rpm doing spray test w/ tb cleaner. got tb rebuild kit (really not thinking its my prob tho)
 

billr

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#8
Well, I would look at the EGR a bit more before attacking the TB. Those electric ones are pretty simple to check with a meter, but it is even simpler to remove it and block the ports in the manifold, to confirm the EGR is the problem.

If you want to try checking the EGR valve electrically, check the resistance of the solenoid winding and check the feedback pot by manually stroking the pintle. If that all looks good, then measure current going to the solenoid coil and measure the feedback pot voltage signal. Using long meter leads and doing that while the engine is stumbling may give some clues.

I assume you don't have live-data handy; but testing/watching MAP, TP, spark advance, ECT, things like that would sure be desirable. Fuel pressure too, but that isn't easy to do with the TBI and need a long hose to watch it "in the act".
 
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#9
A tough one getting info on this setup - the EGR is mounted RS of TB - it is linear BUT system is OBDI. And the feed position back would require a scanner I cannot find out which scanner BUT we really want a no scanner solution to setting pintel position...
Hopefully nick will have the info.
 
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#10
No dont have way to check live data unfortunately. Changed egr again but to no avail. Still kinda thinking fuel regulator but spray pattern seems fine just vac drops off and engine backfires under wot
 
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#11
Changed fuel filter and have little better response. Damn hard fuel lines dont make it easy 2 put fuel press tstr on.
 
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#12
I did not know about WOT issue, backfire, try reposting a description and event timeline - changed EGR then went to shot OR changed EGR thinking that was issue....
 

billr

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#13
Yep, time to regroup. What were the symptoms that caused you to change the EGR in the first place? Do you have somebody else that can put a second set of eyes on this?
 
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#14
Ok got her figured out, after replacing egr and iac, the stupid tps was out of whack causing the stumble. Checked fuel pressure during stumble was 37psi so didnt mess w/ tb rebuild but did change fuel filter.Repositioned tps and got rid of stumble and backfire.... Off to shoot my buck in the a.m. Thanks again everyone really helps having people to bounce ideas off of