90 suburban, 5.7 tbi; hesitation with backfire

Hi, I'm an ASE tech and I used to consider myself a bit of a specialist at solving drivability issues, but I'm stumped on this one. Hopin' someone out there can help me.
The history: This 90 suburban, 5.7 TBI, has been setting for nearly 5 years and was towed to me in this condition. I have drained out the stale gas and chemically cleaned the fuel supply from the tank to through the injectors. The injectors were initially frozen but appear to be in good shape now. I replace the inline fuel filter after cleaning. Old oil was drained, refreshed and the crankcase chemically cleaned and flushed again with new oil and filter. New plug, wires, cap & rotor. Timing set to 0 degrees. After extensive driving the scanner shows no codes and all data parameters are within tolerance (TPS, MAP, EGR, etc). There is a puff of blue smoke on initial startup after sitting overnight showing valve seal leakage. Vacuum circuits have been inspected and repaired as needed. Compression for all cyls are 170-180 psi. Has smooth idle and no engine or valve noise. Upper distributor assembly (magnetic pick-up) shows rust, which I have doused with wd40.
The symptom: The suburban will feel doggy on acceleration. If I accelerate from a stop to about 25% throttle it will act weak or lean and then, after a few seconds of lag, as if the system finally realized that the throttle is indeed open, etc., she will come to life and take off like a new truck. If I punch the throttle, from 0% to a sudden 75%, she will backfire through the intake almost every time. This symptom seems less obvious when the system is in open-loop and is in warm-up mode.
Any ideas on this matter would be greatly appreciated. I've got mechanics block and I need a nudge.
My thanks in advance,
Wes
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I'd check the coolant temp sensor reading on the scanner when cold, and when warming up, look at the fuel pressure also, could be crap in the screens or regulator sounds like a lean condition and in open loop less of an issue of course.

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wrote:

I agree with fuel pressure check but maybe a bad map sensor too that is kinda plugged up and take a while to tell mixture to respond. properly. Varify the output from TPS is smooth too as it rotates through range. ----------------- TheSnoMan.com
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Thanks snoman. I'm going to do a fp and volume check today and if that is good I think I'll spring for a new MAP sensor. I've been using a snap-on 2500 (red brick) scanner to check things but to check the TPS I'll hook up a scope to be certain of a smooth slope. Thanks again!
SnoMan wrote:

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Thanks Shep! Will do and let you know what I find.
Shep wrote:

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Hog Whisperer wrote:

Did you check the tb gasket? initial timing and ignitiom module not an expert but ive worked on alot of old cars just my best guesses hope they help.
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Thanks. The Vac at idle is around 20 so I wasn't thinking about a leak but I will check again while spraying some carb cleaner around the base of the tb. Initial timing has been set and checked twice since, it's ok. I am going to a pull-a-part tomorrow and will pick up a couple ignition mods to do swap tests. Thanks for the great advice!
snipped-for-privacy@gmail.com wrote:

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Ok, I'm a total retard. Upon re-examination of the timing procedure I see that I had not disconnected the ESC line first (brown/black striped wire/connector located on the firewall). Oh, I was only about 15 degrees aft tdc (yikes). The reason that it would finally take off after a lag was due to the knock sensor retarding the timing after detecting knock. Well, now she runs like a striped ape!
Thanks to all who helped kick my brain back in gear!
Retard (formally Hog Whisperer)
Hog Whisperer wrote:

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wrote:

Yes you must disconnect the connector on firewall to set timming. Also if you want it to run really well, set base timing to 6 to 8 BTDC and use 89 octane or better. (I use 8 BTDC and use 89 in winter and 93 in summer on my 89) Do this and it will really run well and MPG will pickup too. THat engine wants to knock with 87 in it and because of this they set base timing at 0 or TDC to limit knock pontentail on 87 octane within knock sensors ablity to control it but everytime it retatrd spark, you are loosing power and MPG. Newer Vortec engine are completely electronic control and can set timing anywhere from 40 ATDC to 40 BTDC on the fly while the TBI motors are more limited because they combine mechanical advance with limited electronic advace too. Either design though benifits from better fuel though the TBI version needs to have based line timing advanced to make the most of better fuel. When I travel to rockies, I set my timing to 14 BTDC while out there for a few weeks. It improves power and responce a lot in thin air because the TBI ECM cannot realy compensate for it well otherwise unless you advance base line timing. ----------------- TheSnoMan.com
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<snip description of violating federal regulations>

Where is the mechanical ignition timing advance on a TBI motor?
(this ought to be good)
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On Fri, 17 Nov 2006 14:00:57 GMT, aarcuda69062
You are such a dumb a$$! Always looking to start something. There is a limited mechanical advance built into distributor itself (my 89 had one when I pulled to to lube shaft bushings and replace some internal parts years ago for PM) along with a electronic advance together with base line timing set with static timing. This is what I call a generation 2 knock control system because the knock control can only remove electronic advance which is in the 15 to 20 degree range depending on model and in reality this electronic advance is nothing more than a replacement for old style vacum advance because it is based on a old style distributor and cannot retard spark below baseline and mechanical timing (you would know this if you really had a clue and guy like you think 87 octane is all you every need too). New Generation 3 and beyond is all electronic (a Vortec 350 in a gen 3 because it still has a distributor though it controled electronically and 99 and later Vortec modular motors are gen 4 with no distributor) See people like you read a few articles make a few post and think they are wizards with this stuff when they would do well to stop flapping theirgums via "keys" on the computer and learn something rather than trying to attack other because of their own deep seated insecurity and inmaturity. ----------------- TheSnoMan.com
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Sno, not being a smart -ass here, but I believe you will find there is no mechanical advance in the 90 est type dist. I assume that is what you referred to being or having a partial mechanical advance system.
wrote:

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snipped-for-privacy@snoman.com says...

<LaLa land snipped>
I'm looking at an exploded view of a 1990 Distributor for the 5.7 List of parts are as follows -Pick-coil -Module -Housing -Spring Retainer -Gear -Pin -Cap -Rotor -Shaft assembly -Retainer -Shield
I see nothing to "mechanically advance" timing. Now who is the dumb a$$?
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I guess you may be right, because in 35 years as a mechanic, I've never heard of "vacum pressure."

Right again.

I don't know what you think you remember seeing inside your distributor, but unless you're referencing some obscure offshore export set up meant to be shipped to Saudi Arabia, there never has been any method or manner of mechanical advance mechanism in -ANY- GM truck TBI system. Truck TBI systems have always been EST with ESC. Don't believe me? All you have to do is look at the VECI label, it is all listed there.

'What you call?' All this indicates is that you haven't ever had so much as one minute of formal education on automotive control systems.

What a crock of malarky. Absolutely all of it.

You have absolutely no way of knowing what I "think" about gasoline octane ratings. I on the other hand know (because you've posted proof) that you have failed in god knows how many attempts to get a variety of engine to run correctly so you resort to half assed band aide kluge fixes, the stupid part is how you brag about them.

A Vortec distributor is electronically controlled? Funny, every one I've ever pulled still had a drive gear at the bottom of the shaft.

But the technical name for it escapes you at the moment, right? (of course it does seeing as you're such a fountain of [mis] information)

So far, three people disagree with you, no one has come forward to support what you've said. What do you suppose -that- means? As for the "deep seated insecurity and inmaturity" comment goes, you keep trotting it out like it is supposed to give you some sort of free pass for all the inane bullshit that you post. You can't even spell vacuum or Immaturity correctly, your butchery of the english language makes you appear illiterate and lazy. My guess is that those traits carry over to the physical jobs that you perform also. That you would confuse my education and training for "insecurity and Inmaturity" merely betrays your ignorance, which apparently, knows no boundaries.
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wrote:

Sounds like a certain V-10 thread all over again! When will the IDIOT ever learn to get a large cup of shut the fuck up!
R
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It certainly does.

Remember Roy, Snojob is a website administrator, that makes him an IDtenT
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Soon Snojob is going to have is spark so far advanced....he's going to have to move all the plug wires on the cap over one cylinder.
skimmer
wrote:

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========Hog Whisperer wrote:

<<snip>>
========== I can help you.................
first..........
BURN......that ASE certificate, rip the patches off yer arm......
stand in front of a mirror.............
and yell at yourself..............................
I'M A FK'N IDIOT FOR THINK'N CRAMM'N FOR A TEST WOULD MAKE ME A GOOD MECHANIC.......... WHEN A L L THE GOOD MECHANICS I KNOW HAVE GREY HAIR AND BATTLE SCARS!!!!!!!!!!!
then.........
take a class in diagnostics.......... you know......... one that will teach you to DOUBLE CHECK YOUR FREEK'N WORK......... BEFORE running through the shop screaming ..........
I'M A CERTIFIED DUMMY!!!!!!!!!!!
fer all the world to hear....for eternity.
:)
ps.... i liked that part about being a drivability Specialist....ASE certified too......
We see ALOT of those in the forums..........
begging for help.
:O
~:~ marsh ~sips his mushroom tea.......reaches down in his lunch box...... pulls his fanger out of the ice tea jar........sews it back on..... gonna need it at work tommorow........that ASE kid is putting in a motor~
^ ^ o o L O
.
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Hog Whisperer wrote:

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