Tbi backfire through intake

[Schurkey]

A compression test is a quick 'n' dirty way to find out if you have excessive lifter preload.

Too much preload = one or more valves doesn't close.

Valve doesn't close = low cranking compression pressure.

Borrow a compression tester from the "loaner tool" program at various parts-stores, or buy a GOOD one as it's pretty-much an essential tool. Don't go shopping at Hazzard Fraught for tools you'd use more than once, or anything that measures something else and needs to be accurate, repeatable, and reliable.

Low (or unreasonably high) cranking compression pressure can also be an indication of a camshaft that wasn't timed properly, either by workmanship or defective parts.

 

[udidwht]

First timer, eh? lol Let the ol' guys tell you about adjusting rocker arms in the freezing cold uphill both ways.
There are a thousand ways to do it, all of them make it easier than all the rest. Personally, I use this method (with 3/4 turn instead of 1/2 on stock-ish motors)

SBC Valve Lash Adjustment on Hydraulic Lifters | 8 Steps

To get the longest life and most performance out of your engine, it's important to have the valve lash set correctly. Here we show you how to set valve lash on a Small Block Chevy using a hydraulic lift camshaft in just eight steps.

www.speedwaymotors.com

The real trick, no matter what method you use to find your cam position, is to find with precision the 'zero lash' adjustment. You have to feel the pushrod, rocker, and lifter pocket by rolling and wiggling the pushrod, and feeling the rocker arm until you know you have no slack, but it's critically important that you do not preload the lifter at all. Then get your 1/2-3/4 turn. Because I'm old, stupid, and never did have enough memory to find my own house on my own block, I clean a shoulder on the rocker arms and use a paint marker to dot the ones I've completed. Then when I'm finished, I roll the motor through 2 more turns to be certain I got them all.

Yep and it is best done with your 'fingers' when feeling for the zero lash.

 

[Rustjunky67]

First timer, eh? lol Let the ol' guys tell you about adjusting rocker arms in the freezing cold uphill both ways.
There are a thousand ways to do it, all of them make it easier than all the rest. Personally, I use this method (with 3/4 turn instead of 1/2 on stock-ish motors)

SBC Valve Lash Adjustment on Hydraulic Lifters | 8 Steps

To get the longest life and most performance out of your engine, it's important to have the valve lash set correctly. Here we show you how to set valve lash on a Small Block Chevy using a hydraulic lift camshaft in just eight steps.

www.speedwaymotors.com

The real trick, no matter what method you use to find your cam position, is to find with precision the 'zero lash' adjustment. You have to feel the pushrod, rocker, and lifter pocket by rolling and wiggling the pushrod, and feeling the rocker arm until you know you have no slack, but it's critically important that you do not preload the lifter at all. Then get your 1/2-3/4 turn. Because I'm old, stupid, and never did have enough memory to find my own house on my own block, I clean a shoulder on the rocker arms and use a paint marker to dot the ones I've completed. Then when I'm finished, I roll the motor through 2 more turns to be certain I got them all.

Yes sir I did all that meticulously to be sure I wasn't preloading any plungers to get zero lash, I put paint on each lifter and push rod to check rotation upon assembly, checked oil flow. So I feel like that should not be an issue but I really don't know my r*e*c*+*u*m* from my retina (of which I only have 1 by the way, old jail fight souvenir), but I followed the path of others with a reputation that precedes them as best I could....
I know a plethora more now than I did a few months ago....
Anyway. Just got her fired off again pretty as you please idled well, timing was good on 0-1°or so.... but iac open causing huge vacuum leak that sucked he wax out my ear through the pipe I was using to verify I only had that one leak.... put in diag mode to close pintle, fire her back up and can't get her to idle without a little steady throttle....
Onward we go....
Any suggestions to save me heartache or hassle?
Thank you sir

 

[Rustjunky67]

A compression test is a quick 'n' dirty way to find out if you have excessive lifter preload

Yessir I'm not afraid to do it just never have.... I've got a buddy that has one I can borrow if needed, and I don't see where it will hurt anything to do it regardless....I have been extremely nervous and SLOW and meticulous with everything I've done to try to not have any issues at start up, yet here we are lol.... however I do feel quite accomplished that I barely hit the key the first time and she fired right up as pretty as you please, seeing as how I had never removed an oil pan, a valve cover, an intake manifold; camshaft and lifters I knew ZERO about..... so at least I have that
I was scared to death to deal with a distributor and timing, and procrastinated like crazy out of anxiety, but now I've got a pretty thorough understanding of how it works, on a 94 tbi 350 at least lol.....
And much of that is thanks to you and others in this forum being generous enough with your time to help out someone like me with nothing but a "thanks until you're better paid" to give ya in return....
So thank you in that regard
I took the cap off at TDC of exhaust stroke (first tdc I came to) and realigned cap to rotor (it was retarded probably 8 degrees from playing with it), so that was good.....
I put a different map sensor from my pile on just because it exhibited some symptoms of needing a change..., and boom she fired right off again and was idling fair, slight miss, timing was good at 0-1° or so advanced, big vacuum leak at IAC, so it looks like a trip to the treasure yard or parts store, I can't believe I haven't grabbed a few of these already‍...
I put in diag mode to get pintle of IAC closed, but it didn't stay and so when I fired her again, I can't get her to stay running absent some applied throttle.... of course IAC isn't doing its job, still having that great big vacuum leak......
Any suggestions on what to do as a temp stop gap just so I can bring her up to temp and see how everything plays together?

 

[Rustjunky67]

One more question I just thought of....
Do I need bypass wire unplugged from the beginning or do I need to leave it plugged in until I reach operating temp and then unplug it?
Also, at what point do I plug it back in and do I do so with it running or not?
Thanks

 

[PlayingWithTBI]

Do I need bypass wire unplugged from the beginning or do I need to leave it plugged in until I reach operating temp and then unplug it?
Also, at what point do I plug it back in and do I do so with it running or not?

You wanna set initial timing when engine is up to operating temp so, it doesn't matter until then. With engine off, disconnect the EST bypass wire, start the engine, adjust the timing, shut off the engine, hook the wire back up, disconnect the battery to reset your MALF code 42 which should set when it was disconnected, start the engine and hopefully enjoy!

 

[Schurkey]

I don't know why the engine needs to be warmed-up to set base timing.

The connector gets disconnected, the computer doesn't control timing. Hot or cold makes no difference.

 

[PlayingWithTBI]

The connector gets disconnected, the computer doesn't control timing. Hot or cold makes no difference.

It does AFR so, you may not be happy with how it's running because of the rough idle. Who knows, it may not even wanna run.

 

[Rustjunky67]

It does AFR so, you may not be happy with how it's running because of the rough idle. Who knows, it may not even wanna run.

So this morning I did IAC reset again just to give it anther chance at maybe I did incorrect procedure, disconnected battery again to be sure no codes were stored, plug bypass wire back in and crank it.....
She runs (with a little added throttle) and u hold her at steady 2k rpm for 30 seconds or so...... ran smooth as you could want.....I started easing off throttle and still good steady running.....
I get to about 700rpm and let off a little and she dies....
Got her fired again and was holding about 1200rpm, she shut off and won't start, even with throttle.....
Indication????

 

[pressureangle]

So this morning I did IAC reset again just to give it anther chance at maybe I did incorrect procedure, disconnected battery again to be sure no codes were stored, plug bypass wire back in and crank it.....
She runs (with a little added throttle) and u hold her at steady 2k rpm for 30 seconds or so...... ran smooth as you could want.....I started easing off throttle and still good steady running.....
I get to about 700rpm and let off a little and she dies....
Got her fired again and was holding about 1200rpm, she shut off and won't start, even with throttle.....
Indication????

You have a tough issue to nail 'over the phone' (which is BTW a game I love to play)
Here's a .pdf that has more information about the TBI than most people know; it's not all applicable to your stock 5.7 setup, but if you know what's in there it can't hurt.

www.gmcmi.com/wp-content/uploads/2013/02/TBI-Initial-calibration.pdf
The first next check I'd make would be TPS voltage. I think stock typical is 0.5 volts, the document linked above calls for 0.58-ish. If your TPS is failing, the computer doesn't know where the throttle is so can't send correct fuel, and the O2 sensor isn't lit yet to help correct.