5.7L TBI stalling and low idle help

[DiGiTaL D]

Connect a scan tool, look at commanded idle speed vs. actual idle speed. Perhaps the IAC or the IAC wire harness is defective.

How old is the O2 sensor?

I am back with some data.

When the engine is cold, the O₂ cross count starts low and runs up to approximately 250. It then rolls back over to a low value, say 14 or 16, and runs back up to around 250 again.

Commanded idle speed does not appear in the available live data.

With A/C not requested (off) idle speed hangs around 650 RPM. With A/C requested (on) idle speed hangs around 700 RPM but I did see it go as high as 725 RPM.

When A/C is requested, idle air motor position is approximately 47.

When A/C is not requested, idle air motor position is approximately 24.

I did notice that once the engine warms up, the O₂ cross count slows way, way down.

When it is in the process of stalling out (A/C on, in gear, stationary turning steering wheel thus loading the power steering pump and drive belt system) the idle air motor position advanced rapidly from 40 to 91 at the approximate point of stall, then topped out at 140 after the engine actually stopped.

 

[blackdeathmessenger]

Have you tried the coolant temp sensor in the intake manifold?

 

[MANIAC]

So I have a similar issue that I've been trying to chase for what seem like a while but here's what I got

1995 K2500 suburban with the 5.7 originally,
When I got the truck it was built to have 393 cubic inches of displacement and it's a stroker motor

I rebuilt the entire thing from the block up
Ring gap set at .021
4.125 bore, 3.625 stroke
New bearings all around

Iskandarian camshaft 252/256

And 487X cast heads that have been ported

And 2 1/2" exhaust all the way down and straight piped
Truck starts and revs but idles so low around 480 to 500 that it won't stay running but for maybe 10 seconds before bogging down and dying I've already checked for vacuum leaks around gaskets and just recently installed 1.5 roller rockers and still have same issue I know they aren't too tight because if I gave them even a 1/4 turn preload it would hang open the valves and I had no compression when it did so it wouldn't start, now i got it to start but can't get it to idle and timing is definitely close enough to start, I have been stumped for about 3 months now and I'm starting to think it might be fuel related but I'm not sure the company who makes the cam says it ment to be set at 16° BTDC and that it's computer compatible but if i set it at that and then plug in the timing adv wire it won't start and it acts like too much advance but if i set timing at zero it won't idle and backfires through my throttle body and as much as i love a good fireball i much prefer it in the cylinder any help would be appreciated thanks also cam info is in the attachment

 

[Schurkey]

won't stay running but for maybe 10 seconds before bogging down and dying

just recently installed 1.5 roller rockers and still have same issue I know they aren't too tight because if I gave them even a 1/4 turn preload it would hang open the valves and I had no compression when it did so it wouldn't start,

Classic symptoms of the rocker arm adjusting nuts being WAY TOO TIGHT. Back off, find zero lash by wiggling the pushrod UP AND DOWN instead of "spinning" it with your fingers.

That, or you've got limited-travel lifters. Possible, not likely.

can't get it to idle and timing is definitely close enough to start

What is the initial ignition timing set to?

I'm starting to think it might be fuel related

What is the fuel pressure at prime, and under load?

How old is the fuel filter? How old is the gasoline?

Who did the computer tuning to get the fuel curve right for forty-three additional cubic inches?

Connect a scan tool, verify EVERY sensor and computer output, and look at the fuel trim numbers.

Assure the EGR isn't stuck "on", leaking exhaust into the fuel/air stream.

 

[MANIAC]

Ok so when I got the truck I was originally told it had nothing done to the fuel system it has (MAXSPEEDING link bar hyd roller lifters) plus it's an OBD1 the only code it's throwing is code 43 which is a knock sensor code and the only sensors that aren't brand new are the o2 and the purge valve solenoid and I bought stainless steel full roller rockers and the weight of the rocker arms was enough to remove valve lash I don't do the spin method of the pushrods because Everytime I tried that method I got it too tight i sinched down the rocker arm nuts and tightened the poly locks the valves are for sure good I can put a video of it on YouTube or some social media platform tomorrow after I get off work also I did replace the EGR valve yesterday and it made no difference the valvetrain should be good I'm either fighting a timing issue or a fuel delivery issue or at least I think I've already gone over the valvetrain well over 30 times in the last 3 months I managed to get the break in for the engine done with little to no problems but before that I again had to fight a timing issue just to get that far so i know it runs its just fickle but I am planning on buying a new efi system in a few weeks because I can't for the life of me figure out why I can't get it to idle above 500 because if i hold the gas pedal it stays running so long as I don't throttle it but even doing that it still surges like its struggling to stay running and I don't currently own the tool to test fuel pressure in any form or access to someone who can tune it for the fuel curve I have asked around a few shops and nobody messes with OBD1 anymore hardly and I definitely don't want do an ls swap because ive already got too much money in this thing

 

[docstoy]

Isn't it a basic rule of thumb to adjust valves to no lash and then back off 1/2turn when going to roller rockers with adjustable rocker arms ? Been long time since messing with non oem performance parts, did rockers come with installation instructions ?

 

[1990Z71Swede]

Classic symptoms of the rocker arm adjusting nuts being WAY TOO TIGHT. Back off, find zero lash by wiggling the pushrod UP AND DOWN instead of "spinning" it with your fingers.

That, or you've got limited-travel lifters. Possible, not likely.


What is the initial ignition timing set to?


What is the fuel pressure at prime, and under load?

How old is the fuel filter? How old is the gasoline?

Who did the computer tuning to get the fuel curve right for forty-three additional cubic inches?

Connect a scan tool, verify EVERY sensor and computer output, and look at the fuel trim numbers.

Assure the EGR isn't stuck "on", leaking exhaust into the fuel/air stream.

Sorry to Hijack the thread.
@Schurkey I have sent you a couple of PM's Have you seen them?

 

[docstoy]

No I was wrong, it is 1/2 to 1 turn after no lash , you should be able to look at performance parts website (jegs or summit) they will have instructions for installation and adjustment depending on what type of (heads,guide plates,lifters) that you have, another thing they mention is pushrod length if going from stock to performance, hope this helps.

 

[MANIAC]

Sorry to Hijack the thread.
@Schurkey I have sent you a couple of PM's Have you seen them?

Fuel is less than two months old, timing when I can get it to run on its own and this is only with the timing advance wire unplugged is at 16° BEFORE TDC I don't currently have a way to test fuel pressure however I think it's running too lean since it's idling so low I did rebuild the throttle body and the fuel pressure regulator on it too so I think that might be my problem but I can't test it till I get home from work I already have a junkyard unit to at least see if that will do anything different being that I don't have a fuel pressure test gauge kit but I want to set initial timing at zero so I can plug the computer in and let it take over but until I get this absurd low idle problem figured out I'm kinda stumped also I'm not sure if it makes a big difference or not but the 487X heads are fully ported and the exhaust is 2,1/2" and cat-less so since I know the fuel system hasn't been upgraded I'm gonna confirm wether or not the pressure regulator is no good and if it is a fuel delivery problem im definitely getting rid of the TBI unit and going either efi or tuned port injection its definitely gonna be one of the two

 

[Schurkey]

FIRST, knock the ignition timing back to something reasonable. About 4 BTDC with the connector unplugged.

As soon as you plug it in, the computer is going to advance the timing "about" 12 degrees. If you start at 4 degrees, and the computer jacks it up another 12, you've got 16 degrees of effective timing.