Vince's TBI woes, progressive revelations, and tbi mods.

Vbb199 said:

Visiting my threads for my scrap heap i see

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me? i was seeing if urban got what he wanted

AuroraGirl said:

me? i was seeing if urban got what he wanted

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Ok

So im back again with a new delimma! Kinda new... been going on as of like 2 months now maybe.

Trucks been running. Flawlessly. Last month i was preparing it to go to the beach and out of the blue i start having intermittent behavioural problems.

-Stumbling randomly at idle
-Changing idle speed randomly
-"Blurbling" on decel or engine braking
-stumbling/misfiring over 3000rpm

I went on a hunt with coil replacement, ohm checking my plug wires and finally replacing them, changing dizzy caps, changing the rotor, checking all 8 plugs, Inspecting for vacuum leaks, zip tying any vacuum ports snugly, watching the injector cone spray pattern, inspecting for exhaust leaks, hooking a vacuum gauge up (16-17" steady at 900rpm), inspecting grounds and hot wires in the engine bay for increased resistance (with a dmm) , ive replace the fuel pump, cleaned and secured the pump ground wire, changed the pump screen, the fuel filter, ive went thru an IAC position reset, replaced the IAC and reset it, checked the ignition timing while in Bypass mode, (it was strangely 5° btdc and it should be 0 per stock spec) lol idk why that happened.

Ive hooked up my ALDL scan tool and monitored the ECM, im seeing steady 13.3+ volts, reasonable ST and LT fuel trims, O2 sensor is oscillating as it should, it enters CL with no codes. Minimal knock acitivity... i know the sensor and calibration for it isnt super advanced though lol

Im at a loss honestly. Which is why im reaching out. Its now gotten so bad it stalls randomly at idle, at a stop light.


At this point the only things remaining to replace is the FPR gasket, and the whole damn distributor, maybe the O2? which i replaced both probably 2-3 years ago. The FPR gasket was part of my TBI reman i did maybe 2 years ago now.

I did inspect the pickup magnet and reluctor wheel in the dizzy and it was rusty, but hopefully my readers know the TBI is non mechanical. It didnt seem nasty, and the shaft play i felt didnt seem bad.


Hopefully someone out there knows something i dont? I final clue i might add is it revs above 3000 in park, just wont climb beyond it under engine load.

Thanks guys

you said the fuel pump and filter are new, but have you checked the fuel pressure? ideally with a gauge long enough to watch while you drive? or will it idle bad just in the driveway and you don't need to drive it to act up?

or a vacuum leak that is worse under load...something like an intake gasket that only misbehaves under load and high rpm?

last thing that comes to mind is a clogged up exhaust? I think my old cat was clogged. I had idle issues and went round and round with it. replaced the exhaust because it rusted apart, not even thinking it might be part of my idle problem, and the stumbling at idle went sway.

Odd how you and AG always reappear at the same time. Good to hear from ya.

I also would suggest checking the fuel pressure, even though the pump is new.

As far as the distributor goes, yeah, that would be a suspect for me. I don't know about the rust on the pickup coil, I've seen some cruddy ones that ran great but also some that caused problems. Typically, an ignition module will either work or it won't... but that's not always the case. Occasionally they do cause driveability issues such as those.

The TBI small blocks love more timing than the factory setting of 0*. 8 to 10 degrees works well. 12 degrees can even work with pump gas in a lighter vehicle with no towing.

SquareRoot said:

Odd how you and AG always reappear at the same time. Good to hear from ya.

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Lol, I never realized that.

I do miss seeing regular postings from @Vbb199 though

Trigger coil in the dizzy?
Even if it reads ~900 ohms, they will still be ef'ed up.

gmbellew said:

you said the fuel pump and filter are new, but have you checked the fuel pressure? ideally with a gauge long enough to watch while you drive? or will it idle bad just in the driveway and you don't need to drive it to act up?

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bucket said:

I also would suggest checking the fuel pressure, even though the pump is new.

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Fuel pressure was 60 something with the ep381 on send side, its now 15 or whatever with the stock pump i replaced it with

Regarding the idle question Glen, you can fire it up and itll idle fine, then out of nowhere change idle speeds for no reason. Like the IAC pintle just suddenly moved for no reason. Perhaps to save it from stalling? I dont know.

gmbellew said:

or a vacuum leak that is worse under load...something like an intake gasket that only misbehaves under load and high rpm?

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80kPa is when it begins breaking up which is also around the 2500-3000 mark, or like 1/4 throttle or something.
its my experience most OEM systems like enrichening the mix by 80kPa, often known as "power enrichment"

gmbellew said:

last thing that comes to mind is a clogged up exhaust? I think my old cat was clogged. I had idle issues and went round and round with it. replaced the exhaust because it rusted apart, not even thinking it might be part of my idle problem, and the stumbling at idle went sway.

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that ol stock muffler and stock cat "fell off" sometime ago for a straight pipe off the Y with a flowmaster 40 on the end. She flows good as far as i know.

bucket said:

As far as the distributor goes, yeah, that would be a suspect for me. I don't know about the rust on the pickup coil, I've seen some cruddy ones that ran great but also some that caused problems. Typically, an ignition module will either work or it won't... but that's not always the case. Occasionally they do cause driveability issues such as those.

The TBI small blocks love more timing than the factory setting of 0*. 8 to 10 degrees works well. 12 degrees can even work with pump gas in a lighter vehicle with no towing.

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Yea i was thinking even with the rust it should still run.. that ign module was part of the replacement i did when i bought the accel distributor, but i suppose i could see it being fowl. Seems like the whole thing just doesnt work when the module dies



As far as timing, yea, i think i had advanced it because the trucks got a chip tune, with more timing and all that, plus i advanced it? Ive never heard it knock once under load with 89 octane, while towing. I just wanted to start from the top and set it at 0 or 2, whatever it was lol

Snoots said:

Trigger coil in the dizzy?
Even if it reads ~900 ohms, they will still be ef'ed up.

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I havent dug that deep, i wonder if i should look into that.

SquareRoot said:

Odd how you and AG always reappear at the same time. Good to hear from ya.

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I actually just made this observation not long ago lol...
Eminem made a song for this

there is some sort of a stall-saver mode that will open the IAC to let in more air to bump idle. I don't know a lot about it, though. it kicks in when RPM drops below a certain target is my guess. the AC also affects IAC position. it opens up more when the compressor kicks on.

when the idle speed changes, does the IAC position also change to let in more air? or maybe a vacuum leak that is coming and going that is bumping idle, and IAC tries to close but can't compensate?

AuroraGirl said:

I actually just made this observation not long ago lol...
Eminem made a song for this

xc_hide_links_from_guests_guests_error_hide_media

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Lol. I'm just a cosmic cowboy on a horse with no name making observations on this journey thru life.

gmbellew said:

there is some sort of a stall-saver mode that will open the IAC to let in more air to bump idle. I don't know a lot about it, though. it kicks in when RPM drops below a certain target is my guess. the AC also affects IAC position. it opens up more when the compressor kicks on.

when the idle speed changes, does the IAC position also change to let in more air? or maybe a vacuum leak that is coming and going that is bumping idle, and IAC tries to close but can't compensate?

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The AC does make the rpm bump like 100 or 150 like youre saying, and i felt like replacing/resetting the IAC was a futile waste of my time. The IAC seems to be operating like it should.


Im going to hook up the ALDL scanner on lunch today and monitor the IAC motor steps and see if they change when idle changes like im hearing

bucket said:

Lol, I never realized that.

I do miss seeing regular postings from @Vbb199 though

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I'll be honest i just dont post much because i havent worked on anything in months and months aside from maintenance items

gmbellew said:

when the idle speed changes, does the IAC position also change to let in more air? or maybe a vacuum leak that is coming and going that is bumping idle, and IAC tries to close but can't compensate?

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So i watched it on lunch with a "cold" start (99° lol) and IAC steps started at like 140 something for high idle, and slowly counted down until it was hanging around like 40-50. I was thinking anywhere between 30-50 counts was ok for IAC operation. At least thats from my tuning experiences.. preferably lower 30s

I did observe that when the idle blipped or acted weird, the counts would go up, then count back down. Nothing wild. Like shoot to high 50s or 60s.

I just picked up a new gasket kit for the tbi. Im gonna look into this FPR.

I did observe something today on the way back. I noticed at like half throttle, it was breaking up at like 2700, approaching 3000...

So i let off the throttle slightly and let it climb in RPM naturally, and it did go to 3000 without a hiccup.

Im gonna toss an extra TPS i have laying around on before i break the seal on my gasket kit.

I didnt test that things sweep with a dmm either
Maybe i should.