350 tbi idle speed

[RanchWelder]

The seals on every fuel pipe connection, including the white plastic seals at the TBI, must be perfect and the connections must be tight. What you describe can be from loose threads anywhere or everywhere, with bad o-rings. Mine were bad at the tank, as well. It would not idle correctly and to keep it running, it hit the converter too hard going into gear. Your fuel system, canister and vacuum lines must be solid. Capable of holding pressure, everywhere.

My fuel lines had green air conditioner o-rings on the steel lines at the tank
When they fell out they were three times the size they started at...

My braided steel lines were rubbed where they cross the transmission case.
Looked like they were hit with an angle grinder. Almost through the plastic inner tubing...

The 3" x 1/4" Fuel tank o-ring must be perfect. Do NOT use Permatex at fuel fittings for bad o-rings repair. That is a Nono... Big flames...

The gas cap must be correct, with perfect seal, no leaks...
The fuel pressure regulator must not have PERMATEX in it by the last person working on your TBI/engine...
Replacing FPR is very tricky. It is easy to ruin it, during install, get pissed off and install it anyways and cause a leak.

A warped manifold will cause this as well. Check your TBI with cleaner...

Corn fuel and the yellow bottle of dry gas is your enemy.
Your injector seals will hate you for using ethanol.

Depending on which HEI you have... the HEI must be in good working order, without a cracked coil or failing Ignition Module. 4500ohms = fail 5000+ohms=good.

The o-ring at the ignition module connector MUST be present on both connectors for 7 wire HEI. (For any wired HEI)... water gets in and causes rust inside the cap and fail. Be surprised how easy it is to lose that lil' green seal, pulling and inserting the HEI connectors... over and over during a rebuild.

The HEI shaft MUST not be swollen with burned oil and MUST move freely. Any resistance will cause the ECM to stumble. Davis Unified costs a fortune because they work. Stick with OEM... please... Every el cheapo aftermarket aluminum HEI I have tried to use had a bad ignition module. GM, ACDelco or Standard Motor Prod only... in, my experience... I have found 1 single strand of copper wire under the cap, connecting the ignition module to the HEI magnetic coil, under the rotor... If your distributer gear is bad and worn, due to this issue, your timing is your issue... My brand new engine had all these broken parts put back on, and every issue stated above... All of them.

Your fuel line o-rings may be 36 years old.

The throttle body shaft must have good bushings... No leaks, spray with carb cleaner and look for stumble....

Then you may start testing sensors...

Just for the sake of argument, how's your transmission shifting lately?
The Governor being stuck towards the drive gear on a 700R4 will cause the converter to surge and the engine to stall... at idle. Just like a bad O2 sensor...

Just replaced my Governor this past week on a brand new fresh built non-lockup 700R4. Less than 1 year old and failure.
The valve locked open causes some weird things...
The Converter shudder was key to diagnosing this issue.
Late or soft shifting into 3-4... Stall at idle...

The 3 wire O2 mod on a relay works great for idle control issues.
Highly recommend this mod before you play with timing and search for idle issues. The $16.00 1-wire O2 sensor is rated for 10,000 miles or LESS.
It sucks. Buy the 3-wire sensor and start again with diagnosis.
The one wire O2 is an annual service part, when used in fleet operations back in the day.

I have contaminated mine twice. You may have accidentally ruined it with engine additives, corn fuel, dry gas... or whatever you keep spraying into your TBI. So be certain it has ground to the firewall or the ECM cannot see it and/or the signal may be intermittent...

Has your TBI been allowed to corrode, under the TBI in the orifices for vacuum flow? Any builder digging in the bone yard has removed a TBI, only to find a full deck of cheese under the darn things.

It may be time to rebuild it thoroughly, which includes, removal, a gallon of solvent and a good cleaning...

How's your injector pigtails? Cracked and brittle, 40 years old?
You can test and clean your injectors using a 9v battery in solvent.
Do not forget the little tiny injector o-rings in the injector rebuild kit.
Buy extra's of the fuel line o-rings... Keep a spare in your truck...
Make certain your fuel filter is good. I have had to change 3 in one year...

Clean EVERY GROUND CONNECTION and use dielectric grease and star washers EVERYWHERE. Add an extra engine to body ground, if you can afford it. Sensor without good ground can ruin your marriage...

I'll bet you 10 cents it's an o-ring... or a weak fuel pump... the crap they sell over the counter is worthless these days... BUY a better pump... run fat wires to your new expensive TPI GM Pump or buy the Edelbrock unit, 43psi.

I'm not trying to overwhelm you, and I hope I'm not going overboard with search and destroy mechanics details. Once you can be certain of every one of these simple steps, you can diagnose anything, with confidence.

Poor fuel pressure causes bad vacuum too.
You could be off 1 tooth at the distributor... Verify top dead center and stick the dizzy after verifying it's not clogged up with gunk and the gear is not badly worn.

Just 1 item listed, could cause the issue you are having. Resist the urge to buy everything new, except the distributor goodies. A bad ignition module will drive you up a wall. Dielectric grease EVERYTHING. Do not over do it... just enough to create a good reliable electrical connection, EVERYWHERE.

Make certain your CAT is NOT blocked full of dirt... if you have one.
EGR must not be clogged, if you have one.

As you carefully inspect and rebuild these parts, you'll learn why everything matters, all the time.

If you exhaust this list, you may earn the respect of every TBI builder on this site, so get started with the cheapest things first.

The thing what cause the space shuttle to go down... O-rings.
It really is Rocket Science, after all.

Good Luck!

 

[RanchWelder]

After you rule out this list, we can consider the cam profile mystery.
You have to confirm these things first, or you are wasting everyone's time...
By establishing a base line, we can move forward confidently.
If you need to cut a hole in the bed, instead of dropping the fuel tank, do NOT burn yourself up. If your seals are bad, you are leaking fuel vapors.
You will see fuel stains on the top of your tank, where they have been leaking...

Try not to hurt yourself or break old plastic connectors, at this point.

The Welder comes out every once in while...

 

[RanchWelder]

Have you replaced the Brake Booster vacuum valve? It needs to be from a quality manufacturer or it will suck. (*Literally... vac leak...)
How old is your Booster... If it's new, disregard the contrarian remarks and move on...

Is your Fuel Canister vacuum port cracked or broken?
They have a rubber diaphragm which goes around the exact same time as the brake booster rubber diaphragm, the EGR, the vac advance, the rubber hoses...
If it has rubber in it, and it's anywhere near original, it is ****. All those little rubber elbows are ****. The pvc 1/8" tubing is ALL ****. Your TBI ports may be clogged with ****... Your MAP may be off a few points and if it is NOT GM or ACDelco, IT IS ****... Plug wires, ACDelco or they are most likely low dollar 8mm ****... Ignition Module, GM or ACDelco or Standard Motor Products ONLY, or else... you get the program...

These old brittle canisters are famous for cracking at the hose connection, just enough to fool you and cause a vac leak... Some are clogged and will screw with emissions due to contamination. They run $100 new and almost ALL for sale as USED are junk... If you require emissions, you'll need this item new, not used or you'll be chasing your tail and fail with a fleabat used canister.

The 3 wire O2 mod means you wire the ground directly to the ground rail with 100% ground DIRECTLY, as is humanly possible, to the ECM... NOT through the grounding of the 1 wire screw threads with carbon contamination, through the headers or manifold to block... NOT behind a gasket and header BOLTS WITH permatex or ANTI-SEIZE blocking your ground...

Any water temp sensor with permatex or teflon is a FAIL. How can it possbly ground with insulation blocking it to ground... ????????????????????????????????????????

See Grounding instructions above ^^^.
When you actually verify, confirm, replace every ground, all the rest becomes test-able... including the ominous ****** ground for the 1 wire O2...and the Temp sensor.

3 Wire O2 sounds like a pain in the ass, after buying the WRONG O2, however, please re-think your position.
Next time, ask before you buy anything new. If you buy sub standard parts, you will hate life and cause more problems than you solve.
Teflon works well when it is NOT causing a ground failure... You cannot count on water flow for ground with corrosion in the coolant system...
Do you use DISTILLED water in your cooling system or tap water? (Your battery like distilled too).
Once the wrong TAP water is used it contaminates and MIGHT create an anti ground film throughout the cooling system blocking ground to sensors.

Most aftermarket parts are cheap because they suck. Buy a known name brand and correct all your grounds, please.

SELF TAX IS THE WORST TAX. We have all bought the wrong parts or from the wrong manufacturer... or bought **** because we screwed up on research...
If you coated your 1 WIRE O2 threads with anti seize, you may have accidentally contaminated the sensor and/or blocked your ground with sealant.
If the angle is wrong, it's full of carbon... again. Straight down or you get clogged...
The 3 wire CAN BE SEALED with anti-seize, because the heater/sensor module grounds through the wiring, not the threads... so your ecm can see it.

Ground problems are 99% cause of needless sensor replacements by new mechanics, throwing money at parts, instead of KNOWING the ground is good and wasting everybody's time with diagnosis trauma. Vacuum= Check. Ground= Check. Now we search for gremlins... When in doubt, RE GROUND EVERYTHING TWICE.

See my SIG pics from my welcome page for my power distribution block. 600 amps from SkyCraft Orlando... Mounted on 3/8" rubber and covered with tire tube splash protector... it is crude and works flawlessly... 3/8" studs, crimped copper lugs, unified ground in parallel. ICE CUBE FUSES...
Fly Back on every relay, doors, windows, headlights... Fat copper wires everywhere... Research flyback diodes, Zener Diodes and flyback resistors.
Resist, relay and flyback: every square body switch...
Unified ground....

Your ground plane will heal everything over time if you fix everything meticulously.

-RanchWelder Jr



Good Luck!