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Discussion Starter · #1 ·
Hope someone can help, at my wits end on this one. I have a 1989 Chevy Silverado 4x4 350 TBI. The first start requires that you put your foot on the accelerator, just enough to get it to start, and then hold it there for about a minute or two while it warms up. It will not start without pushing the accelerator. After it starts and runs for a few minutes, the truck runs PERFECT. No issues at all. It is right at 100k miles. I have replaced all sensors….TPS, MAP, IAC, Temp, fuel pressure regulator, PVC, TBI gaskets, as well as the fuel pump, entire distributor, pick up coils, ignition module, plugs and wires and timing.
It seems the “cold start” is not kicking in and I don’t have high idle for any period of time.
Once the truck is warm, starts easily without pressing the accelerator, runs perfect, no stalling or erratic idle.
What am I missing?
 

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Even though you replaced all those parts, you need to check your fuel pressure with a gauge, and then put a scanner on it to see if your ECM is seeing proper CTS, MAP, IAC, etc. When you mentioned "timing", I take it you set your base timing to zero with the (Black Tan) EST bypass wire disconnected. Usually inaccurate CTS feedback will cause this but, who knows.

I hope you used quality parts and not cheap replacements. Cheap parts can/will exacerbate your issue(s).
 

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I've only had a couple of vehicles with TBI and I only recall having one problem with then, so I don't have a lot of experience.
But lets back up.
Carbs used a choke for cold temp starting, most chokes used a bimetal spring to set the choke. You had to press the gas pedal, this moved a latch that let the bimetal spring move the choke into position. There was a metal tube that was used to bring heat from the exhaust manifold to the bimetal spring to warm the metal.

So the TBI, I don't recall how the choke operated, I remember there was one (well something like a choke) and I remember there was a coolant tube that brought warm engine coolant to the TB to take it off choke.
The TBI operates like all injection systems, the engine computer uses a temp sensor to decide the amount of fuel to be delivered to the intake.

So, a cold engine runs a little rich regardless of the type of delivery system.

What I don't recall is how the TBI raises the idle speed when cold.
Carbs use a throttle cable (mechanical linkage), direct injection uses electrical linkage, and someone please correct me, I think the TBI used mechanical linkage.

If yours uses mechanical linkage, check the adjustments.
 

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Ok try this. Like any throttle body (wet, or dry) there is an Idle Air Control valve, yours may need to be reset.
Key off, you need to short pins A and B on the OBD connector. Use a paper clip opened up to give you the 2 wire ends, bend the wire so the clip is a U shape with the 2 ends about equal length.
Insert the 2 ends into the A and the B sockets.
Turn on the key, don't start the engine.
Give it a minute, or two, turn off the key and remove the jumper from the OBD port.
See if there is any improvement.

Sorry, forgot the picture:

Font Line Material property Parallel Screenshot
 

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So the TBI, I don't recall how the choke operated, I remember there was one (well something like a choke) and I remember there was a coolant tube that brought warm engine coolant to the TB to take it off choke.
Yes there's a vacuum actuated throttle control pot on the 454s but not on the 350s or smaller, they're controlled strictly by the ECM. There is a "stove pipe" coming off the right side exhaust manifold helping to heat the intake air, somewhat. The "Choke" routine is in the .bin (or calibration) of the ECM which will add fuel and IAC counts, to raise RPMs, based on feedback from the CTS, TPS, and MAP sensors. It also has a "Stall Saver" routine where, if your RPMs drop below a certain threshold, it'll open up the IAC to compensate.

Key off, you need to short pins A and B on the OBD connector. Use a paper clip opened up to give you the 2 wire ends, bend the wire so the clip is a U shape with the 2 ends about equal length.
Insert the 2 ends into the A and the B sockets.
Turn on the key, don't start the engine.
Give it a minute, or two, turn off the key and remove the jumper from the OBD port.
See if there is any improvement.
Yes, only with KOEO after the IAC closes all the way, you disconnect the IAC BEFORE turning off the key. Otherwise it'll reset to its "parked position" somewhere ~145 counts. This is for setting your base idle with the minimum idle screw on the Throttle Body. FWIU it shouldn't effect cold weather starts.

I'm wondering if there's a vacuum leak or faulty CTS. Maybe wiring, connections or bad ECM (which is very rare). Anyway, a scanner is your friend!

EDIT: I changed AIC to IAC (Idle Air Control) - my bad!
 

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Discussion Starter · #8 ·
Even though you replaced all those parts, you need to check your fuel pressure with a gauge, and then put a scanner on it to see if your ECM is seeing proper CTS, MAP, IAC, etc. When you mentioned "timing", I take it you set your base timing to zero with the (Black Tan) EST bypass wire disconnected. Usually inaccurate CTS feedback will cause this but, who knows.

I hope you used quality parts and not cheap replacements. Cheap parts can/will exacerbate your issue(s).
CTS, MAP and IAC were replaced with Delco parts from autozone. I suppose one of them could be bad but it runs so good once it’s warm….. The timing and the distributor replacement I had done at a shop, so I’m not sure of the procedure used. The CTS was replaced yesterday, it’s the newest of all the parts but all of them are less then 60 days old.
 

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Discussion Starter · #9 ·
Ok try this. Like any throttle body (wet, or dry) there is an Idle Air Control valve, yours may need to be reset.
Key off, you need to short pins A and B on the OBD connector. Use a paper clip opened up to give you the 2 wire ends, bend the wire so the clip is a U shape with the 2 ends about equal length.
Insert the 2 ends into the A and the B sockets.
Turn on the key, don't start the engine.
Give it a minute, or two, turn off the key and remove the jumper from the OBD port.
See if there is any improvement.

Sorry, forgot the picture:

View attachment 171282
I have done this several times but not since I replaced the CTS. Do you think I should try it again after I‘ve replaced it yesterday?
 

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Discussion Starter · #10 ·
Yes there's a vacuum actuated throttle control pot on the 454s but not on the 350s or smaller, they're controlled strictly by the ECM. There is a "stove pipe" coming off the right side exhaust manifold helping to heat the intake air, somewhat. The "Choke" routine is in the .bin (or calibration) of the ECM which will add fuel and IAC counts, to raise RPMs, based on feedback from the CTS, TPS, and MAP sensors. It also has a "Stall Saver" routine where, if your RPMs drop below a certain threshold, it'll open up the IAC to compensate.


Yes, only with KOEO after the IAC closes all the way, you disconnect the IAC BEFORE turning off the key. Otherwise it'll reset to its "parked position" somewhere ~145 counts. This is for setting your base idle with the minimum idle screw on the Throttle Body. FWIU it shouldn't effect cold weather starts.

I'm wondering if there's a vacuum leak or faulty CTS. Maybe wiring, connections or bad ECM (which is very rare). Anyway, a scanner is your friend!

EDIT: I changed AIC to IAC (Idle Air Control) - my bad!
I don’t have this type of scanner myself but I had a shop scan and everything came back good, according to them. They had the truck for 2 weeks and were baffled. They recommended I take it somewhere else. They have been my go-to shop when I can’t figure things out myself and haven’t failed me in the past.
I‘ve visually inspected the vacuum lines and they look good, would you recommend replacing them just to be sure? Or is there one in particular that controls cold weather starting I should replace?
 

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I‘ve visually inspected the vacuum lines and they look good, would you recommend replacing them just to be sure? Or is there one in particular that controls cold weather starting I should replace?
To find a vacuum leak I use an unlit propane torch and carefully blow it around the base of the throttle body, intake manifold, vacuum hoses, etc. And listen for the change in idle. YMMV
 

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Verify your fuel pressure. It could be the pressure regulator in the TBI.
 
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