At my wits end with high idle issues.

[Art z]

I'mma start by saying that this is my first time ever owning a TBI truck and so far I'm not having a good time with it. I've had both prior gen carbed small blocks and the 5.3 and later lq and ls based small blocks and both have been much more complacent and easy to fix.

Also, before making this thread I did a keyword search and read a few threads and didn't find anything like the issues I'm having.

So here we go.

Truck: 1995 C1500 Silverado W/ 5.7L TBI engine and 4L60e.

What's been done:

Bought the truck with a clogged cooling system running straight water and a blown head gasket.
- Took off both heads and had a valve job done on them and also inspected for cracks.
-used a fel-pro head gasket kit and used new head bolts that were tightened following instructions from a Haynes Repair manual.
-had the intake manifold pressure cleaned and sand blasted and the coolant and egr passages cleaned.

What's going on:

The truck refuses to idle where its supposed to.

Right now it's idling at +/- 1,000 rpm. Since idling high, the transmission has also been shifting much higher in the RPM range. Sometimes the RPM will also rise and drop a bit and then settle at 1k again.

One night, I started it and it was suddenly idling right and it drove and shifted flawlessly. Turned it off and back on and it went back to not idling or shifting right, so I think the late shifting and idling high are related.

I've spent HOURS on forums searching through threads looking for answers and almost always end up with people pointing at the IAC valve or the TBI base plate gasket.

Following the advice of one of these threads I removed the IAC valve and plugged the hole... The truck would not start. I unplugged the hole and let it start and as expected it idled super high. Then, while running I plugged the hole and it idled SUPER low, like at 450-500, which the thread said it was supposed to.

I've changed the IAC three times. Two times with a YCT unit and this last time with an ACdelco one. Still the truck idles at 1k rpm.

I've checked ignition timing with a timing light and its right on the money.

The TBI base plate gasket is brand new as it came with my gasket set.

Also the second time I changed the IAC valve with a YCT unit, the truck idled even higher at 1,500 rpm and I could hear a loud vacuum sound coming from inside the carb. That's why i ditched YCT and went with ACdelco, but still the truck wont idle right.

My Haynes repair manual has nothing on high idle, so I have no idea what else to do.

I've clamped down on the brake booster and crankcase ventilation hoses and it's had no effect on the idle speed. I've checked the vacuum hoses for cracks and haven't found any, but I doubt it's a vacuum leak because of that one night everything worked perfect. A vacuum leak wouldn't just go on and off with an ignition switch cycle.

Any help will be appreciated because I don't know what else to do.

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Sound like it maybe your Throttle positioning sensor it is on the passenger side of your TBI and is held on by 2 or 3 small bolts. I had one of my truck do this same thing and that is what it was. So I would try that next

 

[deve05]

The disc has drivers for your USB interface (which windows 10 already has), not programming software.


94 & 95 were transitional years in which some used 12 pin OBD1 or 16 pin OBD2 connector but were still OBD1, 96 and up were the 1st generation of truly OBD2. Either way, this is only an interface for a programming tool such as TP.

I know i DEFINITELY do not have an OBD2 style connector because when I first got the truck it had a MIL on because the previous owner disconnected the MAP sensor and put a screw in the vacuum line that goes to it..... Yeah, idiot I know. The guy was selling it for cheap due to "misfire and stalling". I wonder why....

Anyways back on subject, I tried connecting my obd2 scanner and found out it wouldn't hook up because the connector is different. Thats when I realised it was an OBD1 truck, not obd2.

This is the connector style attached. So I'm assuming 12 pin??

You must be registered for see images attach

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[PlayingWithTBI]

Anyways back on subject, I tried connecting my obd2 scanner and found out it wouldn't hook up because the connector is different. Thats when I realised it was an OBD1 truck, not obd2.

This is the connector style attached. So I'm assuming 12 pin??

Yes, that's a 12 pin connector. There were no OBD2 trucks until 96 just some pre 96 trucks had OBD2 connectors with OBD1 coms.

 

[deve05]

Thank you to everyone that chipped in! I learned a lot about TBI from you guys in this post alone. Definitely gonna be investing in the diag tools for this gen truck.

However, since I've been very busy with house construction lately and hadn't had time to work on it, I decided to wing it and just follow the gut of some of you guys and ordered a TPS. Changed it out today in a couple of mins and now everything works perfectly. The truck idles right were its supposed to on closed loop, holds a higher rpm on open loop, and the transmission is back to shifting at all the right points. Thank you all so much for the help. Hopefully this post will help some other noob to TBI.

Sent from my moto g(6) plus using Tapatalk

 

[bobby v]

Thank you to everyone that chipped in! I learned a lot about TBI from you guys in this post alone. Definitely gonna be investing in the diag tools for this gen truck.

However, since I've been very busy with house construction lately and hadn't had time to work on it, I decided to wing it and just follow the gut of some of you guys and ordered a TPS. Changed it out today in a couple of mins and now everything works perfectly. The truck idles right were its supposed to on closed loop, holds a higher rpm on open loop, and the transmission is back to shifting at all the right points. Thank you all so much for the help. Hopefully this post will help some other noob to TBI.

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That's great, I wish I could say that, maybe someday
Bobby V

 

[PlayingWithTBI]

Ditto

 

[PlayingWithTBI]

I've been reading and posting some on other threads here and other forums. Here's some things that stand out as far as high idle issues.

Ensure your throttle and cruise control cables are free moving and allow the blades to close all the way to the low idle stop.

Check your fuel pressure at idle - s/b 9-13psi preferably 11-13psi (94-95 7.4 s/b 25-30) and injectors are spraying in an even cone with no drips.

The minimum idle has to be calibrated to 125RPM below desired speed or your IAC may be toast.
TBI tuning-website - Tuning the TBI.pdf

The TPS needs to be calibrated so it reads .58 - .59 volts (as in the TBI calibrating instructions posted above and GearHeadEFI forum), and smoothly climb to over ~4.5 at WOT.
http://www.gearhead-efi.com/Fuel-In...-TBI-with-High-Idle-in-PARK&p=76993#post76993

In the 7747 ECM check your system grounds on pins D1 & A12 connectors and sensor grounds (TPS & CTS) D2 & A11 (from a GM service manual) other ECMs/PCMs have different corresponding pins.
http://www.gearhead-efi.com/gearhead-efi/wiring/747ecm7.jpg

Just changing parts and not calibrating them probably won't help much. HTH

 

[robert lowman]

Id recommend something very simple. Replace the throttle position sensor. Use a ac delco one

 

[Darrell]

My issue was IAC and new vacuum tubing to the MAP

btw... Once I replaced my IAC with an AC Delco part that pretty much fixed it.. After 2 aftermarket cheap-o's !

$20 difference in price, but now I know why.

 

[PlayingWithTBI]

Id recommend something very simple. Replace the throttle position sensor. Use a ac delco one

Hmmm - upon what did you base your conclusion? Do you know something we don't? If you're getting proper feedback from a DVM, scan tool or data logging, why would you replace it?

My issue was IAC and new vacuum tubing to the MAP

Congrats! Did you calibrate it after the new parts? Your success should be helpful for others.