Need help sorting out no idle on new 5.7 TBI engine

Charlie, check the CST by removing the wire connector and probing the sensor terminals with a multimeter set on OHMS. At 210 degrees F. You should show 185 OHMS. If it's higher your sensor is bad. Download the Driveability and emissions manual that's been stickied in one of the forums, I forget which one. If your CST wire connector look's like it's seen better day's replace it too. Check your grounds at that front stud that's right by it also. I believe the ECM is grounded to it.

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The coolant temp sensor(CTS) has much more authority on the fueling tables than it should on many OBDI vehicles(TBI falls in this category). If you have a bad CTS, it can be telling the computer that the engine is at a ridiculously cold temperature and will cause the computer to dump tons of fuel in to achieve the same effect as a choke on a carbureted car. I had this issue on an OBDI Honda and it is apparently a pretty common failure mode on these year range vehicles. OBDII has better checks on reasonable sensor ranges and also doesn't give the CTS as much authority over the fueling tables, so you don't usually run into this issue or you get a sensor fault code on LS or Vortec engines.

 

Shoot im nobody special, just a young guy (26) that knows a tiny piece of the pie!


If youre getting a stinky exhaust that might be your CTS

Tps should show a smooth transition in resistance values (i dont remember what they were)

Like i said before theres a proceedure you can do with your multimeter to check the CTS and IAC voltage and resistance values to verify their integrity.

Your ignition module can also be taken to a auto parts store to be verified..

Just a thought

 

Heat that ignition module up before you test it. They fail when they are warmed up. The module could be bad and still pass if tested cold.

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UPDATE; First my appreciation to all you that contributed your knowledge here. I ran out of ideas and the engine was still running terrible, so i took it to a shop. They discovered the valves were too tight! When I received the long block, it had a big red tag that said "CAUTION; this engine is equipped with adjustable valve train. Check and adjust valve preload if necessary when engine is installed". I did, and i thought the valves were too tight so i loosened them a bit. Well, the important thing is that the engine is now running smooth as silk. I checked the timing, reset IAC again and adjusted idle. I was going to take it out for a spin but as soon as i put it in Drive the engine dies. same in reverse. i am going to check the P/N switch to make sure it is dialed correctly. Hopefully if it is a little warmer tomorrow i fix that. thank you again!

 

Yay!

 

Check your TPS. Should show about 1/2 a volt when at idle. I replaced mine because it went from .68 to 1.28 as the accelerator pedal was pressed slowly with the motor off but the key was on. My truck runs great now. I was able to use a scan tool on mine but you can do it with a multimeter. Just unplug the TPS wire connector and probe the sensor while operating the throttle. The voltage should stay consistent without gap's all the way to full throttle.

Clayton

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thank you Chevyguy, i have checked the TPS and the CTS, both are operating in the proper range.
Since GM does not want people messing with the idle speed, there are no official procedures in the manuals to do that. My question is, once you reset the IAC to fully closed position, disconnect it, turn the engine on, do you set that base idle with the trans in Park? or in Drive? i set it with the trans in Park, and it runs like a Swiss watch, as long as i leave it in Park (or Neutral), but it dies immediately when put in Drive or Reverse. it also died when i turned the AC on, which tells me it is not the P/N switch

 

Set it in drive to between 575-600 rpm's. Set your parking brake and block the wheels so the truck won't roll. Ground A-B terminals on the OBD-1 plug under your dash like you do to read codes. Turn key on but don't start for 30 seconds then turn off and unground the A-B terminals in the OBD-1 plug. Under the hood unplug the IAC wire plug connector. Doing this keeps the computer from adjusting IAC which is now in the parked position. Restart your engine and set your base idle in drive with it warmed up. In park your idle should be around 800 rpm. Turn off the engine and reconnect the IAC wire plug connector. Restart your engine, your idle should be within factory parameters. You may have to drive the truck so the computer can relearn what it needs to. Clear any codes that may have occurred due to unplugging the IAC.

Clayton

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Ooooooh that should make a difference to set the base idle in Drive! thanks so much Clayton.
On a completely unrelated subject I notice you are in Eugene, many years ago i attended that "other" college 40-50 miles north of you, lived in Corvallis and spent a summer working in Astoria. I bought from a graduating student a '66 Mustang 2+2, 289 4 barrel no accessories and 4 speed for $700 (that was in 1975 ) and when i graduated in '77 i sold it for $800 and thought I was so smart! that thing would be worth a fortune today.

 

Your welcome Charlie, glad I could help out. I miss my 67 Camaro, and my 73 K5 Blazer. They were just old cars that ate alot of gas back then. I never figured they would gain the value that they have. Happy Holidays Charlie.

Clayton

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Your welcome Charlie, glad I could help out. I miss my 67 Camaro, and my 73 K5 Blazer. They were just old cars that ate alot of gas back then. I never figured they would gain the value that they have. Happy Holidays Charlie.

Clayton

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