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Old 11-30-2010, 10:45 AM   #1
MotorHead Lon
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90 350 TBI wont idle down.

I have a junkyard fresh 350 in my 90 Silverado. When I put the "new" engine in I swapped my TBI unit the sensors, harness and everything else to the junkyard engine. When I bought the truck it wouldnt idle down when stopping and putting the truck in neutral or between gear changes. The new engine acts the same, so I'm guessing I have a bad sensor. I have had the Check Engine Light come and found a faulty temp sensor, I replaced that and a month later, while a friend was running diagnostics found the temp sensor had failed again, it was reading too hot. I have replaced the temp sensor again, and the truck still idles too high or wont idle down. Other than the temp sensor we found no other codes.

When I swapped the new engine I left the smog pump off and the truck had the catalytic converters removed. Recently sitting at a fast food drive through the truck idled down but I could smell the truck running rich.

Any help puzzling this out is greatly appreciated.
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Old 11-30-2010, 02:14 PM   #2
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Re: 90 350 TBI wont idle down.

More specific information is needed.

Which coolant sensor code or codes were stored? What number?

Does it still set the trouble code when it is driven?

Did you change the ECM?

Is the ECM in the truck the original?

Did the truck have a 350 in it before the swap?

Have you checked for vacuum leaks?

Have you tried pinching off the vacuum hose to the brake booster?
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Old 11-30-2010, 02:16 PM   #3
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Re: 90 350 TBI wont idle down.

Have you tried blocking the IAC port?
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Old 11-30-2010, 02:29 PM   #4
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Re: 90 350 TBI wont idle down.

I dont remember the code, I think I have it written down I will look for it. The first engine was a 350, I did not change the ECM. I used the junkyard engine's intake manifold, and it looked like it had never been off. I have checked for vac leaks and found none.

After swapping engines I did install an open element air cleaner, however the truck ran the same with or without an enclosed aircleaner.

The truck is a factory 350 V-8, with a 5 speed manual transmission. I havent messed with the IAC.

Last edited by MotorHead Lon; 11-30-2010 at 02:31 PM.
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Old 11-30-2010, 02:59 PM   #5
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Re: 90 350 TBI wont idle down.

Check to see what codes are in it now.



Here are some web sites with the basics on retrieving codes, on OBD1 systems, without a scan tool or code reader.

Poor drawing of ALDL but ok instruction + codes
http://www.cfm-tech.com/gm_tbi_trouble_codes.htm

OBDI terminal identification
http://robertpowersmotorsports.frees.../ALDL_plug.jpg

OBDI illustration and instructions
http://www.troublecodes.net/GM/

Good explanation
http://www.syty.org/old/d&e.html
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Old 11-30-2010, 03:11 PM   #6
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Re: 90 350 TBI wont idle down.

I got a 13 code. I left the paper clip in for a minute to see if anything else pops up and it just repeated the 13 code, no signal to computer from oxygen sensor.

Last edited by MotorHead Lon; 11-30-2010 at 03:13 PM.
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Old 11-30-2010, 03:14 PM   #7
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Re: 90 350 TBI wont idle down.

Did it blink out a code 12 before and after the code 13?
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Old 11-30-2010, 03:15 PM   #8
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Re: 90 350 TBI wont idle down.

Yes. Also the shift light blinked on for a second.
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Old 11-30-2010, 03:20 PM   #9
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Re: 90 350 TBI wont idle down.

Look at the wire to the O2 sensor and make sure it is plugged in all the way and did not get burnt off or cut.
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Old 11-30-2010, 03:24 PM   #10
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Re: 90 350 TBI wont idle down.

I'll look at before I leave. Thanks for the help and info, I'll post my progress if I make any.
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Old 11-30-2010, 03:28 PM   #11
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Re: 90 350 TBI wont idle down.

You’re welcome!

I will check back to see if you have new posts on the board later tonight.
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Old 11-30-2010, 03:37 PM   #12
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Re: 90 350 TBI wont idle down.

Thanks again. I probably wont be on again until Thursday.
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Old 03-26-2011, 07:41 PM   #13
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Re: 90 350 TBI wont idle down.

I have had that issue before and it was the fact I had the rong TPS in it.
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