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1993 Chevrolet Truck surging idle at all Temps

Tiny
THEROWEBOATS
  • MEMBER
  • 1993 CHEVROLET TRUCK
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 347,000 MILES
Ok here is my issue I have a 1993 k1500 with a 5.7 tbi when at idle it has a low to the idle. It flacuates between 350 and 1000 rpm. I has so far replaced distributor, cap, rotor, plugs, wires, cts, iac, o2 sensor, map sensor and tps. I have checked and cleaned all of the grounds, reset the timing, check fuel pressure, checked vacuum and checked for vacuum leaks. I have no clue where to go from here any help would be greatly appreciated.
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Thursday, February 5th, 2015 AT 8:08 AM

11 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Make sure vacuum line going ot map sensor isn't soft an sucking together. Check bottom of tbi unit for a vacuum leak with choke cleaner they like to blow the gasket also fuel pressure should be 9-11 psi if lower then you may need a pump or filter. Replace the ect sensor if still a problem then scan for codes.
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Thursday, February 5th, 2015 AT 12:19 PM
Tiny
THEROWEBOATS
  • MEMBER
  • 5 POSTS
Fuel pressure is at 11 psi. Checked the tbi gasket with throttle body cleaner and showed no leaks. Checked the vacuum line to the map sensor and verified that it is not soft and hold vacuum. Already replaced the coolant temp sensor with the 2 wire plug next to the tstat. Tried a new ecm and checked code and the only code that comes up is the 12 code. Any other ideas would be greatly appreciated
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Friday, February 6th, 2015 AT 10:19 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Could be canister purge is opening at idle when it shouldn't be check to seeif it is or not the solenoid may be bad it has a red connector and if I remember right will be under hood area. You can block off the vacuum line and if the surging stops you found your problem. Then check for blocked exhaust if you read 15" or below of vacuum @ idle then suspect that. It also may be silicone contamination on O2 sensor which cause ecm to give a false reading so it makes the engine do what it is. It also may be the wiring going to TCC or dist for bypass is shorted. Other than that no other ideas
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Friday, February 6th, 2015 AT 12:04 PM
Tiny
THEROWEBOATS
  • MEMBER
  • 5 POSTS
I plugged all but the map sensor vacuum and that made no difference. It is getting 20" of vacuum. What are you referring to as the tcc? The distributor is new and the bypass you are talking about is the tan and black wire correct.
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Friday, February 6th, 2015 AT 12:10 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
The canister purge is not themap sensor it normally has a long vacuum hose an dit sits by the fender on the frame block that line and yes you are correct about the by pass
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Friday, February 6th, 2015 AT 12:58 PM
Tiny
THEROWEBOATS
  • MEMBER
  • 5 POSTS
Ok so I plugged that line off when I did the vacuum leak test and it still ran the same. When I did the vacuum leak test the only thing that was still attached that had anything to do with vacuum is the map sensor. Could it be a weak ignition coil or bad injectors? Other information that might help I tried to unplug one sensor at a time to see if it would make a difference. When I unplug the TPS the engine runs good for about 45 seconds and then starts to surg again I went through each sensor and everyone did the same thing besides the IAC it smoothed out and ran good. The IAC is brand new and I just cleaned the throttle body and IAC port. Does that make any sense to you.
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Friday, February 6th, 2015 AT 2:50 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
After you installed the iac I hope you drove the truck because for that to reset to the correct position you have to drive 40 mph then see what happens. From everything iv'e told you I can't think of anything that would make it do what it is besides what I've given you.
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Friday, February 6th, 2015 AT 3:04 PM
Tiny
THEROWEBOATS
  • MEMBER
  • 5 POSTS
Ended up being faulty plugs and wires. Thanks for your help
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Saturday, February 7th, 2015 AT 12:18 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Thanks for letting us know many people don't.
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Saturday, February 7th, 2015 AT 1:55 PM
Tiny
JOHNSMITH777
  • MEMBER
  • 1 POST
I have the same problem but I have replaced plug and plug wires, complete dist and coil. Plus all sensors still poor idle shuts in gear.
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Monday, November 13th, 2023 AT 10:57 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,397 POSTS
It sounds like you have a vacuum leak, please check out these guides:

https://www.2carpros.com/articles/engine-surges

Please use this guide when the engine is cold only:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

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Monday, November 13th, 2023 AT 11:00 AM

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