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1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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Jorwood
Novice
Sep 13, 2016, 12:47 PM
Post #1 of 13
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1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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I have a 1990 K5 blazer with the TBI 350/700R with 174k miles. Recently the car started surging/running rough at low RPM to the point that I stopped using overdrive. I also noticed it being rough just idling forward. I had been getting a code since I had redid the head gaskets intermittently and finally decided to check on it. It turned out I never reinstalled the vacuum line to the EGR valve. Upon reinstallation it seemed to fix everything. Then the motor started loping like it had a big cam with a low idle while I was coasting to a stop. I noticed it doing this as lightly drove forward in a parking lot as well. It never does this just at idle, only while moving. It's also fairly inconsistant as I can't make it do it everytime. I suspect it has something to do with the air idle control valve. I took it off but it doesn't look particularly dirty. I'm not sure how to clean/test it and I don't want to just throw $50 at a new one until further diagnosis... Also I has since started running rough at low RPM again and it doesn't sound very healthy...
Thanks
Jordan
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Discretesignals
Ultimate Carjunky
/ Moderator
Sep 13, 2016, 12:59 PM
Post #2 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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Is it throwing any trouble codes now? If so, what are the numbers?
If you disconnect the vacuum line from the EGR valve and plug the end of the line and drive the vehicle, does your symptoms go away? Idle air control valves usually don't cause idle surging on that design.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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kev2
Veteran
Sep 13, 2016, 1:55 PM
Post #3 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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Agree with colleague - EGR.
do not eliminate it if it is bad.
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Jorwood
Novice
Sep 13, 2016, 8:54 PM
Post #4 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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Well I just reinstalled the IAC and it won't even idle without a little throttle and sometimes it will idle but BARELY I mean I couldn't even hear it at first. It does the loping thing even just sitting now but sometimes it's just smooth. On the drive home it felt like power was starting to cut out... It did throw a code; 33 - MAP (manifold absolute pressure) sensor circuit high (low vacuum)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Sep 14, 2016, 2:39 AM
Post #5 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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? I think it will throw that code for any reason it isn't idling properly. Codes IMO for this with a grain of salt if you will.
Quote from top post ">>I had been getting a code since I had redid the head gaskets intermittently and finally decided to check on it<<"
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So how did that go? A lot of stuff gets moved and what was your reason to blame head gaskets to begin with? Did you send heads out to be surfaced or at least checked out?
Do you have a good clue of general engine condition to begin with? IDK - think I'd recheck work already done and all vacuum areas for leaks then a cylinder balance test on to compression test to narrow down the search for why,
T
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Jorwood
Novice
Sep 14, 2016, 9:28 AM
Post #6 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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Heads were done because it was overheating and pouring white smoke... I installed rebuilt heads and everything was going pretty well until one day it started surging in overdrive.
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Jorwood
Novice
Sep 14, 2016, 10:01 AM
Post #7 of 13
(2078 views)
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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I'll also mention that I have been driving the car for 6 months since the heads were redone with no issues outside of the code 32 coming up for the EGR while on highway runs where it woud test the EGR system... I miss those days back before I reinstalled the EGR line lol
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Tom Greenleaf
Ultimate Carjunky
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Sep 14, 2016, 10:02 AM
Post #8 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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IDK - new if perfect heads on an old lower end can be a battle of one great and one worn and now don't match well. Fine, just plain cylinder balance if possible as it isn't always idling well so next is compression - see if you have a lower end problem. I'd also check valve timing if mentioned sorry I missed that but the miles suggest chains have had it.
Nothing new to me - I frequently pulled chains off without taking the gears off at less miles than this - unfortunately you are 100% sure till looking right at it so if there and not a lower end issue demonstrated just do it - not the world on this,
T
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Jorwood
Novice
Sep 14, 2016, 2:43 PM
Post #9 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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Hmmm I would like to look into other possibilities before going so deep into the engine...
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kev2
Veteran
Sep 14, 2016, 3:07 PM
Post #10 of 13
(2070 views)
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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EGR is so simple on this eng series...
a vacuum line from TB goes to solenoid, ECM opens and closes solenoid, the solenoid sends vacuum when commanded to EGR valve.
So lets check the vac line TB to solenoid and cap/plug it there.
remove EGR and check pintle as well as diaphragm - mouth vacuum should move pintle...smooth opening and not leaking EGR diaphragm,
Another vacuum leak prone area was under the TB esp if there was a backfire...
TB= throttle body- mentioning that TB - always check the injector wires where they pass thru/under air cleaner the wires often pinch and ground out...
only other suggestions checking for vac leaks - use the old school spray trick...
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Jorwood
Novice
Sep 14, 2016, 3:17 PM
Post #11 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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My current issue seems to be that the IAC won't reset... when I took it off I tried starting it while it wasn't installed to see if it was working/moving but it made a godaweful loud sucking sound so I cut the engine after a few seconds... now it wont idle. Any tips on how to reset this thing?
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Hammer Time
Ultimate Carjunky
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Sep 14, 2016, 4:43 PM
Post #12 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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If you had that IAC plugged in when you started the engine, you probably overextended it and will likely have t replace it now.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Discretesignals
Ultimate Carjunky
/ Moderator
Sep 15, 2016, 5:44 PM
Post #13 of 13
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Re: 1990 K5 TBI rough/loping idle only while coasting or light throttle sometimes
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That made me laugh, cause when I was a newbie I thought I could test a stepper motor IAC by plugging it in with it out of the bore. The pintle went shooting across the shop and there was no way getting it back in.
Would be interesting to see what the MAP sensor voltage is with it running after getting the IAC fixed of course.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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