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| Bad Idle after warmup (Page 1/2) |
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Markeeise
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MAY 08, 11:47 AM
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Hello everyone, first time on the Fiero forum, 1988 Fiero GT 2.8L, When I first start the car it idles very good at 1500 and the engine runs very smoothly. As the engine begins to warm up and idle drops down to 900, idle begin to get rough and the engine starts shaking and vibrating a lot more, you can feel the whole car vibrating even in the steering wheel, it is almost like it's missing. This is what has been replaced, IAC, air filter, sparkplugs, distributor cap and rotor, oxygen sensor, temperature sensor, lower and upper intake gaskets, the EGR tube has also been tested working and cleaned out, pretty sure there are no vacuum leaks. At this point not sure what could be the problem. Thanks.
1988 Fiero GT 5 speed manual[This message has been edited by Markeeise (edited 05-08-2020).]
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fierosound
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MAY 08, 12:06 PM
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Reset the ECM to force it to relearn.
Near the battery disconnect the ECM power connector for 5 seconds then reconnect.
Start engine and let it idle for 5 minutes, then take it for 20 minute drive.
If "Check Engine" light comes on, check the codes.
Heres how: http://www.fiero.nl/cgi-bin/main.cgi?ECMCodes
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3.4L Supercharged 87 GT and Super Duty 4 Indy #163[This message has been edited by fierosound (edited 05-08-2020).]
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Markeeise
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MAY 08, 12:15 PM
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| quote | Originally posted by fierosound:
Reset the ECM to force it to relearn.
Near the battery disconnect the ECM power connector for 5 seconds then reconnect.
Start engine and let it idle for 5 minutes, then take it for 20 minute drive.
If "Check Engine" light comes on, check the codes.
Heres how: http://www.fiero.nl/cgi-bin/main.cgi?ECMCodes
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Thank you, I will try that now and get back to you. Update, the car still seems to act the same way after resetting, when resetting does it have to be unplugged for exactly 5 secs or can it be more?[This message has been edited by Markeeise (edited 05-08-2020).]
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theogre
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MAY 08, 02:14 PM
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| quote | Originally posted by Markeeise:
Thank you, I will try that now and get back to you. Update, the car still seems to act the same way after resetting, when resetting does it have to be unplugged for exactly 5 secs or can it be more? |
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Can leave it off longer but likely do nothing.
"Resetting" by plugging plug or removing battery term just kill standby power to save codes and little else.
The "leaning" happens all of the time even w/ engine and O2 hot but ECM operates in Closed mode when O2 is hot.
"Resetting" the IAC is same but needs to drive > 35mph. Until that happens the ECM tries to guess to set IAC and if you have worked on IAC etc then guessing have problem.
Bad idle is often Not Engine problems but Electric problems.
If ECM just thinks have power/ground problems then will try to adjust idle etc. Need ECM Scanner to get ECM data stream that shows what ECM board sees.------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)
The Ogre's Fiero Cave
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fierosound
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MAY 08, 02:48 PM
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| quote | Originally posted by Markeeise:
Update, the car still seems to act the same way after resetting, when resetting does it have to be unplugged for exactly 5 secs or can it be more? |
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I'm guessing no Codes were set?
Oops. I pulled this from Factory Service Manual.
You may want to try it again.
Check wiring for good connections.
Unplug, replug IAC and TPS connectors a couple of times.
How fresh is the gas?
 [This message has been edited by fierosound (edited 05-08-2020).]
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Markeeise
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MAY 10, 11:09 AM
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| quote | Originally posted by fierosound:
I'm guessing no Codes were set?
Oops. I pulled this from Factory Service Manual.
You may want to try it again.
Check wiring for good connections.
Unplug, replug IAC and TPS connectors a couple of times.
How fresh is the gas?
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No codes were set, brand new fresh gas been driving it for 5 months now, might check the TPS and connections.
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Gall757
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MAY 10, 02:06 PM
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| quote | Originally posted by Markeeise:
brand new fresh gas
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does this correspond with the problem?
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fierosound
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MAY 10, 02:15 PM
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Just a bump so people know what we're trying to figure out...
| quote | Originally posted by Markeeise:
When I first start the car it idles very good at 1500 and the engine runs very smoothly.
As the engine begins to warm up and idle drops down to 900, idle begin to get rough and the engine starts shaking and vibrating a lot more,
you can feel the whole car vibrating even in the steering wheel, it is almost like it's missing. |
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So runs fine in when first started in "open loop" when ECM runs off of A/F tables.
Runs rough when engine warms up and goes into "closed loop" and ECM uses feedback from the O2 sensor and others to make corrections to the air fuel ratio.
You been driving for 5 months you say. Has the engine always done this?
Or did this problem start after changing all those sensors/parts you mentioned at the outset?[This message has been edited by fierosound (edited 05-10-2020).]
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Markeeise
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MAY 10, 06:39 PM
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Ever since I went to look at the car once it warmed up it seemed idle was always wandering around 1,100 - 800 and it could never keep smooth constant idle. Hoping that replacing a bunch of sensors would fix the issue but it didn't. But get this, sometimes while I am driving and the engine is hot and I come to a stop it idles perfect right at 900, then il accelerate again and the next stop I come to its rough idle again, it is so mind-boggling. I would like to learn and look into that "closed-loop" you were talking about, it seems if the issue lays there.
again thanks[This message has been edited by Markeeise (edited 05-10-2020).]
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Patrick
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MAY 10, 07:12 PM
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