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  2.5l 5 speed erratic idle, slight shudder while cruising town speed, and stalling.

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2.5l 5 speed erratic idle, slight shudder while cruising town speed, and stalling. by lukeeggers
Started on: 07-15-2017 11:15 AM
Replies: 12 (196 views)
Last post by: lukeeggers on 05-16-2019 10:21 PM
lukeeggers
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Report this Post07-15-2017 11:15 AM Click Here to See the Profile for lukeeggersSend a Private Message to lukeeggersEdit/Delete MessageReply w/QuoteDirect Link to This Post

I bought a fiero that had been sitting for 20 years, and have been restoring it. I have tried everything, but can't seem to get it to run right. Here's what happens. The car will start hard most of the time after it has sat over night then when driving at a constant speed will have a slight shudder. When coming to a stop sign as soon as I push the clutch in it will drop the rpm's to about 500 and often stall when it's cold. Sometimes after it gets warmed up it will fluctuate in its idle, sometimes it will be right around 1,000 rpm's, and other times it will sit at 500 rpm's. I have replaced the ecu, icm, tps, map, mat, fpr, injector, fuel pump, map, o2 sensor, egr valve, and probably more parts I am forgetting about. I am out of ideas on what to try next. No service engine light comes on.

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cebix
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Report this Post07-15-2017 12:14 PM Click Here to See the Profile for cebixSend a Private Message to cebixEdit/Delete MessageReply w/QuoteDirect Link to This Post

IAC? Vacuum leaks? Does you speedo work?

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lukeeggers
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Report this Post07-15-2017 12:27 PM Click Here to See the Profile for lukeeggersSend a Private Message to lukeeggersEdit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by cebix:

IAC? Vacuum leaks? Does you speedo work?


Speedo works well and I have thoroughly checked for vacuum leaks. Now the iac is an interesting topic. I attempted a replacement, and it made the car run far worse. Very high idle. Under closer inspection the spindle of the new iac was much longer. Is there a certain brand of iac I need? Maybe a difference in years I wasn't aware of?

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KurtAKX
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Report this Post07-15-2017 01:41 PM Click Here to See the Profile for KurtAKXSend a Private Message to KurtAKXEdit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by lukeeggers:


Speedo works well and I have thoroughly checked for vacuum leaks. Now the iac is an interesting topic. I attempted a replacement, and it made the car run far worse. Very high idle. Under closer inspection the spindle of the new iac was much longer. Is there a certain brand of iac I need? Maybe a difference in years I wasn't aware of?


The spindle moves in and out (when the IAC is functioning properly) but the shape of the cone is important- not all the tips are same.

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lukeeggers
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Report this Post07-15-2017 02:05 PM Click Here to See the Profile for lukeeggersSend a Private Message to lukeeggersEdit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by KurtAKX:


The spindle moves in and out (when the IAC is functioning properly) but the shape of the cone is important- not all the tips are same.


From what I remember the cone was the same shape, but the distance the spindle moved was different. The original one moved, but the new one moved much further.

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Patrick
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Report this Post07-15-2017 04:13 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by lukeeggers:

The original one moved...


I suspect it's probably okay then. Did you thoroughly clean out the IAC valve seat and air passage?

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cebix
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Report this Post07-15-2017 05:34 PM Click Here to See the Profile for cebixSend a Private Message to cebixEdit/Delete MessageReply w/QuoteDirect Link to This Post

How's your ignition aside from the ICM? I mean coil, cap, rotor, wires, plugs. Cat not stuck? EGR working properly?

[This message has been edited by cebix (edited 07-15-2017).]

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VOS1319
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Report this Post07-17-2017 02:33 PM Click Here to See the Profile for VOS1319Click Here to Email VOS1319Send a Private Message to VOS1319Edit/Delete MessageReply w/QuoteDirect Link to This Post

You didn't say what year your Fiero is! It certainly matters with the ignition parts, (rotoror/cap distributor or ignition module). My first Fiero was an '84 SE, my latest an '88 2.5 w/5spd. The only time I had a really rotten idle was on the '88 when the timimg control from the PROM went kerfluey and left the car to run at 0 degrees spark advance. I changed the IAC in my '84 when I probably didn't have to, and seem to recall some warnings about only installing the critter with the pintle/cone retracted, as the cone bottoms out on the air openning if extended and may be damaged when tightened. Idle should be 1000-1100 rpm warmed up. Most other drivabilty issues occured on acceleration, when missing was caused by everthing from bad plug wires to a bad EGR or a TPS. GOOD LUCK ! Victor

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lukeeggers
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Report this Post07-18-2017 07:15 PM Click Here to See the Profile for lukeeggersSend a Private Message to lukeeggersEdit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by Patrick:

I suspect it's probably okay then. Did you thoroughly clean out the IAC valve seat and air passage?


I replaced the IAC after some research and saw no change. Another point of interest is the MAT sensor. Somewhere between the ecm and the MAT sensor the temperature is reading wrong. Both the MAT sensor and the ECM have been replaced and have tested out okay. A local auto shop ran these tests for me and informed me about the misreading. Any ideas on what causes this?

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Patrick
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Report this Post07-18-2017 10:42 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by lukeeggers:

I replaced the IAC after some research and saw no change.


Ummm... that doesn't answer the question at all. Replacing the IAC valve doesn't do anything about the IAC valve seat and/or air passage.

 
quote
Originally posted by Patrick:

Did you thoroughly clean out the IAC valve seat and air passage?

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KurtAKX
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Report this Post07-19-2017 09:43 AM Click Here to See the Profile for KurtAKXSend a Private Message to KurtAKXEdit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by lukeeggers:


I replaced the IAC after some research and saw no change. Another point of interest is the MAT sensor. Somewhere between the ecm and the MAT sensor the temperature is reading wrong. Both the MAT sensor and the ECM have been replaced and have tested out okay. A local auto shop ran these tests for me and informed me about the misreading. Any ideas on what causes this?


Since 1984-86 don't have a MAT threaded into the intake, I'm going to assume you have an 87 or 88.

If I were you, I would not waste any time on anything else, until your MAT works correctly. (incorrectly inferring an intake temp of -40 is not good)
Do you own a multimeter?

If so, it's time to find the problem with the MAT circuit.
I have a (paper) copy of the 1987 service manual, but if you don't somebody on here created a searchable pdf of it.

I'd start with checking continuity from the MAT connector to the PCM connector.

[This message has been edited by KurtAKX (edited 07-19-2017).]

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lukeeggers
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Report this Post05-16-2019 10:15 PM Click Here to See the Profile for lukeeggersSend a Private Message to lukeeggersEdit/Delete MessageReply w/QuoteDirect Link to This Post

It's been a while, but I thought I should post the solution to my problem. After replacing a number of things mentioned in the form, I was left with two problems, occasional stalling and stumbling when just barely stepping on the gas pedal (first 1/8 of pedal). What was causing these two problems was the MAP sensor vacuum line. The hard plastic line had broken at some point and been patched by sleeving a rubber line over the two plastic halves. The rubber patch did not have a vacuum leak but left about half an inch gap between the two plastic lines where the rubber line was left to span the gap. In this gap, the rubber line was collapsing under vacuum. I fixed it by shortening the rubber patch so the two halves of the plastic lines were touching which keeps the rubber line from collapsing. After this fix, my problems were resolved.

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lukeeggers
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Report this Post05-16-2019 10:21 PM Click Here to See the Profile for lukeeggersSend a Private Message to lukeeggersEdit/Delete MessageReply w/QuoteDirect Link to This Post

Also of note, the issue with the MAT sensor reading incorrectly was caused by engine flooding from the fuel return line being clogged.

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