88 2.5L running issue :( please help (Page 1/5)
gtxbullet NOV 08, 11:30 PM
long story short I put my 2.5L back in my 88 and sold my 3.4L swap to buy a fastback daily driver.
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here's the details:

Starts fine, idles good for a couple minutes

after a couple minutes the idle rpm's sort of wander very oddly. not reving up and down, but gradual

when driving it sometimes wants to stall (and does) while shifting into another gear (manual) or when coming to a stop.

when I'm stopped in traffic the rps kick down then up A LOT, to the point I need to keep my foot on the gas so kids don't think I want to race them...lol

when the engine is running it sounds like it's very slightly putt putting.
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here are the things that are new or have be tested and have ZERO ISSUES:

brake booster (and all connections between the booster and the engine)
fuel pump - new
Pump Strainer - new
fuel filter - new
fuel pressure regulator - new
Every engine seal and gasket was replace (full deconstruction and rebuid - this problem was happening before I did this)
MSD Coil Packs
ignition module - new
Crank Sensor - new
Spark Plugs - new
Spark Plug Wires - new
Power and Ground cables - new
PCV - new
EVAP System has been checked and bypassed - issue doesn't go away
EGR - new
Computer (PCM) has been swapped (1988 5-speed and 1987 5 speed, no change)
no catalytic convertor (though I'm going to be putting a high flow unit on to quiet it down again
Gall757 NOV 09, 08:22 AM
VSS
phonedawgz NOV 09, 08:30 AM
With the engine warm and idling at the correct speed, unplug the IAC. That will freeze the IAC's pintle at that particular spot. Then see if you still have the idle speed error. If so - then why is the ECM commanding a changing idle speed. If not - then what is actually causing the idle speed changes?
fieroguru NOV 09, 09:25 AM
Try clean the IAC passage and the throttle body and backside of the throllle body blade. Over time they get carbon build up and reduce the amount of air that flows through them at idle. When the engine tries to idle too low, then it kicks into stall saver mode and will shoot the RPM's up.

You might also try blocking off the EGR at the manifold... the last new one I had for an 88 2.5 wouldn't seal shut properly and was flowing all the time. Idled like a champ once I blocked it off (welded the port in the EGR housing closed).

[This message has been edited by fieroguru (edited 11-09-2012).]

gtxbullet NOV 09, 10:25 AM
Replaced the IAC as well as unplugged it. no change.
The throttle blade and passage was cleaned prior to reinstalling.
theogre NOV 09, 10:44 AM
1. check battery and alt volts etc. Iffy power will be a problem. Check when engine is cold and hot.

2. get ECM scan tool
Monitor ECM volts. ECM should see battery volts +- 1v max at all times. If not check all grounds etc... G201 and G202 too. See my Cave, Scan Tool Help, Wire Service

2a. Maybe ECM w/ heat issues. ECM won't set a code etc for heat problems... maybe bogus codes. If cooling help, get a new ECM and read ECM Heat

3.
Get stock coils. Car starts and runs but has problem when hot is a likely sign Coils have heat issues.
What brand/type of plugs?
DIS hates "Performance" coils and many types of plugs.
If shield is missing, find a used one or make one.
See my Cave, DIS Ignition

Coil
AIRTEX / WELLS Part # 5C1058
STANDARD MOTOR PRODUCTS Part # DR39 (But not cheap DR39T)

4. You switch ECM PROM... What number on the PROM?
May need PROM update. See my Cave, 87-88 DIS Duke ECM

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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 (It's also at the top and bottom of every forum page...)

gtxbullet NOV 09, 11:02 AM
Problem was still there with stock coils.
Plugs are stock ac delco.

I tried platinums last year and no difference.

The shield is there and I clean it.

I have heatsink compound on the ICM as well.

[This message has been edited by gtxbullet (edited 11-09-2012).]

TejasFiero NOV 09, 01:12 PM
Changed MAT sensor? I hear if that's fouled up it causes idle issues.
hercimer01 NOV 09, 04:11 PM
You shoulda bought my Formula.
fieroguru NOV 09, 04:55 PM
Might check the coil pack connector harness end. I was having random loss of power and sometimes a really low idle that would come and go. Eventually it stayed in the low power mode and was able to limp it home. Never set a code... but this was the culprit: