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Cold start

5K views 12 replies 4 participants last post by  Leeroyj2 
#1 ·
My car has a rough cold start. Fires right away, but dies. Then starts up but really bad surge. Almost sounds like it's starving for air. Warms up a bit then idles perfect at 750 rpm. I have new o2 sensors, iac valve, new throttle body, not sure where to start.
 
#2 · (Edited)
The first thing you might wanna do is test the ect sensor,since its input is used by the ecm to determine when to open or close the iac valve,depending on coolant temp.If the ect sensor tells the ecm coolant temp is hotter than what it really is,the ecm wont open the iac valve because it'll think the engine is at operating temp instead of cold start temp.The ecm opens the iac valve 100% at cold start,so if it doesnt open the iac due to a false ect reading,the engine will immediately stall.

Another thing you need to do is a base idle reset,since you installed a new iac valve & throttle body.The procedures to use for testing the ect & doing a base idle reset are listed in the links below.A base idle reset is important because having the throttle body idle set screw adjusted incorrectly will cause the engine to either stall or idle high to a point that'll take away the iac's ability to control idle.ECM target idle is 672rpms,so youre shooting for a mechanically set idle below 650rpm then when you reconnect the iac harness,the iac will take over idle control from 650rpm up.
BTW- your throttle body doesn't have a small hole drilled in the throttle plate does it?? If yes,that can also take away idle control from the iac because almost 100% of the air (at idle) through the throttle body is supposed to be bypassed around the throttle plate through the iac valve and that hole being present will disrupt this process.The air passing through that hole will make it impossible to set the idle speed below 650rpms mechanically because the hole wont allow you to set idle speed below 750rpms.

One more thing,test the iac harness.Pierce the iac harness wires with a needle then touch your meter leads to the needles or backprobe the harness where it plugs into the valve.Turn the key on, test the red wire to make sure it reads 10.5-12volts.Use the engine block as your ground for the multimeter black lead.Now test the w/lt blue wire with the engine at idle.The red meter lead gets attached to the w/lt blue wire & the black lead gets attached to the red wire.
The meter reading should fluctuate between a voltage less than 12volts but more than .5volts,because the ecm is cycling this ground on/off continuously.If both tests pass,the ecm & wiring to the iac valve are good.

You didnt list what year your car is,so do so in your next post because the iac wiring has a diode inline with it on 86-88 models,but 89-95 models have the diode built into the iac valve itself.

ECT Codes 21/51/61 Troubleshooting

Summarized/Corrected Base Idle Reset Procedure
 
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#3 ·
My car is an 88. My iac only has 2 wires, red and white with light blue stripe
 
#5 ·
So I decided to give everyone what I know about my car and what has been done. I bought the car last year. It is an 88 mustang GT with Mass air conversion. C&l Mass air housing with Ford 19# mass air meter. GT40 heads, explorer upper and lower intake. Bbk headers and h-pipe. Flowaster exhaust. This winter I put in a MSD ignition kit(coil, distributor, 6al2 box). Put new plugs in gapped at .038, new msd wires. Added a new aluminum radiator with electric fan. Then I noticed there was 24# injectors in the car so I replaced them with 19#. When I started the car up it ran bad at cold start but when it warms up it smooths out a little. I replaced both o2 sensors, and ECT sensor. My IAC is new last fall and is clean. The car is spitting out the exhaust pretty bad. Both sides. So I tore upper intake off and tested the new(refurbished) injectors to make sure one wasn't leaking by or stuck open. Tested good. There was some wetness around the vacuum line that goes to the pcv valve. Maybe a leak in the fuel regulator line?
 
#8 ·
my harness does have the diode on it, I have 12 volts coming to the iac valve.
 
#10 ·
Was the idle issue:

1) present when you first bought the car??
Or
2) present before you replaced the iac??
Or
3) not present until after you replaced the iac??

I just found a thread on Four Eyed Pride.com that talks in detail about the iac harness diode vs the iac valve diode.The surging/rough idle that occurred was actually a TSB that Ford put out years ago.The following paragraph is one of the posts from the thread Im posting a link to below.You can read the whole thread (3 pages) if you want,which talks about clipping the harness diode out vs reversing the wires on the iac valve.A few pictures are shown too of the diode and wiring.However the following paragraph describes it pretty well.


(Four Eyed Pride post)
*** Okay, I just want to back up a little back after re-examing all of the posts. I think I may have been misunderstanding the initial post. It sounds as though you have an upgraded IAC that already has the diode built in AND you were using an older style harness that had an inline diode. From what I am gathering, the inline diode was battling with the built in IAC diode so you simply inactivated the inline harness diode to let the built in IAC diode do its thing unopposed. On my bone stock 1986 GT, the only diode that is there is the external inline one. My question now is, do I really have to do anything with this external diode if I am running the stock 1986 IAC? It would seem that the answer would be no. From what I gather, the TSB may have been to address IAC problems if someone upgraded to the newer IAC but left the other diode in place also. In short, the mod you described only needs to be done if the IAC is a newer replacement one and that this really doesn't apply to original stock 1986 IAC, which doesn't have the diode built in.***

(Four Eyed Pride thread link)

Wanted dead or alive: Hanging/Surging Idle..SOLVED!!!!
 
#11 ·
So tomorrow I am going to change back to my stock coil and see if it helps, change out the tfi module to my stock one. I have a question regarding the distributor and timing. I put the new msd distributor in lining the rotor up exactly where the stock one was. Is there anything different from stock to msd?
 
#12 ·
No theres nothing different between the MSD & stock distributer,when it comes to dropping the distributer in the block.If you wanna double check the distributer to make sure its installed correctly,do the following:
1) pull the #1 spark plug & stuff a piece of toilet tissue into the plug hole slightly
2) use Witeout to draw a line over the 0° & 10° btdc mark on the balancer.This helps you see the marks easier with the timing light
3) use a torque wrench/breaker bar & socket to rotate the crank CW until the tissue blows out
4) continue rotating the crank then stop once the 0° mark is aligned with the timing pointer
5) remove the distributer cap & look at the rotors position
6) it should be pointing straight ahead,between the front two upper intake bolts,like shown in the picture below
7) if its pointed correctly,start the engine and let it idle until it warms up,barely loosen the distributer holddown bolt,remove the spout plug,aim a timing light at the balancer then rotate the distributer til the 10° balancer mark is aligned with the timing pointer,tighten the bolt,plug the spout back in.
8) if its not pointed correctly,pull the distributer,rotate the shaft slightly then drop the distributer in until it is pointing in the right direction.Once its dropped in correctly,follow step 7 to adjust timing to 10°
9) once its set to 10° & the spout is plugged in,aim the timing light at the balancer again to make sure the ecm is advancing timing properly.
Timing should stay at a fixed position with the spout plug removed & a advanced/fluctuating position with the spout plug installed.If you see the timing mark bouncing back & forth with the spout plug removed,the balancer ring could be bad or the spout wires could be shorted in the harness.
Make sure youre on the compression stroke too,not the exhaust stroke,when you drop the distributer in the block.If its on the exhaust stroke,it will possibly still run,but it'll run like crap because being on the exhaust stroke puts timing out 180°.
Triple check your plug wire routing on the cap too.The distributer cap adapter has two metal clips that holds the cap on.The #1 plug wire post is two posts to the right of the clip on the left side of the cap.The firing order is 13726548 CCW.

You can also run a cylinder balance test to see if the ecm id's any specific cylinder(s) as having an issue.The procedure is listed below.Once you click the link,scroll down to the appropriate section.


DIY KOEO/KOER/Cylinder Balance Self-Test Procedure
 

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#13 ·
I reset timing last night, I deleted everything I did and put the car back to the way it was when I parked it last fall except for the electric fans and the 24# injectors that were in the car when I bought it. No change.
 
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