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Help diagnose weirdity

2K views 31 replies 10 participants last post by  indy2000 
#1 ·
Stock foxbody efi setup. Sometimes while running perfectly, rpm will hang at around 2k and not go lower than that until I turn off key. Restart returns everything to normal. I have a bit of blowby and wondering if a oiled up 02 sensor could be the cause. I’ve done the usual base idle setup, checked tps, iac etc.
 
#5 ·
I had a similar condition on several occasions with my 1989 GT. Mine was always related to the 10-pin connectors (aka salt and pepper shakers). Next time the idle is hanging, try moving the 10-pin connectors around to see if the engine idle changes. If so, look up 10-pin connector fix.

Dean
 
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#22 ·
Dean is right, I've had this happen a bunch on my '89. You won't notice it cruising down the road, but get to a stoplight and the idle just haaaaaaaangs...

What's happening (from the Quarterhorse logs) is that the TPS signal drops out and the EEC thinks the new idle TPS setting is 0V. Then when it connects again, the EEC doesn't think it's at closed throttle, and pumps up the IAC.

I'd love to find a permanent replacement for the stock connectors. I'd even go with GM parts :eek:
 
#6 ·
the next time it hangs

do NOT shut off the engine, put in park or neutral, e-brake on

pop the hood, get out, walk to the front of the car, open the hood, prop the hood

then

disconnect the IAC connector..........what happens?
 
#7 ·
Thanks for the suggestions. I’m on my 3rd iac and tps, and serviced my 10 pins last year. Servicing the 10 pins did solve some other issues I was having. About the TB butterfly....do you mean like a physical hanging of the valve? I wouldn’t think that would be the issue as a restart clears the problem. Gotta be sensor/electrical/eec related.
Next time it does it I will pull over and start jiggling stuff. Will jiggle stuff on the engine too ;j
 
#9 ·
engine requires air to run

in order for the engine to run at 2000rpm, air must be entering

and dont be jumping to conlusions, that it has to be a sensor or elec issue

start at the source then gain direction....
 
#10 ·
Happened again. Jiggled the 10 pins, no change. Unplugged IAC and idle dropped to normal. Plugged back in idle jumped again. This is a nearly new Motorcraft IAC on it.
 
#15 ·
you may not have the throttle blade in the perfect, "expected" value, it may learn over time or not

the bucking is usually a good indication the blade is not in the correct position

as the IAC, is opening and closing during cruise when it shouldn't

time will tell
 
#17 ·
look up the the throttle body air flow scalar in your ecm

then set it to that number while looking at real time air flow data stream from the maf

or measure you throttle body air from maf data, and input into the ecm

both will require use of tuner with data logging in real time


that is why ford clearly stated to never adjust the stop, as idle is set in the ecm
 
#18 ·
My numerous experiences with erratic/ hanging idle and low speed bucking over the years has lead me to find these conditions as possible sources-

•Voltage too low. RPM increases to speed up alternator.

•Idle RPM too low.

•Vacuum leak.

•Dirty or mismatched MAF.

•Fuel trim incorrect

•Clogged fuel filter/fuel pump issues.

• Bad ground.
 
#19 ·
To add to this, I have seen a corroded positive battery cable cause this as well. It always seemed to happen after an instant heavy load was put on the battery such as the fan turning on.
 
#20 ·
Thanks for the replies. Thought I had it licked, but it happened again tonight. Regarding voltage....might be onto something there. Ever since I did this conversion I have had flickering headlights when at idle. I’ve tried raising idle, but that causes TPS issues. I’m running a 3G alt and have no charging problems and battery voltage is normal. What would be a good way to eliminate this as a cause?
With the help from this great resource of talent and experience, I’ll get this sorted. Thanks guys.


edit: I should mention the battery is in the trunk and I’m using the stock 2awg wire from the trunk to the engine bay.
 
#21 ·
playing with the throttle stop does NOT cause TPS issues..

it causes incorrect air flow with the throttle closed, that leads to idle/cruise issues

you cannot adjust idle with the throttle stop, well UNLESS you

disconnect the IAC, and start adjusting the stop to where you want

but then you lose all the IAC functions...

if you have flickering lights, that might be something you want to address before any idle issues.......
 
#23 ·
Where should I be checking voltages at to find the cause of the flickering headlights. Battery reads normal with engine off ,and running. 12.6 key off, 14.6 to 14.7 engine idling. Measured at the battery posts. Stock fox 5.0 efi (sn95 front drive) with 3G alt added.
 
#25 ·
Checked voltages at different areas of the car tonight. Getting strong DC voltage at the lights and other accessories. BUT, I did a little internet university and discovered that a bad alternator could be putting out small amounts of AC. When that happens the result is flickering lights. I checked for AC with a DVM and got fluctuating readings of .1 to .3 v. I could see where something like this could possibly make the eec grumpy.
I’ll throw on a new alt tomorrow night.
 
#26 ·
you did voltage drop tests on both the power and ground of the light wires?

a voltage drop test, requires the leads to be both on either the pos or neg

a bad ground will give you battery voltage when you just test the positive wire

its a useless test
 
#27 ·
Changed the alt. Flickering eliminated. Yay.
Went out for a spin, came home and noticed a layer of coolant below the oil in my catch can. Bummer. I guess my mysterious bubbling coolant issue is soon to be solved. Hoping it is only head gasket and not a bad block.
Thanks for the help everyone, especially Indy. I like your style- no coddling or small talk, straight to the point.
 
#31 ·
While I agree with you on trying to measure, rather than take a shot with a shotgun, this rarely happens when you are at home, in the garage and you have the leisure of back probing connectors.
It invariably happens on the road and it is rarely convenient or safe to pull over and start diagnosing.
 
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