View Full Version : 90SC drives by itself...

05-27-2004, 09:24 AM
My newly rebuilt engine is in. I'm having idle problems about 5 minutes after it warms. This morning, after replacing the IAC last night, the car will just drive by itself at around 2000 rpm's! When I slow down, the rpms drop to 1000, then as it shifts (AOD) back into first the idle will jump to around 1500 and stay there. If I put it in PARK or NEUTRAL, it jumps to about 2700 rpm's.

The replacement IAC was used, but looked really new (diaphragm was white and clean).

Any advice on where to start looking? Would a mis-adjusted TV cable cause these problems. Also, it is non-EGR. Please help so I can be ready for Carlisle! Mods can be seen in my member page...


05-27-2004, 11:00 AM
Did it look exactly the same as the one you took off? If its a aftermarket, they will do this. Not intended for our cars.

05-27-2004, 11:06 AM
Yes, it is a Ford part, not after market. All the symptoms are of a bad IAC, but now I find it unlikely that the IAC that was on there and working perfectly before I swapped the engine, somehow went bad while it was sitting safely in a box on a shelf and still attached to the tb... Now this second IAC which didn't look very old doesn't correct the problem either...

I really need to take it to the trans shop and have them adjust the TV cable properly, but I can't do that if the idle is throwing all of the shift points out of whack too...

05-27-2004, 11:18 AM
Wow im stumped, only thing I can think of is a bad air leak or sticking TB?

05-28-2004, 08:37 AM
The keep alive memory (KAM) could be a problem. It needs to be cleared when you clean or replace the IAC. However, if after doing this and the problem still presents itself, you'll need to retrieve the DTC's and go through a step-wise process of troubleshooting using a Star code reader and doing the EOEO and EOER tests, and addressing and resolving each of the DTC's in the order they are presented, and following the diagnosis flow charts. These system are too complex, and that's why they put a computer on them, so all we have to do is retrieve the codes and do what the charts say to do.


05-28-2004, 09:17 AM
disconnect the TV cable and see if it allows the car to idle.
if the cable is damaged it may be holding the throttle body from dropping to idle. :eek:

05-28-2004, 09:18 AM
Well, the only codes I get are 41 and 91 and they both seem to refer to an O2 sensor problem. I'd been getting these codes since before the engine swap, and I didn't have these idle problems before... The last time this happened it was the IAC, so I guess I will replace it with a third one and see if there is any change...

Would a bad blower bypass valve cause these problems? Anyone?

05-28-2004, 09:20 AM
Vac leak at the IC tubes......check out this thread

05-28-2004, 09:29 AM
Thanks Justin. I already checked the i/c tubes, they are tight and I'm using tbird88's gaskets. Sprayed contact cleaner (it's flammable) around everything and no change in idle. Maybe I will try again when the car is cold- it idles ok on cold startup, but when warm the idle gets to high to notice much of a change if there is a leak... All of the vac lines are tied with zip-ties, and were checked carefully when the engine was going back together. While it was idling high, the throttle arm was sitting firmly against the set screw, and the tv cable appears to move freely.

~~~~ :mad:

05-28-2004, 09:40 AM
Check the coolant temp sensor(not for the guage)
Maybe its not working when the engine gets warm and the ecu thinks its still cold....
I checked my IC tubes with (quick start) at idle but the vac leak didnt show up.
Look at the lower connection on the IC. If you see any signs of oil down there its leaking.

05-28-2004, 02:24 PM
One other thing- when I unplug the IAC (w/ car running), all of the symptoms are still the same, but the rpms go down a few hundred or so...

05-28-2004, 02:40 PM
Oh, and about that coolant temp sensor- both the guage and the fan work as they should. If the coolant temp sensor was the culprit, wouldn't it cause the fan to either stay on all the time, or not turn on at all? I just don't want to start chasing a lot of unneccessary stuff...

05-28-2004, 02:56 PM
The replacement IAC was used, but looked really new (diaphragm was white and clean). All it takes is 1 tiny crack in the diaphragm which might not even be visible. If it has been handled at all, there's always a chance it could be leaky. You might try another used Ford IAC or try to devised some sort of pressure test for the one thats on there. Try blowing through it from both sides.. if the seal is good, it should not pass air from either direction.

05-28-2004, 07:01 PM
Ok, well I'm keeping my fingers crossed here, but I may have found the problem... I noticed that the idle stop screw seemed a little loose. I'd already set it by backing it off till it's just not touching, then turning it 1.5 turns down. But somehow, when I went to repeat the procedure, it seemed to take 3-4 turns to back it off again. I think it was so loose it was vibrating down while I had my foot on the gas, then when I let off the throttle, the throttle stop arm would hit the now lowered screw, thus keeping the throttle open more than it should be (thus the higher idle). I put some sealant on it so it wouldn't jiggle loose, took it for a ten minute drive, and the idle stayed where it supposed to... Hopefully that was it (I'll know later when I go for a longer drive), and I won't have to go down the rabbit hole chasing this problem! :eek:

05-28-2004, 07:13 PM
mine idled at 2000 for over a week after i had my throttle body off i had to unhook the battery for about 10 min to clear the computer then it idled fine it did take 3 days and about 200 miles to quit stalling at stop signs then it was fine after computer relearned everything