weird check engine problem...

fastford3388 · April 14th, 2007, 04:38 AM

I've got a very strange problem with a check engine light and need some advice. I've got an '89 gt and I had 7 codes when i bought the car and it wouldn't idle without stalling out. the codes included: TPS, EGR position sensor, and a speed sensor. I've fixed all of those things and cleared the codes. Now when I check my codes the read out reads "no codes" but the engine light stays on. The strangest thing about this is that at wide open throttle, the engine light goes off and i feel a boost of performance. Also, sometimes I do smell gas/exhaust when I am idleing at a light but that could be because my exhaust dumps off after my mufflers. any one have any suggestions on what it could be? i was thinking a vacuum leak but I am not sure.

stang fanatic · April 14th, 2007, 09:16 AM

could...not saying is....could be ecm problem. try using a reader that allows you to monitor sensors and various readings.

Dr.Watson · April 14th, 2007, 11:48 AM

yea my lx wouldn't pull codes, bad ecu

angermgt · April 14th, 2007, 05:20 PM

i also have 89 gt. had a rough idle and sometimes stalled. it turned out to be idle air controller(a cylinder looking thing located on the front of the throttle body)take it off (2 little bolts) and clean the ports on tb and air controller with carb cleaner. its quick. cant hurt.

tording92lxnotch · April 14th, 2007, 05:58 PM

are you getting no codes with the KOEO test and the KOER test? In order for the tester to even run the codes for the KOER test you have to have all the codes cleared from the KOEO test. If they truly are throwing no codes then you will get a code 11 which means system passed. If this is the case and you still have a light then you have an electrical issue or your ECU has something going on with it.

fastford3388 · April 14th, 2007, 10:57 PM

but the weirdest thing is that at Wide open throttle, the light goes off and when i slow down, it comes back on. the IAC shouldn't do that because the IAC only works when the car is idleing. How can I tell if i have a bad Engine computer? could the fact that It used to be a 5 speed and now it's an AOD have any thing to do with it?

SALIV8 · April 14th, 2007, 11:09 PM

tranny swap shouldn't be an issue I don't believe because the computer doesn't control shifts, etc... I might try disconnecting the o2 sensrs one at a time( just the connections if u can reach) and see what happens. did u pull the negative cable for a few minutes to reset the ecu?

fastford3388 · April 15th, 2007, 04:18 AM

i have stored the car over the winter with no battery. I'm not getting an O2 sensor code.

SALIV8 · April 15th, 2007, 10:16 AM

o2 sesors control the operating loop of a computer... when they're warm and working properly the ecu goes into closed loop- when they're inop it stays open loop -in a preprogrammed air fuel ratio- which is rich -therefore when you're at wot- that is also preprogrammed air fuel... my thought u might get lucky and find a faulty 02 sensor, or like was said your ecu is fried... but ur code goes away at wot tellin me that operating parameters other than the ecu preprogrammed air-fuel is acting up... its worth a shot...

stang fanatic · April 15th, 2007, 05:10 PM

I could be wrong, but wasnt the aod an electronic od wherer the computer signaled the trans to shift in and out of od?

tording92lxnotch · April 15th, 2007, 05:13 PM

Originally Posted by stang fanatic

I could be wrong, but wasnt the aod an electronic od wherer the computer signaled the trans to shift in and out of od?

I think you may be correct on this. I have always read/heard that you can use an Auto ECU (A9P) with a manual car but you can not use a manual ECU (A9L) with an Auto car. The ECU controls the tranny and the manual ECU is not setup to do this so you get some funky things going on.

stang fanatic · April 15th, 2007, 05:24 PM

in which case that MIGHT or MAY NOT explain the ck engine light on/off at wot and not...but doesnt explain why no code. need to try to read koer codes and see if it shows up or find a way to monitor data while driving

SALIV8 · April 16th, 2007, 01:08 AM

I have an aod out of my 92...there are aode and aod. I guess it would depend on the tranny your prolly right.

fastford3388 · April 16th, 2007, 02:11 AM

I'm almost positive i have an AOD not an AODE. How can I tell? the car also smells like it could be running a little ritch. Could this have any bearing on why the check engine light is on?

nateo · April 17th, 2007, 08:52 AM

how do you know how to tell if a ecm is bad?

stang fanatic · April 17th, 2007, 06:33 PM

I used to hear in the past from several ford 'techs', that more than 3 active codes indicated a bad ecm...couldnt ever verify if statement is correct except to install another ecm to test

SALIV8 · April 17th, 2007, 07:24 PM

Originally Posted by fastford3388

I'm almost positive i have an AOD not an AODE. How can I tell? the car also smells like it could be running a little ritch. Could this have any bearing on why the check engine light is on?

yes it could have a bearing... again it sounds to me that it may be an o2 sensor or both because on start up your ecm has a preprogrammed air/ fuel ratio until it warms up then the o2 sensor(s) give the ecm their reading based on a 5 volt reference. I think its the o2 sensor(s).

SALIV8 · April 17th, 2007, 07:25 PM

Originally Posted by nateo

how do you know how to tell if a ecm is bad?

It can be impossible to diagnose sometimes. There is no set standard that I'm aware of to tell if one is bad or even any tricks... Like fanatic said replace and see.

ken5_0 · April 17th, 2007, 07:36 PM

Bad 02 or bad TPS is my guess.

95opal · April 17th, 2007, 08:05 PM

Pull the codes again. Might also be the EGR cause its only used at part throttle and is inoperative at WOT.

April 14th, 2007, 04:38 AM	#1
fastford3388 Regular 1991 Mustang GT Join Date: Feb 2007 Location: Lisle, IL Posts: 367 iTrader: 0 reviews	weird check engine problem... I've got a very strange problem with a check engine light and need some advice. I've got an '89 gt and I had 7 codes when i bought the car and it wouldn't idle without stalling out. the codes included: TPS, EGR position sensor, and a speed sensor. I've fixed all of those things and cleared the codes. Now when I check my codes the read out reads "no codes" but the engine light stays on. The strangest thing about this is that at wide open throttle, the engine light goes off and i feel a boost of performance. Also, sometimes I do smell gas/exhaust when I am idleing at a light but that could be because my exhaust dumps off after my mufflers. any one have any suggestions on what it could be? i was thinking a vacuum leak but I am not sure. __________________ - '90 Mustang LX-Sold "Debbie" - '89 Mustang GT- Deceased "Lucy" - '91 Mustang GT-70,000 mi "The Unicorn"

April 14th, 2007, 09:16 AM	#2
stang fanatic Enthusiast Join Date: May 2006 Location: El Paso, Tx Posts: 861 iTrader: 0 reviews	could...not saying is....could be ecm problem. try using a reader that allows you to monitor sensors and various readings. __________________ "The gods too are fond of a joke." "If I had only known, I would have been a locksmith. "

April 14th, 2007, 05:58 PM	#5
tording92lxnotch Regular Join Date: Apr 2007 Posts: 165 iTrader: 0 reviews	are you getting no codes with the KOEO test and the KOER test? In order for the tester to even run the codes for the KOER test you have to have all the codes cleared from the KOEO test. If they truly are throwing no codes then you will get a code 11 which means system passed. If this is the case and you still have a light then you have an electrical issue or your ECU has something going on with it. __________________ 92 notch--Shorty headers--Pro 5.0--3G alt--U/D pullies--A/C delete--190 LPH Walbro FP--Compucar N2O kit--FMS alum D/S--Taurus fan--Weld Draglites--Mallory 685 ign Box--dccontrol fan controller 13.0 - 108 mph - 1.9 60' - 90 shot N2O and 175k on the clock

April 14th, 2007, 10:57 PM	#6
fastford3388 Regular 1991 Mustang GT Join Date: Feb 2007 Location: Lisle, IL Posts: 367 iTrader: 0 reviews	but the weirdest thing is that at Wide open throttle, the light goes off and when i slow down, it comes back on. the IAC shouldn't do that because the IAC only works when the car is idleing. How can I tell if i have a bad Engine computer? could the fact that It used to be a 5 speed and now it's an AOD have any thing to do with it? __________________ - '90 Mustang LX-Sold "Debbie" - '89 Mustang GT- Deceased "Lucy" - '91 Mustang GT-70,000 mi "The Unicorn"

April 15th, 2007, 04:18 AM	#8
fastford3388 Regular 1991 Mustang GT Join Date: Feb 2007 Location: Lisle, IL Posts: 367 iTrader: 0 reviews	i have stored the car over the winter with no battery. I'm not getting an O2 sensor code. __________________ - '90 Mustang LX-Sold "Debbie" - '89 Mustang GT- Deceased "Lucy" - '91 Mustang GT-70,000 mi "The Unicorn"

April 14th, 2007, 11:48 AM	#3
Dr.Watson Newbie Join Date: Mar 2007 Posts: 32 iTrader: 0 reviews	yea my lx wouldn't pull codes, bad ecu

April 14th, 2007, 05:20 PM	#4
angermgt Regular Join Date: Nov 2006 Location: new jersey Posts: 154 iTrader: 0 reviews	i also have 89 gt. had a rough idle and sometimes stalled. it turned out to be idle air controller(a cylinder looking thing located on the front of the throttle body)take it off (2 little bolts) and clean the ports on tb and air controller with carb cleaner. its quick. cant hurt.

April 14th, 2007, 11:09 PM	#7
SALIV8 all motor 92 coupe 9.94@136.40 Join Date: Dec 2006 Location: Chicago Posts: 576 iTrader: 0 reviews	tranny swap shouldn't be an issue I don't believe because the computer doesn't control shifts, etc... I might try disconnecting the o2 sensrs one at a time( just the connections if u can reach) and see what happens. did u pull the negative cable for a few minutes to reset the ecu?

April 15th, 2007, 10:16 AM	#9
SALIV8 all motor 92 coupe 9.94@136.40 Join Date: Dec 2006 Location: Chicago Posts: 576 iTrader: 0 reviews	o2 sesors control the operating loop of a computer... when they're warm and working properly the ecu goes into closed loop- when they're inop it stays open loop -in a preprogrammed air fuel ratio- which is rich -therefore when you're at wot- that is also preprogrammed air fuel... my thought u might get lucky and find a faulty 02 sensor, or like was said your ecu is fried... but ur code goes away at wot tellin me that operating parameters other than the ecu preprogrammed air-fuel is acting up... its worth a shot...

April 15th, 2007, 05:10 PM	#10
stang fanatic Enthusiast Join Date: May 2006 Location: El Paso, Tx Posts: 861 iTrader: 0 reviews	I could be wrong, but wasnt the aod an electronic od wherer the computer signaled the trans to shift in and out of od? __________________ "The gods too are fond of a joke." "If I had only known, I would have been a locksmith. "

April 15th, 2007, 05:24 PM	#12
stang fanatic Enthusiast Join Date: May 2006 Location: El Paso, Tx Posts: 861 iTrader: 0 reviews	in which case that MIGHT or MAY NOT explain the ck engine light on/off at wot and not...but doesnt explain why no code. need to try to read koer codes and see if it shows up or find a way to monitor data while driving __________________ "The gods too are fond of a joke." "If I had only known, I would have been a locksmith. "

April 16th, 2007, 01:08 AM	#13
SALIV8 all motor 92 coupe 9.94@136.40 Join Date: Dec 2006 Location: Chicago Posts: 576 iTrader: 0 reviews	I have an aod out of my 92...there are aode and aod. I guess it would depend on the tranny your prolly right.

April 16th, 2007, 02:11 AM	#14
fastford3388 Regular 1991 Mustang GT Join Date: Feb 2007 Location: Lisle, IL Posts: 367 iTrader: 0 reviews	I'm almost positive i have an AOD not an AODE. How can I tell? the car also smells like it could be running a little ritch. Could this have any bearing on why the check engine light is on? __________________ - '90 Mustang LX-Sold "Debbie" - '89 Mustang GT- Deceased "Lucy" - '91 Mustang GT-70,000 mi "The Unicorn"

April 17th, 2007, 08:52 AM	#15
nateo Newbie Join Date: Mar 2007 Posts: 27 iTrader: 0 reviews	how do you know how to tell if a ecm is bad?

April 17th, 2007, 06:33 PM	#16
stang fanatic Enthusiast Join Date: May 2006 Location: El Paso, Tx Posts: 861 iTrader: 0 reviews	I used to hear in the past from several ford 'techs', that more than 3 active codes indicated a bad ecm...couldnt ever verify if statement is correct except to install another ecm to test __________________ "The gods too are fond of a joke." "If I had only known, I would have been a locksmith. "

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April 17th, 2007, 07:36 PM	#19
ken5_0 Regular 1990 GT Convert Join Date: Aug 2006 Location: Prescott WI Posts: 203 iTrader: 0 reviews	Bad 02 or bad TPS is my guess.

April 17th, 2007, 08:05 PM	#20
95opal Hardcore Enthusiast 95 opalfrost gt Join Date: Nov 2006 Location: Connecticut Posts: 1,516 iTrader: 0 reviews	Pull the codes again. Might also be the EGR cause its only used at part throttle and is inoperative at WOT. __________________ 5.0 magazines 94-04 performance mustangs feature car! Fall 2010

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