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Discussion Starter · #1 ·
I am in the beginning stages of my first project car, a 1989 Mustang GT Convertible. I have had a lot of fun times so far. This is not one of those times. My mechanical ability at this point is somewhat limited, so excuse me if I contradict myself or otherwise talk out my butt. :winks

I have been struggling with what I believe is either an electronic issue or a vacuum leak. The car has trouble running at idle/coast. I have read several threads that describe the symptoms of a bad throttle position sensor. I replaced the TPS this weekend, following the instructions on this site. I ended up with a voltage of .983 vDc from the green wire, key on, engine off. This made no improvement (especially the first time, when the replacement part was missing its internal spring and stayed in the WOT position) so I carefully ran the Idle Reset Procedure. The only thing that changed in now the throttle sticks closed because I'm an idiot and backed the idle set screw out to far!

I have looked everywhere for a vacuum leak and can not find one so far. Not that I am an expert. Friday I plan to pick up an OBD-1 scanner (it's worth $30 to me to take a blink-counting error out of the equation), check the timing, and check the vacuum pressure with a gauge.

Any ideas or advice (other than "hire a real mechanic" lol)? Thanks in advance.
 

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when you are driving it does the car try to or does die when stopped? If so that could be your tfi. Now if your idle is really high and stays running high then you have a sticking iac.
 

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Discussion Starter · #3 ·
Yes, it tries to die when stopped as well as when slowing down at idle (jerks like it is running out of fuel). I can't get the idle set much below 1,000 RPM, so it stays running, but barely. I should probably also mention that the transmission is AOD. I was actually starting to suspect the TFI based on some reading last night, but don't know much about it yet.

I will try and find some info on the IAC.

THANK YOU!!
 

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if it was the iac it would be reving really high like 2,000rpm when it should be idling. the tfi wont let it idle it will try to die and have an unsteady idle up and down but still low. When in gear it will or try to die when stopped or slowing down.
 

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Discussion Starter · #5 ·
Yes, it does, but not until it reaches idle rpm. That's when it jerks like it's out of fuel. Doesn't idle that high. I am starting to suspect tfi. Would that affect the whole RPM band or just at idle? If I stand on it, it kinda hauls ass.

Just noticed your avatar. Is that a Regatta Blue / Silver 'vert with a black top? Mine is identical. White leather?
 

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Discussion Starter · #6 ·
(Nope - hard top) :bigthumbsup
 

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Yes, it does, but not until it reaches idle rpm. That's when it jerks like it's out of fuel. Doesn't idle that high. I am starting to suspect tfi. Would that affect the whole RPM band or just at idle? If I stand on it, it kinda hauls ass.

Just noticed your avatar. Is that a Regatta Blue / Silver 'vert with a black top? Mine is identical. White leather?


The fact that its bucking and jerking at low rpm and speed,but kinda hauls ass when you get on it..I suspect your timing is outta whack or your dizzy bolt is loose and the dizzy is "walking around". I would check timing first thing since you havent already.
 

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Check your timing.

Check for vacuum leak? Carb cleaner.

Run your codes.
 

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check the codes
 

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Discussion Starter · #10 ·
Headed out first thing tomorrow to pick up a timing light, a code reader, and a replacement TFI and see if I can start a garage fire looking for leaks :gringreen. If it's not the TFI, I've got one for the road kit. If I get it figured out early enough, I may dive into the nightmare of swapping the heater core and replacing the carpet. Gonna be a long weekend.
 

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Discussion Starter · #11 ·
Narrowing it down. Computer throws a code 31 - EGR voltage out of range (or something like that) and nothing else. Cylinder balance test is Code 9 - pass. I have adjusted the timing to 14° BTC with the spout connector unplugged, which advances to about 20° with the spout plugged back in. Nothing has changed. I bought a new TFI, but have not installed it yet because it didn't throw a code (unless the EGR code is related). I did some more vaccum leak hunting with carb cleaner to no avail. What would be the next logical step (don't say shotgun, 'cause I just might :nono:)? I really hope I don't have to spend $83 on a new EGR valve. Going to read up on salvaging that for now.
 

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Discussion Starter · #12 ·
* Forgot to mention that I pulled the vacuum line off of the top of the EGR and nothing changed. Would this be an indicator that the valve itself is bad / needs cleaning?
 

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Discussion Starter · #13 ·
FOUND IT!!! Ended up being a big vacuum leak under the front of the upper intake. Of course it's a dealer part because of the flare on one end, so I broke out the box of shame and grabbed a vacuum cap for now. Worked like a charm. Thanks again for everybody's input. Never would have found it without the carb cleaner.
 
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