1989 GT Possible air/fuel issue

Tim83

Member
Sep 18, 2016
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Hi folks,

I hope everyone is enjoying the summer. I've been having an intermittent issue with my car and I'm hoping someone can help me figure it out. Under normal driving conditions the car behaves as it should but after some spirited driving, mainly winding it out in the first 3 gears, it exhibits what I can only describe as a lurching in low-mid rpm. The behavior is similar to the low rpm lurching you get with an aftermarket cam. It's almost as if there is a very brief interruption in either air or fuel. When the car exhibits this behavior it also idles high at anywhere from 1000-1250rpm. I ran the codes again and received 91 - lean mixture and the EGR valve code. The sensors and valve were original so I just replaced them but the problem persisted. A subsequent read of the codes also gave me the code for insufficient voltage from the TPS so I replaced that as well. I've also removed and cleaned the mass air meter. At this point I'm still receiving the following codes (key on engine off):

10: I don't believe this is anything to worry about unless I perform the cylinder test
29: Vehicle speed sensor
91: o2 sensor out of range/always lean *sometimes I get this code and sometimes I don't

Does this sound familiar to anyone? Could the VSS sensor be my issue or should I check for something else?

*When I installed the new TPS I was unable to get a read on the voltage. I think my multimeter is no good. I assumed if the TPS code didn't resurface then I'm within the correct parameters for TPS voltage. Am I wrong on this?

*I also bought a fuel pressure gauge and the pressure is normal.

Trickflow HCI
BBK CAI
70mm throttle body
75MM mass air

Thank you in advance!
 

90sickfox

Wasn't a pretty sight...and I've got big hands
SN Certified Technician
Mar 2, 2015
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Hey man...I know it's been a few days but I've had similar issues from 2 different mustangs recently. Both of them ended up being the vacuum line that runs from under the intake, down under the distributor, and to the carbon canister solenoid at the right front frame rail. Once you fix the line ( if it's the line ) disconnect the battery negative for a while. Unplug the idle air control valve and start the car up. Adjust the idle screw to between 650 and 750rpm. Cut the car off and plug in the IAC and restart the car.
 

Tim83

Member
Sep 18, 2016
43
8
18
39
Hey man...I know it's been a few days but I've had similar issues from 2 different mustangs recently. Both of them ended up being the vacuum line that runs from under the intake, down under the distributor, and to the carbon canister solenoid at the right front frame rail. Once you fix the line ( if it's the line ) disconnect the battery negative for a while. Unplug the idle air control valve and start the car up. Adjust the idle screw to between 650 and 750rpm. Cut the car off and plug in the IAC and restart the car.
Thanks, man. I suspect mine may be leaking as well. I’ll keep an eye on it when I do the smoke test.
 
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