Rough/Choppy idle and slight miss

96lhsb

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Alright I have a 96 GT that’s completely stock other than a CAI and straight piped off-road x pipe. I’ve had this issue for the past 2 months and cannot figure it out. The car will almost always start up just fine and make it about a block down the road and then as soon as I let off the gas and let the car idle it will start to idle extremely rough almost like it has a cam until the idle settles down and fluctuates from 900-600rpms. I started off rigorously cleaning the IAC valve which seemed to do nothing, I then noticed a coolant leak where the heater hose connects to the intake manifold on the passenger side and decided to replace the entire intake manifold, the car ran fine for about a day and then the issue came back out of nowhere. I took off the fuel filter and saw brown/orange gas coming out of both ends of the filter so I put it back on the car and drove to auto zone and got a new one however the car ran perfectly fine on the way there until I was heading back when it started up again. I let the car sit for about a month and then dropped the gas tank and cleaned it out completely, letting vinegar soak in it for 2 days because I thought there could be rust in the tank. After cleaning the tank I put a new fuel pump and the new fuel filter in and filled up the tank completely. The car ran perfectly fine for about 2 days and then started up the same issue again. I checked the fuel filter again and it’s clear clean gas from both ends. I’ve checked every COP for spark and replaced all the spark plugs, checked for vacuum leaks with starter fluid, checked 02 sensors and still nothing. I have a check engine light but my OBD2 port isn’t getting any power so I can’t read the codes. So far I’m at a loss I’ve been trying to research as much as I can the past few months and I haven’t been able to come up with anything so far.
 

96lhsb

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I also have a new battery, alternator, and spark plug wires all under 6 months old. The car also has various wiring issues from the previous owner, I’ve been able to fix some of the issues but not all and my speedometer doesn’t work however my odometer does so I know it’s not the worm gear that typically breaks inside the cluster. Transmission was rebuilt about 12k miles ago, 114k miles on the engine.
 

96blak54

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Any moisture, water, coolant, conductive grease that is down in them sparkplug holes, youll get a goofy miss and stumble. Be sure to dry them out real good along with the plug boots.

Also insert a spark plug into each wire boot to be sure their isnt excess rubber pushing the plug off. Or pushing itself off the plug when installed into the plug bores
 

ttocs

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my gas tank got dirty while it was out. I tried to clean it out as well before I put it back in and seemed to struggle with similar problems after. I ended up installing a new tank and still had issues until I cleaned the fuel injectors. IF they are up in age you might consider replacing them if you had bad gas, it went through them and then you can only hope what what ever was bad went through them as well but it normally gets clogged up.
 

96lhsb

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Any moisture, water, coolant, conductive grease that is down in them sparkplug holes, youll get a goofy miss and stumble. Be sure to dry them out real good along with the plug boots.

Also insert a spark plug into each wire boot to be sure their isnt excess rubber pushing the plug off. Or pushing itself off the plug when installed into the plug bores
I went ahead and checked all my spark plug wells for any kind of liquid and found some pooled up in the number 5 well, I think my thermostat housing is leaking slightly from the side since there was some faint white residue in one spot on it. Changed the plug, tested the COP and sealed the housing with some RTV and the car ran a lot better but it still had a slight unwelcomed idle rumble so I borrowed a multi meter from a friend and got my OBD2 port working after some rewiring and got the codes read, both my front 02 sensors are bad which must have happened recently since I’ve checked them before by swapping them on my buddies 97 gt and no check engine lights came on for him and it ran fine. Getting new sensors tomorrow and hopefully it’s just the combination of those 2 things and I’ll have it all sorted out. Thank you so much for telling my about that I wouldn’t have thought about it at all
 

lwarrior1016

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Those front oxygen sensors are a good place to start, those are what control your air to fuel ratio. Are you sure the sensors are bad and it’s not just a wiring problem? A lot of folks don’t pay attention when they do transmission swaps and can damage wires by pinching between the bell housing and the trans.
 

96blak54

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Good info! Even us car wrenching veterans goof and miss things like that. I know i have
 
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Man i hope you get your baby straight faster than me, I've got the 95 gt and she's been giving me hell for almost 2 years, when i think I got it fixed it runs a couple weeks and boom same problem back again, it would be quicker for me to name the things I haven't replaced, I just knew I had it when i found the burnt spots on the ecu, then the iac (i probably did that testing it) rung bad with my meter, then the evap canister purge valve, I'm about to go out and play with the timing now, good luck brother.
 

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