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1988 GTA TPI 350 - rough/light idle (500rpm), rich

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1988 GTA TPI 350 - rough/light idle (500rpm), rich

Old 11-25-2018, 08:31 PM
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1988 GTA TPI 350 - rough/light idle (500rpm), rich

Hi gang,

Seems like most of the guys who were around "in my day" have probably moved on with life, like me, but I've got a problem and now I'm back to ask for your input.


I dropped in a rebuilt 350 TPI setup (using Helm/OEM manuals and all factory wiring, computer and components, heat shrink tubing, no problems here) from a wrecked 1988 GTA 350/Auto into an '86 Trans Am L69/T5 about 5 years ago, and I just went to get the car out after years of it sitting in the garage (I dry cranked it to build oil pressure first).


I have a light start (takes a few cranks, but it's a light/slow start that will die 30% of the time), then it very lightly idles/lopes at 500rpm. It dies easily unless I blip the throttle and then it'll run fairly well around 1500-2000rpm, but will stall/idle very slowly instead of around 800-900 once I let off the throttle. It also smells quite rich, and when I sprayed the engine bay with water after trying to get it running for a few minutes (probably started 8-10 times in 5-6 minutes), the passenger's side manifold was hot and steamed; drivers side was relatively cold, no water evap. Not sure if that helps at all.


How can I rule out bad or stuck injectors, airflow sensor, or whatever else may be going on? I could see that timing could cause rough or light idle, but what about the richness? What codes should I look for? I used to know this stuff years ago when I was working on it all the time, but after being out of it and busy with life, I've forgotten an awful lot. I do know that I wired everything engine-related correctly (engine bay is 1988 harness, body/lights is 1986) so that shouldn't be a problem. I purchased a junkyard 1988 GTA donor and pulled all parts from it.


Back then I dropped in Moates chip and edited the VATS (turned off), and changed to manual transmission (it's got a T5 now, that originally came with the '86 L69). I don't know if there is something I messed up there, but I don't believe so as I used a 1988AKXY codebase. I can upload the moates flashed code here for review if you wish. But I don't know what to do to rule out bad sensors, injectors, or computer.


Thanks for your help.

Last edited by super_kev; 11-25-2018 at 08:35 PM.
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Old 12-02-2018, 04:13 PM
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Re: 1988 GTA TPI 350 - rough/light idle (500rpm), rich

Does your car have a catalytic converter? A plugged cat will very quickly cause running problems.

Could possibly be a sticky IAC valve. If it doesn't open up during idle, the engine's idle speed will drop and probably stall.
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Old 12-04-2018, 06:59 AM
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Re: 1988 GTA TPI 350 - rough/light idle (500rpm), rich

Do you have a CEL on? Have any Codes? You have to start there.

Then I would suggest cleaning the throttle body and IAC Ports, perform the IAC reset, idle speed set, and TPS adjustment. Also, old fuel may have varnished stuff up. I would make sure you got fresh gas with a good cleaner in it and a new fuel filter.

After that, you can see where you are. It may/may not be a major issue with the injectors. But if all the above is taken care of, then you can start looking at other components.
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Old 12-04-2018, 04:10 PM
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Re: 1988 GTA TPI 350 - rough/light idle (500rpm), rich

Agree with the above as first steps, then I think the first thing I would do is pull the fuel filter and drain the tank and lines. Put 5 gallons of premium in and some injector cleaner with a new fuel filter. See if you can get it running long enough to get it to temp by keeping it at 1000 rpms and see if it clears up. I've had injectors stick open (flood the car) and stick closed just will not run...

I would also verify fuel pressure both running and right after shutdown to see how fast the pressure drops.

You could also have sticking valves that are not closing so you may want to do a compression test and put in new plugs.

I would also pull the cap and rotor and clean the contacts or replace them since they are cheap enough.

Then the last thing before I bought new injectors would be check spark at each cylinder.

You can get rebuilt injectors for ~150
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