89 2.8 Idles at 2500-3000 at Cold Startup

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89 2.8 Idles at 2500-3000 at Cold Startup

Postby 89GtuPearlWhite » Tue May 12, 2020 8:06 am

I have a 89 GTU and whenever is start it up and its cooled off it idles up to 2500-3000 RPM Right at startup and sits there. I've noticed that if I put it in drive and move forward and Reverse and move back it drops to 750 RPM. I replaced the TPS Sensor, Map Sensor, New Fuel Injectors, New IAC Valve, done the idle re-learn process 5 different times, and Checked for vacuum leaks (Could have missed one). If I start it up Hot it typically doesn't do it. Im completely stumped any advice would be greatly appreciated.

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Re: 89 2.8 Idles at 2500-3000 at Cold Startup

Postby Rettax3 » Wed May 13, 2020 2:25 pm

Coolant temperature sensor maybe? MAYBE IAT... GM coded those ECMs to be so dependent on coolant temp that a bad sensor can do almost anything to how they run -even preventing them from running at all. Best bet is to get a data cable and see what it is actually reading, barring that though try unplugging the sensor while it is running weird and see what it does. An Ohm-meter can also tell you if it is functional, but I don't recall the specs off the top of my head. I still use an old Snap-On 'Brick' MT2500 to scan my older cars, so I just look at what the computer says the sensors are reading... Other things that effect idle-speed on the '89s are power-steering pressure sensor and A/C request, but neither of these should bump it that high... Good luck, keep us posted.

1989 SuperCharged 3800 Srs-II (First)Six-Speed GTU
1990 Turbo 3.4 5-Speed T-Type
1990 4.0L 4-Cam 32-Valve V-8 5-Speed Indy GTi (Project)
1990 Stock(!) 3.1 MPFI Auto Indy
1995 LA1/L82 4T60E Z-26
1995 3.4 DOHC Turbo 5-Speed Z-26

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Money pit Beretta
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Re: 89 2.8 Idles at 2500-3000 at Cold Startup

Postby Money pit Beretta » Fri May 15, 2020 9:57 am

Had a lot of problems with the idle on my 95. It would be fine and normal for months. Then one day the rpm would crank up to 2500rpm just after the start. It seemed like changing the TPS helped. After the third time the brain kicked in.....there must be something more to this. Inspecting the pig tail that goes to TPS I found that the main wire was pulling out. GM had given the car a short harness, over time engine movement had pulled the wire out of the connecter. Got a pig tail from the JY and that was the end of that problem.
As to the harness the IAT wires are short and make it hard to keep the IAT in my cold air intake. Damn thing is always falling out.

keep'em flying!

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