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85 T Type tune up advice

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My 85 T Type bogs/hesitates/stalls until it gets warmed up. I have tried to set the idle and IAC using my Turbolink scanner. I am not getting anywhere. Before I go any farther, are the idle adjustment and IAC adjustment done via the same screw on top of the throttle body, just below the vacuum lines? Or is there another adjustment screw that I am missing completely? :confused: I'm attaching a pic of the screw in question.

Phil Cooley


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As I understand it, the IAC is where the idle air is set, which is controlled by the computer commands. The idle speed is a function of the ECM "chip" and its application. If the speed is not "to spec", then it commands the IAC to vary things to make it "to spec". That's my understanding of how that works.

The set screw you reference might control the closing of the throttle plate in the throttle body, but it's the IAC that really controls the idle air flow and related idle speed rather than a set screw "travel limiter". For the IAC to work, the throttle blade needs to be basically closed completely at idle, but it might be cracked just a hair to keep it from binding and being hard to open. Typically, there are not any "user adjustable parts" in the throttle body other than the items you can replace . . . IAC, vacuum manifold block gasket, vacuum lines. Therefore, the set screw should not need any adjusting after it leaves the factory.

Have you cleaned the throttle body and ALL of the internal and related "air supply" passages with an appropriate cleaner? How recent is the Oxygen sensor? Any codes in the computer?

Just some thoughts,


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On the carb turbo motors Buick used a "heat grid" which is simply a heated screen that looks like a carb mounting gasket to heat up the incoming mixture when the engine is cold. I dont know if the later fuel injected cars still use the grid but I can vouch for the fact the carb equipped cars do exactly what your car is doing when the grid is non-op.

I dont think idle adjustments, especially to a computer controlled fuel injected engine, will cure your problem.

Check for vacuum leaks. Several small leaks can disrupt manifold vacuum enough to cause the symptoms you are experiencing.

Are the plugs and wires relatively fresh? Basic state of tune? How about the fuel filter?

As has been suggested, look for codes. Be sure the throttle body does not have a buildup of carbon on the butterfly as this can dramatically affect idle quality and part throttle op. If you have a scanner, make sure, by observing real time data, that your temp sensor is reading accurately. If not, there is a possibility the temp sensor is telling the computer the engine is up to temp when in fact it is cold. Just a few bases to cover...

Tom Mooney

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Phil, make sure the throttle body and plate is good and clean, especially the IAC passage ways. Be careful with the IAC gasket when you take it off, can't get them. Once you make sure it's all clean ground the diag terminal and turn key on, (don't start), with the ground and key on go unplug the IAC valve connecter. This is suppose to make your IAC stick all the way out, which is essentally closing the IAC passeng ways when setting base idle. Key off, take ground out and with someones help you have to start the car, put it in drive and adjust that screw you showed till the car will run at 500 RPM +/- 50. When that's done reconnect the IAC harness. You've set the base idle. The GN and the suped up Turbo guys says 600 but my service manual says 500. Also some of them don't even bother with the IAC part of the procedure. Does it work? IDK?

I have a basic tune up procedures from a Hemmings Muscle mag Tom Mooney showed me a few years ago. I'll scan it and email to you.

BTW My rebuilt turbo engine will be ready for me to pickup Saturday!

Edited by sosuzguy (see edit history)
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Phil how long has it been since the EGR was replaced? Mine passed the EGR test in the manual but the spring tension was weak enough when placed under a load during first take off it was weak enough to suck the EGR open and hence I had an unmetered air leak making it too lean..That and a temp sensor that was giving ECM erroneous readings. Mine was doing the same thing!

Edited by sosuzguy
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  • 4 weeks later...

I hadn't been on here in a while. It was good to see you guys' posts... .

Throttle body plate is clean, as is the IAC passage. IAC is new. O2 sensor is several years old but has less than 200 miles. It looks like I'm getting decent O2 readings on my Turbolink scanner.

I have replaced all of the vacuum lines, but it's been a while--I need to double check for vacuum leaks.

I haven't replaced the EGR, so I've ordered one from Rock Auto. I should have it by Saturday, which is when I'll be working on the Riv again. Wish me luck!

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