Any TBI experts here? I need help with my 87

[NASTY-LSX]

Just picked up a 87 GMC. Top end has been rebuilt. No motor problems.
The owner could not get it to run smooth. He said it was probably the timing.
I bit in, I bought the truck. Well, it was not the timing cause it was dead on.
after 1200 RPM it runs well and strong. idle to 1200 RPM it's not smooth running.

New EGR, New balancer, wires, cap and rotor, Timing chain. what else is there to check. Is there a list to go by as a check off list as where to start for trouble shooting. I dont even know if mechanic shops with touch a truck this old with fuel injection LOL.

[BLE 'BURBAN]

throttle position sensor or idle air control motor. Both located on the passengers side.

[Mattchu60]

When the top end was rebuilt - did they use a stock camshaft? If you put a wilder than stock cam into these TBI engines, they idle very poorly.

[OutlawDrifter]

iac, tps, and cts sensors should all be checked. +2 on the cam.

[NASTY-LSX]

Originally Posted by BLE 'BURBAN

throttle position sensor or idle air control motor. Both located on the passengers side.

Both new
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[NASTY-LSX]

Originally Posted by Mattchu60

When the top end was rebuilt - did they use a stock camshaft? If you put a wilder than stock cam into these TBI engines, they idle very poorly.

Stock cam. Never went internal on replacing parts.
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[rfmaster]

Originally Posted by NASTY-LSX

Just picked up a 87 GMC. Top end has been rebuilt. No motor problems.
The owner could not get it to run smooth. He said it was probably the timing.
I bit in, I bought the truck. Well, it was not the timing cause it was dead on.
after 1200 RPM it runs well and strong. idle to 1200 RPM it's not smooth running.

New EGR, New balancer, wires, cap and rotor, Timing chain. what else is there to check. Is there a list to go by as a check off list as where to start for trouble shooting. I dont even know if mechanic shops with touch a truck this old with fuel injection LOL.

Couple of things come to mind:
1) Stock LO5 or LO3 engine should idle around 500 to 550 RPM with base timing set with EST plug open. Once timing is set reconnect EST plug and timing will be bouncing around 18+/-3 deg BTDC, 600 RPM under ECM control. You'll have to reset ECM to clear CEL (engine off, disconnected battery for about 30 seconds).

2) You'll need to reset IAC. ECM does not know position of the IAC so it needs to be reset after repair. To do this you need to put a jumper between pins A and B of the ALDL connector in the cab. This puts the ECM into ALDL test mode. Turn ignition key to run position (not start) and the IAC should drive completely closed. You should hear a buzzing noise from the IAC. If you have the winaldl up and running on your laptop, you should see the IAC went from 145 to 0. Now unplug the IAC connector. Turn the ignition key off. Remove jumper from ALDL plug. Start motor. If the idle is too low you may have to keep it running. On the front drivers side of the throttle body is the adjustment screw. It may be behind a silver plug. If so just use a nail or punch to poke it and remove it. Then use a #20 torx bit to adjust the idle speed to at least 100 rpm less then your warm idle speed. If 600 then set to 500 rpm. You are looking for the lowest consistant idle your motor will do. When set, turn off motor and reconnect the IAC. Done - ECM will now be able to control idle RPM.

3) Check for vacuum leaks - unconnected vacuum hoses will play havoc with idle. Incorrect intake gasket may result in engine sacking air from lifter valley.

4) Verify that timing markings on the 'new' balancer match OEM balancer relative to the crank shaft dowel slot.

//RF

[NASTY-LSX]

Originally Posted by rfmaster

Couple of things come to mind:
1) Stock LO5 or LO3 engine should idle around 500 to 550 RPM with base timing set with EST plug open. Once timing is set reconnect EST plug and timing will be bouncing around 18+/-3 deg BTDC, 600 RPM under ECM control. You'll have to reset ECM to clear CEL (engine off, disconnected battery for about 30 seconds).

2) You'll need to reset IAC. ECM does not know position of the IAC so it needs to be reset after repair. To do this you need to put a jumper between pins A and B of the ALDL connector in the cab. This puts the ECM into ALDL test mode. Turn ignition key to run position (not start) and the IAC should drive completely closed. You should hear a buzzing noise from the IAC. If you have the winaldl up and running on your laptop, you should see the IAC went from 145 to 0. Now unplug the IAC connector. Turn the ignition key off. Remove jumper from ALDL plug. Start motor. If the idle is too low you may have to keep it running. On the front drivers side of the throttle body is the adjustment screw. It may be behind a silver plug. If so just use a nail or punch to poke it and remove it. Then use a #20 torx bit to adjust the idle speed to at least 100 rpm less then your warm idle speed. If 600 then set to 500 rpm. You are looking for the lowest consistant idle your motor will do. When set, turn off motor and reconnect the IAC. Done - ECM will now be able to control idle RPM.

3) Check for vacuum leaks - unconnected vacuum hoses will play havoc with idle. Incorrect intake gasket may result in engine sacking air from lifter valley.

4) Verify that timing markings on the 'new' balancer match OEM balancer relative to the crank shaft dowel slot.

//RF

Thank you RF.. this was the kind of info I needed to hear. I will try all of this and I will report back. thank you..
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[NASTY-LSX]

RF, will running a vacuum test tell me if I have a vacuum leak if so what is normal vacuum at idle Thanks
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