No High Idle - 1989TBI

[1987SierraClassic]

I'm wondering if anyone has seen an issue with the GM TBI where the high idle (cold) does not function. The truck starts/runs, but it stumbles a lot when it's cold. Once warmed up, it's fine. I've replaced pretty much everything except the TBI itself and I think that may be the next step, or at least rebuild it.

I have two trucks with the exact same problem, so it has to be something in that comes along with high mileage.

Anyone seen this one?

Thanks!

[Jonboy]

Check the coolant temp sensor. A failed coolant temp sensor will cause poor idle.

[d10s69c10]

not neccisarliy familiar with this but have you checked your vacume lines intake air tempurature sensor and egr valve?

leaning more towards vacume due to the cold weather. air temp really only tells it how much fuel to throw in and a bad egr will just make a funk idle anyways cold.

[Square84]

a bad idle air control valve would do it also

[1987SierraClassic]

Thank you for all the replies...

The EGR valve is new and there are no signs of vacuum leaks. When I hook up the laptop diagnostic, the ECM temp reading agrees with the temp. gauge and the IAC is stepping (at least the ECM thinks it is). I can put the system into diagnostic mode and the idle steps up to 1000 RPM.

I'm thinking that perhaps there are some blocked passages in the TBI assembly. It could also be leaking air past the throttle shaft which is common in those TBI's.

Most of the sensors have been replaced while chasing this problem.

Does anyone know of a decent source for rebuilt TBI's? I'd like to minimize down time, but I'll rebuild my own if necessary.



Thanks!

[BLE 'BURBAN]

Your engine has 2(two) coolant temp sensors on it. The one located in the D/S head just above the spark plugs is for the gauge reading.
The other one, located at the front of the intake to the P/S of the thermostat housing is the coolant temp sensor for the computer for the T.B.I. unit.
Also I have to agree with SQUARE84 as well.

[ChevyTech]

I can put the system into diagnostic mode and the idle steps up to 1000 RPM.

In the “Data link” mode (also called 10K or special) the IAC controls engine idle to 1000 RPM + or – 50 RPM (if applicable).

Being you see the RPM increase to 1000 RPM the IAC is working.

Any vacuum leaks would cause a fast idle, not a slow idle.

I agree with JonBoy that this sounds like a bad coolant temp sensor.

Let the truck sit over night and use the laptop diagnostic to see what the coolant temp reading is BEFORE you start the truck.

I can post specifications if you want to test the temperature sensor with an ohm meter.

[1987SierraClassic]

Hey, Chevy Tech,

I agree, the ECM is getting a false temp reading. I'll check the ECM temp reading prior to startup and see if it reads ambient...or whatever the 'low' reading is. Maybe a bad ground or something on the sensor.

If you wouldn't mind posting the Ohm test specs, I'll check to make sure that it is within spec. I replaced this sensor on one truck and it didn't make a difference, but...the other truck may be different.

The idle steps up when I engage the A/C compressor, so I'm in agreement that the IAC is working.

I appreciate the help!

[ChevyTech]

It is common for these sensors to fail.

If you see any coolant leaking out the terminals replace it.

To test the coolant sensor, unplug the wire connector and test the resistance across the two terminals of the SENSOR with an ohm meter.

The sensor screws into the intake manifold, on the front passenger side, by the thermostat housing.

Coolant sensor approximate resistance specifications:
177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
667 ohms @ 140 deg. F. or 60 deg. C.
973 ohms @ 122 deg. F. or 50 deg. C.
1188 ohms @ 113 deg. F. or45 deg. C.
1459 ohms @ 104 deg. F. or 40 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.
12300 ohms @ 23 deg. F. or -5 deg. C.
16180 ohms @ 14 deg. F. or -10 deg. C.
21450 ohms @ 5 deg. F. or -15 deg. C.
28680 ohms @ -4 deg. F. or -20 deg. C.
52700 ohms @ -22 deg. F. or -30 deg. C.
100700 ohms @ -40 deg. F. or - 40 deg. C.