88 GMC Idle problem

Will · May 30, 2005

I’m having a problem with my ’88 GMC 1500. First off the problem occurs simultaneously with the service engine soon coming on. The idle on the truck is very erratic to say the least. While sitting at a stop light I have to keep my foot firmly planted on the brake pedal to keep it in place. If I put it in neutral it will rev way up and then down and then way up again. While driving it will almost stall and then pick back up. I’ve had this problem in an Oldsmobile, fuel injected, it was the idle control valve. However, I am not sure if my truck has an idle control valve?

Need any and all help, working on a tight budget.

Thanks.

GM-Tech · May 30, 2005

The first step would be to see what code is stored. I suspect you might get a MAP code, which would not be unusual with that type of idle condition. In that case, the MAP code would probably be a result of the idle, not the cause of it.

Been a veeerrrrrrryyyy long time since i checked a code with a paper clip, but I think you jump from A to B in the diagnostic connector under the dash with the key on, engine not running.

Sounds like it's lean. An IAC doesn't usually cause this kind of problem.

Will · May 30, 2005

The first step would be to see what code is stored. I suspect you might get a MAP code, which would not be unusual with that type of idle condition. In that case, the MAP code would probably be a result of the idle, not the cause of it.

Been a veeerrrrrrryyyy long time since i checked a code with a paper clip, but I think you jump from A to B in the diagnostic connector under the dash with the key on, engine not running.

Sounds like it's lean. An IAC doesn't usually cause this kind of problem.

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How does it flash the codes? Or better yet how do I decode the flashes?

GM-Tech · May 31, 2005

When you ground the diagnostic terminal in that connector, it flashes the check engine line. Each code is two parts. For example, it should flash a code 12, three times, to start. This indicates the computer is capable of communicating, it's not a problem.

It will flash once, pause, then flash twice. Pause again and then repeat the previous. It will do this three times.

After the code 12, it will flash the real trouble code(s) in numerical order and repeat each one three times. If there are no codes, it will just keep flashing the 12 over and over until you unground the terminal.

If you had a lean code, which is 44, after the 12, you'd get 4 blinks, a pause, 4 blinks. That'd repeat three times.

MTRMAN5 · June 4, 2005

I’m having a problem with my ’88 GMC 1500. First off the problem occurs simultaneously with the service engine soon coming on. The idle on the truck is very erratic to say the least. While sitting at a stop light I have to keep my foot firmly planted on the brake pedal to keep it in place. If I put it in neutral it will rev way up and then down and then way up again. While driving it will almost stall and then pick back up. I’ve had this problem in an Oldsmobile, fuel injected, it was the idle control valve. However, I am not sure if my truck has an idle control valve?

Need any and all help, working on a tight budget.

Thanks.

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CHECK THE GASKET UNDER THE TBI, IF IT HAS NEVER BEEN REPLACED TIME TO REPLACE, LOTS OF PROBLEMS WITH 88 89 350'S WITH TBI. SPRAY SOME WD40 AROUND THE BOTTOM OF THE TBI IF THE ENGINE SPEEDS UP AND STARTS TO RUN BETTER THAT IS THE PROBLEM, THREE BOLTS REMOVE IT AND NEW GASKET UNDER 10 BUCKS...

GM-Tech · June 4, 2005

I agree. Replaced a lot of those throttle body gaskets myself. I will say, however, that in most cases, they seemed to cause a high idle and not an irratic idle. But it's been a while and I may not be remembering all the particulars.

MTRMAN5, you might want to try turning off your CAPS lock.

Will · June 4, 2005

When you ground the diagnostic terminal in that connector, it flashes the check engine line. Each code is two parts. For example, it should flash a code 12, three times, to start. This indicates the computer is capable of communicating, it's not a problem.

It will flash once, pause, then flash twice. Pause again and then repeat the previous. It will do this three times.

After the code 12, it will flash the real trouble code(s) in numerical order and repeat each one three times. If there are no codes, it will just keep flashing the 12 over and over until you unground the terminal.

If you had a lean code, which is 44, after the 12, you'd get 4 blinks, a pause, 4 blinks. That'd repeat three times.

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Okay I got a 15 and 45??

So that's Rich Exhaust? and a coolant senor?

GM-Tech · June 4, 2005

Okay I got a 15 and 45??
So that's Rich Exhaust? and a coolant senor?

Yep. And the 15 (coolant sensor) could result in the 45. 15 means the ECM sees a high voltage on the coolant sensor circuit. That means the sensor's not pulling it low so it thinks the engine is cold. Maybe very cold. If there's an open circuit in the coolant sensor circuit, the ECM sees that as -40 degrees.

If it's -40 degrees, the ECM is going to inject a LOT of fuel to get the engine started and keep it running. If it's not really that cold, it's going to run very rich (code 45).

The sensor is in the front of the intake manifold if I recall correctly. Two wires, one yellow, one black. We used to see those wires badly deteriorated right at the sensor connector. Also had the rare case of the sensor leaking coolant, and with the weather tight seal of the connector, it would pump coolant through the harness insulation all the way back to the ECM. That's not very common, but it can happen. Usually, it's the wires insulation falling apart right at the connector.

Anyway, that's where I'd look first. Could be a sensor too.