'89 Jimmy IDLE PROBLEMS

[bassman1]

New to the forums! Just bought a used 89 jimmy and the service light came on recently. Checked codes and solutions were changing the TPS and MAP sensor. It had a very high idle. I did replace both....Seemed to help for that day, however, it returns to the high idle now and then. Battery was diconnected when i replaced the sensors for at least 40 minutes. Any idea's?

[jeffharley]

Originally Posted by bassman1

New to the forums! Just bought a used 89 jimmy and the service light came on recently. Checked codes and solutions were changing the TPS and MAP sensor. It had a very high idle. I did replace both....Seemed to help for that day, however, it returns to the high idle now and then. Battery was diconnected when i replaced the sensors for at least 40 minutes. Any idea's?

I had this problem with a 4.3 throttle body carb.It turned out to be the carb gasket(between carb and manifold)and you couldn't tell till you pulled the carb.The gasket was abuot $6 and instantly corrected the problem.~Jeff~

[bassman1]

Originally Posted by jeffharley

I had this problem with a 4.3 throttle body carb.It turned out to be the carb gasket(between carb and manifold)and you couldn't tell till you pulled the carb.The gasket was abuot $6 and instantly corrected the problem.~Jeff~

Of course that is the last resort I want to do........i just started it and the idle shot sky high.........i disconnected the battery for a few minutes and it brought it down when i tried it again....this tells me it is still something electrical or computer problem.........any more ideas?

[jeffharley]

Originally Posted by bassman1

Of course that is the last resort I want to do........i just started it and the idle shot sky high.........i disconnected the battery for a few minutes and it brought it down when i tried it again....this tells me it is still something electrical or computer problem.........any more ideas?

The reason the idol shoots up so high is because of a vacuum leak and the computer sends more fuel to compensate for the improper mixture.There are several jurnals under the carb that control the air/idol mixture.This is where the gasket dissintegrates.and hense the vacuum leak.Its really not a hard job to get the carb off I didn't want to do this either but after going crazy for about 2 weeks I finally bit the bullet and did it .Can't tell ya how glad I was I did.~Jeff~

[Teaman3]

Originally Posted by bassman1

Of course that is the last resort I want to do........i just started it and the idle shot sky high.........i disconnected the battery for a few minutes and it brought it down when i tried it again....this tells me it is still something electrical or computer problem.........any more ideas?

Carburettor, CPI or TBI ? Engine type and size ?

When the engine is started from "cold", The ECM will set the rpm high by enriching the fuel mixture - same idea as using a manual choke on a carburettor. After about 20 - 30 seconds, the idle should decline, depending on the coolant temperature.

What can make it worse, are leaky/broken vacuum lines.

Intermittent "spikes" of high idle when warm may have several sources of faults:

Poor electrical connections
Temperature sender faulty
O2 sensor signal false
RPM signal false
IAC incorrectly set

Before you spend your cash, spend some time eliminating the simple faults:

Gas tight vacuum lines. Most of the lines you can see. There's one, the interior vent vacuum feed, that is behind the distributor. It's a plastic tube passing through the bulkhead. Check it, it may be fractured. The rest are probably just loose.

Sound electrical connections including chassis grounds. The O2 sensor requires a chassis ground. You may have to provide one, if the existing one via the exhaust is high resistance.

I have a 1988 S15. Most of the connectors had never been cleaned since they were installed. Cleaning them, including the ECM connectors, has made quite a difference.

Disconnecting the ECM causes the current fuel mixture setting to be lost. The ECM will start from a preset ratio and adjust it, within limits, according to the O2 sensor, when the engine is warm, and O2 sensor stable. You should avoid disconnecting the ECM until you really have to.

The ECM relies on electrical signals to run the engine. Dirty/oxidised connectors and high resistance chassis grounds kill signals.

You say you've recently bought this vehicle. One thing I'd advise you to do is buy a spare ignition module and keep it in the vehicle. They fail and then there'll be no ignition.

Enjoy....