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Tim's Auto Repair
Tim's Auto Repair, Mechanic
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Experience:  Have owned a repair shop for 25 yrs.
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1989 GMC truck 350 TBI. loading excessive feul at idle. backfires

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1989 GMC truck 350 TBI. loading excessive feul at idle. backfires until I get moving good then it runs great. Gives excessive feul code. Changed idle air,coolant temp. There is no cataletic converter just a pipe in place of it

Tim's Auto Repair :

Hi,My name is Tim,

I will do my best to assist you

Tim's Auto Repair :

please tell me the code you are getting


45 excessive feul

Tim's Auto Repair :


Tim's Auto Repair :

sorry i missed you

Tim's Auto Repair :

there are a couple of other things you may want to check here

Tim's Auto Repair :

1-the map sensor

Tim's Auto Repair :

2-the ecm[computer]itself

Tim's Auto Repair :

i will give you some tests/checks you can do for the 45 code that may help you find your problem

Tim's Auto Repair :


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Code 45 - Rich Exhaust Indicated

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Wiring Diagram For Code 45 - Rich Exhaust Indicated


Circuit Description:

The ECM supplies a voltage of about .45 volts between terminals "D6" and "D7". (If measured with a 10 megohm digital voltmeter, this may read as low as .32 volts.) The oxygen sensor varies the voltage within a range of about 1 volt if the exhaust is rich, down through about .10 volts if exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below about 315~C (600~F). An open sensor circuit or cold sensor causes "Open Loop" operation.

Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 45 is set when the oxygen sensor signal voltage or CKT 412:

^ Remains above .7 volts for 50 seconds, and in "Closed Loop".
^ Engine time after start is 1 minute or more.
^ Throttle angle greater than 2%. (about .2 volts above idle voltage)

Diagnostic Aids:

Using the "Scan", observe the block learn values at different rpm and air flow conditions"Scan" also displays the block cells, so the block learn values can be checked in each of the ceils to determine when the Code 45 may have been set. If the conditions for Code 45 exists, the block learn values will be around 115.

^ Fuel Pressure - System will go rich if pressure is too high. The ECM can compensate for some increase. However, if it gets too high, a Code 45 may be set. See Chart A-5 thru A-6
^ Leaking Injector
^ Check for fuel contaminated oil.

^ HEI Shielding - An open ground CKT 453 (ignition system reference low) may result in EMI, or induced electrical "noise". The ECM looks at this "noise" as reference pulses. The additional pulses result in a higher than actual engine speed signal. The ECM then delivers too much fuel, causing system to go rich. Engine tachometer will also show higher than actual engine speed which can help in diagnosing this problem.
^ Canister Purge - Check for fuel saturation. If full of fuel, check canister control and hoses.
^ MAP Sensor - An output that causes the ECM to sensor a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the ECM to set a fixed value for the MAP sensor. Substitute a different MAP sensor if the rich condition is gone while the sensor is disconnected.
^ Pressure Regulator - Check for leaking fuel pressure regulator diaphragm by checking for presence of liquid fuel in the vacuum line to the regulator.
^ Check for leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for fuel.
^ TPS - An intermittent TPS output will cause the system to go rich, due to a false indication of the engine accelerating.

Tim's Auto Repair :

If you are satisfied with my answer,Please Rate my answer by clicking the rating button so I can get credit for my work..I'm not always going to be giving you good news, so please don't let this stand in the way of you accepting my answer. .it does not cost you more money and we will still be able to communicate, ,.please understand, i can not see, hear or touch the vehicle and only know what you tell me. Bonuses and positive feedback are appreciated.. if you are not satisfied with my answer, please do not leave bad feed back ,i will gladly opt out and let another expert handle the question. .if i have sent any diagrams, ,please print them. .sometimes you may need to right click the image [,then save picture as]to save it to your computer so you can open it with your picture program...Please ask if you need more help


I put new injectors in today. New EGR ,Pressure reg N feul inj. kit . It runs better. I did this on my own but I asked ya so im a man of my word so I have no problem paying for what I asked for even though I perfomed the work with no answer. Thank You. Rob R Close

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