1987 GMC 350 TBI- hard to start and runs rich - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-29-2011, 04:38 PM
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1987 GMC 350 TBI- hard to start and runs rich

I have a 1987 GMC Sierra Classic 1/2 ton that is hard to start after the engine cools down and seems to be running a little rich. I have rebuilt the TBI (except for the injectors,TPS,and IAC valve). I have also replaced the coolant sensor, fuel and air filter, spark plugs, wires, distributor cap and rotor,oxygen sensor, thermostat, adjusted the timing, and it still burns rich. I have to press the pedal down about half way and crank it for it to start, and if I don't, it will fire and then shut off. The truck registers one code (44) witch means it is running lean, and that doesn't make sense to me. What I believe is causing the problem is the map sensor, tps sensor, or the iac valve. I'm not positive that these are the culprits, so any advice would help.

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Old 01-29-2011, 04:48 PM
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the coolant sensor would be a likely culprit. Seeing that you put in a new one then that isnt your trouble. The map sensor is a pretty good guess actually as that is one of the sensors that control the fuel in open loop (startup and cold engine operation)
You really should get an older scanner and look at the data stream.
The code 44 lean may be due to a degraded o2 sensor not reading much of anything most likely due to contamination.
I have seen throttle bodys that were cracked internally and leaked umetered fuel into the intake.
I am leaning towards a defective ECM actually, but you need to diagnose from the data stream first. Also you should verify that the fuel pressure is correct. It can run rich if it is too hi. A blocked plugged or crushed return line will skyrocket the fuel pressure and cause it to be allways rich

Chances are it starts hard now because the plugs are fouling. Once yo figure it out you will most likely need a fresh set .
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Old 01-29-2011, 05:37 PM
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Thanks for the advice. Come to think of it, my return line does have a kink in it near the connection to the throttle body.
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