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Old 11-04-2009, 02:29 PM
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350 TBI fuel issue

Kind of stumped with my truck (1988 GMC K1500, bone stock 350tbi/700r4)

Went to start it a few days ago, started fine like normal but idle was kind of eratic. I let it warm up to temp and it made no difference.

Long story short, it will not rev up, hold constant RPM, idle, or show a check engine light. I tried unplugging the map sensor and it idled fine, still missed a little and throttle response was terrible. I am aware that unplugging the map increases fuel so obviously the motor is running lean.

Any suggestions or tips would be appreciated.

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Old 11-04-2009, 04:35 PM
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Check the fuel pressure. Check the vacuum lines. These engines are notorious for carbon plugging up the vacuum ports in the throttle body causing it to not get the correct vacuum signal to the map sensor. May also want to check the spark timing.
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Old 11-05-2009, 10:16 PM
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I had some time today to work on it a little, I wasn't able to hook up a gauge to test vaccum but I did pull off most of the lines and non of them seemed to be clogged.

I also bought a kit and rebuilt the fuel pressure regulator. Made no difference.

I pulled off the vaccum line that goes to the power brake booster (Removed from manifold) just for the hell of it and it suprisingly made it run significantly better, not perfect though. Does this usually hint at anything?

So far the FPR, MAP sens, and egr have all been pulled off and cleaned or replaced. The timing was right on the money.

I will check fuel pressure tomorrow but I'm pretty confident its something computer related. Alldata's spec for fuel pressure was 9-13 psi
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Old 11-06-2009, 10:14 AM
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Unplug the coolant temp sensor on the front of the intake, next to the tstat houseing.
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Old 11-06-2009, 10:31 AM
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Did you check the fuel filter? EXACT syptoms of a nearly completely restricted filter.
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Old 11-12-2009, 05:50 PM
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Fuel filter was the first thing I replaced.. I'll check it out again to see if it clogged up. I added 5 gallons of gas right after I started having problems just to make sure that it's not picking junk off the bottom of the tank.

At this point I have replaced the map sensor, checked voltage range on the TPS and IAC, and rebuilt the fuel pressure regulator using both the new spring and the original spring. Also checked the voltage going to all the sensor and all of them were recieving power (around 5 volts) and all grounds were fine.

I am going to replace the map sensor again just to make sure I didn't get a bad one, along with the fuel filter. If that doesn't solve anything then I'm out of ideas.
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Old 11-12-2009, 05:53 PM
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coolant temp sensor.
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Old 11-12-2009, 05:56 PM
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JR, forgot to mention I tried unplugging it.. it increased the idle but still surges and it still has terrible lag when revving up.
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Old 11-12-2009, 05:58 PM
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350 TBI fuel issue

Check codes on the dash ? 88 has flash code,, 327/350hp
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Old 11-12-2009, 06:02 PM
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Had my neighbor scan it while I was at work a few days ago... I don't remember the exact codes he read off to me but they all basically lead to the map sensor.
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Old 11-12-2009, 06:09 PM
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350 TBI fuel issue

I,ve had to replace TB base gasket on those trucks, idle was erratic also 327/350hp
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Old 11-12-2009, 07:20 PM
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What types of symptoms narrowed you down to the tb gasket?
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Old 11-12-2009, 07:32 PM
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The first thing I thought of was the fuel pump but, you haven't had any hard starting problems. The next thing is the throttle body injectors. I would look down the throttle body while it's running with a strong led flashlight. There should be a cone shaped fuel spray coming out of them both. If not, they need replaced. It's not uncommon for injectors to go bad after 20 years.
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Old 11-13-2009, 03:32 PM
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350 TBI fuel issue

Quote:
Originally Posted by Burnsumrubber305
What types of symptoms narrowed you down to the tb gasket?
Hi,, the idle was higher, truck was sluggish, over all DOG.. These gaskets will burn through over a time....Good Luck 327/350 hp
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Old 11-13-2009, 06:16 PM
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Quote:
Originally Posted by sqzbox
The first thing I thought of was the fuel pump but, you haven't had any hard starting problems. The next thing is the throttle body injectors. I would look down the throttle body while it's running with a strong led flashlight. There should be a cone shaped fuel spray coming out of them both. If not, they need replaced. It's not uncommon for injectors to go bad after 20 years.


The fuel, no matter how the truck is running, always comes out in a cone shape. I'm starting to wonder if I have a short to ground somewhere within the map circuit.

As far as fuel pump goes, at first I thought it was too but even if it was slowly dying the motor should still be able to idle, which it's not. I could be wrong but that was just my guess on it.
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