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Old 02-04-2012, 08:29 PM
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89 Throttle body 350 rough idle and hesitation

I'm working on my 89 gmc sierra its got a throttle body 350...i'm having problems with its idling and acceleration...both issues are not a steady issue, it comes and goes but sometimes it idles real rough and when you put it in gear and give it gas to take off it sputters and acts like it doesn't want to take fuel push the gas down a little further and it clears up and takes off most of the time, every once in a while it will kill the engine. like i said though it doesn't do this all the time. as far as what i've done to it so far, its gotton new plugs, wires, cap, rotor button and fuel filter pretty much all of the sensors and electrical components are new or less than a year old so i'm a little stumped as to what the problem might be. Also as of yet the computer has not given any trouble codes.

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Old 02-04-2012, 08:46 PM
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sometime the tbi throttle shafts wear and cause vacuum leaks

also have you checked the tps.. ? hows the spray pattern out the injectors

egr could be acting up..

just because a sensor was replaced don't mean it can't be failing

check for vacuum leaks, cracked hoses /etc
play in the tbi throttle shaft..

smooth signal from the tps. as it travels from open to closed
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Old 02-04-2012, 09:03 PM
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I don't have a meter to check the tps but i did the sensor off of another tbi truck that was running perfectly just so i could eliminate that. With the tps off of the functional truck it had the same exact issue so i'm led to believe thats probably not it. The spray pattern looks fine and the shaft has no play, i've even went around the throttle body with wd-40 and a hose listening for leaks, couldn't find any and i've long since replaced the factory plastic vaccum lines for old school vaccum lines and as for the egr it was taken off last week, its was nice and clean and functioning properly when vaccum was applied to it. I've been working on cars almost all my life so i've got pretty good problem solving skills but sometimes i overlook the little things and thats where this forum is helpful...I don't have a guage to check the pressure but i'm wondering if the fuel pump might be getting weak, but it seems like my problem would be a steady issue if that were the case. I miss having a carb and a few wires running everything, those were the days........Thanks for the suggestions though
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Old 02-04-2012, 10:06 PM
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you test the fuel regulator on the tbi
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Old 02-04-2012, 10:17 PM
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Is there a way to test it without a gauge? Could the regulator be malfunctioning and only do it some of the time?
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Old 02-04-2012, 10:38 PM
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Electric fuel pumps will sometimes go out intermittently, off and on. I'm not diagnosing your problem, but a bad electric fuel pump isn't always a consistent problem. You can always pull it and test it.
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Old 02-04-2012, 10:56 PM
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thank you JJack010 thats what i was wondering...anytime i've had a fuel pump problem in the past it always went out all at once but i've never been sure whether they can show signs of failure before quitting all together. Thanks for the suggestion I'll check into that further tomorrow, being as i've checked about everything else i was leaning towards the fuel pump anyway, the second opinion makes it worth checking out
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Old 02-04-2012, 11:00 PM
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maybe a bad ground for the fuel pump circuit..
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Old 02-05-2012, 01:05 AM
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First off you need a scanner and the 3 shop manual is a must!! I say this because in them they have most of the trouble shoot guides you need! with the scanner you can read the values to see if a component is not in the right value range and may need replacing! Most common problem is the EGR water temp switch and the most common on a OBD1 truck is the ground wires I thing there are 4-6. if there not clean you can get all kinds of weird things happening! Also here is a good site for trucks! www.fullsizechevy.com
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Old 02-05-2012, 08:45 AM
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Thanks for the suggestions Chevytruckguy...I don't have any money to go out and buy a scanner or a manual, i have the basic Chilton manual which is pretty much useless in this situation. I put a motor in it last week so the 3 ground straps that go to the motor are clean as well as the body ground, i'm sure of that much. I'm going to checkout the fuel pump ground and the fuel pump itself and if that doesn't help I'm either gonna sell the truck or send it to the scrapyard, which will likely bring me more than I can sell it for.
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Old 02-05-2012, 02:26 PM
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Quote:
Originally Posted by martee373
Thanks for the suggestions Chevytruckguy...I don't have any money to go out and buy a scanner or a manual, i have the basic Chilton manual which is pretty much useless in this situation. I put a motor in it last week so the 3 ground straps that go to the motor are clean as well as the body ground, i'm sure of that much. I'm going to checkout the fuel pump ground and the fuel pump itself and if that doesn't help I'm either gonna sell the truck or send it to the scrapyard, which will likely bring me more than I can sell it for.
Take a step back! That what I had to do on mine! autozone and checker have scanners you can use for free. In the manual you have they have the values you need. I have one myself before I pickup the shop manual off of ebay!!
So first you need to know is everything working correctly no trouble codes? don't assume! and don't just throw parts at it! My son's truck the egr worked but the port was all carboned up so it was not working! Remember the computer takes the info from the sensor and adjust the tbi!! Its a pain I know! The truck site is great for helping you through the process of getting your truck to run right!!
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Old 02-05-2012, 03:10 PM
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You can get codes with a jumper wire on a truck that old. If the MIL isnt on, there wont be any stored codes.
You need to perform a fuel pressure test. Put a gauge on it, tie it to a windshield wiper and drive it. When it acts up check the pressure, makes it easy .
If you think it could be the pump loses power take a wire and tap into the grey wire back near the tank, then hook it to a small light bulb or test light and put it in the cab where you can see it and the gauge. If the light goes out or dim and the pressure starts to drop and the truck starts to run like crap then it is electrical.Be sure to check and clean the ground wire that comes from the tank harness to the frame.It should bolt to the frame sonwhere close to the tank. It will have a bolt to the frame with a 13 mm head on it. Take it off,wire brush it, sand the frame spot that it touches, put a little grease on it to help keep it from corroding right away, and continue.
If the power stays constant but the pressure drops and the truck runs like crap, then I would suspect the pump or regulator.
If the truck runs like crap and the pressure stays good say at 13 lbs, then you have eliminated the pump as the problem.
You can do the fuel gauge thing first to see if it is fuel pressure.If it is then check the power supply to be sure it isnt the power or ground.
If it is electrical and not the pump, the issue could be corroding wiring or a bad /intermittent oil pressure switch, as the switch powers the pump after oil pressure is made by the engine.keep us posted
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Old 02-05-2012, 04:07 PM
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chevytruckguy, the autozone in town, which we just got about a year ago says they only have a scanner for 96 and newer. and i know the egr ports are open because i put the intake on about a week or so ago before i put the engine in the truck and both the ports in the intake and the head for the egr had some buildup but that was taken care of before it got put back together.

latech, there are no trouble codes to check and yes i know about pulling codes with the a jumper wire, thats how i've always done it i in the past but theres simply no codes to read but which oil sensor are you referring to that is in the circut with the fuel pump? When i put the motor in i actually had both unplugged, they're kinda outta sight out of mind and i simply overlooked them at first and the truck started just fine, coincidentally the oil pressure gauge in the dash pegs out as soon as you turn the key but i've got a manual gauge so i know i have good oil pressure. but which sensor is supposed to have that control?

Took a day of from working on the truck but i'll keep you up to date when i do some inspecting of the fuel pump wiring, and if i can find someone that has a fuel pressure gauge how would i go about hooking it up on this truck? would it screw in between one of the lines and the throttle body??
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Old 02-05-2012, 04:31 PM
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The Oil pressure switch in the back by the distributor has the fuel pump switch and also has the ground path for the Oil pressure gauge.It should have a 3 wire.It may have a 2 wire which would be for the pump and could have a seperate bulb down by the oil fiter for the gauge but I doubt it.
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Old 02-05-2012, 05:55 PM
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Its just a tee fitting off the fuel line with a fuel gauge on top! A gauge is a round 14.00 look on the truck site you can see pic's of them install! Also ebay tbichip is a good source for how too!
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