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Old 12-16-2008, 04:08 PM
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russ130 russ130 is offline
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1988 Chevy 5.7 tbi runs rough cold and....

Ok here is the deal. This truck has a good motor but it lacks power all the time not like barely moves but it won't burn rubber. Now when its cold it runs really rough coughing and sputtering. If you have the throttle just barely open it will smooth out until it upshifts then it does it again until the rpms build. If I push in the throttle even 1/8th of the way in it just sputters and runs like crap. Once it gets to operating temp all these symptoms go away and it runs normal other than I think it should still have more power but there is no sputtering. I figured since it's a engine temp related problem that a new temp sensor might solve this problem. It didn't! The only code I'm getting is a lean mix. So without buying everything do I need an O2 sensor, ecm or eprom?
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Old 12-16-2008, 04:37 PM
m2low m2low is offline
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crazy question, but have you checked all the spark wires? replaed... they could be split when cold, but as the electricity runs through them they heat up and make connection... would also explain bad power and low RPM sputter... once the RPMS up the one cylinder does not make that much difference

also, those TBI injectors have a rubber 0 ring where they seat in the TBI - have you check when its idleing in the driveway take off the air cleaner and see if you are getting a good spary or is kind of dripping out of one of the jets?
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Old 12-16-2008, 04:53 PM
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russ130 russ130 is offline
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Its got new wires and yes I have checked the spray pattern, no drips just a nice round spray. I also should add that the idle surges while in park and neutral but not while in drive or reverse.
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Old 12-16-2008, 05:15 PM
Onebadbowtie86 Onebadbowtie86 is offline
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if its got new plugs, replace the cap and rotor if you havnt done so
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Old 12-16-2008, 05:54 PM
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Quote:
Originally Posted by russ130 View Post
Ok here is the deal. This truck has a good motor but it lacks power all the time not like barely moves but it won't burn rubber. Now when its cold it runs really rough coughing and sputtering. If you have the throttle just barely open it will smooth out until it upshifts then it does it again until the rpms build. If I push in the throttle even 1/8th of the way in it just sputters and runs like crap. Once it gets to operating temp all these symptoms go away and it runs normal other than I think it should still have more power but there is no sputtering. I figured since it's a engine temp related problem that a new temp sensor might solve this problem. It didn't! The only code I'm getting is a lean mix. So without buying everything do I need an O2 sensor, ecm or eprom?
If its lean best to ck fuel pressure and volume.Then let us know. Im prety sure to do this u need a special adaptor that goes in place of the fuel filter on a tbi.It would also be essention to monitor what all ur sensors are doing on a scanner.
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Last edited by brad96z28; 12-16-2008 at 05:57 PM.
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Old 12-16-2008, 06:18 PM
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Cap and rotor are new and so are the plugs. I had originally thought about it being the fuel pump just based on the lack of power. I didn't know about it running bad when cold at first cause it was in the summer but now that it has cooled down it has shown itself. My thought is if it was the fuel pump wouldn't it not be such a big problem in the cold since the motor on the pump would be cooler?
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Old 12-16-2008, 06:28 PM
Onebadbowtie86 Onebadbowtie86 is offline
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you need more fuel the colder the weather, so look into it being the filter/pump
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Old 12-16-2008, 06:32 PM
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I could make guesses all day. U got to start checking things.
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Old 12-16-2008, 06:43 PM
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Originally Posted by Onebadbowtie86 View Post
you need more fuel the colder the weather, so look into it being the filter/pump
I'll bet you are right about it being the filter or pump. I overlooked the fact it would need more fuel when it is cold.
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Old 12-16-2008, 06:44 PM
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One more thing, anyone know off the top of their head what the pressure should be?
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Old 12-16-2008, 06:54 PM
ed39 ed39 is offline
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if you t in off the supply line in the back of the throttle body it should be 9-11psi, but if your truck is surging at idle i would check for a vacum leak, and check over the basics like timing, check the cap (make sure the contact points ware the rotor meets are good)
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Old 12-16-2008, 09:03 PM
Spitz Spitz is offline
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To be honest you shouldnt have to hook a pressure gauge up to it. Any easy and accurate way it to pull the return line off the throttle body and start the truck, If there is enough fuel the fuel should come out of the return line port at a decent flow, if its slow or not much comes out chances are you have a flow or pressure issue. The regulator is spring loaded and if the pressure isnt enough the fuel doesnt even get to the return line.. Its a hell of a lot easier than checking it with a gauge.
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Old 12-17-2008, 06:47 PM
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There is a temperature sensor that tells the computer how to mix fuel/air since the truck does not have a choke. I have had to replace this sensor on a few of my past trucks. It's located on the front top of the engine, next to where the thermostat is in that elbow.If you replace it antifreeze will leak out when it is removed. Mine had 2 wires to it and the plug went into the sensor. If you get a new one, the truck has 2 temperature sensor on it, one goes to the gauge in the dash and one tells the computer the temp of the engine so the computer can adjust fuel/air mix.I do not know if I ever got a code on mine.
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Old 12-17-2008, 07:32 PM
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Yeah I already tried a new temp sensor that was the first thing I tried
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Old 12-17-2008, 07:41 PM
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Quote:
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Yeah I already tried a new temp sensor that was the first thing I tried
Sorry that did not help
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Old 12-17-2008, 08:02 PM
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Quote:
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Yeah I already tried a new temp sensor that was the first thing I tried
Yes but is the computer seeing the correct signal?
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Old 12-17-2008, 08:16 PM
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i just had a similar problem with my 88 it had no power and a surging idle come to find out the wire broke off my O2 sensor i replaced it and it was like night and day
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  #18  
Old 12-18-2008, 01:11 AM
Beater_K20 Beater_K20 is offline
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pull the vac line off of the EGR and plug it. see if that makes any difference. my 94 was doing the same thing, unplugged the EGR, no more problems. though it does throw a CEL after extended periods at highways speeds. going to eventually turn it off in the tune once i get some more chips.
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  #19  
Old 01-03-2009, 07:53 PM
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The EGR is already bypassed. I checked under the truck to see if the fuel filter looked new or not and it was kinda old but also noticed the line being rotted. So knowing that to replace the fuel filter the line will more than likely not make it, and to replace the line I have to drop the tank, and since the tank is already dropped I might as well go ahead and replace the fuel pump and do all this in one shot. So today I picked up a new pump and screen, fuel line and I already had the correct filter for some reason. Tomorrow I will drop the tank and make the repairs. I'll let you all know how things work out Sunday night.
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Old 01-03-2009, 09:03 PM
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Like i said take the air cleaner off and remove the fuel line on the left side (passenger side) and start the engine, jump up on the truck and see how much fuel is coming out of the port on the tbi.. There should be a decent amount of fuel streaming out, if not, you have a pressure problem.. If no fuel is returning you have some sort of pressure or flow issue..

Ps- this is the return line, the pump should make well above what the truck needs, most can create 90psi or more if deadheaded, so if there is no fuel return then there isnt enough pressure and you need a new pump, or a filter that is extremely clogged.
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