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Thread: 1998 5.7 Vortec issues again, requesting help

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    1998 5.7 Vortec issues again, requesting help

    I'm posting again about the 1998 silverado 4x4 k3500 with the 5.7 vortec. I had the posting about the backfiring issues during deceleration. All of your advice was rock solid so I'm looking for more info from chevychase and dirtrider. Not sure how to contact. Tried to add chevychase as friend. At any rate, I did as instructed and the truck is running very nice. Accept now it's really weirding me out. I replaced the MAF sensor and it cured the issues it was having. But the service engine light wont go off so I go back to parts store and scanner says MAPressure low. So I buy a new MAP sensor and change it out (yes battery was disconnected) the light goes off signifying no problems and I get it out on the road home and it goes right back to doing what it did before I changed out the MAF! Kicking, sputtering, backfiring on deceleration! Brand new parts! What gives? I take the new MAP out and put the old one back on and it seems fine again but the service light comes back on and I did this twice with two different MAP sensors both brand new! Should I drive it over ten miles and see if the PCM will relearn to accept new sensor? What do I do? I just want it to be fixed! Why would it go back to doing that with NEW parts?! I just need to know what to do. I'd appreciate the help fellas. Do I need to get something done to the PCM?

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    Senior Moderator chevychase's Avatar
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    Okay Bobby ... you need to check for a vacuum leak. Map works off of vacuum true name for a map / manifold absolute pressure. Could type a whole thread on the map and it,s working.
    Check that vacuum hose to map is good check for vacuum leaks at throttle body intake. All over. A dirty air filter will make maf read wrong .
    Check make sure no air leaks in system down stream of maf. Like you said run for a bit clear codes see what happen.
    But on both the maf and the map all has to be sealed good. It is late for me if need be i can let you know tomorrow how to check voltages on the two.
    Thanks on the friend request. Sometimes hard to explain all this . We here will try our best to get it right .

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    I have also had problems with the cap and rotor on these vortecs they'll crack and will cause it to spark all different cylinders causing it to back fire and will also throw the MAF sensor code

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    If you put new parts on and clear codes then the codes should be gone and never come back unless something is wrong, It keeps giving the MAP or MAF code cause something is still not right. The MAP code saying its low pressure, need to make sure its getting good solid vacuum. Make sure the vacuum line is good no crack or leaks, make sure its a good solid line as the vacuum the MAP needs and wants is a good vacuum and thin lines will be sucked closed, need to check where the vacuum line for MAP connects at the intake or throttle body is getting vacuum, when running remove vacuum line put finger over hole see if it has good suction if not then the line is good and the rest is not could be clogged up. MAP sensor being bad or the vacuum being bad will give the problems you have. I had this happen on a car and took me forever to figure it out, let alone trying to drive the thing home when it wanted to die out on me.
    1994 white ext cab chevy silverado 5.7 rolling 230k miles
    stock with air filter lid flipped,
    open flowing exhaust to a 40 series flowmaster dumped.
    God Bless America, Support our Troops!

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    Senior Moderator chevychase's Avatar
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    What code 105 106 107 108 ?

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    Senior Moderator chevychase's Avatar
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    Done some reading a lot.. my eyes hurt ..without checking some voltages ect. will be hard to track down.
    But with you symptoms... what i came up with.. sounds like it is tps ../ throttle position sensor.. tripping your map issue.
    Classic symptoms .. hesitations ... backfire out the exhaust on decelaration.. running rich.

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    Okay fellas check it out. I replaced the MAF already and it seemed to fix the problem the first time but the service light won't go off. I then had it scanned to tell me MAP input low. I bought a new MAP and replaced it and the service light went out but the truck went right back to it's old ways! I put the old MAP back on and truck runs good but service light back on and MAP code again! I go to oreillys yesterday and hook it to their scanner and it tells me cam position sensor and MAP sensor. I come to find out the CPS is not even plugged in! Connector was down on the transmission casing way down low! Plugged it in and also replaced EGR gasket on advice of store clerk because it could cause vacuum leak. It was barely a piece of metal between the EGR and intake! So anyway truck seems really nice now , but! Service engine light still on and MAP code! Should I try to replace it again and see if it's the problem now that the CPS is connected? To Chevychase, I thought the TPS was only for idle purposes? Will it affect drivability? It idles perfect! Also I have taken the throttle body off and cleaned it completely when I changed out the csfi unit with a new one. There are no vacuum lines going to the MAP sensor it plugs directly into the manifold and the wire clips to it so I'm back to maybe just replacing the MAP and see what happens. Thank you for your help fellas and please keep the knowledge flowing! You are helping me immensely! God bless you all.

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    Check the vacuum line for the MAP.
    1994 white ext cab chevy silverado 5.7 rolling 230k miles
    stock with air filter lid flipped,
    open flowing exhaust to a 40 series flowmaster dumped.
    God Bless America, Support our Troops!

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    It doesn't have one on it dirtrider it just plugs into the upperintake right behind where the connector is for the csfi.

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