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1998 5.7 misfire

1998 5.7 misfire

Postby potsy [OP] » Jan 12 2012, 3:24pm

New here seems like it is a great site! Looking for some help. I have a 1998 k1500 with a 5.7. Every once in a while the truck will miss usually cap, rotor, or cam sensor from what I hve found. I get it home running very terrible almost dies at stop lights. I pulled the cap and rotor, did not look too bad. replaced the cam sensor (always have a spare) still missing very bad. Replaced the cap and rotor as they were lifetime warr anyways. No better. Went to my work got a scanner and had code p0300. Watched and found that 5 and 7 were missing like crazy when at idle. If I would rev it up they would smooth out and #6 would start to miss. I checked the fuel pressure and it is 65psi was late at night so could not go to parts store so I swapped out plugs and wires from cylindels that were not missing. Still the same way. It is a throttle body so I would guess it is getting fuel to all cylinders and the exhaust is extremely rich of gas smell.I dont understand how a coil or egr valve would cause the same 3 cylinders every time as thats what a few other people have told me. Any help would be great!
Thanks
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Re: 1998 5.7 misfire

Postby ASTE25 » Jan 12 2012, 9:50pm

Those engines are notorious for their intake gaskets to develop a laek. A vacuum leak can cause a miss. Since you have checked the electrical system I would start checking for a vacuum leak.
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Re: 1998 5.7 misfire

Postby potsy [OP] » Jan 13 2012, 1:08am

I took the vac line off of the brake booster and plugged it with my hand. I was told at one time that if you do that and the engine dies its ok. But if continues to run that there is a leak somewhere that it is pulling vacuum from. Is that true?
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Re: 1998 5.7 misfire

Postby ASTE25 » Jan 13 2012, 1:21am

No, that's not correct. Take some brake cleaner and spray it around the intake gasket. If the idle speed changes, you probably have a leak. The thing is, it could be leaking on the bottom side in the lifter valley area and the brake cleaner test will not help you detect it there. If you have never had the intake gaskets replaced, it really would not hurt to go ahead and replace them, because if it is not leaking, it will leak. I think Felpro makes some heavy duty gaskets too (metal and rubber instead of plastic and rubber). Be sure, if you do it yourself, to make sure and mark the distributor position really good. Mark both where the shaft meets the engine and what position the rotor is in. Do not rotate the engine once the distributor is out. You must install the distributor back EXACTLY how it was when you removed it or the camshaft position sensor will be out of adjustment.
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Re: 1998 5.7 misfire

Postby potsy [OP] » Jan 13 2012, 1:32am

I did try the carb cleaner trick but no leaks found also I did the intake gaskets about a couple years ago they were leaking coolant.
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Re: 1998 5.7 misfire

Postby ASTE25 » Jan 13 2012, 1:43am

Well, I'm not sure then. As long as you are getting compression, fuel and fire, all should be good. Have you done a compression test?
Jordan
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Re: 1998 5.7 misfire

Postby potsy [OP] » Jan 13 2012, 1:58am

I did got 120-125 on all cylinders. I am confused how the 2 cylinders stop missing and then #6 starts to miss.
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Re: 1998 5.7 misfire

Postby ASTE25 » Jan 13 2012, 2:06am

It does not make a lot of sense to me either. I would just check all the basics again and go from there. I'd probably replace the wires with a new set (I know you swapped them with another set already).
Jordan
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Re: 1998 5.7 misfire

Postby potsy [OP] » Jan 13 2012, 2:20am

I am planning on new wires and plugs and see what happens. Someone thought maybe the distributer? But I don't know.
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Re: 1998 5.7 misfire

Postby ASTE25 » Jan 13 2012, 2:45am

I'm not sure if they have a module in them like the old ones or not. If it does, then that could be suspect.
Jordan
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