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Username Post: 98 Vortec 5.7 P0305        (Topic#59188)
turbowrx 
Member
Posts: 8

Reg: 03-02-04
03-02-04 02:19 PM - Post#402308    

I have a 98 1500 with 106k. The truck has a code of p0305 which is a mis in cyl 5. I have replaced that fuel injector, new plugs, rotor, cap, wires, coil, fuel filter, and the 02 sensors and it wont go away. the truck has a very bad idle but runs good after 35mph. What else should i do to fix this. the dealer said it was the injectors so i replace that but it still didnt do anything. MPG has gone down severely to. Thanks

 
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GTO_Ryan 
Very Senior Member
Posts: 2090

Loc: Bloomington NY
Reg: 11-06-02
Re: 98 Vortec 5.7 P0305
03-02-04 02:23 PM - Post#402309    
    In response to turbowrx

How does the plug look now and how did the old plug look when you pulled it versus the other plugs?

Oh, and did you use quality parts? I kept fouling plug 8, turned out to be a crappy cap I got from advance auto. Switced over to an AC delco cap and its been fine since.
'69 Pontiac GTO 400ci V8 '92 Chevy Blazer '99 Chevy Silverado 5.3L We need an air strike!


 
turbowrx 
Member
Posts: 8

Reg: 03-02-04
Re: 98 Vortec 5.7 P0305
03-02-04 02:25 PM - Post#402310    
    In response to turbowrx

they all looked fine. they all had a little white on the end of them

 
turbowrx 
Member
Posts: 8

Reg: 03-02-04
Re: 98 Vortec 5.7 P0305
03-02-04 02:28 PM - Post#402311    
    In response to turbowrx

I was running an accel truck super tune kit which was everthing but now i am running basic advance auto parts generic stuff and it does the same thing.

 
GM-Tech 
Super Senior Member
Posts: 4157
GM-Tech
Loc: South Florida
Reg: 06-27-03
Re: 98 Vortec 5.7 P0305
03-02-04 04:16 PM - Post#402312    
    In response to turbowrx

Have you checked compression?
.


 
turbowrx 
Member
Posts: 8

Reg: 03-02-04
Re: 98 Vortec 5.7 P0305
03-02-04 10:51 PM - Post#402313    
    In response to GM-Tech

Im going to do a compression check this weekend. What should the compression test numbers be like.


 
GM-Tech 
Super Senior Member
Posts: 4157
GM-Tech
Loc: South Florida
Reg: 06-27-03
Re: 98 Vortec 5.7 P0305
03-03-04 01:43 AM - Post#402314    
    In response to turbowrx

I don't believe there are any specs as to what the compression should be. The important thing is that all cylinders are fairly close to the same reading. Our specs say the lowest reading should be at least 70% of the highest.

If you had an engine that was 120 psi across the board, you probably wouldn't notice anything wrong with the way it ran. But if you had one that was 180 psi on 7 cylinders and 120 on one, you'd have a noticeable problem. That's what you want to look for.

70% of 180 is 126, so if you had a cylinder that was 120, it's below spec. Realistically, if you had 180 across the board and 127 on one, something's going on there even if technically it's within the allowable range.

I'd say you're going to find that your compression is 150 psi or higher, but again, the difference between cylinders is what's important unless they're all 75 psi.
.


 
someotherguy 
Moderator
Posts: 25894
someotherguy
Age: 44
Loc: Texas
Reg: 08-01-03
Re: 98 Vortec 5.7 P0305
03-03-04 08:37 AM - Post#402315    
    In response to GM-Tech

Quote:

I don't believe there are any specs as to what the compression should be. The important thing is that all cylinders are fairly close to the same reading.



Huh? I agree with your second statement but your first one has me puzzled!

I've seen specs for compression test results for all kinds of engines, why would there not be any published for a '98 Vortec 5.7? Maybe I misunderstood what you said but I find it hard to believe there's not a spec published somewhere. The compression figure is a known number when the engine is designed and put into production.. why would this info not exist for diagnostic purposes?

Richard
94 C2500LD / 97 Suburban LS1 / 06 300C SRT8
Check out my truck shop projects


 
turbowrx 
Member
Posts: 8

Reg: 03-02-04
Re: 98 Vortec 5.7 P0305
03-03-04 09:38 AM - Post#402316    
    In response to someotherguy

If the compression is off in cyl-5 would it could a misfire code.

 
turbowrx 
Member
Posts: 8

Reg: 03-02-04
Re: 98 Vortec 5.7 P0305
03-03-04 12:13 PM - Post#402317    
    In response to GM-Tech

Did the compression test tonight.
Cyl 1 @190
Cyl 2 @190
Cyl 3 @190
Cyl 4 @190
Cyl 5 @160
Cyl 6 @185
Cyl 7 @185
Cyl 8 @190

Now Cyl 5 is at 160psi would that cause a CEL (Misfire). I checked the plugs and checked the gap on them and they are correct @.060.

 
Phazer 
Valued Contributor
Posts: 3004
Phazer
Loc: Armpit of the Universe
Reg: 10-10-03
Re: 98 Vortec 5.7 P0305
03-03-04 04:33 PM - Post#402318    
    In response to turbowrx

Back the rocker arm off 1 turn on the intake on that cyl and see if that helps. Saw a lot of 'tuliped" valves on some early 90 5.7s (sustained WOT with vehichle in "D"(cop cars)). I don't know if this a problem on the Vortec or not. Saw some look like wine goblets.
My "To Do List" for this weekend:
Nothing but womens names...
and drugs....

92 Bravada
96 Sunfire
00 Impala
05 Stratus
90 Wrangler
77 Cub Lo-boy
87 Intruder VS700
95 Intruder VS1400


 
Diamond Jim 
Senior Member
Posts: 385
Diamond Jim
Loc: central NC coast
Reg: 02-26-04
Re: 98 Vortec 5.7 P0305
03-03-04 09:12 PM - Post#402319    
    In response to turbowrx

Take a squirt can of oil, and squirt a couple of pumps (at least a 3 or 4 tablespoons) of 30W oil (not 10W-30 or any thing else) into the #5 cylinder through the spark plug hole. Quickly check the compression again. If it is up after the oil is squirted into the cylinder it means you have bad or broken rings. If the compression is still down then you have a valve problem.

Q.E.D
Diamond Jim 98 Burb, 5.7L, 2500LS, 4X4; 80 GMC 383, ½, Longbed; 74 Dually Flatbed 350; 73 Shortbed 350 4X4; 41 Chevy Master Deluxe, 327/375hp Rochester FI; 69 Nova w/6.0L Gen IV.


 
turbowrx 
Member
Posts: 8

Reg: 03-02-04
Re: 98 Vortec 5.7 P0305
03-04-04 10:52 AM - Post#402320    
    In response to Diamond Jim

thanks for the info. I will let you all know how it goes


 
turbowrx 
Member
Posts: 8

Reg: 03-02-04
Re: 98 Vortec 5.7 P0305
03-04-04 12:00 PM - Post#402321    
    In response to turbowrx

well when i put some oil in the cylinder the compression jumped to over 200. so this means there are some bad rings?


 
MikeJ 
Moderator
Posts: 4173
MikeJ
Loc: Ashland, Ky
Reg: 08-21-02
Re: 98 Vortec 5.7 P0305
03-05-04 05:54 AM - Post#402322    
    In response to turbowrx

It could mean that. It could also mean a worn or scuffed cylinder wall or a stuck ring. You could try using some Marvel Mystery oil in your oil and/fuel and see if a stuck ring can be unstuck, or even pour a little ATF or Marvel right in to the cylinder through the spark plug hole and let it sit overnight to see if that might help, again if a ring is stuck. Be sure to crank the engine with the plug out with a rag over the plug hole to purge the oil if you go that route before putting the plug in and running it. Others may differ on my suggestions but I've had similiar problems about compression and a stuck ring was the culprit on a lower mileage engine. Be sure to let others comment before taking off with my suggestion(s).
Mike



 
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