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Misfire On #3 & #5 Cylinders 5.7 Vortec

mfinch79

By mfinch79
in Ask A GM Technician

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mfinch79 Enthusiast

mfinch79

Posted
Posted

Hello, I am new to this forum so thank you in advance.

 

I have a 1999 GMC Yukon 5.7 Vortec. Recently it started to mis one day sitting at a stop light. Lucky for me I do have a scanner so I scanned for codes.

 

Found #3 & #5 cylinders to have misfires.

Replaced plugs & wires, misfire still present.

 

Ran compression test on both cylinders, each cylinder hold 150psi.

 

After running the vehicle some I found that if I rev the engine up between 2500 & 3000 rpms, the mis goes away and the scanner data for misfire goes to 0 and the vehicle runs smooth. However, if I "punch" the gas the mis returns, but holding steady in that rpm range the mis IS NOT present. Let the rpms drop back to idle and the mis returns.

 

With that information I thought certainly I had one of the all too famous poppet style injector problem. After researching and thinking I purchased the complete conversion (SFI to MFI) injection setup.

 

Replaced complete injection spider and mis is still present. At this point I thought just maybe I fouled the new plugs up by moving the vehicle while testing. So I swapped the #1 and #3 plugs, the mis did not move, engine still missing on #3 & #5 cylinders.

 

Please tell me the next step to take. Can the distributor cause a mis just on two cylinders? Couls it be as simple as the cap?

 

Any and all help is greatly appreciated.

 

Thanks,

Michael.

ChevyTech77 Senior Enthusiast

ChevyTech77

Posted
Posted

Make sure the screen, in the distributor base, is clean. Some had the screen, some didn't. I'd put a cap and rotor on it if it has an miles on it anyways (100k+).

 

Possible plug wire issue? Swapped (positions on the cap) or high resistance?

 

What kind of a reading do you get with a vacuum gauge?

mfinch79 Enthusiast

mfinch79

Posted
  • Author
Posted
Make sure the screen, in the distributor base, is clean. Some had the screen, some didn't. I'd put a cap and rotor on it if it has an miles on it anyways (100k+).

 

Possible plug wire issue? Swapped (positions on the cap) or high resistance?

 

What kind of a reading do you get with a vacuum gauge?

 

 

Like I stated in the post I did replace the spark plugs & the wires, nothing changed. I do understand that even something new can have problems, I'm just going off the fact that NOTHING changed with the new plugs & wires.

 

When you speak of the vacuum gauge you are speaking over my head, I'm sorry.

mfinch79 Enthusiast

mfinch79

Posted
  • Author
Posted
If you said the compression was fine, I wouldn't mess with checking vacuum.

 

 

Yes, compression was 150psi on both cylinders.

 

Do you think since I've already got plenty of money tied up in it should I just go ahead & replaced the cap & rotor & rule that out, or do you really think I'll see teltale sign that the cap is bad or good when I take it off tonight?

tylerjenkins92@aol.com Senior Enthusiast

[email protected]

Posted
Posted

Well, being that caps are dirt cheap, and most people change the cap and rotor when they change the plugs and wires, I would've already changed that. Matter of fact I would've changed cap, rotor, and plugs before changing the wires as they don't go bad if they're not rubbing on something. If a wire was bad you could crank the truck up at night and see an ark coming off the wire.

ChevyTech77 Senior Enthusiast

ChevyTech77

Posted
Posted
If you said the compression was fine, I wouldn't mess with checking vacuum.

 

 

A vacuum gauge test will show many things that a compression test, will not. Found

more than a few internal engine issues with a vacuum gauge test that a compression

test never showed. A vacuum test is one of the oldest and simplest tests you can do.

Get a vacuum gauge and hook it to manifold vacuum (PCV and brake booster vacuum supplies

are good spots) and see what the gauge reads and does. Should be steady at about 16-20 inches

of mercury.

 

Sorry I missed the post about wires.

 

You say 3 and 5 were 150 psi. What were the others?

ChevyTech77 Senior Enthusiast

ChevyTech77

Posted
Posted
Note: I didn't say it doesn't show some stuff that comp. testing doesn't, I just said if I was in his shoes I wouldn't worry about doing it, right now.

 

 

Quick and easy, can't hurt. Might have a sacked valve spring or cam lobe issue.

mfinch79 Enthusiast

mfinch79

Posted
  • Author
Posted

Since I'm in it this deep I'm gonna go ahead & replace the cap & rotor and see what happens.

 

I will post back with the outcome.

 

Thank you to everyone and you'll hear back from me shortly.

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