Constant misfire on number 5 cylinder

Maintenance/Repairs
,
1

1993 chevy silverado 2500. 2wd automatic has a consistant mis fire on number 5 cylinder. Changed plug wires plugs etc… would the ecu be causing this to happen?

2

I don’t think it’s the computer. I don’t see how it could be. If it’s always misfiring on cylinder 5, and only on that cylinder, I’d tend to think it’s something mechanical…like maybe a burnt valve or a broken valve spring or something.

3 Likes
3

I agree with @Scrapyard-John , the ecu isn’t at fault. Hook a vacuum gauge up to check for unsteady readings, that might indicate a broken valvespring. Run a dry and wet compression check on the engine to see if 5 is low and if it gets better when a little oil is added. If it does, the pistons rings for 5 are worn. If it doesn’t, one of the valves is likely leaking.

2 Likes
4

my wag clogged fuel injector.

1 Like
5

Would be a good guess, but a 1993 is going to be throttle body injection, so only 2 injectors. If one is clogged, you’ll probably get misfires on multiple cylinders.

I’m assuming this is a gas engine (350 or 454) though, so I may be getting ahead of myself. If it’s a diesel, I’m out lol. I’ve never fooled with them.

6

Thanks SJ,I dont know enough about engines to diagnosed something like that. I just assumed it had to be computer related. Now I’ve got an idea of what to start pricing. It’s only got 92k on it. Arent those vortec engines supposed to be good for like 200k miles?

| Scrapyard-John
June 8 |

  • | - |

I don’t think it’s the computer. I don’t see how it could be. If it’s always misfiring on cylinder 5, and only on that cylinder, I’d tend to think it’s something mechanical…like maybe a burnt valve or a broken valve spring or something.

7

Does that mean you are just going to throw parts at this problem ? Why not pay for a real diagnostic with the understanding that you have to authorize any repairs.

8

Yes sir, Mustangman had some good advice on tests you could run to determine exactly what the issue is. The old 350’s and 454’s, some of them easily ran 200k miles or more, some didn’t. It still may not be a huge issue, though, like a valve spring.

1 Like
9

No.I’ve decided to get rid of the vehicle. More work than I;m willing to put into it. I’ve never cared too much for GM trucks.so it’s an easy decisiion for me. Thanks

| VOLVO-V70
June 8 |

  • | - |

scottadams650_166242:

Now I’ve got an idea of what to start pricing.

Does that mean you are just going to throw parts at this problem ? Why not pay for a real diagnostic with the understanding that you have to authorize any repairs.

10

I think it’s way too premature to throw in the towel on that truck but the choice is yours.

Many years ago I bought one of my antique Harleys as is with the owner stating that the transmission was trashed. Not a big deal as I can fix that.
The transmission was fine. Someone had adjusted the clutch too tight and the clutch push rod had welded itself to the adjuster screw from friction. Snapped it apart, filed it flat, and all was good.
The bike is worth now about 50-60 times what I paid for it.

And as a mechanic I’ve seen many vehicles that were written off needlessly when the real cause was something very minor.
A friend bought a super slick VW diesel once that the owner had thrown the towel in on. The problem? A blown glow plug fuse; a whopping 1 dollar to fix.

3 Likes
11

is it possible there is a Vacuum leak on the intake manifold around that cylinder? just a thought.