Misfires Chevy 1500


#1

I’ve got a 2002 silverado 4.8 motor with about 207,000 miles on it. Been having troubles with misfires for a while now can’t quit seem too narrow it down. I’ve changed coil packs spark plugs wires new fuel pump new intake new fuel filter and nothing has changed. Before I change injectors I was wondering if there was any way I could know for certain what is causing this. Also have cut both catalytic converters out. Any help would be much appreciated thanks. I do also have a small leak coming from my exhaust manifolds but I don’t think that would cause it too misfire that bad.


#2

With that many miles on the engine, have you checked for a stretched timing chain?

Tester.


#3

Are the misfires worst when the truck is started stone cold, in the morning?

If so, does everything smooth out to a large degree after the engine has been running awhile and is allowed to warm up?

If the answer is yes, I’ve got a culprit in mind


#5

Happens continusly warm and cold and doesn’t stop misfiring until I shut it off and park it sometimes it will start at 25 sometimes right when I hit the gas pedal the code scanner just tells me random misfire


#6

How would I check that ?


#7

Remove one of the valve covers so you can observe the valve action.

Have someone get on the crank bolt with a socket/breaker bar and turn the engine over by hand in the normal direction while watching the valve movement.

Now have that person turn the engine over in the opposite direction.

If the crankshaft can be turned more than 5 degrees in the opposite direction before there’s any valve movement, it points to a stretched timing chain.

Tester


#8

Okay I will check that out tomorrow thanks


#9

Hard to say. The exhaust gasses leaks out , true, which doesn’t seem like it would cause much of a problem; but air also leaks in. The pressure at that point is positive some of the piston phases, and negative others, so an exhaust manifold leak will draw some fresh air into the exhaust stream. That fresh air will hit the O2 sensor and make the computer think the engine is running lean, and so the computer will inject more gas than is necessary for the proper mixture. An improper mixture can cause misfires.

Suggest to measure the fuel trims. If the trims are outside the normal range, best to fix all the exhaust leaks before assuming the problem is elsewhere. Otherwise, assuming the compression is good and there’s no other engine diagnostic codes showing, it sounds like you’ve pretty much done all you can do absent and injector cleaning treatment or replacing the injectors.


#10

I have no idea what quality your “code scanner is” . . .

High level scanners will show live data, which will show you which cylinder is currently misfiring. More than that, they will also show you misfire history


#11

I will be getting a better scanner and will check all that when I get it tomorrow thanks


#12

Try cleaning the MAF sensor. if applicable, not too hard, but a good place to start.


#13

Already did that and nothing


#14

Found out cyclinders 1,2,3,5,6,7 were misfiring did and injector test and these are those numbers
58-41 #1 injector
58-36 2
58-43 3
58-39 4
58-44 5
58-40 6
58-49 7
58-40 8


#15

check the EGR valve and gasket.


#16

There doesn’t seem to be any correlation b/t the injector test results and the cylinder misfires. I’m not sure how to interpret those injector test results, how are they performed and what is the objective of that test? 58 seems like an acceptable fuel rail pressure. Misfires are usually caused by a proble with one of these

  • spark
  • spark timing
  • fuel/air mixture
  • compression

It’s a dice toss which one to test first. On a car like yours with over 200 K miles, I’d probably start with a compression test, if it isn’t overly time consuming.


#17

58psi is the pressure when the fuel rail is primed

The second value is the pressure after the individual injectors are “buzzed”

Injector #7 might be partially restricted, because it only dropped 9psi

Conversely, #2 might be flowing too much

Personally, #7 grabs my attention more than #2


#18

Got good compression on all 8 they read 125 130 psi it’s got good spark as well


#19

I consider those to be pretty low numbers, not good

The low compression may indeed be the reason for your misfires

We have quite a few GM trucks of your vintage in our fleet, and with the same engine family as yours. And I’ve seen several examples with persistent misfires caused by low compression, which you have

Call me a sourpuss, but I’m just giving you a possible explanation for your problems

I suggest you follow your original idea . . . dirty injectors.

Disable the fuel pump

Hook up your canister cleaner directly to the fuel rail

Start the engine and let it run until it stalls

Disconnect the canister cleaner

Reenable the fuel pump

If that brings no improvement, then you’ll just have to live with the misfires

It’s hard for anybody to conclusively solve/diagnose your problem without having the truck right in front of them

You’re clearly in an area that has no smog inspections whatsoever . . . correct?

I suspect your misfires clogged both cats, which is the reason you straightpiped the truck, correct?


#20

Ok and yes that is why I cut out the cats


#21

Those are borderline compression numbers, at least for a typical 2002 (i.e. modern) engine. Might be ok numbers for a 1970 engine with an 8.0 compression ratio, but modern engines with compressions in the 9’s and 10’s usually test at 160-190 psi compression. Still with them all testing in the 125 to 130 range, I wouldn’t guess it would case a huge misfire problem, at least not on a warm engine. Did you ever get a chance to check the fuel trims? Of everything you’d said, my first suspect is the known exhaust manifold leak. My second suspect is what RK said above, EGR malfunction. If I had this problem myself I’d replace the PCV valve (if you have one), as it is cheap and easy to do usually, and that part failing open can cause difficult to diagnose random misfires. It will usually throw a lean code though. Lean running combined with borderline compression numbers would augment any misfire problem.