Chasing down a miss

[8T7K5]

I've asked this in another thread, looking for a little more specific info if possible. My 99 Vortec big block has an intermittent miss on #4 and constant miss on #3. Injectors are new Bosch, regulator is new, distributor, cap, rotor, plugs, wires are all new. Upper/lower intake gaskets are new. Fuel pressure and compression are all good. I'm at my wit's end and way beyond my abilities to diagnose this. It seems to run ok when it's cold but once it warms up it gets worse. I've ordered a fuel injector tester to check the injectors on 3 and 4. The injectors came from ebay and I suspect that maybe they're Chinese copies, if those two are the problem. I've contemplated a Sniper but ideally would rather the factory system work like it should because if the injectors aren't the problem I've fixed nothing.

Is it important that the two misfires are directly across from each other? There's no noise coming from the engine, no ticking or anything that might be bad lifters, I just really don't know where to go short of testing those two injectors and hoping they're the problem so I'm open to suggestions.

 

[smdk2500]

If you think it is a injector maybe try moving #3 to say #1 and see if it the problem follows the injector. I dont know how hard it is to move the injectors around on the 7.4s but it is a option to try. If it doesn't move then I would guess the injector isn't the problem.

 

[stutaeng]

Do you have the ability to do an "injector balance test?"

There's a gadget on Amazon that can do this. Well, 2 that I know: an OTC brand that's supposed the pro grade and a cheap knock off for way cheaper.

Some higher end scanners can also do it. The only way I can think of testing injectors reliably though.

 

[8T7K5]

If you think it is a injector maybe try moving #3 to say #1 and see if it the problem follows the injector. I dont know how hard it is to move the injectors around on the 7.4s but it is a option to try. If it doesn't move then I would guess the injector isn't the problem.

It's not very hard just time consuming and you gotta buy multiple sets of gaskets. Fuel rail is easy to get to once the upper intake is off.

 

[8T7K5]

Do you have the ability to do an "injector balance test?"

There's a gadget on Amazon that can do this. Well, 2 that I know: an OTC brand that's supposed the pro grade and a cheap knock off for way cheaper.

Some higher end scanners can also do it. The only way I can think of testing injectors reliably though.

Don't have access to that. The tester I'm getting is simple, need a 12 volt power supply, connect two leads to the two terminals on the injector and it cycles it. My worry is, given that the two missing cylinders are across from each other, is that it's a worn cam lobe/lobes but compression and vacuum are good.

 

[stutaeng]

Don't have access to that. The tester I'm getting is simple, need a 12 volt power supply, connect two leads to the two terminals on the injector and it cycles it. My worry is, given that the two missing cylinders are across from each other, is that it's a worn cam lobe/lobes but compression and vacuum are good.

So you are just energizing the injector to make sure it works? I would swap them and see what happens, as was mentioned. Do the easy thing first! I'm not familiar with the 7.4s; didn't realize you had to remove the upper intake to get to the fuel rail.

I would think a worm cam lobe would make for a bad sounding engine, no?

And you have any CEL codes?

 

[thinger2]

Just so we are all clear on this.
Do you mean that all of the injectors are new or just the ones on 3 and 4?
Even if compression tests good, is there any noticable difference between 3 and 4 and the rest of them?
A leak down test might be in order.
If only one cam lobe was wiped I would expect to see some difference
Swapping injectors around to see if the problem moves will tell the story but is kind of a pain.
If the code follows the injector, Bingo
If not, then its not the injector

 

[SAATR]

An oscilloscope would be ideal, or at least a set of Noid lights. Both can be used to check the signal from the ECM to each injector to verify that they're firing correctly. It sounds like you may have a wiring or ECM issue, given your symptoms and what you've replaced.

 

[8T7K5]

So you are just energizing the injector to make sure it works? I would swap them and see what happens, as was mentioned. Do the easy thing first! I'm not familiar with the 7.4s; didn't realize you had to remove the upper intake to get to the fuel rail.

I would think a worm cam lobe would make for a bad sounding engine, no?

And you have any CEL codes?

No codes. I was thinking that if it were a worn lobe or collapsed lifter/lifters it would be obvious too, just trying to rule out everything that needs to be ruled out.

 

[8T7K5]

Just so we are all clear on this.
Do you mean that all of the injectors are new or just the ones on 3 and 4?
Even if compression tests good, is there any noticable difference between 3 and 4 and the rest of them?
A leak down test might be in order.
If only one cam lobe was wiped I would expect to see some difference
Swapping injectors around to see if the problem moves will tell the story but is kind of a pain.
If the code follows the injector, Bingo
If not, then its not the injector

All injectors were replaced with Bosch injectors off ebay, same style as Five O sells. I'm wondering if maybe I got Chinese copies and 3 and 4 were bad. #3 is at 125, #4 is at 145. Lowest is 125 (1 & 3), highest is 149ish (5). All others were 130/135. I agree it's a pain, but probably what needs to be done short of the injector tester discovering a problem.