Backfire/Studder under acceleration

[spectre6000]

Spectre, you can't see the valve springs with the valve covers off. 5VZ uses bucket lifters, which go over the valve springs. With the cams in the head its hard to get a good look at the valve train. Your idea is a good one just won;t work for this application.

Indeed. I reiterate my disclaimer, I've not dug too deep into one of these yet and most of my experience is with much older engines. Engines haven't changed much since the 1880's, and engines is engines. "Machines just got workins' and they talk to me." (20 points to whoever can cite that quote)

You should still be able to see if one lifter is higher than the rest indicating a sunken seat, should you not? How easy is it to remove the buckets (I've honestly never seen that sort of setup in person and had to do a little quick googling to brush up). If there's a keeper or something that keeps the bucket from going higher than some predefined threshold, a compression test should yield the same result (and wouldn't require dismantling.

T4RFool, do a compression test first ($25 tool and doesn't require a complete teardown).

 

[theizzardking]

personally i'm willing to be his issue has been covered in this thread, but has been missed by his shop, they do this crap all of the time.....how many miles again? the bottom end going awol doesn't seem likely...but hey anything can happen, i would do a compression test , leak down, and oil pressure test before you start tearing into the bottom end and the rest, if the tests are fine, the motor it's self is fine fine, one of these will find an issue internally.....i'm still sticking with a borderline issue with the cat or injectors as that's a more common issue that we've seen on these things, gotta remember the ECU has a variable range for readings, things can give you issues while still being within the ECU's tolerance, i chased my injector for weeks without throwing a code.
that said, i think we're all guessing at this point as none of this is making much sense and there is a very good possibility the other guys are right as well...SN:AFU...good luck bud, keep us posted.

 

[PitbullRescuer]

Compression test or Leak Down test is probably the better way to go.

The buckets have different sized shims on top of them to establish the proper valve clearance. You can't get a good enough look at them with the cams in the heads. And removing the cams just to check is a bit much. This method would work well on a pushrod motor but not so much on these.

And I believe that quote is from Firefly.

 

[spectre6000]

i would do a compression test , leak down, and oil pressure test before you start tearing into the bottom end and the rest, if the tests are fine, the motor it's self is fine fine, one of these will find an issue internally.....

This. It's probably worth noting that my general philosophy with automotive maintenance is that I'm not afraid to completely tear something apart because I know I can get it back together at least as well as it was originally (and cleaner, and for sure torqued to spec, etc.) and I (usually) have the time, facilities, tools, and knowledge to do it correctly.

You can't get a good enough look at them with the cams in the heads. And removing the cams just to check is a bit much.

I did say to remove the cams, and I'd have to go look, but usually it's just a few extra bolts and maybe an endcap/seal (make sure the marks are lined up before AND after; it helps to just turn it by hand to TDC before you start pulling things apart). You nailed me again though with the antiquated engineering... I have dealt with numerous overhead cammed engines, but pushrods are far more common with what I am usually wrenching. The test should still work just fine if he's willing to pull the cams though.

And I believe that quote is from Firefly.

Half credit unless you can tell me which character.

 

[nevada]

kaylee

 

[T4RFool]

so what tools would I need to do a Compression test and Leak Down test? Not familiar with this as this is going into new and unknown territory for me.

 

[spectre6000]

Something like this available at your FLAPS. Just pull a spark plug, screw in the adapter, have someone turn the key briefly and you'll be able to see your compression on that cylinder. Move to the next and repeat. Will expose which cylinder to look at.

The leak down test will narrow it down a bit more specifically, but it requires a bit more equipment. I've never actually done one personally as I've always been able to figure it out with the other information at hand. Might be good since you don't have quite as much old school diagnosis experience. The leak down test essentially puts a bunch of air into a cylinder and measures how fast it escapes. You can generally hear where it's escaping (exhaust hissing = exhaust valve, intake hissing = intake valve, bubbling noises = head gasket, oil pressure relief valve hissing = rings, etc.) and narrow it down.

The trick here, is that once you've figured out which cylinder is giving you fits you're taking off that cylinder head to fix it regardless, so you might as well just take it off. What's wrong from there should be pretty obvious (especially given the degree of trouble you're having). Valves sinking are obvious (fissures in the aluminum around the seat and/or the seat is lower in the head than it should be), rings will leave scored cylinder walls and nasty oil, head gaskets will look corroded and/or there will be carbon deposits, etc.

I was thinking about the oil pressure testing, and it just now occurred to me that these trucks don't have oil pressure gauges! Not used to that... Looks like I have a few little instrumentation additions in my future.

 

[T4RFool]

I'm checking around with a few locals to see if anyone has a compression tester that I might barrow but in the mean time I want to ask about the cat. Would it be worth my wild to cut it out? Could this all be caused by a clogged or partially clogged cat? The reason I ask is because I drilled a hole in the exhaust (in front of the front cat) before left on my trip to see if anything would change with there being a small hole for a release. Should I drill a bigger hole or just leave it be and concentrate on other things?

 

[spectre6000]

Cat does not seem to me to be culpable... If you really think it might be the case though, cut it out (in such a way you can put it back easily).

 

[T4RFool]

I don't want to cut it out unless I know that there is a chance that it is the cause. I got my hands on a compression test kit though, I'll give that a shot first and see what comes of that. The reason that I ask about the cat though is because a clogged cat (as read about) will show similar symptoms as what I'm having but I'm still not entirely convinced that it could be it. Again, grasping at straws...

 

[HCurphy]

Why cut it out? why not just unbolt it after the collector and go for a ride?
(i feel like i remember reading you had some rusty bolts, but breaking those free seems easier then cutting the whole thing out)

i think its worth a shot to try it
(by the way- i have almost the exact same issue you have, or the one you started with at least. However, mine doesn't do it in neutral or park. It has to be under load for mine to show up. I can drive around all day long and never realize it has a miss untill im trying to climb a hill or merge onto the interstate((or if i pull out in front of someone)). Mine starts right at 2800 rpm though, just like yours.)
I'm rooting for you buddy. Good luck.

 

[T4RFool]

So your just suggesting just snapping or breaking the rusted bolts? I guess I never considered that but that would be easier to weld back together than cutting it all the way through.

 

[HCurphy]

that was my thought. cut 'em out, drill 'em out, test it, then shove some quality ss bolts back in there, no welding neccessary (unless it's easier for you to throw a bead on it rather then make a trek in to town for bolts)

edit: you a big venture bros fan? love the avatar. My girlfriend and I were Brock Sampson and Molotov C o c ktease for a halloween party once. Good times.

 

[T4RFool]

you a big venture bros fan? love the avatar. My girlfriend and I were Brock Sampson and Molotov C o c ktease for a halloween party once. Good times.

Yeah, I love that show lol, Brock Samson is an awesome character. I got my wife to watch the first season and she loves it now too lol

 

[PitbullRescuer]

Very important: when doing the compression test, remove all the spark plugs and disconnect the coils wires. (the plug going into the coil.) also, pull the efi relay and fuse. You don't want any spark or fuel during the test.

I usually test each cylinder twice. Do the normal test on each cylinder and record the results. Then, drop a cap full of oil into each cylinder and test again. Record those results too.

 

[T4RFool]

Thanks PitbullRescuer, I was going to ask about that since I haven't done this sort of test before. Another question, is this a 2 person operation? Someone to turn the key and another to look at the gauge or does the gauge stay at its top reading for a bit?

 

[PitbullRescuer]

Thanks PitbullRescuer, I was going to ask about that since I haven't done this sort of test before. Another question, is this a 2 person operation? Someone to turn the key and another to look at the gauge or does the gauge stay at its top reading for a bit?

One man deal. Gauge will stop at it's highest reading so you can get out and look at it. Then you'll have to reset the gauge before you go to the next one.

 

[T4RFool]

Is there a specific number to be looking for or just not worry about numbers and post them up so you guys can have a look, I'll probably be able to do this hopefully on thursday or friday.

 

[nevada]

Is there a specific number to be looking for or just not worry about numbers and post them up so you guys can have a look, I'll probably be able to do this hopefully on thursday or friday.

the numbers really dont matter too much. you just want them to be close. within 10% i think?

if you end up with a cylinder thats really low, then you know theres an issue.

best to just get the numbers, and post them up.

 

[PitbullRescuer]

Minimum of 145 psi with less then 10% variance between the highest and lowest number.