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acerulz
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Apr 12, 2014, 11:14 AM
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hello, I've got an 89 350 tbi chevy that has a misfire. All sensors are tested and verified, all tune up parts are new. I had the timing light on it and noticed the flash became intermittent on multiple cylinders. Wondering if this could be a pickup coil issue, coil, ignition module or distributor issue? Any ideas?
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Discretesignals
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Apr 12, 2014, 11:33 AM
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Re: 350 tbi misfire
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Could be any one of those things.
Does it misfire under load, at idle, or all the time?
When was the last tune-up done including plugs, wires, cap, and rotor?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Apr 12, 2014, 11:34 AM)
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acerulz
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Apr 12, 2014, 1:12 PM
Post #3 of 35
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Re: 350 tbi misfire
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misfires all the time, but its just a stumble. doesn't backfire, just flutters out the tailpipes. still has lots of power tune up components are all new.
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Discretesignals
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Apr 12, 2014, 1:39 PM
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Re: 350 tbi misfire
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You didn't state if this engine was high mileage or not, but usually the first thing you do on an old engine that has misfires is to check the status of the cylinders by doing a compression test using a compression test gauge or doing a relative compression test using a lab scope and current clamp. You want to make sure the cylinders and valves are healthy because you'll be going on a wild goose chase changing ignition and fuel components.
If it has a misfire at idle and it is a steady misfire, you may be able to isolate which cylinder(s) is misfiring. One way is to ground each plug wire at a time while the engine is idling and misfiring. Then see if the tone or rpm of the engine changes when you disconnect and reconnect or ground each wire. The ones that don't make a change is the cylinder(s) that has problems.
The problem with this is that your dealing with high voltage, so you have to be careful not to get zapped.
One way I personally do this is is with a 12 volt test lamp connected to battery neg or somewhere metal on the engine. I slide the tip of the probe between the ignition wire and the boot at the distributor. Don't pierce the wire or the boot. When I do this I want to slide the probe in there so the spark jumps off the plug wire terminal into your test lamp than going through the spark plug. You don't want to damage the terminal or poke holes in the boot or insulation because then you'll need to replace or repair the wire.
You can also hover the test lamp probe against the bottom of the boot as you lift it off the distributor cap. Keep the test lamp probe against the terminal on the cap as you move the wire away. The idea is to have the spark jump into the probe instead of through your body. If you have a set of insulated gloves or plug wire remover, your chances of getting zapped will be less.
Note: Don't just pull a wire off with nothing for the spark to go to. Doing so can damage the ignition coil.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Apr 12, 2014, 1:42 PM)
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acerulz
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Apr 12, 2014, 2:15 PM
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Re: 350 tbi misfire
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engine only has roughly 10000 miles on it and did comp test already and all cylinders are solid. I've also tried grounding plug wires as well but its all over the board. Not specific to just a couple cylinders and not consistent enough to narrow it down to a specific cylinder.
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Discretesignals
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Apr 12, 2014, 2:57 PM
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Re: 350 tbi misfire
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Has it been doing this since it was rebuilt or did this just start happening?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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acerulz
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Apr 12, 2014, 3:20 PM
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Re: 350 tbi misfire
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ongoing since it was built...
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Discretesignals
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Apr 12, 2014, 3:31 PM
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Re: 350 tbi misfire
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One thing that you could try, if you haven't already, is to adjust the valves. If the valves are adjusted too tight, it may cause a flutter or have a rough idle.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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acerulz
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Apr 12, 2014, 4:00 PM
Post #9 of 35
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Re: 350 tbi misfire
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ya been through the valves already. really think its a spark issue just because of the timing light going dead here n there
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Discretesignals
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Apr 12, 2014, 5:36 PM
Post #10 of 35
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Re: 350 tbi misfire
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Using a timing light to figure out random misfiring could be inconclusive. You may end up having to go high tech which means using an ignition analyzer. Maybe viewing secondary voltage patterns can give some direction.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Apr 12, 2014, 5:38 PM)
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acerulz
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Apr 12, 2014, 5:40 PM
Post #11 of 35
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Re: 350 tbi misfire
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just a very mild cam similar to an RV cam and headers and a throttle body spacer. Cylinder ports were ground to match the intake, but nothing wild. other than that its a stock 350
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Discretesignals
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Apr 12, 2014, 5:52 PM
Post #12 of 35
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Re: 350 tbi misfire
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It sounds like you covered most of the basics, but you can go only so far without actually taking measurements and gathering data. I'd say scoping secondary ignition patterns would be important. It can tell you not only how the ignition system is performing, but also what is going on inside the combustion chambers.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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acerulz
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Apr 12, 2014, 6:03 PM
Post #13 of 35
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Re: 350 tbi misfire
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ya I thought about that but unfortunately I live in a small town and the 3 shops here don't have scopes capable of that. so either I travel 200 miles or I play a little diagnostic darts lol.
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Discretesignals
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Apr 12, 2014, 6:37 PM
Post #14 of 35
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Re: 350 tbi misfire
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Do you have a vacuum gauge to measure manifold vacuum and a digital volt meter? Just for the heck, have you checked for trouble codes stored in the ECM? Jumper A to B and count the flashes of the check engine light?
What happened to the engine to warrant a rebuild? Did the engine before the rebuild have the same issue?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Apr 12, 2014, 6:40 PM)
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acerulz
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Apr 12, 2014, 8:54 PM
Post #15 of 35
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Re: 350 tbi misfire
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ya I got a gauge and meter and already checked for codes. I bought it used and automatically thought it was guna need a rebuild so I ordered the kit and tore it apart and found out it was a real low mile motor. so I just gave it a refresh with no machining. so I don't know the history on the motor
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Tom Greenleaf
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Apr 13, 2014, 5:30 AM
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Re: 350 tbi misfire
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Hmmm? Back from an early post you said you hear or notice a "flutter out tailpipe" which is sometimes a good clue without much work to tell if this is random or just picking on one or two cylinders.
Kinda redneck but helps to know if always the same cylinders in part of more testing is just let it idle and with good glove and rag block the tailpipe and you may just about feel if only one or two regularly miss or just one.
Vacuum gauge "Teed" into good manifold vacuum source is also telling with floating readings or a bounce. Timing light jumping or missing if probably inductive pick up on wire is not conclusive as DS already said. Mine can do that with nothing wrong.
True when cancelling plugs ground them. I usually just put a jumper wire direct from plug end of wire to ground so the voltage has a place to go easily not possibly jump to another wire or jump around inside distributor unseen. That with vacuum gauge observations and the rag at tailpipe can help focus in. Also - do or did if replaced the plugs look about the same? Easy to drop one or more or a bad type or batch.
Cam to distributor may have not been happy when installed? Shaft bearings must be good and I think it uses a roll pin to hold the gear to shaft that maybe could be loose or messed up.
The wiring plug ins to distributor could be lousy on the list also.
Valves. Been even older than this year but if you don't set those right it will always be troubles and the usual problem is ticking if too loose or valve doesn't close tight if too tight but you can isolate which one or ones are doing that.
Distributor pick up coil could be bad or just dirty. Air blow it out and while there see if you feel shaft move too much side to side.
Just my few cents for some things that don't require more advanced testing equipment to nail a possible easy problem down without that 200 mile trip or whatever you said.
The non OE cam might be messing up this whole show also and will be difficult to know. Alterations near always just cause problems and don't get better anything. Headers do about nothing for power but people do that or did that all the time. IDK, seems most just leaked gaskets, made hooking up to exhaust a problem spot and routed such that the could burn something being too close than stock,
T
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acerulz
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Apr 14, 2014, 6:27 PM
Post #17 of 35
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Re: 350 tbi misfire
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well been playin with the car tonight and still going back to an ignition problem. tried grounding wires to narrow it down but its real hard to nail it down to what cylinder is messing up. without the use of an oscilloscope and just using my timing light, most cylinders are firing regularly. But some repeatedly don't. I've put the lead close to the plug and close to the distributor and the same cylinders keep acting up. here's what I got by rough percentage of what their contribution is..... #1 25% #8 75% #4 90% #3 75% #6 90% #5 90% #7 50% #2 25%. Again, just going by how the timing light is flashing but I don't really see it being a timing light issue cuz I hooked it up to my other 2 vehicles that are running perfectly and the light never misses a flash. What could be causing this? a bad pickup coil? I really think its somewhere in the distributor area. Any input?
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Tom Greenleaf
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Apr 14, 2014, 9:07 PM
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Re: 350 tbi misfire
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Not quite sure how you applied % values to each cylinder but you get an "A" for the effort. Ya - I think distributor is the likely issue and getting it timed perfectly likely a hassle. Being sure that it's the problem without some extensive testing is going to be tough.
I don't recall doing just the pick up coil on one of these so can't say what troubles that is vs a whole one or the cost difference. Some are troubles some aren't of course.
If I didn't find the shaft being the problem I just might pull a "dart board" shot that a new one would be the fix if it isn't too expensive vs playing around forever.
Other guy here? What would you do with this situation at hand?
T
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Hammer Time
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Apr 15, 2014, 2:29 AM
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Re: 350 tbi misfire
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I seriously doubt the pick-up coil is causing that issue.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
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Apr 15, 2014, 4:30 AM
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Re: 350 tbi misfire
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This is taking a while to diagnose but there's still a clue from back several posts. Quote">>just a very mild cam similar to an RV cam and headers and a throttle body spacer. Cylinder ports were ground to match the intake, but nothing wild. other than that its a stock 350<<"
I'm not so sure that worked out so well,
T
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acerulz
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Apr 16, 2014, 5:29 PM
Post #21 of 35
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Re: 350 tbi misfire
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k so I tried another distributor for sh*ts and giggles and that really didn't do much. So I pulled a valve cover cuz I was going to reset the valves again and I happened to notice by accident that some of the little wire clips for the lifters are out of the lifter. I think I'm getting closer to the problem now, but why would this have happened? Too tight of adjustment?
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Discretesignals
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Apr 16, 2014, 5:49 PM
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Re: 350 tbi misfire
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The only thing I can think of for those clips to pop out would be cheapy lifters or not enough preload. The clips are really there to keep the plungers in the lifters until you preload the lifter. If you run the engine without enough preload, the plunger is going to be banging on the clip. I do believe they make lifters with C clips. I think you can also use cheap lifter and install C clips in them. You can't do valve adjustment with the clips missing..you sure don't want a lifter flying apart.
Now you need to figure out why the preload is goofed. Hopefully the bottom of the lifters are ok. There has been a lot of talk about flat tappets getting messed up during engine break ins because of the lack of zinc in modern oils.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Apr 16, 2014, 5:52 PM)
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Tom Greenleaf
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Apr 16, 2014, 6:06 PM
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Re: 350 tbi misfire
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Didn't re-read whole thread but when a cam was changed lifter probably didn't come out but rather just pull up till they are stuck which is trouble if all OE and then you changed it so IMO it's AFUed.
Again IMO and may stand alone but you can't fruck with OE much since about 1972 and older. It never has proven to me to improve anything desired and becomes the root of making it a boat mooring.
Now you have bits and parts found from doing that so what would you like to do? It got messed with and didn't work out is what I see and left in that state. It's damaged goods to me and now yours,
T
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Discretesignals
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Apr 16, 2014, 6:14 PM
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Re: 350 tbi misfire
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Well, he has to pull the intake anyway. There has to be a reason the preload was lost. If you don't have any bent push rods, rocker studs pulling out, or you haven't misadjusted the valve train, the only explanation is a rounded cam lobes. Rounded lobes or the bottom of the lifters being eaten up would explain the misfiring.
The bad part is if that is the case, that means there is metal circulating in the oil. You really don't want clips down in the pan either because it is possible they could get through the pick up screen.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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acerulz
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Apr 17, 2014, 5:19 AM
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Re: 350 tbi misfire
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I just did an oil change to it and the old oil looked pretty clean so hopefully that will be in my favor lol. Just trying to figure out if I want to do a vortec head swap while the intake is off or if I want to wait for it. If it is a lifter issue then I'm going the distance. No rods are bent and the studs are all intact, so its gotta be either a lifter issue or preload issue but leaning towards a preload issue because the oil was clean. if the lifters and cam were stripping material from them I'd definitely see sparkles in the oil.
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