Thread: need a little help finding a miss

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OK! guys, I have a 94 yukon,FI 350,every so often I get a little miss at the lights or when idling,I've changed plugs and wires,I did notice #4 was carbined
up, it was running great,went out of town to see grandkids,and now when I acccelerate slowly it misses,until OD kicks in or I have to step on it,it sort of feels like a plug wire jumping or arcking,I've had them hook it the machine and they tell me it's OK,could it be a fuel problem or does it sound more like an electrical problem?I was planning on going to get the el camino this week end, but there is no way this will pull a trailer,any help will be be helpful before
I pull all my hair(gray)

Had a similar problem with my 93 GMC 350 the fuel filter was pluged up. Guess I got a tank of dirt somwhere. Had to change filter 4x before it cleared up. Even changed the injectors that wasn't cheap

this is about the 3rd time someone metion the fuel filter,I will change it, my thought was if it's the filter,it would starve for gas at higher rpms more so then low rpms like this one does,I will also try the spraying wires things,I'll have to block the tires,chain it down,set the foot brake,cause it will be the wifey behind the wheel giving it a little gas,I didn't mention we just spend about 600 miles together and the temtation might be there

if it was a fuel supply issue the easiest way to test would be disconnect the vaccum hose to the fuel pressure regulator, fuel pressure would increase.... thing is i really doubt that.... a vaccum leak could cause rough running/missing at low RPMs and drop off at higher RPMs but thats doubtful too..... i'd go with denny on saying its an ignition issue

the places I took it to have the code checked told me their machine don't go back that old,(mine is a 94) it has the carb, type fuel injection,I know there is a name for that but I can remember what it's called, can those be cleaned if so how? also where is the egr.? I know a while back I was having a similar problem,(somewhat) had it checked out and was told to change the oxg. sensor which made a world of differents for awhile,then a couple of months later it started again, this is why it's a hair pulling deal for me seems like the dam thing don't want to go out of town

Just something to think about,fuel injectors that drip or are clogged or not opening the correct duration and amount or even not spraying a good patterns will all test good.The test is just testing whether they are working at all ,not how well they are working.Then the computer will compensate up to a point to keep things at the proper levels by opening the other injectors longer and wider,which makes the other cylinders rich and the one with a bad injector lean or goes the other way and makes all of em lean except for the driping injector cyclinder which would be rich and fouling that plug causing the miss at lower rpm till the rpm comes up and cleans the plug.
A good 100,000 mile tune -up trick is to replace the injectors.

I used to have problems finding a miss----then I started to bathe regularily-----

to test how well fuel injectors are working you can run an injector balance test... you will need a fuel pressure gauge and an injector pulse simulator.... i believe both can be rented from most auto parts stores.... the simulator plugs into the injector you pressurize your fuel system and then turn off the fuel pump.... most GMs have a fuel pump test lead in the engine compartment... so you just have to jump power to it.... run the simulator for a given amount of time on each injector.... and record your findings with the fuel pressure drop that you see on the gauge....between each injector there should be no other than 10 milipascals difference...... if one injector seems to be dumping less fuel then the others, then it is clogged or has a weak coil... injectors can be cleaner properly without replacing by cleaning the screen on it and blasting pressure through the nozzle.... the only way an injector truly goes bad is if the coil inside of it begins to wear out or if the plunger that the coil lifts breaks(which is very uncommon)..when it drops more than not it is cause by the plunger not seating properly, which more than not is debri getting cause in the plunger.. a lot of people replace injectors when they can repair them for MUCH cheaper, they just dont have the knowledge of how to do it.... btw your TBI unit will only have 2 injectors gm v8s have 2 injectors on TBI and everything else only has a single injector.... the TBI isn't a very efficient way of fuel delivery as i'm sure you can tell with 2 injectors powering 8 cylinders.... they tend to wear the coils out faster.... which is why most vehicles run SFI which is very efficient.... a single injector per cylinder and it only shoots once per intake stroke.... the MFI systems would spray fuel ontop of the intake valve before it opened and then spray again when it opened... the puddled fuel wouldn't be atomized well and would be partially wasted....

i know i explained more than you need... but its always helpful to have knowledge that may come in useful another day...

now for O2 sensors getting carboned up... do you have one before and after the cat or just one before... if you've got one after the cat and its getting carboned up then your cat is not functioning... the one before can be cause by rich mixture which can be cause by numerous things.... but a carboned up O2 sensor should actually cause a lean mixture, it will keep telling the computer that its not seeing much oxygen and so the computer will compensate by dropping how much fuel its using... which would then give you a lean condition and possibly a lean misfire.... which makes me think you need to replace the O2 sensor and also fix what is causing your rich condition that causes your lean condition

Originally Posted by brianrupnow

I used to have problems finding a miss----then I started to bathe regularily-----

I knew SOMEBODY was going to take a shot at this one.


Don

i forgot to also ask if it seems rich when warming up.... computer should be in open loop when warming up so that will keep the computer compensating for O2 readings.... notice that a vehicle always runs a little richer when warming up of course because of the choke... even fuel injected cars have a choke system yours would probably be an idle air control valve..... you should be newer than having the warmup injector (i forgot what they called that piece of crap)

yep thats the one... wow those things were trash

I took pictures fo the top of my motor, but I am not able to load them up,the last time I did they came out hugh,so Denny, if you want I can email them to you, but I would need your email address or you can emil me renu3636@hotmail.com ( thanks guys for all the help)