its broken again
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Senior User
Joined: Apr 2003
Posts: 159
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From: nowhere town nj
its broken again
heres the history of my problem, i have an 86 150 4x4 with a 302 efi motor. runs great at idle except for a slight miss. this slight miss becomes a dead miss when i put a load on the truck. at about 2500 maybe 3000rpms i get a back fire and the engine wont rev any higher. heres what ive done. i put new injector on dead cylinder, it has good compression and plenty of spark. new coil on the truck. at idle the truck revs up fine no matter how hard i hit the gas, but under load it backfires. i cant think of anytyhign this could be, im abotu to go check the timing, and vacume thinking either the timing got thrown off somehow or the convertor is clogged. any suggestions would be great.
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Senior User
Joined: Apr 2003
Posts: 159
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From: nowhere town nj
ok i forgot a few things, one my liscence plate is spelled wrong. i put new wires on the truck with the coil. i tried to check the timing last night because that'll cause the backfire and i wasnt able to even find the mark with the pointer. i know in the oast ive been able to do it. any ideas what may have happened?i have real good compression so that makes me think the valves are alright i pulled the valvecover and all the valves were operating properly everythign was tight and solid like it should be.
Cargo Master
Joined: Apr 2003
Posts: 2,408
Likes: 5
From: Parkesburg PA
I like to paint a white / silver / gold line on the timing mark you are looking for (i.e. 12* mark). Then with the timing light on the #1 cyl (passenger side front) check it at idle with the spout connecter removed. If you can't find it, it may mean that the timing chain jumped. How many miles are on the truck and has the t-chain ever been changed?
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Senior User
Joined: Apr 2003
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From: nowhere town nj
i have a white line painted but i cant find it. i thought it was passengers side number one too but when i looked at the cap #1 was labeled and the wire goes to teh drivers side. this in itself could be a problem except its been drivin for 300 miles at least since the wires were installed adn it was runnign fine. my pointer is a big assembly with a ring/hole and than furthur around an edge and pointer kinda thing. should i be lookign throught the hole for the mark? the truck has 191,000 on it i just replaced the chan about 3000 miles ago thinking that had jumped but the distributor pickup went bad. this is one hell of a stumper. is theres a possibility that it could be somethign simple like a modulator or a pickup? u wouldnt hink so becaues i have spark just no power on the cylinder. what woul cause it to backfire through the intake? i assume the valves are all good i have 150psi compression.
More Turbo
Joined: Mar 2002
Posts: 508
Likes: 2
From: Easthampton, Ma.
Lets start by getting the plug wires straight!
#1 cylinder is the front cylinder on the passenger side, the next one back is #2, next is #3, next is #4.
#5 is the front cylinder on the drivers side, next one back is #6, next is#7, Next is#8.
Your distributor spins counterclockwise.
Your distributor cap will have a #1 on it indicating the tower to connect#1 sparkplug wire to.
The fireing order is 15426378
After you connect #1 plug wire correctly, the next tower on the cap (turning counterclockwise) will be the one you connect to #5 sparkplug, next #4, next #2 etc.
After all wires are on correct, clamp timing light clamp on #1 plug wire near sparkplug and set timing.
It is impossible to troubleshoot this problem if the wires are not correct!
Also, what were the compression readings?
#1 cylinder is the front cylinder on the passenger side, the next one back is #2, next is #3, next is #4.
#5 is the front cylinder on the drivers side, next one back is #6, next is#7, Next is#8.
Your distributor spins counterclockwise.
Your distributor cap will have a #1 on it indicating the tower to connect#1 sparkplug wire to.
The fireing order is 15426378
After you connect #1 plug wire correctly, the next tower on the cap (turning counterclockwise) will be the one you connect to #5 sparkplug, next #4, next #2 etc.
After all wires are on correct, clamp timing light clamp on #1 plug wire near sparkplug and set timing.
It is impossible to troubleshoot this problem if the wires are not correct!
Also, what were the compression readings?
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Senior User
Joined: Apr 2003
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From: nowhere town nj
ok so i got the wires all straightened out, i had one and three backwards. now i have the timing set right. and there isnt a miss anymore but there is still a backfire. i guess the only thing left to cause this is burnt or bad valves right?
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Post Fiend
Joined: Nov 2002
Posts: 9,987
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From: Memphis, TN
Running it with the wires mixed could have done all kinds of bad things to the valves, pistons, rings, bearings, & catalysts. The backfiring probably spun the harmonic balancer on its hub, which could be why you can't shoot the mark. Use the pointer, BTW - the hole is for a rare crank position sensor. If you clean that tab, you can see the advance scale stamped into it.
More Turbo
Joined: Mar 2002
Posts: 508
Likes: 2
From: Easthampton, Ma.
If it has good compression, it does not have a burnt valve!
You said it has good compression!
What are the readings on each cylinder?
Let us know so that were not chasing our tails trying to help you!
You said it has good compression!
What are the readings on each cylinder?
Let us know so that were not chasing our tails trying to help you!
Thread Starter
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Senior User
Joined: Apr 2003
Posts: 159
Likes: 1
From: nowhere town nj
im gonna run a compression trest on all 8 tonight, i only checked out the cylinder that wasnt firing. in hte past i did it adn the motor checked out good with all readings being within 10% of one another. i thoght that hole was for a crank sensor, all though there is nothign there. as far as a throttle position sensor, unless its built into the IAC i dont believe there is one. and the motor isfuel injected so ive eliminated all possibilities of carburation issuses. hopefully but late on tonigth ill have numbers to post.
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Senior User
Joined: Apr 2003
Posts: 159
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From: nowhere town nj
ok so this backfire i get in on the drivers side, someone suggested to me somethign abtou teh air bypass valve on the back of the air pump? personally i knwo nothign baotu that air pump system i dont reall yknwo how it works all i know is it dilutes exhaust. to me this seems liek it could cause a bacl fire getting to much air or somehtign in to the exhaust cause a backfire. vacuum at the fuel pressure regulator was 15 inches at idle, this seems low but it was steady. and as the motor revs it stays smooth and consistant. i work at a GM dealer i knwo i know shame on me lol but i had 5 of our top techs standing around today all sayign the have no idea whats wrong with the truck. as far as i know everythign has been checked. if anybody can tell me what i missed than well id say your goor enough to be a gm mechanic if that really says that much lol.
Posting Guru
Joined: Sep 2003
Posts: 1,192
Likes: 1
From: lino lakes, mn
check the AIR pump piping there is an anti-backfire valve in thye big lines going to the heads they rust out disconect the belt to the pump if you can is this changes or eliminates it check the anti-backfire valves