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Backfire, missing, rough idle 2.8L

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Old 10-02-2010, 03:26 PM   #1
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Backfire, missing, rough idle 2.8L

86 s-10 Blazer 2.8L TBI

lost power going home one day, very rough idle, dies when placed under load if you are lucky enough to get it to run long enough....

have replaced;

ECM (mechanic shop placed 30A fuse in the 10A ECM panel)

TPS (was coding)

EST Control Module (42 code, however that stupid f'ing capacitor thing is not in there it came unsoldered and didn't have enough wire to solder back)

currently no codes however the damn thing will not run past a few seconds w/o constantly pumping acclerator pedal, when revving higher you can here misses, and backfires thru TB, runs like absolute dog crap..... if I unplug EST in pigtail inside main harness to set timing, the closer to 10 deg TDC the sh!tter it runs..... I am at a loss here...... this thing makes no sense..... what else could it be????

...would that stupid capacitor not being there cause this?
Old 10-02-2010, 03:40 PM   #2
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Re: Backfire, missing, rough idle 2.8L

I guess the only thing left to replace is the ESC??? would it no throw a code though? perhaps it won't throw the code because it won't run long enough, i.e. go to closed loop mode?
Old 10-02-2010, 06:31 PM   #3
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Re: Backfire, missing, rough idle 2.8L

have you tried doing the timing?
Old 10-03-2010, 08:08 PM   #4
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Re: Backfire, missing, rough idle 2.8L

that capacitor is a ground. IT NEEDS TO BE GROUNDED
Old 02-19-2011, 03:19 PM   #5
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Re: Backfire, missing, rough idle 2.8L

Bro's.... SOS....I am still f*cking with this truck...... I have replaced capacitor in dizzy, TPS, ECM, EST, set the timing to manuf. recommendations on under hood sticker, by manually finding TDC on #1, then back off 10 deg. BTDC...... run terrible, smells rich, stumbles, bogs, sounds like it is mising, fuel pump appears ok.... I am at a loss, what could this be..... no codes..... but runs like shit!! when you push it to the floor stumbles sounds like missing then catches, let off dies at idle.... very wierd any help would be MUCH APPRECIATED!!!
Old 02-19-2011, 03:52 PM   #6
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Re: Backfire, missing, rough idle 2.8L

Okay so all of you are going with the computers. You need to start thinking mechanical.

Do a valve job. I am sure that your valves are starting to become over adjusted.

Its seriously the simple things to fix that you need to do first that is under talked about on these forums.
Old 02-22-2011, 01:15 PM   #7
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Re: Backfire, missing, rough idle 2.8L

Check for huge vac leaks with a spray bottle. Compression test is definitely in order too.
Old 02-23-2011, 06:57 PM   #8
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Re: Backfire, missing, rough idle 2.8L

my truck was doing the same thing when the ICM was bad.its under the dizzy. 30 bucks. in my case it was still throwing spark but apparently not enough to keep it running..
Old 02-24-2011, 12:35 AM   #9
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Re: Backfire, missing, rough idle 2.8L

Here is what I can think of...

After cheking for the basics... such as vaccum leaks with a spray bottle, TBI bolts torque, and intake manifold bolts torque, a compression test, coil test, spark test, valve adjustment... I'd look into the following:

1) I worked on my school secretary's car. It was a 3100 motor and it bogged, sputtered, stumbled stalled, no power, could barely run... and I ended up replacing the in-tank fuel pump and ran perfectly fine (... that 3100 had like no bottom end pull the way my 3.4 iron head does...).

2) It could be the TPS wiring to the ECM... and a clue would maybe be the fact that you were getting a code. I took off the TPS plug on the old 2.8 just to see what would happen, and similar results happened to what you described. Make sure the TPS is et to about 0.48V in idle position... do you know what I'm talking about??

3) ICM under the dizzy... these modules create a whole wack-load of problems when they get old!!!

4) I have gotten a TPS code for a MAP sensor before... and when the MAP goes the motor runs like a bag of....umm....poop...

5) See if you have a toast injector or 2... a injector died on my old 2.8 and again, similar engine problems to what you describe. Look for a nice even spraying cone coming out of both injectors... make sure you don't see any skipped beats (pulses).

6) If your fuel filter is clogged (which I highly doubt... but you never know...) you may be getting a really lean mixture causing it to run rough and sometimes even miss.
Old 02-25-2011, 08:41 AM   #10
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Re: Backfire, missing, rough idle 2.8L

well what was the scoop? did you fix it? sell it ? set it on fire and make smores?
Old 02-25-2011, 02:52 PM   #11
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Re: Backfire, missing, rough idle 2.8L

What jimmykicker said... check your vacuum lines and compression. The problems aren't always electronic. I don't have *any* electronics in my '83 to mess with so I went straight to mechanical issues, and I found the rings were shot. Now it's fixed and it runs like I just drove it off the dealer lot...
Old 02-25-2011, 08:03 PM   #12
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Re: Backfire, missing, rough idle 2.8L

it might have jumped time just 1 tooth will do this. it happened on my 2.8 when it skips a tooth and you go to retime it the mark will be on the degree but still be shitty you might wanna spend the whole hour or so it takes to yank the front apart and check your chain. if it's stretched that will also cause it to run like shit. but it sounds like it might be that it skipped a tooth though. did you hit any bumps or anything in that nature?
Old 02-28-2011, 05:52 PM   #13
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Re: Backfire, missing, rough idle 2.8L

Quote: Originally Posted by chevy<ford
it might have jumped time just 1 tooth will do this. it happened on my 2.8 when it skips a tooth and you go to retime it the mark will be on the degree but still be shitty you might wanna spend the whole hour or so it takes to yank the front apart and check your chain. if it's stretched that will also cause it to run like shit. but it sounds like it might be that it skipped a tooth though. did you hit any bumps or anything in that nature?
Even though I can't agree Chevy is less than Ford, I can agree with this. ↑
I think it's worth pulling the cover and looking. If you haven't done a compression test yet, I'd highly reccommend you do that before anything else.
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