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Frustrating Backfire in T-Body on Rebuild; Backfire
Topic Started: Mar 15 2015, 09:59 PM (1,260 Views)
CaptainMetro
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After finally finishing the rebuild for my 94 XFI, I installed it over the weekend, it started and idled ok. But when opening the throttle it would stumble and violently backfire through the throttle body, and I couldn't run it above 900 RPM, it would just bog down and POP POP BBBLAP BBBLAP!

So, I checked:
cam timing
fuel pressure
Removed old gas and added 3 gallons of premium
compression: OK
plugs: black from spraying carb spray for possible lean issue, OK
removed lower exhaust pipe for possible clogged cat
no DTCs
found vacuum leak (fixed but not the problem)
all grounds clean and shiny
all sensors connected
spark timing @ 10 BTDC
checked TBI spray pattern: OK
different spark plug wires and CAP/ROTOR
opened up ECM and looked for burnt spots: none
The ECM could have been the problem, but nothing definitive

At this point I was really concerned that I had botched the rebuild, but it didn't make sense since compression was good. :banghead Because the leads for my Vantage are bad, my measurement capabilities are limited until Monday, if it weren't for that I probably could have watched the CMP signal fade out at open throttle. I was going to hook up the timing light again to check if spark fell off when revving but I had another distributor on the bench so I put it in, and to my surprise and glee it purred like a kitten and revs up quite nicely. Now I have to fix the crank seal dumping oil (not sure why a brand new seal would blow, the crank bearings are nice and tight) but after that ordeal it's not such a hassle.
Edited by CaptainMetro, Mar 15 2015, 09:59 PM.
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GeoStalker
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"Chicks dig me and guys think I'm cool."

Probably nothing to do with it, but did you burp the radiator system?
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CaptainMetro
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I used an Airlift when I filled the cooling system.
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91 ragtop
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What all did you do to the engine in the rebuild process ?


Ken.........
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CaptainMetro
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Turned crank
Resized rods
Honed cylinders
New valves (stainless exhaust)
New lifters
New exhaust guides
New pistons/rings
Planed head surface
Edited by CaptainMetro, Mar 15 2015, 10:40 PM.
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oregon250exc
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[ *  * ]
What in the distributor is there to go bad? I know the cam shaft position sensor is in there but if that was bad it would have thrown a code? I only ask because my car is doing the same thing.
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freegeo
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Did the engine run ok before the rebuild?

What was done to the lifters during the rebuild?

Have you tried to retard the timing back to 5 BTDC and see if it will help?
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CaptainMetro
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I bought the car without a running engine.
I used new lifters, the other distributor did the trick.
Edited by CaptainMetro, Mar 15 2015, 10:58 PM.
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Hanuman
"The Almighty Grounds Cleaner"

is there grease inside a lifterholding a intake valve open?

mechanicaly the spark cannot occure early enough for a intake back fire without the valve being held open.
the ecm also cant retard the timing this far (not that it matters, the distributor wont be near acontact this early.

valve timing would need to be at least 2 teeth retarded.

go back to your lifters.
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Car Nut
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People, CM clearly states in post 1 & 8 that the car is healed."The other distributor did the trick". He replaced the distributor & it purred like a kitten. I was also thinking lifters until I saw it was running ok now.
Edited by Car Nut, Mar 16 2015, 05:32 AM.
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Hanuman
"The Almighty Grounds Cleaner"

Car Nut
Mar 16 2015, 05:29 AM
People, CM clearly states in post 1 & 8 that the car is healed."The other distributor did the trick". He replaced the distributor & it purred like a kitten. I was also thinking lifters until I saw it was running ok now.
i noticed that, but the time stamps for his edits....distributor was mentioned in the first post.....time stamped 1hour 40 minutes or so before post #8.

im sure he will reply and we can find out for sure
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CaptainMetro
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Hanuman
Mar 16 2015, 12:55 PM
Car Nut
Mar 16 2015, 05:29 AM
People, CM clearly states in post 1 & 8 that the car is healed."The other distributor did the trick". He replaced the distributor & it purred like a kitten. I was also thinking lifters until I saw it was running ok now.
i noticed that, but the time stamps for his edits....distributor was mentioned in the first post.....time stamped 1hour 40 minutes or so before post #8.

im sure he will reply and we can find out for sure
The edit on the first post was the new lifters part.
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Car Nut
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Glad we got that cleared up. :lol
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Hanuman
"The Almighty Grounds Cleaner"

:thumb gla we could help :lol
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CaptainMetro
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Hanuman
Mar 17 2015, 02:01 PM
:thumb gla we could help :lol
Yes, well not directly but your numerous posts about clean grounds caused me to add it to my checklist.
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