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Miller
04-17-2010, 09:02 AM
Car is a 94.. It had run fine, and on my last run at the track , while stalling the converter it started to backfire....

The car idles the same as it did befor. It cruises fine, It can rev up in P or N fine..Stall it against the converter and it breaks up and backfires.



what I'v checked:

cam sensor was changed less then a week befor the track day.

I changed my plugs ( which were like 3 days old ) to new ones gapped @ .035

Ohmed out plug wires and they have the same resistance as a new wire.

I thought maybe because i used leaded gas it may have killed my o2's, so i replaced them.. they were old anyways... - reset battery

changed tune to one with knock sensor off

Held rpms at 3000 in N and then gassed it hard and no backfires which i think would tell me its not a fuel delivery problem.. Pressure is at 42ish lbs at idle..

No detectable movement in the balancer..

the only idea i have left is to drop the exhaust and run it to see if the cats are shot..

David Neibert
04-17-2010, 09:09 AM
Sounds like a bad plug wire.

David

XR7 Dave
04-17-2010, 09:35 AM
Held rpms at 3000 in N and then gassed it hard and no backfires which i think would tell me its not a fuel delivery problem.. Pressure is at 42ish lbs at idle..


Pressure is high. Sounds like you may have blown off the vacuum lead from the FPR. Does it still rise with boost/drop with vacuum?

Miller
04-17-2010, 09:55 AM
its an adjustable fpr.. it was set a little higher for the 60lbers... i have checked the vac line to the fpr and everythings in place. i have removed hose 2 check 4 gas and nothing.. my guage is attached to the rail so its hard to see what its doing.. i had the gf watch it 4 me she said it did what i asked but i will double check that i guess

Miller
04-17-2010, 09:57 AM
Sounds like a bad plug wire.

David

even with all the wires ohming out good ? .05 ohms each wire.. a brand new wire i had around had same resistance..

David Neibert
04-17-2010, 12:32 PM
even with all the wires ohming out good ? .05 ohms each wire.. a brand new wire i had around had same resistance..

Yes..even if they test good with an ohm meter they can still miss fire under a load. Suggest a set of Taylor wires. Do a search here for the part number.

David

Miller
04-17-2010, 12:46 PM
i have accel wires on it now... i will pull a new set off of another car and try it out...

Miller
04-18-2010, 01:55 PM
Wellll I changed out the wires to a new set i had on another car, and didnt fix...

I also loossened up the exhaust to make a nice big leak 9 and ( broke one of the studs :( ) thinking maybe a cat was clogged.. no bueno...

unplugged cam sensor and no change..

XR7 Dave
04-18-2010, 10:09 PM
But you didn't verify fuel pressure is rising with boost....

Along with what David said, if you have a cracked insulator on a spark plug it will do the same thing.

Oh, and not trying to use Damon's old SD coil pack are you? :p

Miller
04-18-2010, 10:27 PM
i checked FP today and it went down with vac and broke up 2 much to go into boost.. so i assume its working propperly by default.. ?

no SD coil packs ! i used one from a 89 5spd i have...

idk what else to do guys.. iv checked everything I can think of.... DIS is the last thing on my list............ I dont want to fix it by replacing everything in the car..


the spark plugs that were in the car when it happened were 3 days old.. i replaced them with new and had no change...

it seems to break up more violently at lower rpms then at higher rpms

AutoMachinist
04-19-2010, 04:17 AM
Incorrect spark timing can cause back firing, but so can incorrect valve timing.

Backfiring under a load is a symptom of bent pushrods (which is very likely for stock pushrods being pushed to their limits).

Other valvetrain backfiring can be caused by incorrect valve adjustment, worn cam lobes, or other faulty parts such as collapsed or broken lifters or worn/broken rocker arms.

To diagnose this problem, listen to the valve train with a stethoscope or long screwdriver. Listen to the rockers by placing the screwdriver on the valve covers and putting the handle up to your ear. Also listen to the intake and exhaust sides.
Listen for tapping, rattling, or clanking.

It also helps us to know if the backfiring is in the exhaust or in the intake tract.

If you have unplugged the cam sensor and see no difference, perhaps you were sold or picked up a faulty cam sensor.

I have also seen backfiring in the exhaust for having too small a spark plug gap.


Theodore

P.S. Anybody know why my ARC could be stuck in FIRM MODE. 94 SC Auto.

the-big-e
04-19-2010, 10:26 AM
I'm gonna go out on a limb here....:eek:

Vacuum leak.....:rolleyes:

Most likely an intake gasket or vacuum hose came off....:(

Get about 3 feet of vacuum hose.....

Put one end in your ear and probe around the intake and IC connections with the other end while the engine is running.....

If there is any leak, you will hear it.....:cool:

Miller
05-22-2010, 11:05 AM
figured i'd just update this with a solution. Ended up the MAF was rubbing a bit against the DIS wiring harness and i guess made a bad connection on some of the wires.. I streightened out the harness and all is well now..