backfiring under BOOST?

[stageberg]

posting for crossbreed... got a 91 fwd tsi basically stock new plugs normal stuff has a 20g thats about it anyway it runs awesome very lil idle surge but we no thats a vac leak but when you go to boost it starts to backfire! what can cause it? any help would be much appreciated thanks

[2fst-gst]

Check base timing. Also check the exhaust cam that is lines up. Unburnt fuel getting into the exhaust, which makes me think a timing issue.

[stageberg]

ok so i guess its like the turbo spools but the car has to guts did a compression check 140 across the board lil back fire gonna check timing anyway but wouldnt the timing be off make the car well wouldnt we notice it runing funny...or like if just the the intake or exhaust cam are off would you notice it?

[getsideways]

If you have a vac. leak doesnt that equate to a boost leak under pressure :P
When troubleshooting a problem its very hard top find one particular one when other problems are present , maybe fix that first before addressing the backfire

[stageberg]

If you have a vac. leak doesnt that equate to a boost leak under pressure :P
When troubleshooting a problem its very hard top find one particular one when other problems are present , maybe fix that first before addressing the backfire

the vac leak isnt coming from anything that would efect the boost...(emission canister) anyway we no it has nothing to do with it cuz it dont boost at ALL like bent valves(my 92 is wut we comparing to it had bent valves) but 2fst-gts makes good point just wondering if u can have exhaust or intake cam off by one tooth and not notice it

[stageberg]

could it be that temp sensor... cuz doesnt that effect the air fuel ratio by the temp of the engine?

[mechacode]

What mods do you have? What mods did you do right before the problem started happening?

[1slow92]

Check your plugs and wires.

[2fst-gst]

could it be that temp sensor... cuz doesnt that effect the air fuel ratio by the temp of the engine?

When the temp sensor is not functioning the ecu will add fuel thinking the motor is very cold, in turn you will be running very rich. Just like on a cold start your WB02 will read 10.xx - 11.xx until the motor gets warm. This can be adjusted with ECMLink if you have that. Wouldn't hurt to try a new sensor.

If you are off by one tooth on the exhaust or intake you may not notice something is wrong. It's no different than having adj. cam gears. In fact it's an old school method of adjusting the exhaust cam for better spool up, etc. Guys would move the cam 1 tooth in the direction they wanted. My memories a little cloudy on how many degrees each tooth is on our cam gears, but I want to say like 3*-5* per tooth.

We really need to know a mods list. I log of this happening would be even better. Do you have a WB02? A MAP?

Fix the vac leak
check the base timing
check the cam timing
check plugs/wires

Let us know the outcome.

[stageberg]

the only mod is a socket eprom... stock chip now and a 20g turbo thats it...vac leak fixed still have problem. we got the car with the trans tore out guy was gonna convert his SUPER clean 94 tsi fwd to manual but ran outta money to pay shop to do it...thats all it idles realy good tho lil spittin and spudderin so im assuming timing is off were checkin hopfully today

[BoostinPower]

Has this been something thats been happening for a while gettin progressivly worse? or did you just change anything on the car recently before it started doing it?

[2fst-gst]

Just read back through everything....what is the boost at?

[Keith H]

Probably fuel cut. Do a boost leak test.

And get rid of the evap can and all unecessary vacuum lines. Makes things way easier.

[stageberg]

running stock boost and he got the car with trans out so obviously couldnt here it run... we no the guy was trying to convert to manual cuz well everything that was manual related pedals ecu ect was pulled out...anyway it idles Beautifuly so thats y were at a loss... its not a boost leak we no that...and he dont wanna get rid of vac. lines he says you need them(i proved to him u dont but he kinda hard headed but oh well not my car)

[Keith H]

Is it fuel cut? Or a backfire?

its not a boost leak we no that.

How do you know? Did you do a BLT?

[stageberg]

yes did blt and no leaks and its like ur runing with no turbo on a turbo motor... like the waste gate is disconnected but the turbo spools up just no power

[Integra 26R]

This is quite the post. First of all, do you really have a backfire? Because a true backfire comes back out the intake. You sound like you're talking about a missfire because that is usually what happens with problems under boost. You said it spools but makes no power and that it has 140 compression. So the first thing I will say is that your engine is junk. 149 is the service limit. Other then that, if a boost leak test was truly good and your fuel system is running the right pressure and you have good gas then it is probably an ignition problem. Usually plugs or wires for missfires under boost.

[Keith H]

^Service limit is 129. But i've had DSM's with less than 129 that ran great and actually made decent power.

[mechacode]

Engine in the colt was 121 when I put it in year and a half ago and was 121 (one cylinder at 120) a week ago. I've heard of engines as low as 90 still running (barely) but 100 is about as low as you would ever want to get down to.

[Integra 26R]

I had one with perfect 90 across the board. Ran like a piece of shit until about 2600 rpms. Even kept up with a GT Mustang and the car was otherwise stock. Actually the factory service manual says 114 for a 91 turbo. Haynes says 149 and considering new they should be around 180 new, I would consider anything under 140-150 junk.

But if you must know, the engine I just pulled out of my Talon was 60 60 60 49. And it was still faster then most of the cars around. Idle surged like crazy and blew oil everywhere though.

[jakea]

the only mod is a socket eprom... stock chip now and a 20g turbo thats it...vac leak fixed still have problem. we got the car with the trans tore out guy was gonna convert his SUPER clean 94 tsi fwd to manual but ran outta money to pay shop to do it...thats all it idles realy good tho lil spittin and spudderin so im assuming timing is off were checkin hopfully today

does it feel like he is hitting the brakes when it "backfires"? I'd bet its knock from underfueling and wacked timing. 450s can't hold a 20g,especially on stock maps

[stageberg]

fixed it... the idiot (not crossbreed) old owner put a non turbo ecu (as extra with car) told us it was turbo ecu that he traded us for some parts for his other cars... so the ecu was not reading the extra air and shit anyway we fixed it.