lee101315

Im having a misfire that has me scratching my head. Any prolonged time under full boost over 4000rpm (more than 2 or 3 seconds), there is a lot of backfiring, black smoke, and power loss.

I did notice the tachometer bouncing around while it was backfiring.... Under normal driving its fine. Its also been intermittent, with days in between events, for the past two months. Now, its here to stay and wont go away. Here is what I have changed in the past week:
Air Flow meter
Both reference sensors ( lindsey harness, so contacts are good )
Spark plugs
Chip sets
Fuel pump ( 3 months old )
Dist cap, rotor, and wires were done 7 months ago
TPS sensor is also 7 months old.

All vacuums are new and correctly routed. Does anyone have any suggestions? Thank you.

elargentino

Close the plug gap to 23 thousandths first.
Tell me if that yields no result.

lee101315

The set the gap to .25", and even changed plugs, but got no result. Its not as much as a misfire as it is a violent exhaust backfire, stopping all forward progress until I release the throttle. It will back fire, smoke, and slow down until I ease off of the accelerator.

elargentino

If your car has all stock contols, most likely you are getting overboost protection because there is a problem somewhere.

lee101315

I really wish it was, but i havent had the over boost protection in 4 years. I was running autothority chips until I installed a k27/8 with lindsey chips last week...Niether of which solved my problem. It feels ignition related...

hot-J

edit.........

Jeff N.

When I had a similar problem, it was the coil going bad. Apparently sufficient zap off boost but not enough on boost. If you have a buddy with a 951 nearby, try a quick swap and see if that doesn't fix the problem.

lee101315

That was a great idea Jeff, I swapped in another coil from my other 951, but it still has the same misfire.

elargentino

Well - let's not be hasty.
Now you need to check the fuel pressure regulator, which I am sure you know needs hold up (you new pump, by the way - good idea to replace both at the same time, similar to the hydraulic system) 58 psi at 1 bar positive.
The Volkswagon tool/fittings is easy to do this with at the front of te rail, or swap in another regulator.

elargentino

Actually.....hmmm - let's be sure that your new chips are written for the fuel pressure you have mechanically set on your car.
2.5 - 2.5
3-3 ?
I am sure you know that the chip is set for a specific static pressure

Jeff N.

Interesting. Here's my post trying to sort out my issue. Sounds like parallel universes.

https://rennlist.com/forums/944-turb...ackfiring.html

schip43

Hmm you said 4000 rpm? If you also hear a sound similar to rushing water? Then I would look at the hose from the TB to the inter cooler hard pipe.

elargentino

That's an excellent point too.
Check it really well - because positive pressure will push it off, but under vac. it will suck closed again.

Rock

Sounds like Knock Protection (Overboost).

Probably doesnt do it in first or second, mainly in 3rd-4th-5th.

How much boost are you running? You may be off tune causing the DME to kick in the knock protection.

When this happens, does it usually happen in a higher gear? Does it feel like the car just hits a brick wall and loses all power, then if you try to get on the throttle it just puts and misfires. Next time it happens, try to put the car in neautral, turn the car off and back on while you're rolling. See if the problem goes away. If it does you're hitting knock protection.


Edit- Whoops someone already suggested that.


I had a problem like this with my Cabriolets old turbo engine. When I drove it down to Cincinnati it did this. It went away at some point through the drive, but I never did figure out what it was.

Black51

^^^+1

I'd also try setting the spark plug gap closer to .23 as well.