Ok, I'm guessing it is a vacuum leak... Just rebuilt an engine for my '91 non turbo to replace the one that I had turbocharged for about 2 years. Before that it was all stock with all the emissions stuff except cats and smog pump. I had removed the 5/6 port sleeve valves completely. In that form it ran great. I then took all the stock fuel injection off and ran with the turbo and programmable fuel injection. Broke many parts, never ran as smooth as Mazda's fuel maps.



Current engine is mostly S5 6 port with TII intermediate housing, TII exhaust inserts, and pretty good sized street porting. I replaced the 5/6 port sleeves with a pressed in aluminum ramp similar to Pineapple's - only it presses into the housing not the sleeve. I left all the emissions off this time around - no solenoids, excessive vacuum lines, etc. Kept BAC, intake temp, water temp, airflow, and tps. Removed extra throttle butterflies (yes, I plugged the holes!), dashpot, and thermowax.



Engine starts hard, and will not idle or run below about 2000. Trying to hold a steady no load speed it surges a lot. Put a load on it and it runs great; pulls very well. It's also popping and backfiring a lot - acting very lean at low speeds. Basically it acts just like a vacuum leak!



Problem is I've had the manifolds off about 6 times looking for it and can't find it! One of the annoying things is you have to keep revving it to keep it running and so I can't even get out of the car and under the hood while it's running. There is good whistle coming from the intake too - might be because of the the throttle body mod??



Any suggestions???



Thanks!

Jason

 

I have kinda of the same problem on my S4 na but cant figure it out.

 

That definatley sounds like a vacuum leak. Did you use gasket maker on the blockoff plates? Do the fuel injectors have the correct grommets on them?

 

Yep, all the injectors have the correct seals, and the block off plates are all sealed up, too.



Another variable that I'm wondering about is the engine harness - this one came out of a parts car, and the computer in that car had been under water at some point. I cut my original harness for the 2 body/dash loom connectors when I went programmable. So, I guess I'll put it back together and give it a try.

 

also check you intake track, make sure there's no leaks by clamps, or cracks in the plastic/rubber.



My wife's car surged at steady throttle, did fine while accelerating, and tried to cut out on idle, and it turned out to be a spark plug wire.

 

did you make sure to tap and plug the holes left from when you did the TB mod?



if it still persistes get a friend with a steady foot to hold the pedal down so the car will stay at or around 2000RPM. Then get some carb and choke cleaner and begin to spray it around the engine bay when you get near the vac leake the clener will be sucked into the engine and the rpms and sound of the engine will change.



this will help you get close to the leak