I'm about to take my biss out and put a new biss in from another throttle body. hopfully it does not leak.

1) Do another boost leak test. When you fix a major leak, more minor ones tend to pop up. This won't solve your problem, but it's a good idea anyway.

2) If you have a datalogger, record ONLY the front O2V. It should be cycling between 0v and between 0.6-0.9v. If not, replace it. If you have replaced your stock narrowband with a 'narrowband simulator' from your WBO2 setup, make sure it's set up properly.. if it's 'stuck' or reading too high a voltage (like if it's giving the ECU the full 0-5v output) then the ECU will be convinced that the car is running painfully lean, and dump as much fuel in as possible.

3) Fuel pressure should be set with the boost/vacuum reference line disconnected and plugged. Sounds like you're a bit over the stock setting, assuming a 10psi fuel pressure drop at 20in/Hg idle vacuum. That would definitely cause you to run a bit rich.

4) Replacing the BISS o-ring would not cause this. In fact, boost leaks make the car run rich. Sealing one would make you run leaner.

5) When setting the timing/adjusting the BISS, do not forget to ground the appropriate connectors on the firewall/diagnostic port, so the ECU will go 'hands off' and stop trying to adjust it for you. Make sure your timing AND base idle are set properly. One being off WILL affect the other.


I'd honestly go through the engine bay and see if there are any huge boost or vacuum leaks, or if any lines were forgotten to be re-attached. What's the vacuum read at, at idle?

I understand leaking under boost will cause rich a/f, but won't a leak at IDLE cause the car to idle lean? I.E. Air getting in from the leaky BISS that's not being metered??

I'll check my front o2v with my logger this weekend. If I remember right, it was jumping around between .2 and .8v It's the normal o2 sensor in the front. My wideband o2 is just before the Cat.

All vac lines are connected. I'll reset my fuel pressure again. Which pins on the diagnostic port do I need to ground while adjusting the BISS?

My vac reading on my boost gauge says about -11, so I'm sure I still have a few leaks. Also, I only have a shitty little 12v compressor which can't seem to pressurize the intake path more than 2psi or so.

jlado13 said:

I understand leaking under boost will cause rich a/f, but won't a leak at IDLE cause the car to idle lean? I.E. Air getting in from the leaky BISS that's not being metered??

I'll check my front o2v with my logger this weekend. If I remember right, it was jumping around between .2 and .8v It's the normal o2 sensor in the front. My wideband o2 is just before the Cat.

All vac lines are connected. I'll reset my fuel pressure again. Which pins on the diagnostic port do I need to ground while adjusting the BISS?

My vac reading on my boost gauge says about -11, so I'm sure I still have a few leaks. Also, I only have a shitty little 12v compressor which can't seem to pressurize the intake path more than 2psi or so.

Click to expand...

After you changed the BISS, did you do another boost leak test? If not, do one; the BISS may be leaking worse than before. And is the vac reading -11psi or -11in Hg? Read the gauge, because some have vaccuum in (in Hg) but boost in psi. If you did repeat the boost leak test, what were the results? Should hold 20psi and leak down 1psi every few seconds or longer on a warm motor.

As for grounding pins, I vaguely recall that you can fry your ECU on a 2g if you go grounding pins. Grounding stuff as I recall is for 1g only for base timing and ISC. On a 2g, you need some snazzy tool to adjust the ISC. The BISS can be adjusted with a screwdriver.

Oop, my mistake on the diagnostic port grounding. No idea how to get a 2G to go 'hands off' in that case, especially with a six-bolt swap... or how to set the base timing on the CAS for that matter, given that stock 2G timing isn't user-adjustable.

I've honestly never seen a boost/vac gauge that measured vacuum in anything other than in/Hg... and 11 in/Hg is extremely low. Should be around 20 in/Hg, give or take two or three depending upon altitude/air density. Sounds like a major vacuum leak somewhere. Testing for vac leaks is much like boost leak testing (or more precisely, exactly like 'proper' boost leak testing). You warm up the engine, shut it off, attach your boost leak tester and open the throttle while you pressurize the intake system. If it won't hold any air, you take a 1/2" ratchet and manually turn the crank until you hit a point of zero valve overlap (all your compressed air was going through a cylinder and out the tailpipe, before). Note that TDC has valve overlap on either 2 or 3, but as I recall roughly 30 degrees past is when overlap ceases on stock cams. At that point, just take your soapy water and spray around the intake manifold (especially where it bolts to the head, as it's VERY easy to have not quite torqued down one of the fasteners enough, especially on the underside) and look for bubbles as normal. As an added benefit, with the manifold pressurized, the stock CBV ('bov') isn't open, so there's no need to block off the recirculation tube or jump to the conclusion of a 'leaky' CBV being an issue.