348 idle adjustment. U-tube manometer what range required?

Hello fellow member's of the brotherhood,
Here is some quick background to my problem; My car is a 1990 348tb with Bosch 2.5 motronic ecu's.

My car is ticking over at around 1300 rpm (Just fitted a new SS sports exhaust and connected new O2 sensors, car was running without O2 sensors connected before and was running bad when warm, felt down on power. ( It still idled at 1300rpm though) and hence without the O2 sensors connected the ecu was running in open loop. When I connected up the old O2 sensors (which would have been stuffed) the car idled at 2000rpm after the re-learn period (started at 1200rpm and rose to 2000rpm during re-learn but never came back down). I have removed the old O2's and connected the new ones up the same way as I connected up the old ones. After the car re-learned the tickover dropped to its current level (again rose to 2000rpm but then fell to 1300rpm). I am therefore assuming that I have connected them up correctly.

Just to clarify my car is from Dubai and the ba$%£ds have messed with everything removing the O2 connector plugs so it wasn't a simple case of plug and play, more like splice and hope!). I have definately got the heater wires right and I am sure the signal and earth are also wired correctly now as the car feels a great deal more responsive when warm.

Anyway to cut to the chase I want to adjust the air bypass screws on the intake plenum to adjust the idle back to 1000 rpm. However the workshop manual advises the use of a u-tube manometer to balance both sides of the V8. It neglects to mention what scale or range of manometer is required. Can any of you guys advise?

Thanks
D

 

The u-tube manometer range doesn't matter, as you are just making sure that both sides of the engine are equal.


But for adjusting the idle air bypasses on the throttle bodies, you don't require a manometer. Unscrew the lock nut, then fully close the idle air screw on both sides. Once fully tight, start unscrewing the idle air screw the same given amount on both sides (start with 2 full turns each).

Then hold the screw in place while you tighten down the lock nut on each side.


If the idle is high, loosen the lock nut and then tighten (the idle adj screw) each side down an equal amount (typically 1/4 turn increments), then tighten the lock nut again.

If the idle is low (or the motor stalls when you brake to a stop), loosen the lock nut and then unscrew (the idle adj screw) each side down an equal amount (typically 1/4 turn increments), then tighten the lock nut again.


A few pics of this process are on the 348 Primer link below.

 

Also, there are other things that can cause a high idle such as a gummy throttle cable (WD-40!) or a stuck floor mat on top of the gas pedal, etc.

 

Is the 'middle' butterfly open during idle?? If it is open then the vacuum hose going to it sould be disconected, to isolate the intake manifold sides from each other during TB adjustment.

 

You should ALWAYS use a manometer to snyc the air bypass screws. Before you do that you need to start first with checking that the 2 throttle body butterflies are sync'd properly, THEN fine tune the flow/balance by adjusting the bypass screws. Motorcycle parts suppliers will have a manometer that will work for the 348 they are very simple to use and very accurate.