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Discussion Starter · #1 ·
I am tuning my friends L67 swapped 2002 f-body and we have run in to an issue where the car idles very rough and will surge to over 2300 rpm before we shut the engine off. We hooked up a vacuum gauge to the intake tube and it showed basically no vacuum. I think the issue could be the throttle body, since we are using the factory f-body PCM (tuned) and DBW Throttle Body. The HPTuners data log shows the throttle body position is 20% at idle, with the MAF reading about 1.8 lb/min of airflow, or 3700Hz if my calibration is wrong. My question is, what should the idle TPS and MAF readings be for a DBW throttle body on an L67? is there any way to adjust the throttle body in HPTuners if it is too high? Thanks in advance for any help.
 

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The intake tube isn't going to have much vacuum. The vacuum is created by the throttle blade limiting the air drawn into the S/C and lower intake by the pistons pulling air past the valves, which is why the MAP sensors reference the intake manifold.

That MAF reading corresponds to a high idle, so I'd work towards seeing why the DBW is trying to be so open, and hunt down potential vac leaks, as well. You can also swap a V8 DBW TB on to get rid of that choke point ahead of the S/C, so it's not trying to suck thru a straw (exaggeration, but think like that for everything before the S/C).
 

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1998 GTP coupe. All stock and rust free. That will change soon.
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you should be checking engine vacuum on the engine / intake manifold side of the TB blade not the atmosphere side. What's the MAP reading? Normally at idle you should see about 18" of vacuum. I"m not sure how the DBW works, does it also have an idle air controller that bypasses a closed TB blade or does it simply drive the blade open to maintain idle? because 20% position seems pretty open for idle.
 
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The MAP reading for that level of vacuum should be around 20-40kPa
I think there's no IAC, so a low throttle % is the idle setting, like on Series 3 cars.
 

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Discussion Starter · #5 · (Edited)
Thank you for the replies, but I think I should clarify a couple things I missed in my first post. The vacuum readings were taken after the throttle body, and showed no more than 4-5". There is also no IAC in the DBW throttle body. The reading of 1.8 lb/min was at about 950 rpm, which is actually below the target idle RPM at the time. We also had the idea of putting the L67 cable throttle body back on and connecting its MAF to see if manually adjusting the throttle position would fix the issue. We will try this, and also look for any vacuum leaks.

Edit: The MAP reading at idle is around 44 kPa, and when the engine is off it reads 52 kPa. This does seem wrong because atmospheric pressure should be around 100 kPa. Maybe it has the wrong MAP sensor?
 
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