I am trying to troubleshoot a few issues with my STR. I have a small stumble at idle and I think it may be a vacuum issue. I have a nice code scanner that reads data in real time but I am not quite sure what I should be comparing my data too. My average data looks something like: MAP 15 "HG, TPS 3-5%, long/short term fuel trims around 3% and 0% respectively. O2 sensors all switching nicely within range.
If I am interpreting this correctly, my intake vacuum is a little low (only 15" of vacuum), and the ECM is compensating for this by closing the throttle plate further. But again, I am not sure what I should have for baseline readings due to this being supercharged etc.
Does anyone have a rough guestimate as to what my numbers should look like on a 4.2 S/C motor?

 

I'm sure the fuel trims will tell you if you have a vacuum leak ! and 3% is near perfect .
i was watching mine only last night and its boosting 4psi map at idle 3%tp 15%load at 750rpm in gear warm !
i will be checking again on a different scanner soon to see if this is a fact will let you know what i find , I'm sure it should indeed be vacuum but its reading pressure so may be
dont compair to mine at this point in time .

 

Have you changed plugs? Due at 100K miles.

Have you monitored your mis-fires?

Be aware the STR has a well known vacuum leak from the swivel elbow that provides vacuum for the power brakes. This has a strange internal O-ring that gets baked until it's rock hard and can't seal any more. This elbow is located on the cast elbow at the back of the super charger on the passenger side. You really can't see it until you know what to look for.

I have posted a very detailed post on this with a cheap and simple fix. The problem is as with most things on the STR is access to the elbow. I made mine leak when I changed the hose under the super charger and after much searching found the XKR guys had also run into this problem and that's where I finally found the problem as well as the solution.
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Thank for the reply, its good to know I probably dont have a vacuum leak. I finally found my issue. I got a diagnosis code for cylinder 7 misfire yesterday. I put in a new coil after a little troubleshooting, problem solved. This new coil also fixed my transmission issue which I have detailed in another thread. I am really happy that a 40$ coil fixed my transmission's sporadic shifting. Its amazing how sensitive the electronics in these cars are.

 

Glad to hear you got it fixed! So nice when it's a cheap and easy repair. We don't get too many of those for sure!
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