Hey guys, I'm looking for some advice. I swapped a DBC 2002 5.3L into a 1995 Silverado and it won't come down from 2k rpm at idle. Runs perfect besides that. Our first thought was a vacuum leak so I replaced the injector o-rings and resealed the intake manifold, neither helped. When the intake was off i inspected it for cracks but i didnt see anything wrong with it and its a new feltpro gasket. What I realized was unplugging the MAP drops the idle to 800 or so, it's a little lumpy but it idles closer to where it should be. I swapped the MAP out with a know good one, still no change. We have also tried tuning it for a 4.8L instead, thinking that could be a possibility but that didnt help. Anyone have any ideas besides a vacuum leak? I will test for vacuum leaks around the manifold today with carb cleaner, maybe the intake is warped or something. Thanks in advance!

 

is the throttle body closing all the way, also depending on how you ran your pcv system it could sucking to much air there, could be a faulty iac valve or it may need to be retuned for the swap these are just a few suggestions of where i would start looking there are so many different things it could be

 

Yes, the butterfly is closed all the way and the TP reads it as zero. The PCV routes directly to the vacuum port kind of centered on the top of the intake. The rear vacuum port goes directly to the brake booster. Btw it doesn't throw any codes when the map is plugged in and idle is at 2k rpm

 

Ehhh, wouldn't this tell you that the MAP is wrong? Maybe you are trying a 2-BAR MAP? Or one from a different application? If the "good one" used for the swap test was the same kind you'd get the same (wrong) result. Best to check and see what MAP is required and try that one...

 

I replaced the map sensor with one from my 2006 silverado daily driver. Shouldnt it be the same? From the map readings it looked like the vacuum was low. If that was the case wouldnt it add fuel to compensate, which would raise the idle? Sorry you have to bare with me, I'm still learning and I don't have any diagnostic software to help troubleshoot like my tuner did. Thx!

 

MAPs come in 1-BAR, 2-BAR, etc. Factory MAPs are usually 1-BAR as they only see vacuum. The 2-BAR and up are made for vacuum and pressure (boost). Should all 1-BAR Maps be the same? Possibly... The sensor itself might be the same, but the wiring and how the ECM interprets the signal may not be.

 

Hook a scan tool up to it that will show live data and see what the pcm is showing for speed. Had this same problem on my little brothers truck before we hooked up a speed sensor to the tcase and then again when he had the wires for it too close to others. The pcm was seeing that the vehicle was moving and had the idle raised up to compensate.

Moved the wiring and it went from showing anywhere from 3 mph to 10mph down to zero at idle and dropped the idle to right where it should be.

 

I will give it a shot. I don't have a VSS hooked up because I'm running a th350 trans.

 

You need one for the pcm to see speed. Little brothers truck has a th350 in it also. We got an adapter from summit that went inline with the speedo cable hookup on the tcase and gave a electrical speed output. It isn't right as far as speed goes but all the pcm needs is to see zero mph when not moving.

 

Without the VSS hooked up, the PCM should be seeing zero MPH therefore not altering the idle. There's always the possibility though. Check and see if the two MAP sensors have the same part number on them. If the symptom went away when you unplugged the MAP to start with, that would leave me to believe the problem lies there. If you put a known good MAP on it and you still have issues, check the wiring.

 

Before you go screwing around with the map sensors and such hook up the scan tool and see what the pcm shows for speed like I said above.

No speed sensor input did NOT show 0 mph on little bros truck, it usaully showed somewhere around 5/6 mph.

The speed sensor wires are green and purple and they are twisted around each other. Make sure these are routed away or shielded from any other wiring or they will pickup the interference and show movement to the pcm.

 

We checked maf, map, short term fuel ratio, iac counts, etc on his truck trying to figure this out looking at live data and just happened to stumble on the speedo sensor data.

 

Sounds good, what kind of a scanner would I need to retrieve that data? What idle speed were you guys seeing?

 

we were using autotap(scanner program for a pc) or a tech2. There are too many to list now that will show live data.

His wouldn't go below 1200 or so fully warmed up. Cold start was 2000rpm's.

 

Anyway to wire it to trick the ECM into seeing 0 movement?

 

i think you need to retune your idle timing tables since you are running a 350 trans i did a 5.3 swap a few months ago and mine did not idle correct ontill i adjusted the tables

 

Only tuning we did on the little bros truck was to disable the vats.