High MAP readings at 10% Throttle

[Philip Parker]

Hey everyone - have a head scratcher here.

Just rebuilt a GM 454 (7.4L L29) with some aftermarket heads and the stock EFI system. I got some very gracious help with the tune here that has really improved drive-ability but I have one specific problem that I'm stuck on.

The truck Idles ok and shows what I assume are normal kpa readings for the idle. Once in closed loop and warm the kpa will get excessive with ~10% throttle input at 1200 rpm and the ecu will pull timing like crazy. Truck basically creeps along due to retarded timing because the ECU assumes I'm pulling a stump out of the ground at full throttle at 1200 RPM.

I assume the MAP gets priority over the TPS

Replaced the MAP and the problem still occurs - unplugged the MAP and the problem goes away.

I have not smoked the intake to look for leaks yet because the LTFTs look fine. Its a fresh build, and I build quite a few engines. All Fel-Pro gaskets, everything aligned very well, and all torque specs were done in sequence with my snap on digital wrench. I will say this though: the upper manifold bolt torque spec is 11 ft lbs, which seemed really light. Lower is 35ish I believe.

Here is what is so confusing: Why is 10% throttle getting me such a huge dip in vacuum, and do I need to adjust my timing table that drastically for low throttle input? Cam has a 114 degree LSA.

Is the 22 year old ECU getting goofy?

[dhoagland]

Post up your Tune file and a Log showing it occurring and you will get some response

[Philip Parker]

Here is a log for the problematic area. 1200rpm column in timing advance.big blue scan # 6.hpl

[dhoagland]

This might not be anything at all. But look at your TPS vs your RPM gain. It seems like it might be lagging, you open the TPS, your kpa raises (vacuum drops) your Air flow increases but your RPM stays static.
I'm not real familiar with these, but can you log your injector pulse width or duty cycle? I'm wondering if there might be a table to synchronize these 2 and it might get rid of your lag.
Log shows you really don't get any rpm increase until about 12% throttle.

Post up your Tune and I'll see if something sticks out to me

[Philip Parker]

What you're seeing is exactly the drivability issue I am experiencing. That's a very keen observation. As I increase normal throttle input from a dead stop, the ecu drastically retards timing as the table dictates because the KPA gets into the 65-80 region. I altered the timing for these sectors and it made no change. I'm starting to think (and this was suggested by a tuner I have worked with in the past) that the ECU is starting to get weird and the signal from the MAP is being interpreted incorrectly.

If I unplug the MAP is drives great.

[dhoagland]

Okay
With key on engine off what is your MAP reading?

Can you scan MAP voltage (which is what ECM is seeing) and compare it to the voltage at the sensor itself?
If you had access to a scope I'd like to see the singature wave form at the sensor vs ECM to see if there is any signal loss, interference, ETC...

(I just saw your other thread, you already have some pretty heavy hitters in on this).

[Philip Parker]

On vacation for a week but I will look at map reading key on engine off and see if I can rig up a way to see voltage from map next weekend. Is there a way to see that within HP scanner of do I need to tap into it with a DMM?

[2xLS1]

What vehicle is this in? What transmission, rear gear ratio, and tire size?