Poor idle, poor intake manifold vacuum

[scartrouble]

1996 3.4L SFI:

engin rough idle hot - surge/stall cold - low intake manifold pressure (4"), some backfire on stall

questions
1) I disconnected the crankcase position sensor and started the engine. Runs smoother, but stalls out. Does not generate error code. Should it run? Should it generate a code?
2) I disconnected the camshaft position sensor and started the engine. No change. No code. Should it generate a code?
3) At idle with only 4" Hg vacuum, should the MAP generate an error?

Complete to date:
new lower intake manifold gasket
Compression tested, plugs replaced
new timing chain and gears
all emission/fuel systems/sensors tested for correct signal levels
All vacuum lines tested, each blocked and engine started - no change

I goin in circles now ...

[maxwedge]

Was this a result of the work done or it was this way before, BTW 3.1, correct?

[scartrouble]

It is a 3.4L - it is an APV but nothing on that thread in weeks- see http://www.automotiveforums.com/t764...1996_3_4l.html for more details, sequece, tests - I am 4 weeks into repairs.
All was fine until "just in case" it's leaking into the engine I had a mechanic do the lower manifold gasket replacement ... then the fuel pump replacement ... after that everything was by me to try to get it running correctly again - what a nightmare - the only vehilce I have that fits the whole family.

Any answers to the 3 questions would help.

In general I want to know what I can unplug and it should run and what I can unplug that should generate a code (like EGR runs disconnected and should generate code).

Thanks for replying,
Scott

[maxwedge]

Possible that the intake gasket slipped out of place causing a major vacuum leak. BTW a propane torch ( small hand held) can be used to locate and confirm vacuum leaks by adapting a hose over the nozzle, and holding it over suspected areas, including the throttle body intake, this will richen the engine causing it to smooth out when you are near the leak, tank is not lit of course. Something was done wrong in the repair process, focus on those steps. Cam could be out of time also. I assume no bent valves from the pushrods being installed wrong, correct?

[scartrouble]

Thanks for the suggestion!
I tried the propane tank with a hose and ran it around where the plenum connects to the lower manifold and the front lower manifold-to-block seals that can be accessed. The back lower sections I can not get at very well, but no change in idle was noted.

FYI lifters were checked twice by original mechanic, and I checked and replaced the timing chain and gears and the camshaft position looked OK with the marks on the gears all lined up accoring to the book.

So I am now looking at the original work as the source of the issue. The Haynes instructions on the upper manifold removal for the APV indicate to position the purge and vacuum canister solenoids out of the way - where are these on a 3.4L SFI ... there are no pictures in the book. I this refers to the EECS valve it is below and behind - I did not think this could be the source, but I remember the mechanic had to replace a piece of pipe on this valve assembly - could it be a source? Where does it connect to the manifold?

[maxwedge]

Both valves are in the proximity of the upper intake with vacuum lines and connectors,but that would be a small leak, not enough to drop it that low, take the oil filler cap off and get some propane in there and see what happens, if the gasket was leaking vacuum on the inside of the intake the crankcase would have vacuum in it and doing what I said would show some reaction. Again this advice all assumes correct cam timing and good all around compression.

[scartrouble]

Thought I would give you the last word on the low vacuum ...

The major issue was exactly what maxwedge suggested - I went back to the original work done when the low vacuum started and sure enough - the upper manifold gasket had a serious leak at the back of the front center intake port. The propane gas did not identify this but I could not get the gas directly to the spot.

I still have very rough very low idle but it runs without stall or backfire

Other things I have found out that others may like to know:
-if you detach the ERG wiring connection - car runs and does generate error
-extreem low vac issue (4-6" Hg) did not generate MAP sensor error
-disconnecting IAT generates an error - runs the same with worse mileage
-disconnecting MAF it runs - no error generated
-cross-connecting the CPS with the ECT (same connector in same area)peggs the temp gauge in the dashboard full hot and the engine backfires as a result, but seems to do no permanent damage to either sensor
-a high pitch whistle at low idle when hot (like a blade of grass between the thumbs) can be caused by insufficient torque on the bolts connecting the throttle cable section of the air intake to the upper intake manifold
- to get to the lower back bolt of the previous component, save an hour by removing the coolant line - even if you have to cut it off
- the AIC set range is VERY forgiving - Spec is 1.0 to 1.25 - mine functions fine at 1.125 and 1.380, but I still have a low idle condition