Newly Rebuilt Engine Stumbles
Rather than tacing onto EELFALCON's the "EA running problems" thread I thought it was time for a new one; I'm getting desperate.
I recently rebuilt the 3.9L MPEFI engine in my 89 NB Fairlane. In doing so, I fitted new engine mounts, gaskets, rings, bearings, oil pump, distributor drive gear, timing chain and overhauled the hydraulic valve adjusters. Cylinderes were ridge reamed and honed. It runs like a car with and old style vacuum advance where either the vaccum hose has come off or there is a vacuum leak
Engine looses revs, hestitates or stumbles at tip in or intitial acceleration from rest or from any closed throttle to acceleration situation (ie going around a corner) in any gear and neutral and park. Continual throttle blipping at idle causes backfiring in inlet manifold. Spark plugs have dry black coating (rich mixture?). No scan codes (now), clean exhausts, good idle.
TESTS AND FIXS TRIED SO FAR (the ones I can remember):
New plugs, points HT leads and distributor cap and rotor.
Checked TFI earthed.
Checked no water in fuel and filled with fresh premium unleaded.
Checked for vacuum leaks by spraying CRC, water, detergent and carb cleaner at inlet manifold head joints. Nothing happens.
Checked all vaccum hoses connected and have suction.
Checked with vacuum gauge - within specs but lower than I expected.
Checked cannister purge valve by pinching vaccum line; no change.
Checked fuel pressure; within specs.
Cleaned ISC valve (foam air filter oil gunked) this cleared code 13 that I did get initially.
Checked compression at 6 cylinders (worrying I'd lined up the ring gaps by mistake).
Removed timing cover and double checked cam timing.
Reset base timing (with scan tool connected and checked with two different timing lights)
Reset base idle.
Checked TPS - resistance varies with throttle and voltage within specs.
Checked ISC valve - opens and closes; voltage within specs.
Checked MAF as far as I could - removing and blocking vacuum line changes engine behaviour. Voltages within spec and continuity with ECU.
Cleaned blue oil gunk from foam filter out of throttle body.
Pulled each lead from each plug in turn; engine revs fell in all cases.
Checked ECU earthed.
Checked alternator output.
Listened to injectors opening and closing with stephoscope and also checked with noid lights. Even plug colour and plug lead pulling test suggest they are behaving equally.
Swore at engine and kicked wheel (I didn't even feel better).
Called on divine assistance from various gods and dieties (no response yet).
Found a silly reason to blame the wife (felt worse - guilts).
Tried retail therapy at local auto shop; felt more guilty about unwarrantewd spending.
Had a beer - helped for a little while.
Please don't be afraid to suggest the obvious or question the thoroughness of my checks. I feel I have missed something obvious and simple like a hose all plug I reconnected improperly or not at all. I promise not to be insulted by even the most obvious questions or suggestions.
My next step will be to fit a new thicker inlet manifold gasket in case there is a leak I can't detect and re-run all the test again/