4.9 timing is hard! Idles rough, stumble on acceleration

[Capt Fiero]

Well try this. Get the timing to what you "think" is correct. Then swap in a diffrent map sensor. It really sounds like the ECM is playing hell with something.

What caused you to change the timing in the first place?

Its kinda funny but I have never timed my 4.9 ever. I bought the motor with the dist shaft installed. I never changed it. I wired it, and changed to a new MSD coil and rotor and installed it. But never changed the base timing. I have been driving it since last June and the only time I had a problem was when my MSD Box wiring came loose. But aside from that the car has ran flawlessly.

[Wipeout]

I wish I could say the same! I've timed this car probably over 50 times now. Argh! I swapped the cap and rotor since the first one looked burnt and was pitted where the rotor contacts the center pin. I checked the firing order and did it just like the Chilton's manual said to (it was off two posts before). Then, to get it to start I had to make the coil "point" at the left rear wheel - so it's rotated 45 degrees south from west now. It still runs OK with the ALDL jumpered, and like crap without. With the ALDL jumpered and the car running well it says it's about 25 degrees BTDC. As you get close to 10 degrees BTDC it starts missing. If you set it to 10 degrees BTDC then pull out the ALDL, it won't even start. Urg!

I was certain I checked MAP voltage and it was fine. I have to find the AC adapter to my laptop then I can check again.

[Capt Fiero]

Someone has got to know off hand what the ECM changes when in diagnostic mode.

I thought the only major difference was the ECM advances the timing when in diagnostic mode. You might want to take it back to 10' retarded of what you think is correct then remove the ALDL and see what happens.

? Can anyone tell me if the caddy balancer can walk? In the Fiero it is a major problem that older balancers become out of whac due to the marks getting moved. (balancer is not solid one piece and can shift) If the caddy one can shift and has shifted it would cause your problems.

I am not sure what else to tell ya.

[Night Wolf]

when the ALDL A&B pins are jumped, it puts the car in "default" mode, which basically turns off most all sensors and uses factory values. This is done when setting the timing because it makes the timing 10* BTDC (IIRC) and you adjust the timing from there.

[Wipeout]

The Caddy balancer can walk since it's on top of a rubber grommet, but I checked and when the #1 cylinder is at TDC, the notch on the pulley lines up with 0 degrees.

[Wipeout]

By the way, does anybody know which sensors are and aren't affected by set timing mode? If the MAP were to affect the timing somehow, that could be screwing it up. The timing is NOT right when in set timing mode and set to 10 degrees BTDC, but it still runs pretty well. At 10 BTDC it sounds too far retarded. At 20 degrees BTDC it runs better.

[HUF]

Yes, ECM uses data from MAP to set ignition timing.

[Wipeout]

OK here's an update. I've replaced the MAP sensor with one from my roommate's 4.9 and it mostly fixed the stumbling on acceleration situation (still does it but not as bad), but the idle is bad as ever, and I still can't time it. I'm using the right cylinder/plug wire for sure, and when I shine the timing light on the balancer, it runs the best at approximately 20 degrees BTDC. The engine idles fine with the ALDL jumped/set timing mode (I believe for you guys you have to do something with the heater controls to enter Set Timing mode). However, when I time the engine to 10 degrees BTDC you can hear the exhaust get a little more bassy, and it starts missing, like the timing is retarded too far. If you then turn the vehicle off, exit set timing mode, and crank it, it won't even start unless you jump the ALDL. If you crank the distributor back to approximately 20 degrees BTDC it will start without the car in set timing mode, but idles continually worse as it warms up. Argh!