doctormdm

My son and I recently completely rebuilt his 3.1 L (LHO) MFI V6 engine (1990 Firebird). It started but ran terrible with vacuum of about 12. Shop manual calls for valve preload of 1.5 turns. Most sites say half to three quarter. We reset to about little over half turn. Idles okay, vacuum at 15 but if you touch the gas peddle, it tries to die. Car has new cam, lifters and rocker arms. Heads rebuilt but most parts the same. The coil, disstributor, EGR, MAP all new. Any ideas if it is the valve lifters or something else? Mike

AM91Camaro_RS

you said its a new cam...is it a stock replacement? Hopefully it didn't wipe out the lobes on the cam. Did you lube them real good? Also, sometimes it takes a while for the computer to readjust (shouldn't be to this extent, though). what's the timing set at?

Maverick H1L

iTrader: (8)

If you're having trouble setting the valves, get a compression gauge. Turn the nut a little bit and then test the compression. Remember to reset the gauge after each test, writing down the result if you have to. Stop and back up a bit when the gauge starts to show a loss in compression. 1.5 turns is WAY too high, as I've found. Seems to me that when I set mine that way I had serious valve float and the engine wouldn't even run. But this was with my 2.8 that barely ran anyways...

Don't forget to disconnect the gray connector on the ignition coil so you aren't blowing a compressed air/fuel mixture into your gauge (or your face from the open spark plug ports).

doctormdm

The cam is a stock replacement and we lubed it pretty well. The computer does adjust but it still runs terrible. The timing is at 10 BTDC in drive but it flutters a little. The compression test from the second post is a good idea - I have been avoiding it but easier than removing all the intake stuff to get to the rocker arms.

Thanks, MikeM

AM91Camaro_RS

just to verify, you did unplug the connector near the a/c box and stuff before you set the timing, right?

doctormdm

No didn't disconnect the plug - which one is it or what does it go to? MikeM

AM91Camaro_RS

there's a single wire connector by the ac stuff (pass. side), don't quote me but I'm wanting to say tan wire with a stripe, maybe. Anyway, I'm sure someone else will chime in on that. It has to be disconnected to properly set the timing. Once you get your 10* set, simply plug it back in and the computer can then adjust timing on its own.

Maverick H1L

iTrader: (8)

It's a tan wire with a black stripe. Should be in the larger part of the harness, after the flattened loom (I think, been a while since I unloomed my harness and relocated the C146 connector). All it is is a connector with a tan wire on each end by the blower case that comes out of the main harness that appears not to have much purpose...

doctormdm

I followed up and looked at the shop manual (which I now distrust) and it say to jumper the A and B terminal on the diagnostic block under the dashboard. Has anyone used that approach or is the tan wire the better way to go? MikeM

RBob

iTrader: (1)

Don't jumper A&B, that is for older vehicles.

Do disconnect the EST/BYPASS connector. And the car can be left in neutral for this. No need to put it in drive. To get the timing close, set it for about 22* BTDC with the connector still connected.

Then open the EST/BYPASS connector. That will help in keeping the engine running when you open the connector. And set the timing with the engine up to operating temperature.

Note that with a flat tappet cam it should be broken in. Otherwise you stand a chance of wiping a lobe. Proper break in dictates 2200 RPM for 20 - 30 minutes. NO IDLING until broken in.

So to break in the cam without setting the timing first: connect a timing light (#4 works, center drivers side). Start the engine and immediately bring it to 2200 RPM.

Using the light, twist the distributor until the timing is about 28* BTDC. Then continue to run the engine for the prescribed 20 - 30 minutes at 2200 RPM.

After that time the engine may be idled down and the actual timing set with the EST/BYPASS open.

RBob.

doctormdm

Thanks for all the great help. I know how to properly set the timing and the engine now idles very smoothly. I really thought this was THE problem, but the engine still wants to stall if I touch the gas pedal. If I feather it I can get it to run around 2000 rpm and up, but sputters between 1000 and 2000 rpm. The vacuum is steady but at 15. I plan to check the compression this weekend. Any other suggestions would be greatly appreciated. MikeM