Autowolf80

I have a 91 Lumina Z34 3.4L DOHC that I just did a cam carrier gasket and timing belt on. I have replaced the vaccum lines to make sure it wasn't a vacuum leak, also the intake gaskets are new. It now is hard to start. If you give it a little gas when cranking it it will start and idle fine. If you tap the throttle while its running it has a dead spot just off idle then it will rev normally. In gear with moderate throttle it bogs through accelleration untill about 3,000 rpms then goes fine. I also noted a up and down spike in the rpms at around 3,000 to 4,000 when in park or neutral.

Any suggestions on what this may be?

maxwedge

Welcome to AF, the up and down spike may be a fuel cut of by the pcm, at 4k. Many things can cause this vacuum leaks, cam timing incorrect, lean fuel mixture, skewed tps.

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sad-lumina-owner

The engine is supposed to stall at around 4000 rpm in park, to prevent revving with no load, at least on my /94 3.4L.

(Many's the time I lay awake at night DREAMING I had a lean mixture I could top off with HHO generator for an extra 10 mpg, but thats another story).

If this happened right after changing the timing belt, I'd suspect you got one of the cams off by one tooth the wrong way...its a messy job taking all that off again, but this should be checked out...

The timing marks on my 3.4L weren't totally accurate and I had to make a half-tooth compromise, but got lucky, when changing my belt.

If you did it once, taking off the covers again and checking the marks won't be so bad.

Have you looked at the idle screw (yes there is one on mine near the throttle). I cranked mine up. I think the PCM adjusts for it anyway, but this helped mine start, having a slightly faster idle setting. Be careful because the screw is a hex key (on mine anyway) and is difficult to reach and incredibly stiff.

I found mine also would go into open loop especially on cold mornings, burning rich and running rough, until I tried this trick:

Turn the key on for about 30-40 seconds before cranking. If your oxygen sensor has a heater, this warms it up. The O2 sensor has to reach 600 degrees to work properly, and if it isn't putting out signal the PCM goes into open loop until it warms up.

When I did this, the car would start easier and run better.

I'm not sure if your /91 has a ECM/PCM or an OBD-I system for pollution control, but this may be part of the problem.

Also try replacing the PCV valve, that helped my car run better too (see my thread here on where it is).

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- but I can only afford one this big.

Autowolf80

I am really thinking I got the timing off a tooth or two. I had a friend with a scanner try to check the ecm today and it won't communicate with the scanner. i can check the codes with a jumper wire tho. its not throwing any codes atm. I rechecked the vacuum lines and can't find any leaks there. MAP sensor is new so its not there. Dunno how old the TPS is. So I guess I'm set to redo the timing belt again on friday. need to change the left bank cam carrier to head gasket also it seems. I found a couple drips of oil in this seam today. heh should have jut done them both instead of right first.