engine still heasitating and stalling.

[Skeith]

My lumina with the 3.1L MPFI engine has problems with hesitating when accelerating from stop. It stumbles when the pedal is depressed, sometimes to the point where I have to back off the gas to prevent it from stalling, other times it will stall regardless. However it is only when giving it gas from stop. once it gets past its stumbling fit, it will accelerate normally without stumbling or running rough.

It also lopes during idle, but the loping is reverse to the stumbling, it lopes after its been running for a bit but idles strong when its started cold.

This problem only happens from a cold start. After its been running for 5 or so min, it wont do it, or if it does it is very rare.

I am so confused and don't know where else to go from here. maybe you guys can give me some ideas.

So far to remedy the problem, I have checked and replaced vacuum lines, I cant find a leak, testing the vac from the break booster line on the manifold It sit idle at about 14.5 - 15 inHg.

spark plugs have been changed, cables have been tested and check out ok, even tried a set from the old engine and the problem was the same.

Map sensor was tested, changed the canister purge solenoid from the EECS system. cleaned out the IAC and its passages, checked and double checked vac lines, oil is good, tranny is good, ran injector cleaner through the fuel system.

Tested the crank case for vac/pressure to rule out the PCV system, cleaned rebuilt and tested the EGR valves.

And just yesterday changed the coil packs an ignition control module.

Problem still persists.

Tomorrow im changing the fuel filter if I can get the old one off.

and other than that the only thing I can think of is the TPS and the MAF sensors.

Anyone have any other ideas? I am running out of options. what do I do?

[robertisaar]

you have a MAF on a LH0?

and is that vacuum reading when it was cold or when it was hot? it's slightly lower than expected.

i almost want to say it's fuel sticking to the ports until the engine gets warm enough for it not to happen... that could explain hesitation as a momentary lean spot.

[Skeith]

sorry my mistake its an air intake temp sensor, the vac reading stays about the same whether its hot or cold. its pretty stable so sticking valves don't seem to be the problem. Before I changed the coil packs (i just took the ones off the old engine) it seemed like it was missing when I was holding throttle at cruising speed, that symptom is gone now. i have been getting mixed info about normal operating vac pressure. I always thought it was around 20 inHg but reading online and in my haynes manual it states that between 14 and 16 is normal so I am not sure.

I also re seated the air intake plenium to rule out vacuum leak through the gaskets, that did not change anything.

Timing seems good, The timing mark on the harmonic balancer holds perfect 12 o'clock but skips a teeny bit when the idle lopes. but it is momentary for a split second, most of the times the timing strobe doesn't even reveal it. So i believe that is a result of the ECM adjusting electronic timing for the change in idle RPM.

[robertisaar]

my wife's tired as hell LH0 still pulls 18" of vacuum after almost 180K miles.

as a test spray some carb cleaner or flow a little bit of propane around both the LIM to head surface and the TB to plenum surface when it's acting odd... if it seems to makes it worse or better(really, any change will indicate something), then i would look at replacing those gaskets.

and yes, the timing changing like that is normal, if the ECM detects the engine going underspeed or overspeed, it will add/remove advance to try and keep it stable along with IAC movement.

[Skeith]

I tried the propane with no sucess, tried it on the plenium gaskets followed the vac lines etc and didn't notice anything change.

[Skeith]

I changed the TPS, no effect, changing the fuel filter later today. also re-checked the vacuum its hovering right at 17 inHg.