Brian21

Hey guys, been awhile since I've been on here, but I'm having a problem with my car. It's a 91 firebird with the stock 305 in it. Atleast I'm pretty sure it's all stock.

Anyways, as soon as the car warms up, it starts missing pretty bad. It isn't noticeable when driving or taking off at all, only at idle. When it's cold, it runs a little rough, but doesn't seem to miss. I think it might be as soon as it hits closed loop that it starts.
I failed smog, and I think this is probably why. I failed both the 15 and 25mph on NOx and HC's. My catalytic converter is only about a year and 3 months old.

So here's what I did before smog. Full tune up, including spark plugs all gapped properly, brand new plug wires, cap, rotor, pcv valve, air filter and oil change. My old plugs all looked pretty good too, tan in color. I put my timing light on each one of my plug wires with the engine running and I don't see any fluctuations or indication that one cylinder isn't sparking properly. Timing is set right at 0 degrees. IACV is adjusted properly too. I checked the spray pattern on both my injectors, it looks pretty good, a nice cone like spray with no drips or anything. When the engine misses, you can see the spray pattern change a little though.

Does anyone have any ideas? I'm trying to get it fixed ASAP because my tags already expired.

Thanks.

gregsz-28

iTrader: (1)

Have you tested your fuel pressure?

I have a similar problem in my truck, and the question I was asked that led me to the solution was "how's your compression?"

So, have you done a compression test?

Brian21

That's one thing I still need to check, I need to rent the guage. If it was fuel pump related, wouldn't it do it cold as well though?

gregsz-28

iTrader: (1)

Yes, I would think it would actually do it worse when cold, since it is burning more fuel then, if the problem was fuel pressure related. You mentioned a problem with how the injector is spraying though, which would indicate an injector or fuel pressure problem, and if fuel pressure is good, that would indicate a problem with the injector.

Brian21

I was thinking, when I first got the car, the oxygen sensor was dead, so I tossed a bosch sensor on. I've heard alot of bad things about those though. Is it possible that the oxygen sensor is giving false readings and then pulling fuel for a split second causing the miss/stumble? I guess I should check the voltage first to see what it's doing..

Man, I really wish I had a laptop so I could datalog.. This would be much easier.

Thanks for the reply, all advice is appreciated.

gregsz-28

iTrader: (1)

Not likely, esp. if you have a new O2 sensor on it. I usually won't buy Bosch products, but I do buy Bosch O2 sensors and have never had a problem with one, and Autozone gives you a lifetime warranty on them anyway.

Brian21

Well, it's not brand new, maybe 2 years old.. I bought it when I first got the car. I'll throw a voltmeter on it tomorrow if I get a chance to see what I get.