I have a 94 with the 302, manual trans and have been chasing an idle problem for a couple weeks now. It usually lopes really bad at idle but runs fine sometimes. Also when I am slowing down and rmps drop it tends to buck and stumble. The only code I get is 172 - Ho2 always lean. Checked all vac lines and replaced any that were questionable. No vac leaks I can find. No exhaust leaks. Fuel pump and filter were replaced last year. TPS was in spec.

Here's what's been done recently

New IAC - failed voltage test and was stuck open.

New EGR valve - got fail code for low EGR flow and was clogged up.

New Alternator - bad bearing

New smog pump

New belt and tensioner

New O2 sensor - to make sure the 172 code wasn't just due to a bad O2 sensor.

I am planning to fix a leaky oil pan gasket next weekend and will have to take the intake plenum off to raise the motor. So, I will check the injectors etc. while I'm there. I am also planning to put in new plugs and wires , Cap and rotor, fuel filter(even though it's fairly new) and PCV valve.

Any other suggestions of where to look for a cause. I haven't been able to narrow down a source and I'm getting tired of throwing parts at it.

 

Try seafoam

 

Check fuel pressure and injector flow. A vacuum test would be helpful. You may have a leaking valve. And check for exhaust leaks.

 

Already tried seafoam. No help there.

Also did a pressure test on the fuel system and it was all in spec.

As far as an exhaust leak, I have tried to see if I can find one, but can't really tell if/where it might be leaking. I can smell some exhaust when I'm under the hood so I'm looking at that now.

I got a real good quote a while back from a custom exhaust place to have a whole new exhaust redone, but didn't have the money at the time. I've got a little more doe now and may just take it down there and have them redo the whole exhaust. At least they'd be able to tell me if the problem is the exhaust. Besides, a custom exhaust has been on my wish list for a while and this is as good of an excuse as any to get it done.

 

I would test the map and what is the engine vac.

 

Map sensor tested fine. And seems to be working. Vac lines are all good now. Replaced a few when I replaced the EGR valve. Fixed any leaks I found.

Going to talk to the exhaust guys now.

 

Do as you please.

 

Did you verify the egr pipe was good. The exhaust smell from under the hood is suspect.

 

Well I figure out a few things. Had an exhaust leak on both sides. My motor mounts were shot and the rocking was tearing my header off. So I replace the motor mounts and had the shop fix the headers.

I still have a slight miss. I think the exhaust leak was causing the O2 sensor to throw the 172 lean code and so the pcm was riching up the mixture and probably fouled some plugs. Going to change the plugs and check the timing this weekend. As well as fixing a leaking oil pan gasket, woo hoo.

It's running better with the exhaust fix and hopefully will be totally cured this weekend.

Thanks for all the help.

 

I have had the same problem and been chasing for along time. I used a high quality code reader with realtime values displayed. I tested the fuel pressure and was in spec. Oxygen sensors would last about 1000 miles before quit working.

I replaced my fuel injectors with reconditioned and shortly after I had a fuel pump fail. My O2 sensor voltages were still low following replacement of the injectors. Once I replaced the fuel pump the voltage was 0.5 to 0.6 Vdc

It has been about 2500 miles and all seems ok, will check sensor values when I get a chance to see.

Long term fuel trim of +/- 25 sets check engine light