So this makes no sense to me. The other day I took my z28 out for a lil spin. I probably drove around 50 miles that day on it.

It started up fine. The fast idle was set up to something like 1250-1300 rpms or so and then kick down to 550-600 rpms once its all warmed up. And it acted fine before I went out and drove it.

After I came home that day and was ready to put it in the garage, I notice it idling at barely 500 rpms and struggling to keep it there. I was getting ready to back it up into the garage and went to throttle it a bit (its a manual) and it stuttered quite a bit.

Once I got it into the garage, it seemed to smell perhaps a tad rich for some reason. It seemed to be firing on all 8 pistons.

I have no idea why when I came back it acted the way it did. I highly doubt the carb got loose and adjusted itself out of whack. What I should have done before I turned it off was check if it was still in closed loop. I mean I could still hear the MCS clicking, so I know the solenoid was working to some extent.

No codes popped up on the dash or anything. I'm wondering if it magically came out of closed loop or maybe my timing got messed up?? But I have no idea how or why.

What do you guys think?

 

Maybe your long drive knocked some debris in your gas tank loose and is now clogging your fuel filter.

 

It's unlikely your timing or adjustments changed, but you can check your distributor to see if it's still tight.

Visually check everything, every connector and vacuum line. Most likely something came loose or dislodged.

 

i vote vacum line

 

fixed.

 

Thanks for all the posts guys.

I know this might sound really ridiculous, but the carb was actually out of adjustment for the weather. And to be more specific, it was the idle air bleed that was out of adjustment.


When I was in the garage, I simply just started her up and took a vacuum gauge to it and hooked up a dwell meter. Before messing with anything, the car had a hard time idling and it appeared to not want to go into closed loop at all because the dwell was at like 12%. And it wasn't till then that I turned the IAB screw all the way down and out 4 turns that it started to vary dwell.

I was initially shocked and thought how this could make such a big difference. But I guess driving it in 30-40 degree weather can make THAT much of a difference in the mixture! I mean I adjusted it last summer with probably an average temp of like 70-85 degrees.

Speaking of which...I should probably get a new filter. I just have one coming off the hard lines on the 3/8" before it enters the fuel pump. Hasn't been changed in...who knows how long :X

Sounds like a good idea.

Yeah that's why I hooked up my vacuum gauge to see what the engine was pulling in along with the dwell. The cap feels real tight on there. I was almost scared if one of my plug wires were melted or something.

In the picture you can see it pulling in about 19" with about 44% dwell. So not BAD I suppose, right? I mean it would vary 18-19" vacuum with 40-50% dwell idling at 550-600 rpms. Does that sound like a healthy 305?

 

Yes, that's pretty good vacuum.

Glad you got it fixed but I'm a bit confused. The ECM will compensate the mixture due to weather, so it seems odd that adjusting the air bleed did anything.

 

Well I'm guessing because the air is more dense when its real cold out, i.e. 30F. And when I originally adjusted it last summer, the temperature was probably around 80F. So then density difference from 80F-30F=∆50F.


The density difference would be almost a 100th lb/ft^3 difference from the two temperatures. (.081-.074=.007 which is .01-ish lb/ft^3)

I'm just kinda looking it over and pondering about it. I don't see how that could be such a DRASTIC change. Yes, I would imagine the ECM SHOULD compensate for such things.


I'm sure someone with a more scientific background than me can chime in and have a better answer.


Simply because density is dependent on temperature and here on earth, elevation as well. I also think the air speed the engine is pulling in could have an effect on it as well. Oh and also the driving conditions when the car pulls air from highway speed or slower when it rolls up from the hot engine. Agh! Too many variables to think about!

http://www.engineeringtoolbox.com/ai...ity-d_771.html

 

If the O2 hasn't been replaced in a while it may be time. As it ages and approaches failure it will begin reading lean.

The Baro sensor is designed to compensate for changes in ambient. The ECM takes a reading from it on start up and adjusts blocks accordingly.

 

I actually just replaced the O2 sensor in it last summer. I think I put an NGK one in.

But yeah, that's right. The barometer sensor should also compensate for that. Technically there's like two I think. One on the left hanger close to the fan box and the other all the way towards the brake booster. But I think that one is basically used for manifold pressure since it has a vacuum line to it.

Is there a way to tell if the barometer sensor is going bad even tho it might not throw a code?

 

Correct. The baro sensor is near the fan. The other is called a VAC sensor and has the vacuum line (hard, thin plastic for clearer signal). Don't know if you can test it other than checking its reading with a scanner?

 

Now do you know what type of scanner I need to use? Like perhaps a snap-on scanner? Because the OBDII capable scanners with live data don't seem to work all that much. Like one generic one said my car wasn't even in closed loop even tho my dwell was varying.

In order to have varying dwell, don't you need to have the car be a high enough temperature and readings from the O2? Maybe it was just a crappy reader.

 

I use an Autoxray 6000. Don't know which others may work.

I wouldn't worry about troubleshooting the Baro sensor too terribly much. These pieces rarely go bad. Just make sure the dwell is varying correctly and responds to air/fuel changes-like choking the airhorn.

The coolant temp sensor threshold is pretty low for closed loop. The critical piece is O2 temp. It needs to get high enough to begin sending voltage signals.

 

Mk..I see. Well maybe I can find a scanner that will work with my car to just check it out.

And then we'll see if I have to make more adjustments this summer.