Effect of Barometric Sensor

Jeff Higgins · 03-18-2005, 10:39 AM

What are the symptoms of a barometric sensor on an MFI system starting to go bad? The reason I ask is that sometimes my car will "hunt" at idle. It idles just fine at around 900 rpm, then will spike up to a rather rough 1100 rpm, then drop back down and smooth out. I've checked the advance mechanism along with the vacuum retard and they both seem to work fine (I'm running a Pertronix Ignitor too, by the way). I've checked and cleaned the warmup solenoid and its hot air hose, and they seem fine too. If it's not the barometric sensor (I suspect it's not), what else should I check? MSD box maybe?

Grady Clay · 03-18-2005, 11:11 AM

Jeff,

I have never seen the aneroid fail but that certainly doesn’t mean it isn’t possible. I would expect the engine to quit running from way too lean (like fuel shut-off) if it did fail. I don’t think there can be a “partial failure;” it either works or it has totally failed from the chamber being compromised.

Many times when the idle hunts, the throttle bodies are trying to “settle in” to their rest position. Of course there can be all sorts of other causes. How much of the CM part of CMA have you done?

Best,
Grady

Jeff Higgins · 03-18-2005, 12:02 PM

Grady,
I've been through the whole CMA a couple of times just for fun and to familiarize myself with the system. Everything that is checked and adjusted there seems to be fine. When the idle started doing this, I did check to see if the rack fully returns to rest, and it does. Watching from the outside it does not appear that anything external changes or moves. If I hook up a timing light and watch the advance when this happens, it does advance as the rpms spike. I attribute that to the vacuum retard being overidden by increased intake flow, and centrifigal advance starting to kick in. When I mentioned in my first post that this happens "sometimes" I probably should have been more specific. It can go for days without this happening. I thought I was noticing a pattern, where it would not do this mid-day, in the (relative) heat and lower humidity, then it would start in the evening as the temp dropped and humidity came up. That's what led me first to the warm-up solenoid, which I dissassembled and cleaned, and now to the barometric sensor. It sounds like if that is an either working or not proposition, with no intermitant failure mode, I need to keep looking. Off idle, even when the idle hunts, it runs perfectly. You mentioned it could be a number of other things. Is there any order in which to look? I like to begin my investigation with cheap, and work my way towards expensive, if you know what I mean. Adjustments are "cheap", MSD boxes are "expensive", for example. Thanks for the help.

Early_S_Man · 03-18-2005, 12:20 PM

Jeff,

I doubt that the aneroid cell is the problem ... my best SWAG is that the throttle butterfly shafts and linkage are dirty, stiff, and binding. Further, when left idling for a lengthy time [more than just a few seconds] ... the vibration of the engine running will tend to 'shake' the throttles loose from the binding condition, and to a final, normal idling position at 900 rpm.

If the throttle bodies aren't spotless and clean ... I suggest that you remove them as a complete assembly, then soak the entire assemblies in Berryman's Chem-Dip overnight or longer, and rinse and blow out with compressed air. If you can't immediately tell the difference in how the throttles return ... repeat until you can!

Jeff Higgins · 03-18-2005, 01:16 PM

Warren,
Thanks, I'll take a good look at them. As far as I know, they were rebuilt with the engine a few years ago. They had about 15k on them when I bought the car, and I have subsequently put just over 20k on it for a total of about 35k. I'm not exactly sure what "rebuilt" means in connection with the throttle bodies. To me in means eliminating all slop by re-bushing, making sure all orifices are clear, they operate bind-free, etc. I keep in touch with the P.O., who provided a huge pile of receipts reflecting the work performed. So I suspect they are in relatively good condition, but a good cleaning might be in order anyway. The funny thing is, when the idle "hunts" like this, it goes both up and down. The way I understand what you are saying is that once they settle down (or getted sucked down) to a final, normal position, they probably can't just re-open on their own to speed the idle back up. Maybe if they are really rattling around in there they could; I'll have to look. Thanks.

Jeff Higgins · 03-21-2005, 05:36 AM

Quick update and another, hopefully related question: I cleaned up the throttle bodies, which were really pretty darn clean to begin with. It was fun and gave me something to do, but didn't solve the problem. The question is, what is the thing that looks like a sending unit that screws into the left side of the crank case ventilation cover to the right of the MFI pump? It must be MFI specific, as I don't see it in photos of CIS or carburated engines. The reason I ask is that the two wires connected to this unit had rotated down and were making (probably intermitant) contact with its brass base. When I corrected this little problem, all my problems with the hunting idle went away. I'm thoroughly confused. The guy I bought the car from told me it was an "rpm sensor" for the fuel shutoff solenoid. Checking my manuals, they show an "rpm transducer" in the electrical panel inside the left rear of the engine compartment. They also say it gets its signal from the points or coil. Looking at photos in Wayne's book on rebuilding engines, it appears as though there is nothing under the crank case vent cover but an internal baffle cast into the case halves, probably meant to keep oil from splashing up through it. So what the heck is this sensor?