Reader Question: S10 MAP Sensor Debacle?

Here’s the latest reader question,along with my reply! 

BaDnOn writes: I’ve had a problem for some time that I need to get back to correcting at some point. I can’t pass the (Phoenix) emissions test with my 1994 Chevy S10. The only problem it seems to have (indicated by thrown code 33) is the MAP sensor.

Here’s the kicker:  I’ve tried to replace the part about 7 times now, and none of the parts I buy or exchange work. And I don’t just mean they don’t fix the problem. I’ve tested every one I’ve bought, both on the truck and in the lab. The returned voltage simply doesn’t vary with pressure. The truck computer feeds it the necessary 5 volts fine, it just seems like the sensor just doesn’t sense.

Testing the part sans truck means pulling vacuum on the port while the part is being fed the appropriate 5 volts (from a transformer). I still haven’t found one that functions. Some don’t allow the truck to run at all, but if the (fixed) return voltage is in the proper range, the truck will start; but when it “learns” that the part doesn’t function, it defaults to some preprogrammed protocol and throws the check engine light and code. The truck runs well enough then, but seems to run rich, and won’t pass emissions. I’ve even built a MAP sensor, which actually works when adjusted right and allows you to drive, but it still needs some fine tuning and won’t quite pass emissions. Ever hear of anything like this? People think I’m crazy, but I can demonstrate!  ny advice?  I’ve even thought of just feeding an artificial atmosphere of 75 percent argon and 25 percent oxygen into the intake, so it would burn completely all the fuel, and not generate any NOx byproducts, but I’d need to hide a fairly large gas cylinder somewhere under the truck or something, haha.

My reply: It’s funny you should mention this. My friend Tim, who is a professional mechanic and runs a repair shop, was dealing with a similar issue with an ’86 Fiero recently. In the Pontiac’s case, it wasn’t an emissions problem but the problem was the same. A critical part (clutch linkage-related) had broken and the replacements – several of them – did not fit/function correctly. Apparently, GM had stopped making this particular part and the replacement/aftermarket piece was made so poorly as to be essentially useless.

This is not uncommon with cars from the late ’80s and early-mid ’90s that are getting pretty old but not yet considered “classics” and so haven’t got the aftermarket support that cars from the ’70s and ’60s, etc. have got.

He solved the problem by finding a supplier that specializes in Fiero parts and makes certain hard-to-get (or hard to get good) items like this one.

Electrical parts are of of course an additional problem. Have you tried finding a good used MAP sensor from another S10 or Chevy from the era with the same engine?

Another option – if it is one in AZ – is to get Antique tags for the a S10. In some states (mine, for example) you can get Antique tags once the car is 25 years old and once you do, it’s exempt from smog and saaaaaaaaaaaaaaaaaaaaaaafety inspections. The catch is you’re technically not supposed to drive it regularly but unless you literally use it for daily commuting you can “get away” with driving it pretty much whenever you like.

My ’76 TA – its single GM pellet-style cat long gone, with a modded 455 under the hood – would never pass smog, but it’s no worries because it’s an Antique!

. . .