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'91 Sunbird. Sparkplug change = Check engine light?

Picked up a 1991 Sunbird at the end of this summer to use a a junk-college car.

I noticed that, when sitting idle, it would misfire. A lot. As in, I could use the dashboard to shake paint cans if the urge struck me.

I pulled the spark plugs (which were all kinds of horrible, cracked insulators, huge gaps, you get the idea) And replaced them with a shiny new set of Plats (As I’ve been told old GMs don’t like other types).

Now, my Check engine light won’t go off (code 33, Map Sensor out of range, high. Is the only error code it gives) and it revs really high at idle… but doesn’t misfire. (So I guess I “Fixed it”)

I’m pretty sure the MAP sensor isn’t broken (As the exhaust smells fuel-rich, which, I think, does mean high MAP…)

I did a few other things after the light came on (new air filter, it needed it anyway, and new PCV valve because… yeah, same reason)

As you may have guessed, I’m not exactly very car-savvy, but I do Google excessively…

Is this one of the ol’ “You need to disconnect your battery to reset the computer” type of things? Does a ‘91 Sunbird even have a computer that complex?

And, if so, Anyone have a good way of getting rusted terminals that some idiot stripped into smooth, round posts out of a fekkin’ battery?

(Actually, I’ll need to know that sooner or later anyway)

If not, what else could it be? Bad plug-wires? One of my hoses/tubes busted?

Thanks for any help,

A faulty MAP sensor could be causing the problems you describe. What the MAP sensor does is, it takes the place of the accelerator pump that used to be found in carburated engines, and replaces the vacuum advance that used to be found in engines that had a distributor. So if the MAP senor is faulty, it can cause the engine to have a fast idle and run rich.

You can try unplugging the MAP sensor to see if it improves the way the engine runs. When this is done, the computer will go to a default value for the MAP sensor and the engine may run better.

So, unplug the MAP sensor, see if the engine runs better. And since there’s a MAP sensor code, most likely the MAP sensor needs to be replaced.

As far as removing the rounded battery terminals, I’ve had great success using the Craftsman Bolt-Out damaged bolt/nut remover.


Make sure that the manifold vacuum source to the MAP sensor is connected to the intake manifold and that the tubing is intact.

A MAP sensor measures your manifold absolute pressure. With the key on and engine off (KOEO) it should read about 4.5 volts. With the engine running and under high load/low manifold vacuum/high MAP conditions, about 3.5-4 volts. With engine running and low load/high vacuum/low MAP, around 1.5-2 volts. As you can guess, a high MAP reading at idle will cause it to run rough and rich, with considerable exhaust odor.

Does the MAP sensor fasten directly to the intake manifold, or is there a vacuum line connecting it to the manifold? A rotten or broken vacuum line will do it. For a directly mounted sensor, the grommet it fits into must be good. If it is visually OK, the MAP sensors used on these cars are not very expensive. Otherwise you’re looking at some troubleshooting for an open or shorted wire somewhere. You will also get high MAP if the camshaft timing is off, from the timing belt jumping a tooth or two.

Unless by some chance you pulled a vacuum line or wire loose while doing the spark plug change, this work has nothing to do with your problem.

Well, I tried unplugging the MAP sensor, if anything, that made it worse.
With the MAP sensor plugged in, I’d turn it on, it’d start normal, rev way up for a while, then rev back down and smell gasy.

Unplugging the MAP sensor just made it rev high all the time, no eventual slowing. And same error code.

I’ll be sure to give those ratchet heads a try,


Unless by some chance you pulled a vacuum line or wire loose while doing the spark plug change, this work has nothing to do with your problem.