Stinks, vibrations, and loud noises in a 1996 Geo Prizm

Hi all,

I’ve been getting into more DIY since my handle was broken a while back and I fixed it, even diagnosing and fixing a broken brake/shift interconnect switch, power windows, and (still) working on the A/C on a friend’s Mercury Grand Marquis. Due to the recent successes, I now wish to fix my own car, (which is a 1996 Geo Prizm)! However, my problems are threefold.

The first is a bad smell that comes out of the air vents whenever the A/C is NOT on (unless it has rained recently, then the A/C stinks too) and sometimes when the heat is on (though it is slightly different, and more chemically-smelling). The worst is the rain smell and the no A/C or heat smell.

The second is a lot of vibration coming from the engine, that causes the whole car to rattle including loose dash parts and glove compartment items, pets, and even people. While it’s a generally bearable problem, I would really like to fix it as the noises can drive me insane…

The last issue is the loudness of the engine. While it was a nifty feature early on when I could make my friend’s Toyota Camry seem like a toy car in comparison, it makes highway driving much less enjoyable, especially once it reaches 70 MPH, and since I’ll be going to college (UCF!) relatively close to home, there is a possibility I might be doing more long distance highway trips, which makes this a high priority item! I do have an idea of what might be wrong, though. I hear a loose exhaust manifold could be the problem.

Thank you all who have read this far! Any tips, advice, tests, or solutions for any (or all =) of the problems are greatly appreciated!


I think you have a broken motor mount. It is common to this car in combination with your age and desire to outperform other small engine cars. It is easy to fix and diagnose. Google or whatever it.
The stink I have no idea.

Smell 1: Sometimes mold grows on the AC evaporator. Spray a lot of Lysol into the air inlet while the fan is on maximum. If it’s a constant problem, you can reduce it by turning the AC off (but blower still on high) a minute before you shutdown each time.

Smell 2: You could have a small heater core leak. You should have it pressure tested. Breathing anti-freeze is bad. If you can’t fix it, then at least bypass the core.

Noise: Do you have an aftermarket exhaust and/or intake? If not, have you looked for exhaust leaks?

Vibration: When were the plugs and other normal turn-up items last done? Motor mounts should also be checked.

Agreed. I think the vibration is an engine mount. PS - This can cause the exhaust manifold to crack, which would account for the loud noise, especially in a car that has an aged exhaust system.

Thanks for the info, guys. I’ll see what I can find on fixing those.

I read about the mold thing, but I heard the Lysol doesn’t last that long, do you know of any solutions that work for longer periods of time? I’ll try the blower thing, though.

How do you do a pressure test? Does it, per chance, use the same equipment as an A/C pressure test? Also, one day with the heater on I noticed a lot of…not really smoke but air that was quite visible in the sunlight when the smell was the worst.

No, I have the originals for both. My dad told me to feel above the exhaust manifold for air leaks which I did and felt something, and then to try plugging the tail pipe with a potato and seeing if it kills the car or not. Are there other ways of testing? Could it be the intake?

I’m honestly not sure, but the last time I had it in to the dealer for a regular tune up and other repairs was about 4 months ago, but the vibrations have been there for at least two years.

Thanks for the help!

Just to follow up for anyone who happens upon this and has the same question (I hate how there’s never a follow up!):

The stink just hasn’t come back, but I figured out that it was because at the time I was doing mostly night driving where dew would form on the evaporator and cause any mold/fungus/bacteria inside to release a smell, but blowing regular air seems to dry it out fine (however, a more permanent solution is a spray to kill these things, though I never tried it).

The vibrations, my mechanic confirmed, were from bad engine mounts. While there is no visible damage to them (i.e. cracks, tears, dryness, etc.) they told me you can tell as the engine moves around more than it should when they jerk it a little (or something like that =P).

The loudness hasn’t been diagnosed yet but they suspect it might be a small crack in the exhaust (they recommend against plugging the tail pipe as this can make any crack blowout).

Hope that helps anyone who stumbles upon this!

A really late, second follow up. It turns out some stink actually was a heater core leak. Whether that was present (or maybe just imperceptibly so) before, I’m not sure. But I had to replace the heater core on my car as the windows were fogging up at night and it smelled horrible with the heat on. However, it was extremely difficult getting the heater core housing out myself. If heat doesn’t matter to you (which it did not matter much to me, and if I knew how hard it was I would do this…), just bypass the heater core.