Avalon Transmission Problems?


diceccon
11-09-2010, 03:14 PM
I have a 1998 Toyota Avalon with 75,000 miles and earlier today what sounded like a slipping transmission began. I bought the car about 3,000 miles ago, so changing the transmission fluid wasn't quite present in my mind. While trying to shift, it will lag and eventually make a popping noise. When I rev the engine in neutral, everything sounds fine. The popping sounds like it's coming from the back of the car, but I'm afraid to go out and test it.

What should I do about this? This is my first car and it's pretty vital to my job.
Also, I have insurance, but I don't think that it covers this kind of thing.

Brian R.
11-10-2010, 10:15 AM
Have you changed the transmission fluid?

jdmccright
11-10-2010, 01:29 PM
First, check the transmission fluid level and its condition. If it is low, look underneath for any leaks. It may have been topped off when you bought it, but leaked out as you drove. If it is still full, then check its color and odor.

Fresh transmission fluid will look a clear red or slightly orangish color. If it looks more pale red, clear, gray, or brown then it is due to be changed. If it has turned opaque, then it has been oxidized due to overuse...definitely time to change it!

Next is its odor. Transmission fluid doesn't smell great when fresh but it does have a distinctive odor compared to engine oil. Buy a quart and take a whiff to get familiar. Overused fluid will have a pungent, burnt smell to accompany its worn out color. Again, this is the sign of fluid left in too long...change it!

If this is your case, I will suggest you do NOT go and have your transmission power flushed in hopes of getting rid of all the bad fluid immediately. This procedure has been known to cause transmission failures shortly thereafter. Instead, have the tech change the fluid and in-pan filter. Drive the car for a few hundred miles, then have the fluid changed again. A few hundred more, and change it again.

Doing it this way allows any built up particles and varnish to slowly be removed or redissolved back into the fluid. Power flushing causes these particles to become dislodged en masse and pushed into the intricate valves, seals and passages inside.

Good luck and let us know how you fare.

diceccon
11-10-2010, 02:32 PM
I took it to a mechanic, and after further self-inspection I determined that the problem was more likely the bolt holding the engine in place, which made sense with the various problems due to the shaking upon acceleration. I'll post what it actually is once I find out, but for the moment we're not looking at a transmission problem.

jdmccright
11-10-2010, 02:39 PM
It could be a broken engine or transmission mount, but it could also be a broken flywheel flexplate. Let's hope for one of the former.

diceccon
11-10-2010, 10:57 PM
Ok, I just got it back from the mechanic's and it turns out the problem was in the ignition system. Apparently the electrical systems for two of the cylinders (out of six) were faulty, and the sparks were not firing. All in all everything cost me $450, though looking at the parts I was charged nearly $10 for a single spark plug, pretty steep but oh well I'm glad to have my car back so quickly.

Why would my car be making deep mechanical popping noises with acceleration and what seemed like trouble switching gears, from only an ignition problem?

jdmccright
11-11-2010, 08:28 AM
Yet another case where it is hard to diagnose from afar...a noise can be described a thousand different ways.

I don't know how your engine's electronics are laid specifically, but there are sensors galore to help the engine's computer, called the electronic control module or ECM, keep tabs on what's going on under the hood: how much air is going in, at what temperature and pressure, when to squirt the gas and how long to squirt, firing the spark plugs at the right millisecond when the crankshaft is in just the right position while spinning at 3,000 rpms, then also making sure no fuel is wasted, and any that is is taken care of by the catalytic converter, etc, ad nauseum. In short, it is a complex electronic dance that can easily be disrupted by one malfunctioning part.

In your case, it could have been caused by failing spark plug coils (they can be pricey), though to have two fail at the same time is unusual. But having that happen throws off the timing for the other sensors, notably the vehicle speed sensor and the crankshaft position sensor that helps determine what gear you should be in for the engine speed, load, etc. The ECM will compensate the best it can to continue to allow driver operation albeit at a reduced power, speed, etc., in an attempt to limit any potential for engine damage.

Another possible explanation is that the electrical system that controls your spark plugs may fire more than one plug at a time simultaneously. This doesn't hurt the cylinder getting the extra spark and can help scavenge any leftover fuel. The caveat is that if that circuit fails, you've got not one but two cylinders not working well, if at all.

But we're glad you got an answer and thanks for posting back with the solution.

diceccon
11-11-2010, 09:34 AM
Thank you very much,
Apparently the car was in need of a major tune-up, I've only just received the car, and with only 75,000 miles I'm guessing that it's never had one. Indeed both of the spark plug coils failed, I have no idea why, but everything's over and it's back to normal, or better than normal because they performed the tune-up.
(You're right on the wasted gas - just before the problem really kicked into gear I was getting a dismal 17 mpg, where it should be getting around 25)

jdmccright
11-18-2010, 10:51 AM
If it's likely never had a tune up, then I would highly suggest you consider having the timing belt changed soon if not immediately. I'm not sure of the mileage interval for your car, but 75k miles is up there. Check your owner's manual supplement for the maintanance interval.

whopperdad
05-24-2013, 10:24 PM
Timing belt change is not due until 90,000 miles

Brian R.
05-26-2013, 06:07 PM
I doubt anyone interested is still reading this thread.

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