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95 Geo Prizm "Bucking" Acceleration--4 Mechanics and still not fixed! Please Help!

I love this car, and she’s been great except for this problem that started up a few months back. I’m not really sure what else to do at this point, because I don’t want to just paying mechanic after mechanic, to not fix it. I got my permit with her when I was 15 (now I’m in graduate school) and I’m not ready to let go! Here is the complete story (sorry if it’s lengthy, I just didn’t want to leave any important info out, because a lot of work has already been done):

1995 Geo Prizm (~118,000 mi)

Aug 2010

-I noticed that at higher speeds on the highway, it started ?bucking,? only occasionally, while accelerating

-I took to mechanic (found on CarTalk files, had used a few times before) and he test drove it on highway (witnessed problem)

Said it was probably just spark plugs and wires, and that I could wait to replace

-A few weeks later, while driving on the highway, the ?bucking? was much worse, and it lost power (failed to accelerate) a couple of times, decreasing in speed from 70 to 20, before it would accelerate again

During this the engine light blinked on and off a few times

Sep 2010

-I took back to the same mechanic, he test drove it again (witnessed failing problem)

Said that it must be the distributor (he quoted me ~$400-500).

When I called back several days later, he said he couldn?t find the part because my car was too old

-I just took the car back at this point because I felt a little suspicious

Nov 2010

-I took the car to a 2nd mechanic (also found on CarTalk files), who test drove it (only local, and didn?t get it to ?buck?) and did a full diagnostic for $100

Gave me a long list of recommended repairs: clean grounds, replace battery and negative terminal, replace ignition coil, spark plugs and wires, and distributor cap and rotor?totaling over $800

-I had my dad check the repair history, and the spark plugs and wires, and distributor cap and rotor had been replaced by my dad?s trusted mechanic less than 2 years ago

-I took the car back and brought it to a friend?s boyfriend, who is a diesel mechanic

Replaced the battery (it was time) and cleaned all the grounds (but problem persisted)

Checked all the vacuum lines, unplugged them (problem persisted)

Added fuel stabilizer (problem persisted)

Took out the Throttle Position Sensor, tested it, it tested bad; bought a new one, it tested good?replaced it in my car (problem persisted)

Confirmed that the condition of my spark plugs and wires were fine

-In the end, he couldn?t eliminate the problem, but his recommendations were to check: fuel filter, fuel pump, and ignition coil, and to perform a fuel pressure and compression test

-So I took my car to a 4th mechanic (recommended by the parts supplier of my dad?s mechanic back in TN) and he test drove it (witnessed problem)

Thought it was the catalytic converter (it wasn?t the problem)

Tested the fuel pump/fuel pressure (fine)

Looked at the fuel filter (clean)

Found that the rear O2 sensor was faulty, replaced it (problem persisted)

Thought it was the distributor base, replaced it (problem persisted), so he returned the part and put the old one back in

Performed a compression test and all were within an acceptable level

-Now the problem has gradually gotten worse so that even when driving local, the car consistently bucks after driving for 5-10 minutes (once the car has warmed up)

In all of this time, especially with the check engine light blinking there must have been some error codes that someone has pulled from the computer. Everytime that check engine light goes on it records clues to problems. This should be an OBD-1 system which means actual code readers are harder to find. But there is probably a way to pull the codes without a reader. Maybe ask the friend’s boyfriend if he can look it up and do it for you.

If you can get error codes, post them. They should be in the form of 2 digit numbers. Just post the numbers - don’t bother with what anyone said about them.

Don’t use a mechanic who drives you car, experiences the bucking, thinks its spark plugs, and says you can wait to replace them. Crazy.

Just because the distributor cap & rotor are only 2 years old, don’t assume that there isn’t a problem with them.

Your two most likely issues are going to be spark or fuel supply. The fuel pressure was checked but was it checked just sitting static, or was it checked under load? (As for fuel filters, they’re so cheap that I hope a new one went on even though the old one “looked” ok). Many times ignition coils can be tested - yours needs to be checked while hot.

Although the blinking check engine light makes me doubt it, has anyone ruled out a transmission issue? Perhaps with the torque converter clutch? Is the bucking under heavy acceleration or light or either one? Will it do it if you really stomp on it enough to cause it to downshift?

Anyway, see what you can come up with for error codes.

The second mechanic I used said that the engine codes coming out all had a “5v” reference in common, and that’s how they justified reccommending that I replace every part that had anything to do with a 5V reference.
Ok, I just looked up the codes the second mechanic listed in the notes for the diagnostic:
41-TP sensor open or short code
22-ECT open or short code
12-no RPM signal code
24-IAT open or short code
31-map open or short

The TP sensor was replaced after this by my friend’s boyfriend, but I don’t know what those other codes refer to.

Thank you so much!

Also, it’s a pretty heavy bucking. And earlier, when it first started, it would buck more often when I was trying to accelerate quickly (but now it does it all the time after warming up). And no one has checked the transmission to my knowledge, but I think the transmission fluid was checked, but I could be wrong.

Those older cars may not have had a code that tells that the engine control computer, now called the PCM, is dead. It may be OK but the bucking is a bad sign.

The other possibility is with the engine mounts/transmission mount. Try rocking the car back and forth with the hood open and see if the engine moves a lot, transmission should be in park.

If the engine is rocking like crazy, the engine mounts need attention. That isn’t the official way to check them but it sometimes works.

I just read the post again and the forward O2 sensor could be contaminated. Those things caused loss of power in older cars and the driver would have to keep pushing the gas pedal down, trying to compensate and the car would then surge forward, responding to the throttle position sensor’s input. It would repeat over and over.

I favor changing the engine control computer if it costs less than $200. The forward 02 sensor will hopefully cost less than $100.

To attempt to check the computer, disconnect the battery for a minute, with ENGINE OFF. If the problem then goes away for a few days or even hours, the computer is definitely bad.

If possible, you might post the compression readings. There is an annoying tendency to have readings that are claimed to be acceptable (even mechanics claim this) when in reality those readings are not good at all.

What needs to be done is make absolutely sure there is no mechanical fault with the engine before throwing parts at it.

I know it might still be faulty, but the front O2 sensor was replaced within the past 2 years.
I have another friend who also suggested that it might be the ECM–thanks for the advice on how to check it!

Any popping back into intake? Only happens once in closed loop or at operating temp?

41-TP sensor open or short code
22-ECT open or short code
12-no RPM signal code
24-IAT open or short code
31-map open or short

Which ever mechanic noted these as all being on the same 5V reference circuit is right. Why this would lead one to say to replace everything having to do with that circuit I don’t know. To me, it just means find out what the problem is. I’m not saying that’s easy, but I’d say there’s a chance that this 5V circuit problem is your problem.

Has anyone bothered to inspect the wiring? It is possible, as pleasedodgevan2 noted that the PCM itself if bad. But you might just have a chafed wire somewhere. This can be a difficult hunting expedition. You say that you’re a grad student (but not in what) - perhaps now is a good time to become acquainted with a voltmeter if you’re not already.

So, I took a little hiatus in figuring my car troubles out during the several snow storms that hit the northeast this past month, due to the fact that my car was parked (off street, luckily) encased in approximately 4 feet of snow.

Now that we’ve had a few warm days, I’ve been able to get my car out and drive it. Here’s the strange thing: it’s a lot better. After roughly 2 months of sitting, most of the time under a pile of snow, it’s gotten a lot better. It takes much longer for the “bucking” problem to appear (20+ min driving, as opposed to 5-10), and then when it does, it’s much more subtle, and inconsistent (every once in a while, while accelerating, rather than constant) than the last time I drove it.

I tried disconnecting the battery to test the ECU. Here’s what I (actually, my boyfriend) did: engine off, disconnect negative, disconnect positive. Then we tried to drain the engine of any electricity (what one website said you needed to do, because the engine could store residual electricity) by pressing brakes, turning on the lights, etc., but it already had no power at all. So we left it for about 30 min or so, then came back and reconnected (positive first, then negative) and took it out on the identical drive I did before, and it performed the same. Much better than 2 months ago, but still had intermittent, subtler bucking that started after 20+ min driving.

I was thinking about calling a Toyota dealership, because they might have better access to spare ECUs, and asking how much it would cost to check the ECU. But now I’m not sure.
I also don’t know what the fact that it’s better after sitting for 2 months indicates about the problem either?

Any further ideas?

Sorry I let it slide for the past month, but all your advice was really helpful, and I’d love any more feedback and advice about what to do about this mystery!!!

(and I’m a grad student in Biology, so no, I’m not all that familiar with a voltmeter, unfortunately!)

I would highly recommend that you replace the spark plugs, the spark plug wires, the distributor cap, and the rotor in the distributor cap all at the same time.

They are over 2 years old, and will display the exact issues you are having when the wires start to go bad.

You can go the extra mile, and have a compression test done on the motor, and also listen to all 4 fuel injectors to ensure that they are all making the proper clicking noise when the injectors are being opened and closed, but the important thing is to cross off the spark plug wires as being the culprit.


Sure sounds like Catalytic converter. A bad catalytic will kill an O2 sensor.

One more last cheap resort before doing any real work. Look for the ground wire from engine to frame or fender. It connects from metal on the engine to the car body. It is sometimes near the alternator. Remove the body end and wire brush or scrape the rust/dirt/paint on the end of it. If there is paint where it connects, scratch some of it off the body/fender and reconnect it.

That type of ground can cause problems if there isn’t a good connection. Old Toyotas used to stall if the AC was turned on with a bad ground.

Is the check engine light still on and are all of these codes still present?

41-TP sensor open or short code
22-ECT open or short code
12-no RPM signal code
24-IAT open or short code
31-map open or short

Has anyone checked the wiring - especially that 5V ref circuit?

Ok, some answers and more questions!
The catalytic converter was checked (replaced) by mechanic #4 and it didn’t fix the problem.
A compression test was also done by mechanic #4 (I don’t remember the numbers, something about 200, 180, 180, 200–or inverted, 180, 200, 200, 180–but I’m pretty sure he said it was slightly lower in the middle two, and that that was normal)
Also, the check engine light is NOT on. It only flashed on a few times when I had a couple of really bad drives on the highway, but then it’s been off since then.
I don’t know if those codes are still in the ECU, but the most recent thing I tried was resetting it, by disconnecting the battery, so if that was successful, I would assume no.
And the third (friend’s boyfriend) mechanic visually checked all the wires and vacuum lines, and said they looked fine (no cracks, etc), but I don’t believe he checked them with a voltmeter. He also said my spark plugs and wires looked completely fine.

I will try the ground wire as soon as I get a chance and post the results.

I am still thinking about the ECU, so I was considering buying a cheap one from ebay, and replacing it myself. The major problem I can foresee with that is if the ECU has broken other components in the car, it might still drive poorly, even if I fixed the root cause. Does anyone have any advice about the possibility of taking it to a mechanic (try out a 5th one this time) or a Toyota dealer to check it out and replace any bad components after I’ve replaced the ECU? Or should I just take it in and let someone else do the ECU (and pay way more)?

Has anybody warmed the motor up and put it on a scope?
And was the coil replaced? The igniter?

A scope will show the mechanic the actual waveforms to the sparkplugs. It’ll show voltages, duration, ramp times, waveforms (normal or poor), erratic pulses, and lots and lots of good stuff. A tech could even use a heat gun on the components to see if it has any effect on the spark pulses.

And, was the fuel line pressure monitored while the engine was acting erratically? If there’s something wierd going on, like if the charcoal canister is saturated and the tank can’t breath in readily causing a vacuum to develop and cause the fuel pump to struggle as the gas is pumped out, you’d need to be monitoring the fuel pressure to find that.

Your symptoms are typical of an ignition that’s become heat sensitive or of difficulty maintaining fuel supply. Proper diagnostics will determine which is at fault, or if it’s something else like an erratic CPS. I hate to say it, because I know you’ve tried a number of shops, but you need to find a good diagnostician.

And I’m also wondering of that recommendation to change the coil was correct and ignored.

With regards to the coil, it was suggested by the second mechanic, but when I asked the fourth mechanic about it he said that my car didn’t have a separate ignition coil, that the coil was the same as, or part of the distributor. The fourth mechanic did try replacing the distributor base (which he said in doing so, he was effectively replacing the coil too), and the problem persisted.

Also, the fourth mechanic checked the fuel pressure and said it was normal, but I’m not sure under what conditions he checked it. I know the car was driving bad with the fourth mechanic, but I’m not sure if he was trying to get it to “buck” while checking the pressure, because I don’t know how that test is done.

I haven’t heard of this scope thing before, so I’m assuming no one did this on my car yet. Is this something that I could do myself if I had the tool?

I know I need to find a good diagnostician, but I’ve been putting it off because I don’t want to waste more money/time and have my car still not fixed by yet another mechanic. Does anyone have any suggestions for really good, trustworthy mechanics in the greater New Haven, CT area??

Your car, which is a rebadged Toyota Corolla, does indeed have the coil mounted under the distributor cap on the distributor base. The coils are individually replaceable, but it requires removal of the distributor and many shops just replace the whole assembly. Since that’s been done, we’ll move on.

A “scope” is an oscilloscope. They actually show a voltage “trace” over time, an actual signal. When the coil fires its voltage spike to the spark plug, if it’s healthy the “baseline” will be clean leading up to the discharge, a clean spike will occur reaching a specific “peak” voltage, that will endure for a specific time, and it will decay in a specific time and manner. And the pulses will occur with consistance every so many milliseconds depending on the speed of the engine.

A scope can measure voltages, currents, events, and all manner of stuff. These capabilities can be used to check voltage to a coil primary and all mode of stuff electrical.

Old scopes do these things by ultimately controlling a sweeping stream of electrons fired from an electron gun onto the phosphor-coated back of a cathode-ray tube, but new technologies enable other methods of display. To use one requires training and an understanding of exactly what’s happening in the scope itself. As you can perhaps tell from even my very rudimentary description, it’s not something intuitive.

I have no recommendations for a good diagnostician, but can suggest that you ask everyone you know and work with. Perhspa one of them would know.

I finally took my Prizm to a fifth mechanic (SKF Auto in New Haven CT–HIGHLY recommend!!) and Enrico did a lovely diagnostic job and found the faulty crankshaft position sensor. He spent 4 hours of labor, using the oscilloscope, testing the signals of different components, finally finding this sensor, after trying a couple other parts that didn’t end up being the problem. Just wanted to let everyone know my Prizm runs beautifully now. And thank you for all the advice!

I never knew that a car with a distributor would also have a separate crankshaft position sensor. How odd.

I’m glad your car is back to running perfectly.