Sensitive accelerator response after 8 miles

This is about my aunt's car, but it's got me so scratchy-heady I had to join up here to see if anyone can guess the issue...

 

1992 Legacy L AWD

 

She told me every time she drives 8 miles, the engine starts to "jolt" or "chug." The Check Engine light's been on for who-knows-how-long. I'm thinking, "8 miles? Maybe that's when the car is properly warmed up."

 

Check Engine light--couldn't retrieve engine codes; replaced the O2 sensor anyway. Check Engine light goes away.

 

My aunt calls me back a couple days later: she drove the car 8 miles, the Check Engine light came on again and she's having the same symptom: "chugging." I ask her, "Is it hesitating when you try to accelerate?" She says no, it just starts to chug.

 

Check Engine light--this time we get a code 35; I replaced the purge valve circuit/solenoid. Check Engine light goes away (though it briefly gives me a code 22). Now I take it for a joy ride out to the country with my girlfriend as my witness. It's running beautifully, idling beautifully at stops. Then blamo!, I feel a sudden change in its response, look down to find the Check Engine light on, and the trip-meter is at 8 MILES!

 

I would not call the behavior "chugging" or "hesitating" or even erratic. Rather, the accelerator seems to become overly-sensitive: touch it and it jumps, lay off and it lags--like it's starving for gas. So, if you move the pedal at all, you get a jerking response.

 

Now, of course, comes the punchline. We get home, reset the Check Engine light, and it refuses to give up a code or stay lit.

 

Any ideas?

Something may be on it's way to give up (fuel pump or ignition coil is my first ideas), but also drain the tank because it can be water somewhere.

 

Temperature may play a role here - when something gets warm enough it fails.

 

It can of course also be a simple thing like a cracked hose or if you have an EGR valve that it sticks in open. Try to disconnect the EGR control hose/connector and see what happens.

^^^+1

 

Check all hoses. Vacuum leaks can cause all kinds of funky problems.

 

Also, try warming the car up in the driveway before you go for a drive. Start the engine and let it reach operating temperature at idle, just sitting there. Then when you take it out keep an eye on the trip-odometer and see if the problem still occurs after 8 miles. If it occurs immediatly after you start driving it the problem obviously has to do with the engine being warm, like a cracked EGR hose. If it still happens after 8 miles it might have something to do with a speed sensor.

 

Check the transmission. If its an automatic and the problem happens after 8 miles it may have to do with the speed sensor. The "Chugging" may be the transmission not shifting into 4th gear. If its stuck in 3rd throttle response would appear to be acute and inconsistent, and the car would decelerate faster when you let off the gas. When the problem starts to happen, throw the gear lever in "3" and see if anything changes. If nothing changes and you're driving at adequate speed, the car is stuck in 3rd gear. You could also try this with "2".

 

Another possibility is that there is an issue with the locking torque converter. The car is equipped with a locking torque converter, I'm not sure how they're supposed to work because I've read different things online but the one in my 92 Legacy locks in 4th gear above 45MPH once the car is warmed up. Anything other than the lightest throttle inputs will cause the torque converter to unlock, causing a slight jerk and a spike in RPM's. When the converter re-locks there is another jerk followed by a sudden drop in RPM's. Once the converter is locked the car will jerk slightly upon slight throttle inputs, and decelerate faster when the throttle is removed. Thus is the nature of the locking torque converter. You'll know when the converter is locked because the RPM's won't change at all according to throttle inputs, unless enough throttle is applied to cause the converter to unlock, which is when you'll notice a spike in RPM's and a slight jerk. It might not be what you're experiencing, and it shouldn't cause a check engine light, but its worth looking into.

In that case it may be time to check the oil in the gearbox as a first step. If the level is too low or too high it can cause trouble.

Thanks guys for your input. It's a stick, so I don't know if the torque converter applies, or the gearbox oil.

 

I forgot to mention in my story that, before going for our "joyride" I let the car idle while the whole family ate lunch, so it was plenty warm and purring like a kitten before we took off--idling at around 600 rpms.

 

I realize these details would have been very helpful, so sorry for leaving them out.

 

So... nice and warm... it drove perfectly for the first 8 miles, through various starts and stops, and on long stretches at urban highway speeds (40-50mph). Then, 8 miles, something triggered the issue.

OK, so it's not any torque converter because they are ONLY on AT gearboxes.

 

What it can be is speed sensor in the gearbox, but also check the oil level in the gearbox just in case. Notice that the gearbox won't easily heat up when you idle the car. OK, it will get some heat from the engine and some from the exhaust but it may not heat the location of the speed sensor.

 

However it may be something completely different that's related to the driven distance. Even something wrong with the ECU or a grounding issue.

 

And do measure the voltage in the car over the battery, it may be that there is something wrong with the alternator and as long as it's idling it won't go bad but when you are driving it suddenly turns into overvoltage and messes up everything.

I don't understand exactly what your suggestion is re the "voltage in the car over the battery." Please explain. Also, are you saying the alternator may be working erratically? And if so, is it something easily measured if I take it to an auto parts store, idling in the parking lot? Or would it always be generating "overvoltage" but only appearing to cause bad effects at certain times?

It may be that the voltage regulator of the alternator stops working and causes the system voltage to be too high.

 

Using a volt meter to measure the voltage over the battery should with the engine running give you something around 13.8 to 14.4 volts. Not much above 14.4 volts.

 

It's a simple test just to rule out that there is a problem with the voltage. Measure before driving and measure when the problem appears.

 

But when you get the problem what happens if you turn off the car and turn it on right away - has the problem disappeared for another 8 miles or is it still there?

If you don't have a volt meter or multi-meter go to Autozone, they'll help you out. Just make sure to have them check the voltage at idle and while holding the RPMs at about 3K RPMs or so.

 

 

And like ehsnils said, try turning the car off after the problem happens and see if it comes back right away, or only after you've driven another 8 miles.

Check the operation of the o2 sensor. Its not as common on OBD 1 vehicles as it is on the OBD ll to have the o2 sensor cause this type of problem , but who knows...it might.

One of the symptoms of an o2 sensor failing on an OBD ll is the engine will start to "stumble" after 8 miles , give or take a couple thousand feet. Almost like a timer. It may or may not leave a code . Not sure if the sensor on a OBD l will cause the same result or not , but its worth a look .

 

Good luck.

Ya know, I was thinking of this thread on my way into work this morning.

 

With a heated O2 sensor it would make sense that performance drops off after 8 miles. Worth checking out, I know its a little late to post this, but I figured I'd add my revelation.

Thanks for keeping with this. It's been a while and I still don't have answers, but I haven't heard back from my aunt. She's retired and doesn't drive the car that often. Our strategy is for her to drive it until it starts bucking, then turn it off and start it up again to see if the symptoms persist. I know that she's driven it a few times since, reached the 7.8 - 8 mile mark and it started doing its thing, she just hasn't pulled over and restarted. I must say though, it definitely seems to be the result of something being activated, not in the least gradual.

 

Also, the O2 sensor was the first thing I replaced, as noted in my original post. It's OEM.