My car started revving automatically after being jump started.

October 2, 2011 12:01 PM

My car is revving automatically. Can I fix this myself?

I've got a Fiat Palio 1.6 (with fuel injection). I left the interior light on for four days which drained the battery. I got someone to help jump start the car and immediately the following problem started.

The car revs automatically from 1500 rpm to about 4000 rpm and then back to ~1500 etc. This happens every few seconds, independently of the accelerator pedal.

I tried disconnecting the battery and reconnecting it, but the problem persists. I checked the fuses and they all seem ok.

Any ideas of things I can try before sending the car into the dealer?

posted by Gomez_in_the_South to travel & transportation (10 answers total)

This happens when you're idling? If yes, it's something with the idle bypass valve methinks. Could be any one of:

1. Your idle bypass valve is sticking
2. The engine maps controlling the bypass valve screwed up
3. The stepper motor controlling the bypass valve is wonky or its electrical connection is a little wonky
4. Your engine temp sensor to the ECU is fucked, and you need to replace it. If your check engine light isn't on, it's probably not this.

Something tells me #3 is most probable and easiest to fix (you might've inadvertently torn the insulation on the bypass valve wire while boosting) . So, to rule that one out, pop the hood, look at the wiring going to the bypass valve. If the insulation doesn't look pristine, replace the wire entirely, or splice in new wire where it looks destroyed. If the connector looks somewhat standard, you should replace that too. You can pick up wiring, splicing equipment and the connector at an auto parts store.

I used to take care of all the electrical wiring on a race car team, and 90% of the time, the problem was a bad connector or a shitty wire.

If that doesn't do the trick, it's best to take it to the dealer, because armchair diagnosing can only go so far :)
posted by Mons Veneris at 12:16 PM on October 2, 2011

Yes, it happens when idling as well as when engaged in gear (it is a manual transmission).

I don't think any insulation was torn when connection the jumper/booster leads. I guess that it was some electronics that were damaged when connected to the other car. I'll try to find the bypass valve and check it's wiring.

The check engine light is not on.
posted by Gomez_in_the_South at 12:34 PM on October 2, 2011

The batter boost is just one thing that happened to your car. It also sat for four days. Perhaps something happened in your gas tank (condensation, sediment getting snorked up) and is now messing with your fuel injection. Perhaps moisture has condensed on an electrical connection and is either interfering with a sensor or with getting spark to the cylinders.
posted by zippy at 1:01 PM on October 2, 2011

Get the battery fully charged, first, or borrow a good battery and try it.

Check for dislodged vacuum lines under the hood, or hoses like PCV that
might have been disturbed when you got the jump when your car was
stalled.

The fuel injection duty cycle can interact with the battery voltage (low
battery voltage, slower actuator operation, so compensatory higher
duty cycle).

You might have to perform some kind of throttle calibration on the on the
the Engine Control Unit, as durng its lengthy lack of voltage it has almost
certainly reset itself. See if there is any procedure that is recommended in
the shop manual for this model of car.

Get a scan of diagnostic codes from the ECU, using something like an
ODB scanner. It might be able to give you a clue what was happening.
posted by the Real Dan at 3:01 PM on October 2, 2011

I've had this problem before with a completely different car. Because it only happened intermittently, it never got fixed due to the cost involved in diagnosing an intermittent fault. The general consensus at the time was that the ECU had been fried, probably by jump-starting and that it was interpreting signals from one or more sensors incorrectly. Replacing the ECU was too expensive (a second-hand one would have been cheaper, but these are often worse than the problem you started with).

To eliminate simple problems, go over every connection to the engine carefully and look for loose/faulty connections (as Mons Veneris says, these are the cause of almost all electrical gremlins in cars) before spending any money on mechanics. If you can't find anything, you need to take the car to a mechanic that can diagnose the ECU when the fault occurs.

Usually, fried electronics as a result of jump-starting happens to the car that is the donor, due to a surge from the alternator when the car being jumped starts, but this doesn't seem to be the case here. It's possible that the fault is unrelated to the jump-start.
posted by dg at 5:17 PM on October 2, 2011

the Real Dan: "You might have to perform some kind of throttle calibration on the on the Engine Control Unit, as durng its lengthy lack of voltage it has almost
certainly reset itself. See if there is any procedure that is recommended in
the shop manual for this model of car."

I think this is a good first step. The ECU probably needs to relearn/recalibrate the Idle Air Control (PDF) and/or throttle position sensor settings, due to losing power.
posted by ArgentCorvid at 5:51 PM on October 2, 2011

Unless Fiat is totally incompetent the ECM should have detected that the battery was disconnected, and set the learned idle air flow to a default value that is slightly higher than what would be expected to be seen in most of this engine's population.

The only recalibration involved would be to start the engine, let the it warm up fully, then put your foot on the brake, turn the A/C off, and put the transmission in drive. The ECM should step the IAC valve until the RPM is under control at the idle speed. It should remember this value for subsequent idling. On a manual push in the clutch instead of doing the auto trans drive step. Make sure you keep your foot hard on the brake pedal until it has learned the idle air.

This assumes that nothing else is wrong other than the dead battery.

What you are describing sounds more like an idle air leak, with the ECM shutting fuel off when it thinks idle RPM is too high (4000 RPM), then turning fuel back on when RPM falls low again. It is also possible that your engine has a slight air leak that the ECM had learned out which was lost when the battery went dead.
posted by rfs at 8:01 PM on October 2, 2011

You might want to get your alternator checked because it might be surging?
posted by dottiechang at 10:06 PM on October 2, 2011

I appreciate all the answers. I will update this thread when I find out what the problem was.
posted by Gomez_in_the_South at 12:22 AM on October 3, 2011

I started the car again this afternoon (about 16 hours after I had the problem) and it seems to be back to normal.

I'm a bit surprised as nothing else has changed yesterday evening, but I'm not complaining.

Thanks again.
posted by Gomez_in_the_South at 8:13 AM on October 3, 2011