Battery died, now my RPMs are way up

97 XJ, 4.0 AW4 190k miles. Stock except hellas, cb, 3" lift and tires.

Left my interior light on for 2 days...Jeep didn't start. I jumped it with my mom's car and drove it around. Volt meter was reading 14 when engine was on, and 2 notches under 14 when just on accessory (what it usually reads). As soon as I started it up, I noticed it was running high. Sitting in my driveway on park, engine somewhat warm, RPMs were 2500. In drive, it felt like it wanted to go and go, currently keeping around 1500-1250 (engine warm) at 35-40mph and idle. Definitely noticeable higher idle speed, and its harder to brake. I have to put the pedal to the floor to make the jeep stop moving. Any idea what's going on here?

updated symptoms from tonight: -between 10 and 0 when braking, it sort of lurches and then the RPMs shoot up a little, then go back down.
-from a stop, jeep will accelerate by itself (including upshift) even up a hill. I got to 20 before I had to slow it back down. Did NOT touch the gas pedal
-Idle in drive seems to be around 1300RPM (stopped at a light). In Park, it shoots immediately up to around 2300.
-faint smell of barbeque (literally smells like charcoal) coming from somewhere.

The ASE Master Tech in our club told me it was probably the computer resetting after the battery dying and it should go away after some running. he recommended idling it in the driveway for a few minutes. Tried this, it didn't go down.

I did search, didn't really find a lot on this but more on vacuum leaks.

i know you already tried letting it idle for a few minutes, but how long did you run it exactly? im just saying because it usually takes me around 7 minutes for my computer to reset itself and idle correctly after a dead battery. this is on my 97 4.0

I would agree on the PCM reset.

Have you charged the battery? After a full discharge you should charge it.

One problem with alternators--if there isn't enough voltage available it can't be energized to charge, unlike a generator.

If your voltage is still too low you will get erratic operation of things like, well, your idle.

Ah i didn't count, maybe 5 mins? enough time to move a car or two around and get ready to go where i was going.

I'll try that tomorrow but this thing is really sucking the gas outta me.

Anyone in the NAC got a battery charger i could borrow?

Gotta look at "cause and effect". If this all started after the battery incident, it most likely is the cause.

Draining an older battery can sometimes by the death of it. LOAD TEST your battery to be sure it is still good. Many parts stores will do this for you for free in the hopes of selling you a new one if you need it.

You really should buy a battery charger; that is something we all use at one time or another. They aren't too expensive.

Not sure if this is related, but I know with my jeep, and a friends jeep, that when the battery dies, the 4.0 does not like to run, until it gets up to temp, or about 7-10mins of running. Except, we have had the opposite problem... the 4.0 wants to die. We have had to sit in the vehicle, and rest a foot on the gas pedal to get it to warm up. once it warms up, they are fine.

I know on my jeep, I am running a good red-top optima battery, and I highly doubt that my alternator is bad. Im curious as to what snail mentioned: takes some time for a computer (PCM) to reset?

As much as I try to prevent a battery from draining, for some reason, when I have her in the garage for a few days, it just dies... not sure why... Is there a more cleaver way to reset this PCM than to just letting it run for 7 minutes??

An extended high rate of charge such as from a dead battery can play hell with an old braided grounding strap from the rear of the head to the firewall.

^^ X2. My SWAG would be that the engine is a little above the frame voltage wise right now, resulting in the TPS putting out a higher voltage than it should from the ECU's viewpoint, so the ECU thinks you have your foot on the gas. Try measuring the voltage from the frame to the engine block - positive meter probe on the block, negative on the frame. If you measure more than a few tenths of a volt try replacing the ground strap, you probably should anyways.

Thanks, will try those things myself if I don't see my mechanic tonight (got a few little things to fix for various reasons). Where on the frame do I put the probe?

My 2000 doesn't seem to be as finicky after sucking the battery flat. A good recharge, reconnect, starts up first time, no need to let it idle or do anything, runs perfectly.
Maybe they finally got something right in time to discontinue the range?

MahnkenVille said:

Thanks, will try those things myself if I don't see my mechanic tonight (got a few little things to fix for various reasons). Where on the frame do I put the probe?

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Anywhere clean and paint/rust free. I'd probably measure from an unpainted bolt head anywhere along the firewall.

Make sure you don't let the probe wires get into the path of the serpentine belt...

It did start just fine, not having the needs-gas-pedal-to-idle problem. It seems to have vanished mostly today. It did it this morning and afternoon, but on my drive home from practice it didn't. Parking it at the gas station, it did jump up to 1k in Park, but didn't do it again and it is perfectly normal in drive again.

problem solved?

Keep an eye on it and see how it goes. I'm thinking your battery is gaining charge and this may eliminate the symptoms.

Complete discharge is tough on a battery. Get a charger.

kastein said:

^^ X2. My SWAG would be that the engine is a little above the frame voltage wise right now, resulting in the TPS putting out a higher voltage than it should from the ECU's viewpoint, so the ECU thinks you have your foot on the gas. Try measuring the voltage from the frame to the engine block - positive meter probe on the block, negative on the frame. If you measure more than a few tenths of a volt try replacing the ground strap, you probably should anyways.

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SWAG! haha :thumbup:

Im going to check that out. Good info. Thanks!

EDIT: I'm assuming this test should be done when engine is running?

Correct.

For your information, that's a SCIENTIFIC wild-arse guess :laugh3:

kastein said:

For your information, that's a SCIENTIFIC wild-arse guess :laugh3:

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Yuppers! One of the few things I learned in college! :lecture:

Just thought I'd fill in on some results.

My friends XJ, that also suffers from stalling out after a drained battery, is now confirmed to have a bad ground connection. We wanted to just test it real quick, so I held a 12awg wire on the intake manifold and attached it to the negative post on the battery. Jump started it, had a second or two of hesitation, then ran just fine! Amazing... So, we both have to make a permanent fix soon...

Thanks so much for the info!