Grand Cherokee dying off throtle. Diagnose ?

livi · 08-10-2007, 11:40 AM

Jeep Grand Cherokee '96. V8. 110k miles.

Lately it has started a bad habit of dying when I come off the throttle at low speed, say taxing up to a stop light. I can usually start it right up and continue. Never happens when I am on the throttle at speed. Only when I let go coming up to a stop.

It starts up fine, with a nice and even idle. Drives well. I have noticed that it tend to happen mostly before the engine is completely warmed up and particularly if I have not driven it for several days or only for short unhealthy trips the last days. After longer trips (20-30 miles) it does not happen, although the idle is still lower than it used to be.

Someone told me it could be the 'idle engine' that needs cleaning out. I did that but it did not help. Another tip said that the V8 is very sensitive to batteries going bad, that the current to the 'idle engine' is not strong enough. I have no problem starting the engine from cold so I thought my battery is still good, but maybe I am wrong.

The car is otherwise serviced and maintained as scheduled and generally problem free.

What do you guys think? Any help much appreciated as usual!

berettafan · 08-10-2007, 11:59 AM

did you check for codes?

there are a couple excellent bbs for jeep products and, as you would expect, they are full if info on error codes and such. in our '01 JGC you turn the key on, off, on, off, on and the mileage display will give you codes. yours might be different. you can then look up the codes online.

livi · 08-10-2007, 12:04 PM

Did not think of that. Great idea. Thanks!

Rot 911 · 08-10-2007, 12:12 PM

Marcus, the Jeepsunlimted forum is the PelicanParts board of Jeeps. This link will take you right to the tech board for the ZJ/WJ Grand Cherokees and you can post your question there: http://www.jeepsunlimited.com/forums/forumdisplay.php?f=51

berettafan · 08-10-2007, 12:16 PM

that's the spot! those threads saved me several hundred bucks recently when a transmission error code came up.

livi · 08-10-2007, 03:32 PM

Thanks for your help - exactly what I need!

john70t · 12:03 AM

At first glance I'm leaning more towards:
1. A bad throttle position sensor(TPS-which gives the throttle butterfly angle to the computer), or......
2. A bad manifold absolute pressure sensor(MAP-which gives the engine intake vacuum requirements--very basic fuel/air mixture), or...
3. Unknown: vacuum leak in one of the hoses or internally, dirty/clogged throttle body due to backfires, worn plugs, bad computer, or ?????

The fact it runs better when warm/hot seems to me (at first glance) like there is a systemic lean problem[edit:especially off-idle] with the mixture when cold, which the computer is compensating after it goes into closed-loop mode (when the computer starts looking at the O2 sensor for minor corrections).

#1 can be checked by unplugging it (keyoffengineoff) and using an Ohm meter across the terminals while opening and closing the TPS/butterfly.
#2 can be checked by backprobing the signal return (keyonengineoff) while applying vaccuum with a Mityvac to the supply hose.

john70t · 08-10-2007, 11:16 PM

Not knowing any better, computers do lie sometimes......take with grain of salt.

livi · 08-10-2007, 11:57 PM

Thanks John!

lm6y · 08-11-2007, 06:52 AM

Being a '96 it's an OBD2 system so the key off/on a certain number of times won't work to retrieve a code.

I'm guessing that there are no Auto Zones in the south of Sweden to run a free OBD2 Scan. The only way to pull a code is with a scanner (if there are any codes).

Unfortunately my scanner is in North America, and your problem is in Europe. My scanner cable isn't that long!

I'm guessing it has the 5.2l ? Should be too old for the 4.7l.

livi · 08-11-2007, 07:05 AM

Thanks for offering though!

Yes, it is a 5.2 L.

I suppose my second guess, about the battery being a tad low, can be ruled out as an explanation.

Tobra · Tobra's Garage

Gotta get a reader, plugs in under the dash. I have seen them for $99, but as stated, you give them your credit card number and they will loan you one and many auto places in US. OBD code reader is a handy thing to have, especially if you have a few cars on which to use it.

livi · 08-11-2007, 07:19 AM

All this computerized electronic mystery stuff on modern cars is killing me. I have a tough time understanding the rather simple design of the Carrera, but this is like reading Greek to me.

Thanks so much for all your suggestions! I suppose it is just a matter of tucking in my tail and scoot off to the nearest Chrysler hacienda..

lm6y · 08-11-2007, 09:46 AM

You know Markus, This sparked a few brain cells that haven't fired in a while.

A few weeks ago, I replaced a battery in a friends 98 Dakota R/T with the 360 (5.9l). It wouldn't idle at all with the bad battery.

It also wouldn't start with out a jump from another vehicle though.

Got a known good battery you can put in there for a test? Worth a shot if you do.

lm6y · 08-11-2007, 09:52 AM

Forgot to add that Tobra's right. There needs to be a code reader right next the feeler gauges in the toolbox these days.

Look for one that has real - time sensor monitoring. That's helped me more than actual codes being set. Get to know what the data is telling you, and it will point you in the right direction.

Good luck with it!

livi · 08-11-2007, 10:40 AM

Thanks! That is good info!