Cuts out when coasting

[jeepnutz]

I have a 2002 Ford Explorer with the V8, it just recently started with a new problem. When accelerating everything seems to be fine but when I'm coasting either down hill or coming to a stop it cuts out..... anyone have an idea what this might be?
thanks

 

[swetrid]

Could be vacuum leak or iac valve, double check all the pcv fittings and hoses also.

 

[jeepnutz]

Thanks for the response. I'll try those tomorrow and post back with my results.....thanks again!

 

[Pontisteve]

The IAC valve acts as a dashpot when you're cruising down the road. It opens up all the way while cruising. This way, when you let off the gas to coast down, it slowly closes the IAC until you finally come to an idle. Without this design, the motor would stall as you quickly let off the gas. Since this is exactly what's happening to you, I would pull off the IAC and clean the carbon out of it thoroughly with carb cleaner, and small brushes or whatever. Clean it good and reinstall, and see if that cures your problem.

If not, look into checking the TPS with a voltmeter.

 

[jeepnutz]

Fantastic! This sounds exactly like the problem, since the other things I tried didn't work... however I had a mechanic run a OBD and it showed that my CAT was worn out. (probably part of the problem why my "check engine" light was always on). Not sure if a OBD would pick up a bad IAC.... Thanks for the feedback :

 

[Pontisteve]

IAC's dont' show up when pulling codes, as far as I know.

The engine has 4 oxygen sensors. 2 pre-cat, and 2 post-cat. The 2 precat ones adjust the fuel curve to perfection. The 2 post-cat ones generally are only for determining if the cat has cleaned up the exhaust, compared to the pre-cat readings. If the cat is bad or missing, the front and rear sensors would read the same, and the computer would know that no emissions reduction has been accomplished, so it will spit out a bad cat code, which is generally a rear o2 sensor code.

The catch here is the sensor. Is it actually detecting a bad cat, or is the sensor itself wrong? Could be either. You could probably check exhaust temps before and after with a laser temp gun to help determine a clogged cat, or even an exhaust backpressure tester could be used. Another simple way might be to swap the rear o2 sensors side to side. If the problem code suddenly switches sides with the O2 sensor, the problem must be the sensor since the cat didn't change sides too. If you swap sensors, and the problem code still comes up on the same side, there must be a cat problem.

Knowing the exact code your mechanic found would be helpful here.

 

[jeepnutz]

Well I can see you know much more about it then I do. I feel a little quilty to go back to my mechanic and get the exact code number... he seems to frown on me everytime I do so and come home and try and fix the problem myself. I don't know much about OBD's that a consumer can buy but I'm looking at this one. It may save me some money in the long run.

http://www.obdcom.com/index.htm

 

[Pontisteve]

I've looked at buying one like that before too...

http://www.scantool.net/

The ELM 327 chipboard that these guys use (possibly a newer design by now) is widely used. Just avoid the China copies. The software you use can change how good your results are here, so consider that too. Good news is that most of that software is compatible with any ELM 327 tool, so the brand software doesn't necessarily have to be from who you buy the tool from. Cheaper to buy it all at once though, and scantool.net comes pretty highly recommended I think.

What I don't like about these is that they generally do not have "enhanced" data. Enhanced means OEM type data, instead of the typical generic OBD2 data, which is lower resolution and doesn't cover everything. Get Enhanced any time you can. I think with scantool.net, you can actually add the enhanced data on, by buying it per manufacturer (GM, Ford, Dodge).

The great part about this setup is that a laptop (required) has a lot more computing power than most traditional scan tools. This makes the refresh and sample rate much faster, and also makes datalogging possible. This can be very handy. Lugging a laptop around in a car is not so handy though.

The Innova Equus line of scantools is worth a look too! It would be very handy if your scan tool did AC, ABS, Body Modules, Trannies, etc. and not just engines.

But without getting carried away, one of those OBD loggers is a cheap way into high speed OBD2 datastream. Buying one of these is money in the bank, as cars are not ever going back to the days of carburetors! And the OBD2 standard seems to be a long lasting thing. Can't go wrong with an OBD2 scan tool! Buy it.