Funny Problem, Limp Home Mode Solves Everything

[jimilane]

Special problem...

This is on a 1990 LX

Car will not start the first time when cold. It takes a few attempts to start, then it starts fine. but when the O2 get going, the car wants to die...and it does sometimes.

If I pull off the MAF sensor power, the car goes into "limp home mode" and runs fine. If I start with a cold car and just pull the MAF sensor connector loose, the car will start and idle perfect. If I shut it down and reconnect the MAF sensor, then engine stumbles and you can hear it sucking air. Connecting and disconnecting the MAF is only to get it in Limp Home Mode. The sensor works fine and everything about the sensor tests fine.

Disconnecting the EGR sensor put the computer in limp home mode too.

It's my understanding that if the check engine light is on, the computer is in limp home mode. Please correct me if I'm wrong.

My questoin is....

What sensors are cut out of the loop when in limp home mode, and which sensors are operating.

It's really funny how it idles perfectly with the check engine light on. Then when you fix that, it runs bad.

Any help will be appreaciated!!!!

Thanks

Jim

 

[jrichker]

Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Walmart.

Or for a nicer scanner see http://www.midwayautosupply.com/detailedproductdescription.asp?3829 – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $35.

 

[jimilane]

I use an Actron Scanner the $400 one. It works great.

The only trouble code I have are ones for stuff that isn't there or disabled. I have other Mustangs, this one is giving me a fit.

I have no air machine, so that comes up with a few "codes". The EGR is blocked off at the heads. The EGR position sensor is working fine. I'm still gettng a code 84 and that's it for Key on , engine off.

I' need the help with what sensors are taken "out of the loop" when in limp home mode. I think that will help me figure this one out. The car starts and idles fine in limp home mode. It has to be a sensor that is put back in line when I reconnect the MAF. I have tried to find information on this for a few days now. I don't have the shop manual for this car so that makes it harder.

Thanks,
Jim

 

[jimilane]

Hey Guys,

If you run the key on, engine off test without the car warmed up. Shouldn't you get some codes for the Intake Air Sensor and the Coolant Temp Sensor??????

I just did a "cold" code pull and found no problems.....except the code 84.

I think there should be other codes for the ACT and ECT sensors....

What do you think?????

Thanks,
Jim